endo and exo Ring fusion in the Diels–Alder reaction of 1-(2,4,6-trialkylphenyl-)3-methylphospholes with maleic acid derivatives

Diels–Alder reaction of the title phospholes and N-phenylmaleimide afforded, surprisingly, a mixture of endo and exo fused cycloadducts with the P-aryl substituent anti to the double bond giving, after oxidation the corresponding P-oxides. The P-center of the exo ring fused P-oxides was found to be inverted under the conditions of the oxidation. The cycloaddition of triisopropylphenylphosphole with N-methylmaleimide or with maleic acid anhydride gave the corresponding endo fused phosphanorbornenes affording stable products after oxidation. Relative stability of the possible isomers was evaluated experimentally and by quantum chemical calculations.