Homologated amino acids with three vicinal fluorines positioned along the backbone: development of a stereoselective synthesis

Backbone-extended amino acids have a variety of potential applications in peptide and protein science, particularly if the geometry of the amino acid is controllable. Here we describe the synthesis of δ-amino acids that contain three vicinal C–F bonds positioned along the backbone. The ultimately successful synthetic approach emerged through the investigation of several methods based on both electrophilic and nucleophilic fluorination chemistry. We show that different diastereoisomers of this fluorinated δ-amino acid adopt distinct conformations in solution, suggesting that these molecules might have value as shape-controlled building blocks for future applications in peptide science.


Synthetic procedures and characterization of intermediates Amino acid 6a
Pd/C (~6 mg) was added to a solution of 40a (12 mg, 0.031 mmol) in dry acetic anhydride (~3 mL).
The mixture was stirred under H 2 atmosphere for 5 h. The reaction mixture was filtered through celite To a stirred mixture of imide 46a (10 mg, 0.023 mmol), dichloromethane (0.6 mL), acetonitrile (0.6 mL) and water (0.72 mL) was added sodium metaperiodate (98 mg, 0.46 mmol) followed by ruthenium chloride hydrate (~3 mg). The resulting mixture was stirred at room temperature for 5 days.
The mixture was filtered through celite (ethyl acetate wash) and the filtrate was concentrated in vacuo.

Bis-nitrile 14
A mixture of Et 3 N (0.2 mL), TsCl (23.1 mg, 1.2 mmol) and DMAP (1.0 mg, 0.1 mmol) were stirred in anhydrous DCM (2 mL) at room temperature. Then alchol 11 (15.0 mg, 1 mmol) was added in one Tosylate 48 (13.0 mg, 0.04 mmol) was stirred in DMSO (2 mL) at 80 °C and then potassium cyanide (9.1 mg, 0.14 mmol) was added and continued stirring for 4 h at 80 °C. The reaction mixture was allowed to cool to room temperature and EtOAc (3 mL) and water (3 mL) were added. The organic layer was separated and the aqueous layer was extracted with EtOAc (3 × 3 mL). The crude mixture was subjected to column chromatography (15% EtOAc/hexane) to give bis-nitrile 14 as a white solid To a vigorously stirring solution of 49 [5] (83.1 mg, 0.258 mmol) in CH 2 Cl 2 (40 mL) at 0 °C was added HOBt (52.3 mg, 0.387 mmol) and DIC (48.9 mg, 0.387 mmol). This solution was allowed to warm to room temperature over 1.5 h before N,O-dimethylhydroxylamine hydrochloride (37.8 mg, 0.387 mmol) was added. The resulting suspension was stirred for 13 h at room temperature before it was diluted with 10 mL of 1 N aqueous HCl. The layers were separated and the aqueous layer was washed with CH 2 Cl 2 (10 mL). The combined organic layers were washed with H 2 O (10 mL) and a 1 N aqueous solution of  A solution of 58 (53 mg, 0.19 mmol) in CH 2 Cl 2 (15 mL) was cooled to -78 °C and a stream of ozone was bubbled through the mixture until a deep blue color persisted. Oxygen was then bubbled through the solution for an additional 10 min to remove excess ozone. Dimethyl sulfide (0.2 mL, 3.8 mmol) was added to the solution, which was then stirred and warmed to room temperature over 1 h. The mixture was then concentrated under reduced pressure, the crude mixture with MeOH (4 mL), KBH 4 (102 mg, 1.9 mmol) was added at ambient temperature. After 30 min the reaction was quenched with To a stirred mixture of compound 49 [5] (33.1 mg, 0.10 mmol), CH 2 Cl 2 (3 mL), methanol (1 mL) and DMAP (10.2 mg, 0.082 mmol) at 0 °C was added DCC (23.3 mg, 0.113 mmol) over a 5 min period.
After a further 5 min at 0 °C, the reaction mixture was stirred for 3 h at room temperature. The formed precipitate was removed by filtration, and the filtrate was washed with 0.5 N hydrochloric acid (2 ´ 10 mL) and saturated aqueous sodium bicarbonate solution (2 ´ 10 mL). During this procedure some additional precipitate formed, which was removed by filtration of both layers to facilitate their separation. The organic solution was dried over anhydrous sodium sulfate and concentrated in vacuo.

S-20
Fluoroepoxide 38b [1] A solution of triphenylphosphine (2.9 g, 11.2 mmol) in anhydrous tetrahydrofuran (25 mL) was cooled to 0 °C. Diisopropyl azodicarboxylate (2.28 mL, 11.2 mmol) was then added dropwise and the mixture was stirred for 20 min. A solution of p-nitrobenzoic acid (1. 8 g, 11.2 mmol) in anhydrous tetrahydrofuran (22 mL) was then added dropwise and stirring was continued for a further 20 min. A solution of 37 (2.0 g, 5.2 mmol) in dry tetrahydrofuran (25 mL) was added via cannula. The mixture was slowly warmed to rt and stirred overnight. The reaction mixture was then concentrated onto silica and purified by flash chromatography (dichloromethane). The resulting partially-purified solid product was washed with methanol (10 mL) to provide ester 59 [1]  Difluoroalcohols 39a and regioisomer [1] Triethylamine trihydrofluoride (1.2 mL) was added to a microwave tube containing compound 39a (100 mg, 0.28 mmol). The mixture was subjected to microwave irradiation at 150 °C for 15 min. The reaction mixture was then diluted with dichloromethane and quenched with saturated aqueous NaHCO 3 . The mixture was extracted with ethyl acetate (25 mL) and dichloromethane (30 mL). The organic layers were combined, dried over MgSO 4 and concentrated onto silica. The crude product was S-22 purified by flash chromatography (dichloromethane→dichloromethane/ethyl acetate 10:1) to provide a mixture of compounds 39a [1] and its regioisomer as a pale brown solid (44 mg, 46% combined yield).
Data for 39a: [1]  Data for the regioisomer of 39a: [1]  Difluoroalcohol 39b and regioisomer [1] Triethylamine trihydrofluoride (1.0 mL) was added to a microwave tube containing compound 38b (50 mg, 0.14 mmol). The mixture was subjected to microwave irradiation at 150 °C for 10 min. The reaction mixture was then diluted with ethyl acetate (15 mL) and quenched with saturated aqueous NaHCO 3 (10 mL). The phases were separated, and the aqueous portion was extracted with ethyl acetate (3 ´ 25 mL). The combined organic layers were dried with Na 2 SO 4 and concentrated onto silica. The crude product was subjected to flash chromatography (dichloromethane→dichloromethane/ethyl acetate 10:1) to provide an inseparable mixture of compounds 39b [1] and its regioisomer as a brown solid (total 23.5 mg, 45%).
Data for 39b: [ Trifluoroalkane 40b [1] DeoxoFluor (2.10 mL) was added to a plastic reaction vessel containing 39b (mixture of regioisomers, 64 mg, 0.170 mmol). The resulting mixture was immediately heated to 70 °C and stirred under nitrogen atmosphere overnight. The reaction mixture was cooled to room temperature, diluted with DCM, and quenched with saturated aqueous NaHCO 3 . The mixture was extracted and the aqueous layer was  S-27

NMR simulations of 6a/b
Expansions from the simulated and experimental NMR spectra of 6a are displayed below. From these simulations, a set of J values was obtained (see Table overleaf). It is acknowledged that the match between simulated and experimental NMR spectra is not perfect, which places some uncertainty on the true J-values. 1 H NMR: 19 F NMR: