PI-FACIAL STEREOSELECTIVITY IN THE DIELS-ALDER REACTIONS OF BENZENE OXIDES

The Diels-Alder reactions of N-phenylmaleimide and dimethyl acetylenedicarboxylate with benzene oxide (1,3,5-cyclohexatriene 1,2-oxide, 3) and its more substituted derivatives 1,2-dimethyl-1,3,5-cyclohexatriene 1,2-oxide (7) and 10-oxatricyclo[4.3.1.0]deca-2,4-diene (11) in a kinetic manner gave exclusively products of addition anti to the plane-nonsymmetrical oxygen. The structures of the adducts were determined unequivocally by nuclear Overhauser enhancements in their H-1 nuclear magnetic resonance spectra and by X-ray crystallographic methods. The pi-facial steroselectivity was rationalized in terms of unfavorable orbital interactions, steric hindrance between the dienophile and the syn face of benzene oxide, and sigma-donation by the oxygen.