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

The chiral non racemic N-Cbz-trifluoropyruvaldehyde-N,S-ketal la (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 alpha-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 (SS)-5 and -ephedrine (S,S)-6. (C) 1997 Elsevier Science Ltd.