
==== Front
J Org Chem
J Org Chem
jo
joceah
The Journal of Organic Chemistry
0022-3263
1520-6904
American Chemical Society

39189689
10.1021/acs.joc.4c01070
Article
Synthetic Efforts toward the Synthesis of a Fluorinated Analog of 5-Aminolevulinic Acid: Practical Synthesis of Racemic and Enantiomerically Defined 3-Fluoro-5-aminolevulinic Acid
https://orcid.org/0009-0002-9015-2413
Pashikanti Gouthami *‡
Chavan Lahu N. †§
https://orcid.org/0000-0002-2223-6618
Liebeskind Lanny S. ‡
https://orcid.org/0000-0002-3438-1185
Goodman Mark M. †§
† Department of Radiology and Imaging Sciences, School of Medicine, Emory University, 1364 Clifton Road NE, Atlanta, Georgia 30322, United States
‡ Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, United States
§ Center for Systems Imaging, Emory University, 1841 Clifton Rd NE, Atlanta, Georgia 30322, United States
* Email: gpashik@emory.edu.
27 08 2024
06 09 2024
89 17 1217612186
01 05 2024
15 08 2024
08 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by/4.0/ Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained (https://creativecommons.org/licenses/by/4.0/).

In 2017, the FDA authorized 5-aminolevulinic acid (5-ALA) for intraoperative optical imaging of suspected high-grade gliomas. This was the first authorized optical imaging agent for brain tumor surgery to enhance the visualization of malignant tissue. Herein we report the synthesis of a racemic and enantiopure fluorinated analog of 5-ALA, i.e., 3-fluoro-5-aminolevulinic acid (3F-5-ALA). We anticipate that these studies will provide the foundation for the future construction of a fluorine-18-labeled 5-ALA PET tracer to be used for functional and metabolic imaging of gliomas.

Winship Cancer Institute 10.13039/100011621 NA Common Fund 10.13039/100015326 1 R21 CA230108-01 document-id-old-9jo4c01070
document-id-new-14jo4c01070
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pmcIntroduction

Brain tumors represent a prominent category of severe and life-threatening malignancies on a global scale. Within this category, gliomas emerge as the most prevalent primary neoplasms within the central nervous system (CNS), constituting approximately 80% of malignant brain tumors.1 Their molecular and cellular heterogeneity promotes proliferation, invasion, and treatment resistance.2 Therefore, noninvasive imaging technologies that reveal a tumor’s molecular and metabolic information are needed for subtype-specific and individualized treatment for gliomas. Despite the fact that magnetic resonance imaging (MRI) is the clinical gold standard for imaging gliomas due to its superior spatial resolution and soft tissue contrast, its molecular and metabolic imaging applications suffer from low sensitivity.3 In contrast, positron emission tomography (PET) appears promising and offers insight into the molecular biology of tissues across the body.4 However, the currently used fluorodeoxyglucose (FDG) PET has low specificity and is unsuitable for imaging brain tumors due to significant FDG uptake in normal brain tissue.5a,5b In contrast, [11C]methionine is valuable for PET imaging in oncology, particularly for gliomas, due to its ability to trace tumor metabolism, but its short half-life poses logistical challenges.5c Currently, there is a significant unmet demand for highly diverse and infiltrating molecular imaging probes for gliomas.

5-Aminolevulinic acid (5-ALA) is the first natural precursor metabolite in the heme biosynthesis pathway; it is typically produced from succinyl-CoA and alanine.6 Protoporphyrin IX (PPIX) is a fluorescent red molecule that represents the final metabolite before heme production.6 During the synthesis of PpIX, 5-ALA is the rate-limiting intermediate step, and oral administration of 5-ALA causes selective accumulation of 5-ALA and PpIX within the pathological tissue, particularly Glioblastomas (GBMs).7 Its preferential accumulation within glioma cells is associated with decreased amounts of the ferrochelatase enzyme and impaired cellular clearance by an ATP-binding cassette transporter (ABCB6).8 It has been demonstrated that 5-ALA-induced PpIX fluorescence is highly selective (98%) for neoplastic tissue in patients with known glioblastoma multiforme.9 Exogenous administration of 5-ALA causes selective accumulation of 5-ALA and PpIX in gliomas, enabling tumor cell-specific intraoperative identification of the tumor margin for tumor resection with optical devices and photodynamic treatment of high grade gliomas (HGGs),10 both of which have been FDA-approved for the treatment of gliomas. Even though ALA-mediated PDT offers several benefits, it has been utilized mostly for superficial lesions due to poor light penetration within the tissue.11,12 The combination of PET imaging and a radiolabeled form of 5-ALA can overcome the constraints of optical detectability and provide insight into diagnostic and therapeutic techniques (for presurgical planning purposes) with 3D rendering capacity at the molecular level throughout the body.

The possible PET radioisotopes suitable for 5-ALA are 11C, 13N, and 15O. To date, Goodman and co-workers have successfully synthesized [13N] 5-ALA in high radiochemical yield (65%) and validated the rapid tumor-specific uptake of [13N] 5-ALA in rats bearing intracranial 9L glioblastoma.13 However, the extremely short half-life of the 13N tracer limits its use for further applications. The longer half-life of 18F compared to those of 13N and 11C (110.8 min for 18F vs 9.98 min for 13N and 20.3 min for 11C) makes it the ideal radionuclide for therapeutic PET imaging; hence, we envisioned producing a fluorine-18 derivative of 5-ALA. In order to develop a stable 18F PET radioligand of 5-ALA, fluorine-18 can be introduced by substituting hydrogen at the 2- or 3-carbon positions of the ALA core, as illustrated in Figure 1. There have been reports of nonradioactive forms of 2-fluoro-5-ALA (2F-5-ALA) and 2,2′-difluoro-5-ALA.14 Since 2F-5-ALA is an inhibitor of 5-aminolevulinate dehydratase (Ki = 0.5–2 mM), it can only be used to determine the production rate of PpIX and is not suitable for PET imaging. Based on the inhibitory effect of 2F-ALA to aminolevulinate dehydratase, we chose to define and synthesize a nonradioactive version of 3-fluoro-5-ALA (3F-5-ALA) and evaluate its optical characteristics to determine if fluorination at 3-position affects glioma cell uptake and metabolism to produce PpIX. In this manuscript, we disclose our synthetic efforts for constructing 3-fluoro-5-ALA and its enantiomers as a prelude to future disclosures of 18F analogs of 5-ALA.

Figure 1 5-Aminolevulinic acid (5-ALA) as the lead compound for 18F PET radiotracer development

Results and Discussion

Our first approach to 3-fluorinated 5-ALA began with the substitution of the bromine atom of the requisite functionalized levulinate 1 (prepared by a known protocol15) with nucleophilic fluoride (Scheme 1). However, many attempts to fluorinate bromide 1 under the required basic conditions, including (CsF/tBuOH, KF/18-crown-6/ACN, TBAF/THF, etc.) were thwarted by the formation of the olefinic byproduct via elimination of HBr, undoubtedly facilitated by the presence of the ester group.

Scheme 1 Attempts to Fluorinate Levulinate 1

Based on these initial observations, we concluded that the oxo and carboxylic acid functionalities of 3-F-5-ALA should be revealed later in the synthetic scheme to favor SN2 formation of the desired fluorinated product over the competitive β-elimination product since these activating groups enhance the formation of a double bond. We thus sought out appropriate protecting groups for the oxo and carboxylic acid groups that could be exposed post fluorination. After an extensive investigation, we were led to protectively embed both the amino and oxo functionalities of 3F-5-ALA within an oxazole precursor16 as depicted at the top of Scheme 2 (I–II–II). We anticipated a gentle fluorodecarboxylation17 to introduce the fluorine atom (bottom of Scheme 2) followed by a hydrolytic reveal of the 5-ALA skeleton.

