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

The enolates of (+)- and (-)-menthyl acetate, formed by transmetallation from lithium with chloro(cyclopentadienyl)bis(1,2:5,6-di-O-isopropylidene-alpha-D-glucofuranosato-O3)titaniUM [CpTi(DAGO)2Cl] (1), react with benzaldehyde from the Re-face with high stereoselectivities (95 and 92% diastereomeric excess) when 12-crown-4 is used during the transmetallation. Use of the chiral cyclopentadienyltitanium-alpha,alpha,alpha',alpha'-tetraaryl-2,2-dimethyl-1,3-dioxolan-4,5-dimethanol (TADDOL) complex (3) gives lower diastereoselectivities (58 and 40%) but the Si-face of benzaldehyde is attacked preferentially. Transmetallation of the 2-methylprop-2-enyl moiety to CpTi(DAGO)2Cl gives an allyltitanium reagent (2) which reacts with benzaldehyde from the Re-face with 88% enantiomeric excess.