Cross Couplings of Cinnamates to Benzo‐Fused Oxanorbornadienes

Abstract

AbstractPotential 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 CH3CN 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.