N-CBZ-TRIFLUOROPYRUVALDEHYDE N,S-KETAL - ABSOLUTE STEREOCHEMISTRY ANDADDITION OF GRIGNARD-REAGENTS - HIGHLY STEREOSELECTIVE ENTRY TO TRIFLUORO ANALOGS OF EPHEDRA ALKALOIDS

The chiral non racemic N-Cbz-trifluoropyruvaldehyde-N,S-ketal la (enan tiomeric excess up to 74%) is a new trifluoro 3-C building block, whic h has been reacted with several Grignard reagents, stereoselectively a ffording the corresponding secondary carbinols 2. The N,S-ketal stereo centre, whose absolute stereochemistry has been determined by X-ray an alysis of the alpha-phenylpropionate 3, is able to provide excellent s tereocontrol. Highly stereoselective p-tolylthio group displacement af forded the N-Cbz-phenyl derivative 2d, transformed into trifluoro-nore phedrine (SS)-5 and -ephedrine (S,S)-6. (C) 1997 Elsevier Science Ltd.