π-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 1 H nuclear magnetic resonance spectra and by X-ray crystallographic methods. The π-facial stereoselectivity was rationalized in terms of unfavorable orbital interactions, steric hindrance between the dienophile and the syn face of benzene oxide, and σ-donation by the oxygen. Key words: cycloaddition, Diels-Alder, syn-anti, π-facial stereoselectivity, benzene oxide.