PI-FACIAL SELECTIVITIES IN CYCLOADDITIONS TO NORBORNYL-FUSED AND NORBORNENYL-FUSED PARA-BENZOQUINONES

The stereochemistry of the Diels-Alder cycloaddition of several dienes to the facially perturbed dienophiles 2,3-norbornenobenzoquinone (3) and 2,3-norbornanobenzoquinone (4) has been examined. Unambiguous structural proof for the adducts formed has been obtained from complementary 1H and 13C NMR spectral data and in two cases through X-ray crystal structure determination. While 1,3-cyclopentadiene, 1,3-cyclohexadiene, and cyclooctatetraene exhibit preference for addition to 3 from the bottom side, the stereochemical outcome is reversed in their response to 4. 1,3-Diphenylisobenzofuran and 1,2,3,4-tetrachloro-5,5-dimethoxycyclopentadiene engaged 3 from the top side with marked selectivity, which is further enhanced in their reaction with 4. The observed stereoselectivities seem to be essentially controlled by steric interactons at the transition state. Model calculations provide support for this interpretation. © 1990, American Chemical Society. All rights reserved.