Mechanism of chiral Lewis acid mediated enantiotopic group-selective ring cleavage of cyclic acetals derived from meso-1,2-diols

Diastereoselectivity and enantioselectivity of chiral oxazaborolidine-mediated ring-cleavage reactions of meso-2,4,5-trisubstituted 1,3-dioxolane acetals with a trimethylsilyl ketene acetal were investigated in detail and discussed in terms of a mechanism involving a contact ion pair as a product-forming intermediate. Both diastereomeric 2-phenyl derivatives syn- and anti-11a gave the same ring-cleavage product 13a. However, the reaction of 2-phenylethynyl derivatives syn- and anti-11b proceeded almost stereospecifically, giving rise to 13b and 14b, respectively. In all of the reactions, isomerization of diastereomeric acetals was not observed. On the basis of these results, it was deduced that the dissociation of a: Lewis acid-acetal complex is the rate-determining step, and the resulting ion pair intermediate undergoes either isomerization to a diastereomeric acetal or attack by a nucleophile depending on the structure of the acetal. The possible enantioselection at the product-forming step was ruled out. It was proposed that the observed enantioselectivity is determined by enantiodifferentiating coordination of the acetal oxygen atom by the chiral Lewis acid.