Scheme 2 Decarboxylative Fluorination Approach

Thus, we began our plans with the construction of a functionalized derivative of oxazole that would be amenable to decarboxylative fluorination (7 in Scheme 2). In accordance with the existing literature, commercially available ethyl oxazole-4-carboxylate 3 was selectively brominated to give ethyl 5-bromooxazole-4-carboxylate 4.18 This underwent SNAr displacement with the sodium enolate of malonate 5, yielding the triester intermediate 6 in 76% yield. One-pot hydrolysis of the tertiary butyl esters of 6 followed by decarboxylation to the monoacid 7 proved challenging due to overdecarboxylation resulting in the formation of 9. Fortunately, when compound 6 was treated with a 1:1 TFA/CH2Cl2 mixture for 30 min at room temp, monoacid 7 was produced in 58% yield (Scheme 2).

With the requisite monoacid 7 in hand, we focused on the crucial decarboxylative fluorination reaction. Unfortunately, numerous attempts to fluorinate 7 with electrophilic17a,17b and nucleophilic fluorinating17c agents were futile, all resulting in the formation of the decarboxylative product 9. We postulated that the electron-deficient oxazole ring might activate the benzylic carboxylic acid 7 toward decarboxylation leading to 9.

As an alternative, a straightforward approach for synthesizing rac-3F-5-ALA using electrophilic fluorination of intermediate 11 with Selectfluor followed by decarboxylation under acidic conditions is depicted in Scheme 3. N-Cbz γ-amino-β-keto ester 10 (Scheme 3) is readily accessible in two steps from commercially available Cbz-glycine.19 Treatment of N-Cbz γ-amino-β-keto ester 10 with NaH and benzyl bromoacetate in tetrahydrofuran led to the formation of keto diester 11 in 62% yield along with the recovery of starting material. Keto diester 11 was fluorinated using Selectfluor in the presence of sodium hydride (76% yield). The resulting product was then refluxed with trifluoroacetic acid in dichloromethane to afford monofluoroketoester 13 in 85% yield. Finally, hydrogenation of the monofluoroketoester 13 in a mixture of THF/aq. 2 M HCl (4:1) solvent system furnished the desired rac-3-fluoro-5-aminolevulinic acid hydrochloride (3-F-5-ALA·HCl) 14 in 75% yield.

Scheme 3 Synthesis of rac-3F-5-ALA

Having established a successful route for the synthesis of racemic 3F-5-ALA, we then turned our attention to the construction of enantiopure 3F-5-ALA. A literature search revealed four examples describing the synthesis of 5-ALA by ring-opening of succinic anhydride with different nucleophiles (N,N-diphenylmethylidene glycine ethyl ester, KCN, ethyl hippurate, and methyl nitro acetate).20 Based on this precedent, it was anticipated that both enantiomers of 3F-5-ALA would be accessible via the regioselective ring-opening of the appropriate enantiomeric 2-fluorosuccinic anhydride 15 with N,N-diphenylmethylidene glycine ethyl ester in the presence of a base, followed by acidic hydrolysis to give 3F-5-ALA·HCl (14a or 14b), as depicted in Scheme 4.

Scheme 4 Synthesis of 3F-5-ALA Enantiomers from Fluorosuccinic Anhydride 15

The synthesis started with the preparation of S-fluorosuccinic anhydride 15 (Scheme 4), which is readily accessible from l-aspartic acid, in two steps.21 Unfortunately, all attempts reacting 15 with any of the nucleophiles20 mentioned earlier, including the use of different solvent systems and moderating additives (MgBr2 or ZnCl2), did not provide any discrete ring-opening products.

We next examined a model reaction adding ethyl isocyanate to succinic anhydride following Wilfred’s strategy,22 as depicted in Scheme 5. Interestingly, an oxazole intermediate was formed, which after hydrolysis with 6 N HCl, afforded 5-ALA in 64% yield for two steps (Scheme 5a). Thus, this set of reactions provides a new, high-yield synthesis of 5-ALA·HCl 21 from readily available, inexpensive starting materials. Unfortunately, fluorosuccinic anhydride 15 did not survive under the reaction conditions (Scheme 5b).

Scheme 5 A New Approach to the Synthesis of 5-ALA

Next, a different strategy for the synthesis of R- and S-3F-5-ALA was envisaged from 2-deoxyribose, as shown in Scheme 6. In this approach (the Scheme shows only one of the enantiomers), the enantiomers of 3F-5-ALA would be obtained by overoxidation of 27 using Jones’ reagent followed by reduction of the azide to the amine in an acidic medium (Scheme 6). The precursor 27 would be derived from the nucleophilic fluorination of 26, the latter produced from 2-deoxyribose in four steps.23 As in our previous strategy, the addition of various enolates to fluorosuccinic anhydride proved difficult (Scheme 4). However, contrary to the previous approach, protecting the 2-deoxyribose as cyclic acetal did prevent the elimination and provided the necessary framework.

Scheme 6 Proposed Scheme for the Synthesis of 14a from 2-Deoxy-d-ribose

As outlined in Scheme 6, the new synthesis commenced with the preparation of the known intermediate 25 in three steps. Thus, following the protocol detailed by Gottschaldt,23 commercially available 2-deoxy-d-ribose was treated with catalytic amounts of sulfuric acid in dry methanol, and furnished the cyclic acetal 23 (74%) as a mixture of α+β anomers. In practice, both anomers were utilized since the anomeric center is oxidized later in the synthesis. Conversion of compound 23 into the corresponding 5-O-tosyl-intermediate 24 (78%) was readily achieved under standard conditions, and this was then treated with lithium azide in DMF to afford the 5-azido-derivative 25 in 80% yield.23

Subsequent treatment of 25 with triflic anhydride in CH2Cl2 in the presence of pyridine at 0 °C afforded the highly unstable triflate 26, which was used without purification in the next reaction. Attempts to fluorinate triflate 26 using CsF/tBuOH24 or KF/18-crown-625 were unsuccessful, instead leading to the formation of unidentified products. However, reaction of the triflate 25 with TBAF/THF did produce the required product 27 but never in yields better than 10% (Scheme 6). Although we were able to access the fluorinated cyclic acetal 27 using this route, the poor reaction yield precluded its viability. We therefore investigated amino ribose derivatives 30a toward nucleophilic fluorination followed by oxidation, as detailed in Scheme 7.

Scheme 7 Attempts to Fluorinate Amino Ribose Derivatives 30a and 30b

Thus, tosylate 24 was treated with potassium phthalimide to produce the protected amino ribose derivative 29 (Scheme 7), which was then treated with triflic anhydride to produce the unstable triflate 30a, which could not be purified. Efforts to replace the triflate moiety of 30a with fluoride led to the formation of olefinic byproducts. We hypothesized that a more stable leaving group might survive under the reaction conditions and produce the desired product. Consequently, compound 29 was treated with TsCl in the presence of DMAP to produce a stable tosylate 30b in 60% yield (Scheme 7). Then, to obtain the desired fluorinated product 31, we explored the conversion of 30b under various nucleophilic fluorination conditions (CsF/tBuOH, TBAF/ACN at 80 °C and CsF/DMSO and KF/18-crown-6 in ACN 100 °C), but the reaction resulted in either recovery of 30b or formation of elimination byproducts.

Once again, we needed to turn to an alternative synthetic strategy. At this point, it was clear that elimination to olefinic products under the basic nucleophilic fluorination conditions was the most significant obstacle to achieving our synthetic goal. Consequently, we focused on developing an efficient alternative strategy for synthesizing enantiopure 3F-5-ALA derivatives by maintaining strictly neutral or slightly acidic reaction conditions. We thus turned our attention to construction of the ketonic moiety of 5-ALA using the pH-neutral reaction conditions of the Liebeskind–Srogl cross-coupling reaction as shown in Scheme 8.26,27

Scheme 8 Successful Synthesis of R- and S-3F-5-ALA derivatives (14a and 14b)

To explore this potential, we conducted reactions using fluoro thioester 35 with alpha aminostannane 37 (Scheme 8b). The required ketonic precursor 36 was prepared from the known alcohol 33, which can be obtained from commercially available benzyl l- (or d-) aspartate according to a literature procedure28 (Scheme 8a).

