Boron-Mediated Aldol Reaction of Carboxylic Esters

概要

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Boron-mediated aldol reaction of carboxylic esters is described in detail. Contrary to the gener-al belief that carboxylic esters are inert toward the conventional enolization conditions, propionate esters are shqwn to have adequate reactivity on the boron-mediated aldol reaction. More impor-tantly, the stereochemical course of the aldol reaction can be controlled by the judicious choice of the enolization reagents. Complementary anti- and syn-selective asymmetric aldol reaction of structurally related chiral esters are developed. Also, novel double aldol reaction is discovered with acetate esters, which provides a precursor to the synthesis of chiral triols of C3-symmetry. Extensive NMR experiments lead to characterize the first carbon-bound boron enolates and the novel doubly borylate enolates as intermediates of the double aldol reaction. A plausible mecha-nism of the double aldol reaction is proposed.