N-Cbz-Trifluoropyruvaldehyde N,S-ketal: Absolute stereochemistry and addition of Grignard reagents. Highly stereoselective entry to trifluoro analogues of Ephedra alkaloids

Article abstract of DOI:10.1016/S0040-4039(97)00209-8

The chiral non racemic N-Cbz-trifluoropyruvaldehyde-N,S-ketal 1a (enantiomeric excess up to 74%) is a new trifluoro 3-C building block, which has been reacted with several Grignard reagents, stereoselectively affording the corresponding secondary carbinols 2. The N,S-ketal stereocentre, whose absolute stereochemistry has been determined by X-ray analysis of the α-phenylpropionate 3, is able to provide excellent stereocontrol. Highly stereoselective p-tolylthio group displacement afforded the N-Cbz-phenyl derivative 2d, transformed into trifluoro-norephedrine (S,S)-5 and -ephedrine (S,S)-6.