Pinacolyl boronic esters as components in the Petasis reaction

The multicomponent reaction of boronic esters with imine or iminium species (generated in situ by reaction of amines with glyoxylic acid) has received little attention in the literature despite the current interest in the corresponding reactions of boronic acids (Petasis reaction). Moreover, the use of boronic esters in this reaction is particularly attractive since chiral esters could act as auxiliaries in a novel enantioselective process. We set out to establish whether boronic esters were general substrates in the Petasis reaction. Pinacolyl boronic esters were selected for study as a model substrate for more complex homochiral boronic esters because of their ease of synthesis and chemical stability. We found that pinacolyl boronic esters do not react with imines derived from primary amines and glyoxylic acid under standard conditions. By contrast, imines derived from secondary amines and glyoxylic acid react readily with vinylboronic esters but less readily with heteroaryl- or arylboronic esters.