Diastereoselective alkylation of 8-phenylmenthyl phenylacetate : aggregated lithium enolate versus naked enolate

It is shown that monoalkylation of 8-phenylmenthyl phenylacetate using lithiated bases leads to poor or no diastereoselectivities (50/50 to 69/31) and high yields (75 to 98%) while alkylation using tBu-P4 (a strong and cation free base, known to provide «naked» anion) leads to high diastereoselectivities (92/8 to 98/2) and high yields (65 to 95%). It is postulated that, in the case of phenylacetates, the degree of aggregation of the lithium enolate is responsible of the poor diastereoselectivities