Transition-metal mediated exo selective and enantioselective Diels-Alder reactions for the preparation of octalones with unusual stereochemistries.: Reactions of 2-cobaloxime substituted 1,3-dienes with cyclohexenone in optically active Lewis acid catalyzed [4+2] cycloadditions

2-Cobaloxime-E-1,3-pentadienyl complexes (cobaloxime = pyridine(dimethyl or diphenylglyoxime)(2)Cobalt (III)) react with cyclohexenone in enantioselective Lewis acid catalyzed [4 + 2] cycloaddition reactions. These cycloaddition reactions produce octalones with cis ring junctions through exo transition states. The octalones are removed from the cobalt via demetallation reactions which replace the cobalt with a hydrogen and provide pyr(dmg)(2)CoMe which can be recycled into the synthesis of the starting dienyl complex. The enantiomeric purity of the demetallated octalones was determined by gas chromatographic analysis. (C) 1999 Elsevier Science S.A. All rights reserved.