摘要
Potential exo-specific [3+2] cascade cyclizations between o-Br-substituted cinnamates (CM−Br) and benzo-fused 7-oxa-norbornadiene (Bz-OND) by Pd-mediated catalysis were explored both in racemic and asymmetric methods. A systematic study on Pd(II) catalysts, silane additives, carbonate bases, solvents, chiral diphosphines within a reaction temperature range of 60–80 °C were carried out. The best combination was between methyl o-Br-cinnamate and Bz-OND catalyzed by Pd(OAc) 2 /dppf and /DM-BINAP, respectively, in CH 3 CN at 80 °C. The desired racemic and optically active Bz-OND-fused indanes bearing 1-carbonylmethylidene (CBM) unit were thus afforded in 95 % and 71 % yields and with enantioselectivities of up to 43 % ee. The optimal asymmetric recipe can be applied to alkyl, aryl cinnamates, 4-substituted, and 5-substituted o-bromo-cinnamates. Exclusive hydrogenations from the exo-face of the CBM alkene units can be readily done by Pd(0)/charcoal in EtOAc, affording the product complementary to the direct Rh(I)-catalyzed cyclization.