Fluorination of compound 33 in dichloromethane at −20 °C with DAST resulted in the formation of required product 34 in 48% yield along with the dehydrated olefinic product. However, switching the solvent from dichloromethane to chloroform and maintaining the reaction temperature at 0 °C throughout resulted in a significant improvement in the yield to 69%. Thus, compound 34 was synthesized in 3 steps with one chromatographic purification in 63% overall yield. Substrate 34 was subsequently converted into thioester 35 in a two-step sequence involving hydrolysis of the methyl ester and coupling the resulting acid group with 4-methylthiophenol using EDC·HCl (Scheme 8a). While we attained thioester in 39% overall yield over 5 steps, protection and deprotection of the acid in Scheme 8a were considered inefficient and unnecessary.

Therefore, we slightly modified the scheme by directly coupling carboxylic acid 32 and 4-methylthiophenol using DCC to furnish the thioester 36 in 75% yield (Scheme 8b). Treatment of alcohol 36 with DAST led to the formation of the fluoro thioester 35 (73%) with inversion of configuration (confirmed by X-ray crystallography as shown below). Benzyl tri-n-butylstannylmethylcarbamate 37 was synthesized from tri-n-butylstannylmethyl iodide in three steps following the literature precedent.29 Then, a pH-neutral Liebeskind-Srogl cross-coupling using 2 equiv of [copper(I) diphenylphosphinate (CuDPP, 2 equiv)27 in THF at 50 °C provided the desired ketonic coupling product 13a in 91% yield. Compound 13a was then subjected to deprotection hydrogenolysis in an acidic medium to furnish the desired R-3-F-5-ALA 14a in 75% yield. Separately, S-3F-5-ALA 14b was synthesized from benzyl d-aspartate using the same synthetic Scheme 8. The stereochemistry of the R- and S-3-F-5-ALA isomers was assigned by X-ray crystallography, as shown in Figure 2.

Figure 2 X-ray crystal structures of (a) R- and (b) S-3-F-5-ALA derivatives. Atom labels are as follows: white = hydrogen, black = carbon, red = oxygen, blue = nitrogen, light green = fluorine, and dark green = chlorine.

Conclusion

This study reveals novel and scalable synthetic methods for producing racemic and enantiopure 3-fluoro-5-aminolevulinic acid hydrochloride (3F-5-ALA). The synthesis of racemic 3F-5-ALA·HCl (14) involves a six-step process utilizing electrophilic fluorination, resulting in a satisfactory overall yield of 16% from N-benzyloxycarbonylglycine (Z-Gly-OH). Additionally, both enantiomers of 3-F-ALA·HCl (14a and 14b) were successfully constructed in a five-step procedure starting from benzyl L and d-aspartate, respectively, using a Liebeskind–Srogl cross-coupling as the crucial step. Notably, this enantioselective synthesis provided an overall yield of 34%. Moreover, we demonstrate a scalable approach for synthesizing 5-ALA from widely accessible, inexpensive starting materials in 2 steps in 64% yield. The biological evaluation of the 3-F-5-ALA derivatives will be pursued separately and published elsewhere.

Experimental Section

General Information

Unless otherwise noted, all reagents were used as received from commercial suppliers. All reactions were performed under a nitrogen atmosphere and in flame-dried or oven-dried glassware with magnetic stirring. All solvents were dried before use, following the standard procedures. Reactions were monitored using thin-layer chromatography (SiO2). TLC plates were visualized with UV light (254 nm), iodine treatment, or ninhydrin stain. Column chromatography was carried out using silica gel (60–120 mesh and 100–200 mesh) packed in glass columns. NMR spectra were recorded at 300, 400, 500 MHz (1H) and at 75, 100, 125 MHz (13C), respectively. Chemical shifts (δ) are reported in ppm, using the residual solvent peak in CDCl3 (1H, δ = 7.26; 13C, δ = 77.16 ppm) as internal standard, and coupling constants (J) are given in Hz. HRMS were recorded using ESI-TOF techniques.

1,1-Di-tert-butyl 2-Ethyl 1-(4-(Ethoxycarbonyl)oxazol-5-yl)ethane-1,1,2-tricarboxylate (6)

A suspension of sodium hydride (65.3 mg, 2.73 mmol, 2 equiv) in anhydrous DMF (1 mL) was stirred and treated dropwise at room temperature with a solution of diethyl malonate 5 (824 mg, 2.73 mmol, 2 equiv) in anhydrous DMF (2 mL). After 10 min, when gas evolution had ceased, the mixture was treated dropwise with a solution of oxazole 4 (300 mg, 1.36 mmol, 1 equiv) in anhydrous DMF (2 mL). The mixture turned orange and was stirred and heated at 100 °C in an oil bath for 1 h. The mixture was diluted with water and extracted with EtOAc (2 × 10 mL). The organics were then dried over anhydrous Na2SO4 and concentrated in vacuo to afford a yellow oil. The residue was chromatographed on silica, eluting with 10% EtOAc in hexanes (Rf = 0.5) to give the title compound 6 (457 mg, 76%) as a colorless oil. 1H NMR (600 MHz, CDCl3) δ 8.22 (s, 1H), 4.34 (q, J = 7.2 Hz, 2H), 4.16 (q, J = 7.2 Hz, 2H), 3.36 (s, 2H), 1.47 (s, 18H), 1.35 (t, J = 7.1 Hz, 3H), 1.25 (t, J = 7.1 Hz, 3H). 13C{1H} NMR (151 MHz, CDCl3) δ 169.5, 164.9, 161.0, 160.2, 144.5, 133.5, 83.8, 61.1, 60.8, 59.4, 38.5, 27.7, 14.2, 14.1. IR (neat): υmax 1733, 1369, 1141, 1109, 1023, 840, 734 cm–1. HRMS (ESI) Calcd for C21H32O9N (M + H)+: 442.2071. Found 442.2075.

4-Ethoxy-2-(4-(ethoxycarbonyl)oxazol-5-yl)-4-oxobutanoic Acid (7)

To a stirred solution of 6 (150 mg, 0.34 mmol) in CH2Cl2 (0.6 mL) was added CF3COOH (0.6 mL). After stirring for 30 min at room temperature, the reaction mixture was evaporated under reduced pressure at room temperature. The crude residue was recrystallized from Et2O/hexanes to afford the monoacid 7 (56 mg, 58%) as a white solid. 1H NMR (600 MHz, CDCl3) δ 8.21 (d, J = 1.5 Hz, 1H), 4.47 (t, J = 7.1 Hz, 1H), 4.41–4.30 (m, 2H), 4.19–4.11 (m, 2H), 3.23 (ddd, J = 17.4, 7.4, 1.9 Hz, 1H), 3.13 (ddd, J = 17.4, 6.9, 2.3 Hz, 1H), 1.36 (td, J = 7.2, 1.7 Hz, 3H), 1.24 (td, J = 7.1, 1.7 Hz, 3H). 13C{1H} NMR (151 MHz, CDCl3) δ 170.4, 160.8, 144.4, 133.7, 61.5, 61.4, 41.0, 34.0, 14.3, 14.1. IR (neat): υmax 1727, 1317, 1177, 1110, 1022 cm–1. Melting point: 76–78 °C. HRMS (ESI) Calcd for C12H16O7N (M + H)+: 286.0921. Found 286.0919.

