Regioselective enamine formation from oxonia-boranuida-betaines and their application in asymmetric Michael reactions

beta-Diketonato borate betaines 2 are readily accessible by the reaction of beta-dicarbonyl compounds 1 with BF3.OEt2. Reaction of borates 2 with L-valine diethylamide (3a) gives almost exclusively the exocyclic enamines 4 as the kinetic products. In contrast, direct, acid catalyzed conversion of beta-diketones I with the chiral auxiliary 3a yield the endocyclic enamines 5 as the thermodynamic products. Both enamines 4 and 5 give products with quaternary stereocenters in high selectivity in copper-catalyzed asymmetric Michael reactions with methyl vinyl ketone (8). But interestingly, exo- and endocyclic enamines are complementary with respect to stereochemistry of the subsequent Michael reactions since they give stereocenters with opposite configurations. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.