Development of a Safe and Practical Synthesis of Enantiomerically Pure (S)- and (R)-N-Boc-3-(Trifluoromethyl)piperazines Enabled by Aza-Michael Addition of Optically Pure 4-Phenyl-2-Oxazolidinone to 3,3,3-Trifluoro-1-Nitropropene

Abstract

(S)- and (R)-N-Boc-3-(trifluoromethyl)piperazines are attractive building blocks for the rational design of drugs. Until now, their chiral syntheses were unknown. Exploration of three synthetic approaches via chiral 3,3,3-trifluoropropane-1,2-diamines yielded a scalable route that has been used for multigram synthesis. Two routes were not amendable for scale-up due to low yielding, tedious purification, limited availability of reagents, and safety issues. The successful and practical route relied on a modified process to mitigate the safety issues for the synthesis of (E)-3,3,3-trifluoro-1-nitroprop-1-ene, which was used as a stock solution for the highly diastereoselective aza-Michael addition of optically pure 4-phenyl-2-oxazolidinone. Boc protection of an amino group allowed subsequent transformations to chiral N-Boc-protected 3,3,3-trifluoropropane-1,2-diamine under mild conditions, without the need for chiral chromatography. The amidation of chiral N-Boc-protected 3,3,3-trifluoropropane-1,2-diamine with 2-chloroacetyl chloride, followed by intramolecular cyclization and subsequent reduction afforded enantiomerically pure N-Boc-3-(trifluoromethyl)piperazines.