4-Benzyl 1-(tert-Butyl) 2-(((Benzyloxy)carbonyl)glycyl)succinate (11)

To a stirred solution of 10 (1.0 g, 3.2 mmol, 1 equiv) in tetrahydrofuran (10 mL) was added to a suspension of sodium hydride (60% dispersion in mineral oil, 117 mg, 4.8 mmol, 1.5 equiv, prewashed with hexanes) in tetrahydrofuran (10 mL) maintained at 0 °C under a nitrogen atmosphere. After 20 min, a solution of benzyl bromoacetate (0.62 mL, 3.9 mmol, 1.2 equiv) in tetrahydrofuran (5 mL) was added dropwise, and the mixture was stirred overnight at room temperature. The resulting mixture was quenched with saturated aqueous NH4Cl solution and extracted with EtOAc (2 × 15 mL). The combined organic extracts were washed with saturated brine, dried over MgSO4, and evaporated under reduced pressure. The residue was chromatographed on silica, eluting with 30% EtOAc in hexanes (Rf = 0.4) to give the title compound 11 (963 mg, 65%) as a pale-yellow oil. 1H NMR (600 MHz, CDCl3) δ 7.41–7.31 (m, 10H), 5.43 (t, J = 5.1 Hz, 1H), 5.15 (s, 2H), 5.14–5.06 (m, 2H), 4.51 (dd, J = 19.6, 5.5 Hz, 1H), 4.24 (dd, J = 19.6, 4.6 Hz, 1H), 3.96 (dd, J = 9.0, 5.5 Hz, 1H), 3.08 (dd, J = 17.8, 9.0 Hz, 1H), 2.94 (dd, J = 17.8, 5.5 Hz, 1H), 1.46 (s, 8H), 1.42 (s, 1H).13C{1H} NMR (151 MHz, CDCl3) δ 200.2, 171.1, 166.3, 156.0, 136.2, 135.4, 128.6, 128.5, 128.4, 128.3, 128.2, 128.1, 83.5, 67.0, 66.9, 52.2, 51.1, 32.4, 27.8. IR (neat): υmax 3379, 1714, 1455, 1246, 1146, 697 cm–1. HRMS (ESI) Calcd for C25H29O7N23Na (M + Na)+: 478.1836. Found 478.1844.

4-Benzyl 1-(tert-Butyl) 2-(((Benzyloxy)carbonyl)glycyl)-2-fluorosuccinate (12)

To a suspension of NaH (60% dispersion in mineral oil, 75.6 mg, 3.15 mmol, 1.5 equiv, prewashed with hexanes) in anhydrous THF (5 mL) was added β-keto ester 11 (0.96 g, 2.1 mmol) in THF (10 mL) dropwise at 0 °C under argon. After 30 min of stirring at this temperature, the reaction mixture was warmed to room temperature and stirred for 2 h. Then, a solution of Selectfluor (0.82 g, 2.31 mmol, 1.1 equiv) in DMF (5 mL) was added through a syringe. After stirring for 12 h at room temperature, the reaction was quenched with water and extracted with ethyl acetate (2 × 15 mL). The residue was subjected to silica gel flash chromatography, eluting with 40% EtOAc in hexanes (Rf = 0.55) to give the title compound 12 (758 mg, 76%) as a colorless oil. 1H NMR (400 MHz, CDCl3) δ 7.41–7.29 (m, 10H), 5.22 (t, J = 5.3 Hz, 1H), 5.16–5.04 (m, 4H), 4.47 (dd, J = 5.2, 2.6 Hz, 2H), 3.46 (dd, J = 33.2, 17.7 Hz, 1H), 3.20 (dd, J = 17.7, 14.9 Hz, 1H), 1.46 (s, 8H), 1.40 (s, 1H). 13C{1H} NMR (101 MHz, CDCl3) δ 200.31 (d, J = 28.7 Hz), 167.9, 162.9 (d, J = 23.5 Hz), 156.0, 135.6 (d, J = 140.2 Hz), 128.9 (d, J = 18.3 Hz), 128.7, 128.6, 128.5, 128.4, 128.2, 128.1, 97.5 (d, J = 199.8 Hz), 85.4, 67.3, 67.0, 47.9 (d, J = 5.1 Hz), 39.4 (d, J = 21.6 Hz), 27.6. 19F NMR (376 MHz, CDCl3) δ −170.3 (dd, J = 33.2, 14.9 Hz). IR (neat): υmax 3409, 1732, 1519, 1249, 1154, 697 cm–1. HRMS (ESI) Calcd for C25H28O7NF23Na (M + Na)+: 496.1742. Found 496.1742.

3-(4-(Ethoxycarbonyl)oxazol-5-yl)propanoic Scid (20)

Ethyl isocyanoacetate (200 mg, 1.76 mmol, 1 equiv) in dry dimethylformamide (4 mL) was treated with a solution of 1,8-diazabicyclo[5.4.0]undec-7- ene (396 μL, 2.65 mmol, 1.5 equiv) followed by a solution of the succinic anhydride (228 mg, 2.2 mmol, 1.3 equiv) in DMF (2 mL) and the mixture was heated at 80 °C in an oil bath for 6 h. It was poured into water, extracted with ethyl acetate (3 × 5 mL), the extract was washed with water, dried over Na2SO4 and evaporated. The desired compound 20 obtained as a colorless liquid (271 mg, 72%) was carried forward without further purification. 1H NMR (800 MHz, CDCl3) δ 7.76 (s, 1H), 4.32 (q, J = 7.2 Hz, 2H), 3.32 (t, J = 7.5 Hz, 2H), 2.73 (t, J = 7.6 Hz, 2H), 1.33 (t, J = 7.2 Hz, 3H). 13C{1H} NMR (201 MHz, CDCl3) δ 176.9, 161.7, 157.7, 149.4, 127.4, 61.2, 31.3, 21.2, 14.2. HRMS (ESI) Calcd for C9H12O5N (M + H)+: 214.0710. Found 214.0709.

5-Amino-4-oxopentanoic Acid Hydrochloride (21)

To a stirred solution of 20 (271 mg, 1.27 mmol, 1 equiv) in THF (1 mL) was added 5 mL of 6 N hydrochloric acid and stirred at 100 °C. After 4 h the solvent was removed on a rotary evaporator to obtain a tan solid. The crude material, upon crystallization from ethanol/Et2O (TLC: nBuOH:H2O:CH3CO2H, 12:5:3, Rf = 0.3), provided the desired compound 21 (189 mg, 89%) as colorless needles, respectively. The spectral details are consistent with the reported literature.30

(2R,3R)-2-(Azidomethyl)-3-fluoro-5-methoxytetrahydrofuran (26)

The alcohol to trifluoromethanesulfonate conversion procedure was adapted from our prior work.31 To a solution of 25 (200 mg, 1.16 mmol, 1 equiv) and pyridine (205 μL, 2.54 mmol, 2.2 equiv) in CH2Cl2 (2 mL) was cooled to 0 °C. A separate vial containing trifluoromethanesulfonic anhydride (390 μL, 2.32 mmol, 2.0 equiv) in CH2Cl2 (2 mL) was cooled to 0 °C, and this mixture was added dropwise to solution of 25 with vigorous stirring. The mixture was stirred at 0 °C for 15 min, then diluted with hexanes (4 mL). A white powder precipitated and was filtered away, and the supernatant was concentrated at 0 °C to give 220 mg of crude 26, colorless oil and was used directly without further purification.

(2R,3R)-2-(Azidomethyl)-3-fluoro-5-methoxytetrahydrofuran (27)

To a solution of crude triflate 26 (220 mg) in THF (4 mL) was treated with tetra-n-butylammonium fluoride in tetrahydrofuran (1 M, 2.9 mL, 2.9 mmol) at 0 °C. The reaction mixture was stirred at room temperature for 2 h under a stream of N2. The mixture was diluted with water (10 mL) and EtOAc (10 mL) and the phases were separated. The aqueous phase was washed with another portion of EtOAc (5 mL) and the organics were collected and dried over Na2SO4, filtered, and concentrated. The residue was chromatographed on silica, eluting with 10% EtOAc in hexanes (Rf = 0.6) to give the title compound 27 (20.2 mg, 10% yield over two steps) as a colorless oil. 1H NMR (600 MHz, CDCl3) δ 5.25 (dt, J = 5.8, 3.0 Hz, 1.5H), 5.16 (ddd, J = 5.8, 3.2, 1.4 Hz, 0.5H), 4.16 (dddd, J = 26.7, 7.2, 5.5, 3.2 Hz, 1H), 3.61 (ddd, J = 12.9, 7.3, 1.6 Hz, 1H), 3.51 (dd, J = 12.9, 5.5 Hz, 1H), 3.40 (s, 3H), 2.48 (dddd, J = 30.6, 15.4, 5.8, 1.4 Hz, 1H), 2.26 (dddd, J = 30.4, 15.5, 5.8, 2.8 Hz, 1H). 13C{1H} NMR (151 MHz, CDCl3) δ 104.2, 92.8 (d, J = 183.2 Hz), 78.7 (d, J = 19.3 Hz), 55.5, 49.2 (d, J = 11.3 Hz), 40.8 (d, J = 22.3 Hz). 19F NMR (565 MHz, CDCl3) δ −192.03 (dtd, J = 56.9, 30.6, 26.7 Hz). IR (neat): υmax 2098, 1270, 1178, 1088, 984 cm–1. HRMS (ESI) Calcd for C6H10O2N3FNa (M + Na)+: 198.0649. Found 198.0651.

