Palladium-catalyzed allylation of α-hydroxy acids

Mandelic and lactic acids are converted to the 1,3-dioxolan-4-ones by treatment with acetone dimethyl acetal. Deprotonation followed by treatment with an allyl acetate and a catalytic amount (1 mol%) of palladium catalyst afforded the allylated dioxolanones, which could be hydrolyzed to the corresponding alpha-allyl alpha-hydroxy acids. The lithium enolate of the dioxolanone of mandelic acid was also coupled with methallyl, cinnamyl, geranyl and (E)-1-methyl-2-butenyl acetates. The zinc enolate of the dioxolanone of tactic acid reacted smoothly with allyl acetate in a catalyzed reaction whereas no detectable reaction was observed when the lithium enolate was used. This appears to be the result of complications arising from the enhanced basicity of the lithium compared to zinc enolate. Various attempts were made to achieve enantioselectivity using chiral ligands on the palladium catalyst. The zinc enolates were found to provide better results although the enantioselectivity was only modest, about 30% enantiomeric excess being the best result obtained. Chiraphos proved to be the best optically active ligand of a variety tested