Facile and Stereoselective Synthesis of Non-Racemic 3,3,3-

A highly stereoselective enantiodivergent synthesis of non-racemic 3,3,3-trifluoroalanine 7 is reported. The methodology is based on the reduction, by 9-BBN or DIBAH, of the chiral sulfinimine 3 derived from ethyl trifluoropyruvate, followed by acidic hydrolysis of the resulting diastereomeric sulfinamides 4 and 5.


Introduction
The rapidly expanding interest in the field of peptidomimetics is prompting organic chemists to develop novel and efficient stereocontrolled approaches to rare and unnatural amino acids.An extremely intriguing class of unnatural amino acids is represented by those incorporating one or more fluorine atoms [1].This interest stems from the peculiar biomedicinal and pharmaceutical properties of fluorinated substrates [2], as well as from the considerable synthetic challenges connected with the preparation of these molecules in stereodefined, non-racemic form [3]. 3,3,3-Trifluoroalanine (7) (Scheme 2) and its derivatives have attracted a remarkable deal of interest as suicide inhibitors of a number of pyridoxal-phosphate dependent enzymes [4].Incorporation of 7 into small peptides has been achieved, and some of the resulting oligomers have been found to possess interesting biological activity [5].Although several preparations of racemic 7 have been described following the seminal work of Steglich [6], a very few syntheses of non-racemic 7 are available [7], and only recently its absolute configuration has been clarified [8].To date, a straightforward method for the synthesis of non-racemic 7 from readily available or commercial starting materials, such as trifluoropyruvic esters, is lacking [9].In this paper we describe the successful accomplishment of this goal.
A reasonable transition state (TS) 8 (Scheme 2) accounting for the high stereoselectivity observed with 9-BBN is based on the hypothesis that the sulfinimine (S)-3 is geometrically homogeneous and thermodynamically stable with the sulfinyl and the bulky trifluoromethyl group in trans position with respect to the C=N bond, as strongly suggested by theoretical ab initio calculations supported by NMR spectroscopy [15].In line with the previously proposed TS for highly stereoselective 9-BBN reductions of ketone derived sulfinimines [16], the boron atom should coordinate the sulfinyl oxygen giving rise to a chair-like TS.As a consequence, the hydride predominantly attacks the Re face of the C=N bond producing the diastereomeric sulfinamide 5 with overwhelming preference.Scheme 2. Key: i) HCl conc., reflux, overnight.ii) Dowex 50W-X8.
In summary, we have developed an extremely facile, straightforward, stereoselective and enantiodivergent approach to non-racemic 3,3,3-trifluoroalanine, which is now readily available for further biochemical studies and incorporation into peptidomimetics.

General
Chemical shifts (δ) are reported in parts per million (ppm) of the applied field.Coupling constants (J) are reported in Hertz.Me 4 Si was used as internal standard ( δ H and δ C = 0.00) for 1 H and 13 C nuclei, while C 6 F 6 was used as external standard (δ F = −162.90)for 19 F nuclei.Peak multiplicities are abbreviated: singlet, s; doublet, d; triplet, t; quartet, q; multiplet, m; etc. Anhydrous solvents were obtained by distillation from sodium (THF, benzene) or from calcium hydride (dichloromethane, diisopropylamine).In all other cases commercially available reagent-grade solvents were employed without purification.Grignard reagents were purchased from Sigma/Aldrich/Fluka Company.Reactions performed in dry solvents were carried out under nitrogen atmosphere.Melting points are uncorrected and were obtained on a capillary apparatus.Analytical thin-layer chromatography (TLC) was routinely used to monitor reactions.Plates precoated with E. Merck silica gel 60 F 254 of 0.25 mm thickness were used.Merck silica gel 60 (230-400 ASTM mesh) was employed for flash chromatography (FC).

Reduction with DIBAH and ZnBr 2
A solution containing the sulfinimine (S)-3 dissolved in anhydrous THF (1.5 mL), obtained as described above from the same amount of reagents, was treated with zinc bromide (108 mg, 0.48 mmol) and stirred at r.t. for 30 min.The mixture was cooled to − 70 °C, then a 1.0 M n-hexane solution of DIBAH (0.58 mL, 0.58 mmol) was added dropwise while stirring.After 15 min.the reaction was quenched and worked-up as described above.Purification by FC afforded a 2:1 mixture of the two diastereomers 4,5 with an overall yield of 58 %.

Reduction with NaBH 4
A solution containing the sulfinimine (S)-3 dissolved in distilled methanol (1.5 mL), obtained as described above, was cooled to − 70 °C, then NaBH 4 (22 mg, 0.58 mmol) was added in one portion.After 10 min.the reaction was quenched with saturated aqueous NH 4 Cl and extracted with ethyl acetate.The collected organic layers were dried over anhydrous sodium sulfate and the resulting crude mixture purified by FC (n-hexane/ethyl acetate from 80:20 to 70:30) affording a 2:1 mixture of two diastereomers 6 (overall yield 66%) resulting from addition of methanol across the C=N bond of (S)-3.

Reduction with 9-BBN
A solution containing the sulfinimine (S)-3 dissolved in freshly distilled THF (1.5 mL), obtained as described above starting from 0.58 mmol of iminophosphorane (S)-2 and 0.58 mmol of ethyl trifluoropyruvate, was cooled to 0 °C, then a 0.5 M THF solution of 9-BBN (1.27 mL, 0.63 mmol) was added dropwise.The reaction mixture was stirred under nitrogen at the same temperature for ca. 2 hours, then worked-up by adding 5 µL of methanol in order to destroy the excess of 9-BBN [13], then the solvent was removed in vacuo.The crude was redissolved in 1.5 mL of diisopropyl ether, filtered and the resulting solution purified by FC, affording the two diastereomers 4, 5 in 1:20 ratio with an overall yield of 78 %.

Direct synthesis of (R)-(+)-3,3,3-trifluoro-alanine 7
The first part of the synthetic procedure was performed using the same conditions described above for the preparation of the sulfinimine (S)-3, the only exception being the use of 1.13 mmol of the (R)enantiomer of iminophosphorane 2. After addition of 9-BBN (1.24 mmol), the reaction mixture was stirred under nitrogen (0 °C) for ca. 2 hours, then the solvent evaporated in vacuo.The crude was redissolved in 5 mL of conc.HCl and stirred overnight at reflux [6d].The reaction mixture was diluted with water and, after addition of diethyl ether (2 mL), vigorously stirred for 1 hour.The two layers were then separated, the organic phase washed with two portions of a 10% solution of HCl, and the collected aqueous phases concentrated in vacuo and loaded in a Dowex 50W-X8 column.Elution with 7.5% aqueous ammonia afforded 61 mg (38% overall yield) of free (R)-3,3,3-trifluoro-alanine 7.