(2R,3R)-2-((1,3-Dioxoisoindolin-2-yl)methyl)-5-methoxytetrahydrofuran-3-yl 4-Methylbenzenesulfonate (30b)

To a solution of 29 (200 mg, 0.72 mmol) in CH2Cl2 (8 mL) at 0 °C, was added DMAP (107 mg, 0.86 mmol, 1.2 equiv) followed by TsCl (163 mg, 0.86 mmol, 1.2 equiv) and the reaction was allowed to reach room temperature and stirring was continued for 12 h. After the completion of reaction, the mixture was concentrated on vacuum and chromatographed on silica, eluting with 30% EtOAc in hexanes (Rf = 0.5) to give the title compound (186 mg, 60%) as a white solid. 1H NMR (600 MHz, CDCl3) δ 7.89 (dd, J = 5.4, 3.0 Hz, 2H), 7.76 (dd, J = 5.5, 3.0 Hz, 2H), 7.73–7.66 (m, 2H), 7.32–7.25 (m, 2H), 5.05 (dd, J = 5.2, 0.8 Hz, 1H), 4.94 (ddd, J = 7.7, 2.9, 1.7 Hz, 1H), 4.54 (ddd, J = 8.3, 5.6, 2.9 Hz, 1H), 3.82 (dd, J = 14.0, 5.7 Hz, 1H), 3.72 (dd, J = 14.1, 8.0 Hz, 1H), 3.34 (s, 3H), 2.42 (s, 3H), 2.24 (ddd, J = 14.9, 7.7, 5.2 Hz, 1H), 2.05 (ddd, J = 14.9, 1.7, 0.9 Hz, 1H).13C{1H} NMR (151 MHz, CDCl3) δ 168.2, 144.8, 134.1, 133.5, 132.0, 129.8, 127.9, 123.4, 104.5, 81.0, 80.3, 55.1, 39.2, 38.6, 21.7. Melting point: 120–122 °C (decomposes). HRMS (ESI) Calcd for C21H22O7N32S (M + H)+: 432.1111. Found 432.1116. [α]D20 + 680.77 (c 1.10, CHCl3).

4-Benzyl 1-Methyl (R)-2-Fluorosuccinate (34)

To a stirred solution of diethyaminosulfur trifluoride (0.277 mL, 2.1 mmol, 1 equiv) in CH2Cl2 (2 mL) cooled to 0 °C was added dropwise to a solution of 33 (500 mg, 2.1 mmol, 1 equiv) in CH2Cl2 (5 mL). The mixture was allowed to reach ambient temperature for 1 h and water (5 mL) was added cautiously to the vigorously stirred solution. The organic layer was separated, washed with sat. NaHCO3 and sat. NaCl, dried (MgSO4), and evaporated under reduced pressure. The residue was chromatographed on silica, eluting with 30% EtOAc in hexanes (Rf = 0.5) to give the title compound 34 (348 mg, 69%) as a colorless oil. 1H NMR (600 MHz, CDCl3) δ 7.43–7.33 (m, 5H), 5.34 (ddd, J = 47.2, 6.6, 4.8 Hz, 1H), 5.20 (dd, J = 19.1, 12.2 Hz, 2H), 3.04 (dd, J = 24.6, 4.7 Hz, 1H), 3.03 (dd, J = 23.5, 6.7 Hz, 1H). 13C{1H} NMR (151 MHz, CDCl3) δ 169.0 (d, J = 22.8 Hz), 168.5 (d, J = 3.2 Hz), 135.2, 128.6, 128.5, 128.4, 85.1 (d, J = 187.4 Hz), 67.2, 52.7, 37.4 (d, J = 23.0 Hz). 19F NMR (376 MHz, CDCl3) δ −191.11 (dd, J = 47.6, 23.9 Hz). IR (neat): υmax 3379, 1714, 1455, 1246, 1146, 697 cm–1. HRMS (ESI) Calcd for C12H13O4F23Na (M + Na)+: 263.0690. Found 263.0691. [α]D20 + 17.82 (c 1.10, CHCl3).

Benzyl (S)-3-Hydroxy-4-oxo-4-(p-tolylthio)butanoate (36)

To a stirred solution of hydroxy acid 32 (3 g, 13.3 mmol, 1 equiv) in dry ethyl acetate (2 mL/mmol) was added 4-methylbenzenethiol (1.74 g, 14 mmol, 1.05 equiv) and 1-hydroxybenzotriazole (2.37 g, 19.9 mmol, 1.5 equiv) at 0 °C followed by N-dicyclohexylcarbodiimide (2.73 g, 13.3 mmol, 1 equiv). The mixture was stirred for 24 h at room temperature. At the end of the reaction, a few drops of 50% acetic acid in ethyl acetate were added. The mixture was filtered through Celite, and the organic phase was washed with NaHCO3 solution and brine, dried over MgSO4, filtered, and concentrated in vacuo. The residue was chromatographed on silica, eluting with 20% EtOAc in hexanes (Rf = 0.4) to give the title compound 36 (3.3 g, 75%) as a yellow solid. 1H NMR (600 MHz, CDCl3) δ 7.43–7.33 (m, 5H), 7.29 (d, J = 6.6 Hz, 2H), 7.25 (d, J = 8.0 Hz, 2H), 5.23–5.14 (m, 2H), 4.67 (dd, J = 6.6, 4.2 Hz, 1H), 3.06–2.91 (m, 2H), 2.40 (s, 3H). 13C{1H} NMR (151 MHz, CDCl3) δ 201.3, 171.2, 139.9, 135.2, 134.6, 130.1, 128.7, 128.5, 128.4, 123.2, 74.4, 67.1, 38.4, 21.4. IR (neat): υmax 3477, 1734, 1700, 1455, 1236, 1169, 808, 697 cm–1. Melting point: 78–80 °C (decomposes). HRMS (ESI) Calcd for C18H18O423Na32S (M + Na)+: 353.0818. Found 353.0814. [α]D20 + 53.24 (c 1.10, CHCl3). 99% ee; Chiral HPLC analysis of the product: Daicel Chiralpak IA 250 × 4.6 mm 5um column; hexane/2-propanol = 90/10, detected at 210 nm, Flow rate = 1 mL/min, Retention times: 14.6 min (S), 18.5 min (R).

Benzyl (R)-3-Fluoro-4-oxo-4-(p-tolylthio)butanoate (35) from 4-Benzyl 1-Methyl (R)-2-Fluorosuccinate (34)

A stirred solution of 34 in THF (1 g, 4.2 mmol, 1 equiv) was refluxed with 5% sulfuric acid (10 mL) for 3 h. The reaction was cooled and extracted with ether (3 × 10 mL). The combined organics were dried over MgSO4 and concentrated in vacuo. The desired acid intermediate obtained as a colorless liquid (706 mg, 75%) was carried forward without further purification. 1H NMR (600 MHz, CDCl3) δ 7.74–7.60 (m, 5H), 5.64 (ddd, J = 47.1, 6.5, 4.6 Hz, 1H), 5.53–5.43 (m, 2H), 3.38 (dd, J = 5.5, 4.4 Hz, 1H), 3.34 (dd, J = 5.5, 3.9 Hz, 1H). To a stirred solution of crude acid (706 mg, 3.1 mmol, 1 equiv) and 1-hydroxybenzotriazole monohydrate (506 mg, 3.7 mmol, 1.2 equiv) in dichloromethane (15 mL) at 0 °C was added 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride (710 mg, 3.7 mmol, 1.2 equiv) portion wise. After stirring for 30 min, 4-methylbenzenethiol (458 mg, 3.7 mmol, 1.2 equiv) was added to the solution, and the mixture was allowed to warm to room temperature and stirred for 12 h. The reaction was quenched with saturated aqueous NaHCO3 solution and extracted with ethyl acetate (2 × 10 mL). The combined organic layer was washed with brine, dried over Na2SO4, and concentrated. The residue was chromatographed on silica, eluting with 20% EtOAc in hexanes (Rf = 0.5) to give the title compound 35 (0.85 g, 82%) as a pale-yellow gummy oil.

