STEREOSELECTIVE TITANIUM-MEDIATED ALDOL REACTIONS OF MENTHYL ACETATE ENOLATES, AND ALLYLATIONS WITH BENZALDEHYDE

The enolates of (+)- and (-)-menthyl acetate, formed by transmetallati on from lithium with isopropylidene-alpha-D-glucofuranosato-O3)titaniU M [CpTi(DAGO)2Cl] (1), react with benzaldehyde from the Re-face with h igh stereoselectivities (95 and 92% diastereomeric excess) when 12-cro wn-4 is used during the transmetallation. Use of the chiral tetraaryl- 2,2-dimethyl-1,3-dioxolan-4,5-dimethanol (TADDOL) complex (3) gives lo wer diastereoselectivities (58 and 40%) but the Si-face of benzaldehyd e is attacked preferentially. Transmetallation of the 2-methylprop-2-e nyl moiety to CpTi(DAGO)2Cl gives an allyltitanium reagent (2) which r eacts with benzaldehyde from the Re-face with 88% enantiomeric excess.