THE ALDOL-TYPE CONDENSATION OF LITHIUM, BORON AND TITANIUM ENOLATES OF ISOCARANYL ACETATES WITH BENZALDEHYDE - THE METAL AND CHIRAL AUXILIARY EFFECTS

A new approach to chiral enolates by using a chiral ester (isocaranyl acetates) and a chiral organometallic reagent (boranes and titanates) is reported. The effect of this double enantiodifferentiation and the effects of the isomerism of chiral auxiliary (2- and 4-isocaranyl) and that of the metal (Li, B, Ti) on the enantioselectivity of aldol cond ensation of 2- and 4-isocaranyl acetates with benzaldehyde are discuss ed. Lithium and boron enolates of isocaranyl acetates add preferential ly to the si-face of benzaldehyde while the addition of analogous tita nium enolates occurs preferably on the re-face of the aldehyde. The en olborinate which gives the highest ee value is that obtained by transm etallation reaction of the lithium enolate of (+)-2-isocaranyl acetate with (-)-di-4-isocaranylchloroborane. Obversely the enoltitanate prep ared from (-)-4-isocaranyl acetate and the LDA-(+)-di-2-isocaranoxy(ch loro)cyclopentadienyl titanate system is that which gives the highest enantiofacial discrimination. The chiral auxiliary effect has been use d to plan the synthesis of (+)-4 alpha-acetoxy-3 beta-trimethylsilylox y-trans-carane, the titanium enolate of which gives (R)-(+)-beta-hydro xy-beta-phenylpropionic acid in high enantiomeric excess.