1H NMR (600 MHz, CDCl3) δ 7.44–7.36 (m, 5H), 7.32 (dd, J = 8.2, 1.8 Hz, 2H), 7.27 (d, J = 8.0 Hz, 2H), 5.48 (dddt, J = 47.8, 7.9, 3.9, 0.9 Hz, 1H), 5.23 (s, 2H), 3.18–2.95 (m, 2H), 2.42 (s, 3H). 13C{1H} NMR (151 MHz, CDCl3) δ 197.5 (d, J = 28.0 Hz), 168.4, 140.3, 135.3, 134.7, 130.3, 128.7, 128.5, 128.4, 121.9 (d, J = 5.5 Hz), 92.0 (d, J = 190.2 Hz), 67.2, 37.8 (d, J = 22.1 Hz), 21.4. 19F NMR (565 MHz, CDCl3) δ −187.8 (ddd, J = 49.1, 29.0, 21.0 Hz). IR (neat): υmax 1737, 1697, 1262, 1168, 807, 696 cm–1. HRMS (ESI) Calcd for C18H17O3FNaS (M + Na)+: 355.0774. Found 355.0771. [α]D20 + 78.57 (c 1.10, CHCl3). Assuming >98% ee; Chiral HPLC analysis of the product: Daicel Chiralpak IA 250 × 4.6 mm 5um column; hexane/2-propanol = 90/10, detected at 210 nm, Flow rate = 1 mL/min, Retention times: 7.7 min (S).

Benzyl (R)-3-Fluoro-4-oxo-4-(p-tolylthio)butanoate (35) from Benzyl (S)-3-Hydroxy-4-oxo-4-(p-tolylthio)butanoate (36)

To a stirred solution of diethyiaminosulfur trifluoride (0.57 mL, 4.36 mmol, 1.2 equiv) in ethanol-free dry chloroform (15 mL) cooled to 0 °C was added dropwise to a solution of 36 (1.2 g, 3.63 mmol, 1 equiv) in chloroform (20 mL). The mixture was stirred for 1 h, and an additional solution of diethyiaminosulfur trifluoride (0.5 equiv) in ethanol-free dry chloroform (5 mL) was added at 0 °C. After 30 min, water (1 mL) was added cautiously to the vigorously stirred solution. The organic layer was separated, washed with saturated sodium bicarbonate solution, and saturated brine, dried over MgSO4, and evaporated under reduced pressure. The residue was chromatographed on silica, eluting with 20% EtOAc in hexanes (Rf = 0.5) to give the title compound 35 (880 mg, 73%) as a pale-yellow gummy oil. The spectral data of compound 35, obtained by this route, are identical to those of compound 35 synthesized from benzyl (R)-3-fluoro-4-oxo-4-(p-tolylthio)butanoate (37), including optical rotation.

Benzyl (±)-5-(((Benzyloxy)carbonyl)amino)-3-fluoro-4-oxopentanoate (13) from 4-Benzyl 1-(tert-Butyl) 2-(((Benzyloxy)carbonyl)glycyl)-2-fluorosuccinate (12)

A solution of 12 (500 mg, 1.0 mmol, 1 equiv) and trifluoroacetic acid (0.8 mL, 10.5 mmol, 10.0 equiv) in dichloromethane (10 mL) was heated at reflux overnight. Remove solvent and purification by flash column chromatography on silica, eluting with 40% EtOAc in hexanes (Rf = 0.4) to give the title compound 13 (335 mg, 85%) as a white solid. The spectral profile of compound 13 is identical to 13a in all aspects except for its optical rotation.

Benzyl (R)-5-(((Benzyloxy)carbonyl)amino)-3-fluoro-4-oxopentanoate (13a) from Benzyl (R)-3-Fluoro-4-oxo-4-(p-tolylthio)butanoate (35)

To a stirred solution of 35 (500 mg, 1.5 mmol, 1 equiv) and benzyl tri-n-butylstannylmethylcarbamate 37 (1.37 g, 3.0 mmol, 2 equiv) in dry THF (20 mL) was added copper(I)diphenylphosphinate (0.84 g, 3.0 mmol, 2 equiv) at room temperature under nitrogen. The reaction was heated at 50 °C for 2 h. After cooling, the suspension was filtered through Celite and washed with ethyl acetate. The filtrate was washed with water, and the combined organic layers were washed with brine, dried over Na2SO4, and concentrated. The residue was chromatographed on silica, eluting with 40% EtOAc in hexanes (Rf = 0.4) to give the title compound 13a (511 mg, 91%) as a white solid. [α]D20 + 21.70 (c 1.10, CHCl3). 99% ee; Chiral HPLC analysis of the product: Daicel Chiralpak IA 250 × 4.6 mm 5um column; hexane/2-propanol = 75/25, detected at 210 nm, Flow rate = 1 mL/min, Retention times: 13.9 min (S), 15.3 min (R).

1H NMR (400 MHz, CDCl3) δ 7.43–7.31 (m, 10H), 5.34–5.28 (m, 1H), 5.28–5.19 (m, 1H), 5.18–5.12 (m, 4H), 4.54–4.37 (m, 2H), 3.22–2.98 (m, 2H). 13C{1H} NMR (151 MHz, CDCl3) δ 204.05 (d, J = 25.7 Hz), 168.52, 156.17, 135.63 (d, J = 190.0 Hz), 135.00, 128.75, 128.70, 128.63, 128.58, 128.56, 128.43, 128.30, 128.26, 128.22, 128.16, 128.12, 91.35 (d, J = 186.2 Hz), 67.32, 67.11, 48.34 (d, J = 4.4 Hz), 37.18 (d, J = 21.6 Hz). 19F NMR (376 MHz, CDCl3) δ −197.09 – −197.83 (m). IR (neat): υmax 3412, 1716, 1520, 1248, 1167, 738, 697 cm–1. Melting point: 58–60 °C (decomposes). HRMS (ESI) Calcd for C20H21O5NF (M + H)+: 374.1398. Found 374.1396.

(±)- or (R)-5-Amino-3-fluoro-4-oxopentanoic Acid Hydrochloride (14 or 14a) from Benzyl (±)- or (R)-5-(((Benzyloxy)carbonyl)amino)-3-fluoro-4-oxopentanoate (13 or 13a)

To a stirred solution of 13 or 13a (300 mg, 0.8 mmol, 1 equiv) in a 4:1 mixture of tetrahydrofuran (12 mL) and 2 M aq. HCl (3 mL) was added 10% palladium on carbon (85 mg, 0.8 mmol, 1 equiv). The reaction vessel was purged with hydrogen and evacuated three times, and the solution was then stirred under a hydrogen atmosphere at 30 °C in an oil bath for 2 h. The suspension was then filtered through Celite, and the residue was washed with a small amount of water. The filtrate was concentrated under reduced pressure to obtain a tan solid. Then, add 2 mL of ethanol to the crude material and heat the mixture to boiling using a sand bath. Once the solution is fully dissolved, slowly add diethyl ether drop by drop until the solution reaches a cloud point and allow the solution to stand until crystals grow. (TLC: nBuOH:H2O:CH3CO2H, 12:5:3, Rf = 0.35), provided the desired compound 14 or 14a (111 mg, 75% yield) as colorless needles, respectively. 1H NMR (600 MHz, DMSO) δ 12.87 (s, 1H), 8.44 (s, 3H), 5.40 (dt, J = 46.1, 4.8 Hz, 1H), 4.07 (t, J = 14.4 Hz, 2H), 3.01 (t, J = 5.4 Hz, 1H), 2.97 (d, J = 4.8 Hz, 1H). 13C{1H} NMR (101 MHz, DMSO) δ 202.3 (d, J = 24.1 Hz), 170.9, 91.8 (d, J = 181.4 Hz), 45.3, 37.3 (d, J = 21.4 Hz). 19F NMR (376 MHz, DMSO) δ −193.34 – −198.21 (m). IR (neat): υmax 3022, 1724, 1488, 1397, 1106, 993, 853, 578 cm–1. HRMS (ESI) Calcd for C5H9O3NF (M + H)+: 150.0561. Found 150.0561. Melting point: 138–140 °C (decomposes). [α]D20 + 7.02 (c 1.10, H2O).

Note: A similar route was followed for the synthesis of the other antipode, (S)-(−)-5-amino-3-fluoro-4-oxopentanoic acid hydrochloride (14b) starting from d-benzyl aspartate. The NMR data of all the compounds in this series were exactly matched with the other enantiomeric series. The optical rotations also showed the same magnitude but with an inverse sign.

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.joc.4c01070.Copies of the 1H and 13C spectra for all new compounds, X-ray crystallographic data for 14a and 14b, and HPLC chromatograms of 36, 35, and 13b (PDF)

Supplementary Material

jo4c01070_si_001.pdf

The authors declare no competing financial interest.

Acknowledgments

The authors acknowledge The Winship Cancer Institute’s Discovery and Developmental Therapeutics (DDT) Seed Grant Program and thank Dr. John Bacsa for his assistance with X-ray crystal structures and Mr. Wesley Pullara for his contribution in acquiring HPLC data for the chiral intermediates.
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References

Goodenberger M. L. ; Jenkins R. B. Genetics of Adult Glioma. Cancer Genet. 2012, 205 , 613–621. 10.1016/j.cancergen.2012.10.009.23238284
Eder K. ; Kalman B. Molecular Heterogeneity of Glioblastoma and its Clinical Relevance. Pathol. Oncol. Res. 2014, 20 , 777–787. 10.1007/s12253-014-9833-3.25156108
Di Gialleonardo V. ; Wilson D. M. ; Keshari K. R. The Potential of Metabolic Imaging. Semin Nucl. Med. 2016, 46 , 28–39. 10.1053/j.semnuclmed.2015.09.004.26687855
a Chen K. ; Chen X. Positron emission tomography imaging of cancer biology: current status and future prospects. Semin. Oncol. 2011, 38 , 70–86. 10.1053/j.seminoncol.2010.11.005.21362517
b Conti P. S. ; Lilien D. L. ; Hawley K. ; Keppler J. ; Grafton S. T. ; Bading J. R. PET and [18F]-FDG in oncology: a clinical update. Nucl. Med. Biol. 1996, 23 , 717–735. 10.1016/0969-8051(96)00074-1.8940714
c Kelloff G. J. ; Hoffman J. M. ; Johnson B. ; Scher H. I. ; Siegel B. A. ; Cheng E. Y. ; Cheson B. D. ; O’Shaughnessy J. ; Guyton K. Z. ; Mankoff D. A. ; et al. Progress and promise of FDG-PET imaging for cancer patient management and oncologic drug development. Clin. Cancer. Res. 2005, 11 , 2785–2808. 10.1158/1078-0432.CCR-04-2626.15837727
a Barentsz J. ; Takahashi S. ; Oyen W. ; Mus R. ; De Mulder P. ; Reznek R. ; Oudkerk M. ; Mali W. Commonly used imaging techniques for diagnosis and staging. J. Clin. Oncol. 2006, 24 , 3234–3244. 10.1200/JCO.2006.06.5946.16829647
b Pauwels E. ; Ribeiro M. ; Stoot J. ; McCready V. ; Bourguignon M. ; Maziere B. FDG accumulation and tumor biology. Nucl. Med. Biol. 1998, 25 , 317–322. 10.1016/S0969-8051(97)00226-6.9639291
c Ninatti G. ; Sollini M. ; Bono B. ; Gozzi N. ; Fedorov D. ; Antunovic L. ; Gelardi F. ; Navarria P. ; Politi L. S. ; Pessina F. ; Chiti A. Preoperative [11C]methionine PET to Personalize Treatment Decisions in Patients with Lower-Grade Gliomas. Neuro-Oncology 2022, 24 , 1546–1556. 10.1093/neuonc/noac040.35171292
Franken A. C. W. ; Lokman B. C. ; Ram A. F. J. ; Punt P. J. ; van den Hondel C. A. M. J. J. ; de Weert S. Heme Biosynthesis and Its Regulation: Towards Understanding and Improvement of Heme Biosynthesis in Filamentous Fungi. Appl. Microbiol. Biotechnol. 2011, 91 , 447–460. 10.1007/s00253-011-3391-3.21687966
Valdes P. A. ; Kim A. ; Brantsch M. ; Niu C. ; Moses Z. B. ; Tosteson T. D. ; Wilson B. C. ; Paulsen K. D. ; Roberts D. W. ; Harris B. T. Delta-aminolevulinic acid-induced protoporphyrin IX concentration correlates with histopathologic markers of malignancy in human gliomas: the need for quantitative fluorescence-guided resection to identify regions of increasing malignancy. Neuro-Oncology 2011, 13 , 846–856. 10.1093/neuonc/nor086.21798847
Matsumoto K. ; Hagiya Y. ; Endo Y. ; Nakajima M. ; Ishizuka M. ; Tanaka T. ; Ogura S. Effects of plasma membrane ABCB6 on 5-aminolevulinic acid (ALA)-induced porphyrin accumulation in vitro: tumor cell response to hypoxia. Photodiagn. Photodyn. Ther. 2015, 12 , 45–51. 10.1016/j.pdpdt.2014.12.008.
Stummer W. ; Novotny A. ; Stepp H. ; Goetz C. ; Bise K. ; Reulen H. J. Fluorescence-guided resection of glioblastoma multiforme by using 5-aminolevulinic acid-induced porphyrins: a prospective study in 52 consecutive patients. J. Neurosurg. 2000, 93 , 1003–1013. 10.3171/jns.2000.93.6.1003.11117842
a Stummer W. ; Tonn J. C. ; Goetz C. ; et al. 5-Aminolevulinic acid-derived tumor fluorescence: the diagnostic accuracy of visible fluorescence qualities as corroborated by spectrometry and histology and postoperative imaging. Neurosurgery 2014, 74 , 310–319. 10.1227/NEU.0000000000000267.24335821
b Stepp H. ; Beck T. ; Pongratz T. ; et al. ALA, and malignant glioma: fluorescence-guided resection and photodynamic treatment. J. Environ. Pathol. Toxicol. Oncol. 2007, 26 , 157–164. 10.1615/JEnvironPatholToxicolOncol.v26.i2.110.17725542
c Teng L. ; Nakada M. ; Zhao S. G. ; et al. Silencing of ferrochelatase enhances 5-aminolevulinic acid-based fluorescence and photodynamic therapy efficacy. Br J. Cancer. 2011, 104 , 798–807. 10.1038/bjc.2011.12.21304523
Suzuki C. ; Kato K. ; Tsuji A. B. ; Kikuchi T. ; Zhang M.-R. ; Arano Y. ; Saga T. Synthesis and in Vitro Celluar Uptake of 11C- Labeled 5-Aminolevulinic Acid Derivative to Estimate the Induced Celluar Accumulation of Protoporphyrin IX. Bioorg. Med. Chem. Lett. 2013, 23 , 4567–4570. 10.1016/j.bmcl.2013.06.025.23830504
Rodriguez L. ; Batlle A. ; Di Venosa G. ; MacRobert A. J. ; Battah S. ; Daniel H. ; Casas A. Study of the Mechanisms of Uptake of 5-Aminolevulinic Acid Derivatives by PEPT1 and PEPT2 Transporters as a Tool to Improve Photodynamic Therapy of Tumours. Int. J. Biochem. Cell Biol. 2006, 38 , 1530–1539. 10.1016/j.biocel.2006.03.002.16632403
