Regioselective Enamine Formation from Oxonia-Boranuida-Betaines and Their Application in Asymmetric Michael Reactions

β‐Diketonato borate betaines 2 are readily accessible by the reaction of β‐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 β‐diketones 1 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)