NITROALKENE INTER [4-(-)-2,2-DIPHENYLCYCLOPENTANOL AS THE CHIRAL AUXILIARY(2] INTRA [3+2] TANDEM CYCLOADDITIONS .7. APPLICATION OF (R))

Chiral enol ethers derived from (R)-(-)-2,2-diphenylcyclopentanol ((-) -3) have been found to provide high levels of asymmetric induction in tandem inter [4 + 2]/intra [3 + 2] nitroalkene cycloadditions. The chi ral auxiliary (>97% ee) is readily prepared in three steps from diphen ylacetonitrile employing an asymmetric oxazaborolidine-catalyzed boran e reduction. Either enantiomeric series of tandem cycloaddition/hydrog enolysis product 7 is available from the chiral auxiliary of a single absolute configuration by judicious selection of the Lewis acid promot er, Ti(O-i-Pr)(2)Cl-2 (98% ee, (-)-7) or MAPh (93% ee, (+)-7). Propeny l ethers derived from (-)-3 undergo endo selective [4 + 2] cycloadditi ons (2.3/1 - 8.2/1) in the presence Ti(O-i-Pr)(2)Cl-2; however, exo se lective [4 + 2] cycloadditions (10.2/1 - 54.8/1) are observed in the p resence of MAPh.