Asymmetric Aldol Reactions Using Boron Enolates

Abstract The directed aldol reaction allows the construction of new carbon–carbon bonds in a regio‐, diastereo‐, and enantioselective manner. The kinetically controlled, boron‐mediated aldol reaction is particularly powerful for the efficient synthesis of β‐hydroxy carbonyl compounds. Compared to other metal enolates, the boron–oxygen bond in boron enolates is relatively short which, on addition to aldehydes, leads to tight cyclic transition states and highly stereoselective carbon–carbon bond formation. Moreover, variation of the steric demands of the ligands on boron allows discrimination between competing transition states. Chiral auxiliaries attached to the boron enolate are frequently employed to control the relative and absolute stereochemistry of the aldol products. Asymmetric reactions using chiral ligands on boron are also possible and these produce useful enantiomerically enriched adducts. The utility of boron‐mediated aldol reactions has been demonstrated in numerous total syntheses of complex polyoxygenated natural products, and several of these are highlighted in the Application to Synthesis section of this chapter. Several reviews of the directed aldol reaction are available, including the Organic Reactions chapter by Mukaiyama in 1981. 9 The material covered in this review concerns only the asymmetric formation of β‐hydroxy carbonyl compounds using boron enolates and surveys the literature from 1981 until the end of 1995.