Pippin A. B. ; Voll R. J. ; Li Y. ; Wu H. ; Mao H. ; Goodman M. M. Radiochemical Synthesis and Evaluation of 13N-Labeled 5-Aminolevulinic Acid for PET Imaging of Gliomas. ACS Med. Chem. Lett. 2017, 8 , 1236–1240. 10.1021/acsmedchemlett.7b00311.29259740
a Leeper F. J. ; Rock M. The Synthesis of a Fluorinated Analogue of 5-Amino-Laevulinic Acid, a Potential Inhibitor of Porphyrin Biosynthesis. J. Fluorine Chem. 1991, 51 , 381–396. 10.1016/S0022-1139(00)80192-X.
b Takayanagi H. Preparation of 2,2-Difluorolevulinic Acid Derivatives as Contrast Agents for Magnetic Resonance Imaging (MRI). JP 09067323 A, 1997.
Zavozin A. G. ; Ignat’ev N. V. ; Schulte M. ; Zlotin S. G. Synthesis of thiazole derivatives bearing an incorporated Z-5-aminopent-3-enoic acid fragment. Tetrahedron 2013, 69 , 6975–6980. 10.1016/j.tet.2013.06.060.
Suzuki M. ; Iwasaki T. ; Miyoshi M. ; Okumura K. ; Matsumoto K. New Convenient Syntheses of α-C-Acylamino Acids and α-Amino Ketones. J. Org. Chem. 1973, 38 , 3571–3575. 10.1021/jo00960a028.4780827
a Spencer A. R. A. ; Grainger R. ; Panigrahi A. ; Lepper T. J. ; Bentkowska K. ; Larrosa I. Transition Metal-Free Cross-Dehydrogenative Arylation of Unactivated Benzylic C–H Bonds. Chem. Commun. 2020, 56 , 14479–14482. 10.1039/D0CC06212J.
b Wang F. ; Nishimoto Y. ; Yasuda M. Insertion of Diazo Esters into C–F Bonds toward Diastereoselective One Carbon Elongation of Benzylic Fluorides: Unprecedented BF3 Catalysis with C–F Bond Cleavage and Re-Formation. J. Am. Chem. Soc. 2021, 143 , 20616–20621. 10.1021/jacs.1c10517.34766748
c Huang X. ; Liu W. ; Hooker J. M. ; Groves J. T. Targeted Fluorination with the Fluoride Ion by Manganese-Catalyzed Decarboxylation. Angew. Chem., Int. Ed. 2015, 54 , 5241–5245. 10.1002/anie.201500399.
Jin B. ; Dong Q. ; Hung G. (Haisco Pharmaceutical Group Co., Ltd.). Hemoglobin Modifier Compounds and Uses Thereof. WO 2017218960 A1, 2017.
a El Khatib M. ; Jauregui L. ; Tala S. R. ; Khelashvili L. ; Katritzky A. R. Solution-phase synthesis of chiral O-acyl isodipeptides. Med. Chem. Commun. 2011, 2 , 1087–1092. 10.1039/c1md00130b.
b Katritzky A. R. ; Wang Z. ; Hall C. D. Synthesis of achiral and chiral N-protected γ-amino-β-ketones and β ketoesters. ARKIVOC 2008, 2008 , 26–36. 10.3998/ark.5550190.0009.a04.
Li S. ; Yang M. ; Wu Q. ; Chen-Wu Q. ; Zeng Z. ; Xue Q. ; Xie P. ; Li Y. (Southern Jinmin Pharmaceutical Research Center of Fujian, Ltd.). Method for preparation of 5-aminolevulinic acid hydrochloride. CN 109796353 A, 2019.
Olah G. A. ; Welch J. T. Synthetic Methods and Reactions XII. Preparation of α-Fluorocarboxylic Acids from α-Amino Acids via Diazotization in Polyhydrogen Fluoride/Pyridine Solution. Synthesis 1974, 1974 , 652 10.1055/s-1974-23388.
Armarego W. L. F. ; Taguchi H. ; Cotton R. G. H. ; Battiston S. ; Leong L. Lipophilic 5,6,7, % tetrahydropterin substrates for phenylalanine hydroxylase (monkey brain), tryptophan hydroxylase (rat brain) and tyrosine hydroxylase (rat brain). Eur. J. Med. Chem. 1987, 22 , 283–291. 10.1016/0223-5234(87)90265-0.
Koth D. ; Fiedler A. ; Scholz S. ; Gottschaldt M. Synthesis of Different 3,5-Diazidofuranoses: A New and General Synthesis Pathway. J. Carbohydr. Chem. 2007, 26 , 267–278. 10.1080/07328300701540175.
Kim D. W. ; Ahn D.-S. ; Oh Y.-H. ; Lee S. ; Kil H. S. ; Oh S. J. ; Lee S. J. ; Kim J. S. ; Ryu J. S. ; Moon D. H. ; Chi D. Y. A new class of SN2 reactions catalyzed by protic solvents: facile fluorination for isotopic labeling of diagnostic molecules. J. Am. Chem. Soc. 2006, 128 , 16394–16397. 10.1021/ja0646895.17165796
Silva S. L. ; Valle M. S. ; Pliego J. R. Nucleophilic Fluorination with KF Catalyzed by 18-Crown-6 and Bulky Diols: A Theoretical and Experimental Study. J. Org. Chem. 2020, 85 , 15457–15465. 10.1021/acs.joc.0c02229.33227195
Cheng H.-G. ; Chen H. ; Liu Y. ; Zhou Q. TheLiebeskind–Srogl Cross-Coupling Reaction and Its SyntheticApplications. Asian J. Org. Chem. 2018, 7 , 490–508. 10.1002/ajoc.201700651.
a Li H. ; Yang H. ; Liebeskind L. S. Synthesis of High Enantiopurity N-Protected α-Amino Ketones by Thiol Ester-Organostannane Cross-Coupling using pH-Neutral Conditions. Org. Lett. 2008, 10 , 4375 10.1021/ol8018456.18759432
b Li H. ; He A. ; Falck J. R. ; Liebeskind L. S. Stereocontrolled Synthesis of α-Amino-α′-alkoxy Ketones by a Copper-Catalyzed Cross-Coupling of Peptidic Thiol Esters and α-Alkoxyalkylstannanes. Org. Lett. 2011, 13 , 3682–3685. 10.1021/ol201330j.21675755
Hu X. D. ; O’Brien M. ; Ley V. S. Continuous Multiple LiquidLiquid Separation: Diazotization of Amino Acids in Flow. Org. Lett. 2012, 14 , 4246–4249. 10.1021/ol301930h.22866828
Kobayashi H. ; Eickhoff A. J. ; Zakarian A. Synthesis of 2-Aminoazoles from Thioesters via α-Heterosubstituted Ketones by Copper-Mediated Cross-Coupling. J. Org. Chem. 2015, 80 , 9989–9999. 10.1021/acs.joc.5b01558.26378626
a Kawakami H. ; Ebata T. ; Matsushita H. A New Synthesis of 5-Aminolevulinic Acid. Agric. Biol. Chem. 1991, 55 , 1687–1688. 10.1080/00021369.1991.10870797.
b Ha H.-J. ; Lee S.-K. ; Ha Y.-J. ; Park J.-W. Selective Bromination Of Ketones. A Convenient Synthesis Of 5-Aminolevulinic Acid. Synth. Commun., 1994, 24 , 2557–2562. 10.1080/00397919408010567.
Pickel T. C. ; Pashikanti G. ; Voll R. J. ; Yu W. ; Zhang Z. ; Nye J. A. ; Bacsa J. ; Olson J. J. ; Liebeskind L. S. ; Goodman M. M. Synthesis, Radiolabeling, and Biological Evaluation of the trans-Stereoisomers of 1-Amino-3-(fluoro-18F)-4-fluorocyclopentane-1-carboxylicAcid as PET Imaging Agents. ACS Pharmacol. Transl. Sci. 2021, 4 , 1195–1203. 10.1021/acsptsci.1c00062.34151209
