Synthesis of Boc-protected cis- and trans-4-trifluoromethyl-d-prolines

Abstract

Both Boc-protected trans- and cis-4-trifluoromethyl prolines were synthesized starting from L-serine simultaneously. In our synthetic route, the key intermediate 4 was obtained through the reaction of Garner's aldehyde 1 with ylide 2 followed by trifluoromethylation with FSO₂CF₂COOMe–CuI. After hydrogenation followed by reduction of 4, the alcohol 5 was obtained in low diastereoselectivity, however, the two diastereoisomers could be separated easily by flash chromatography in the following steps. The bromide 8b obtained from the alcohol 5 in a straightforward fashion could not afford the desired cyclization product because of the strong electron-withdrawing properties of the trifluoromethyl group and the low ability of bromide as a leaving group. Instead, mesylation of alcohols 12a and 12b followed by treatment with potassium bis(trimethylsilyl)amide (KHMDS) afforded the desired cyclization products 13a and 13b respectively, which were transformed into Boc-protected cis- and trans-4-trifluoromethyl-D-prolines in a straightforward fashion.