Reaction of vinylcarbenoids with aldehydes: formation of vinylcarbonyl ylides followed by ring closure to oxiranes and dihydrofurans

Rh2(OAc)4-catalyzed reaction of vinyldiazo compound 1 in the presence of benzaldehydes 2a–2c gave a mixture of isomeric vinyloxiranes 3 and 4, and sterically unstable (E)-dihydrofurans 5, but not stable (Z)-dihydrofurans 6. However, the reaction in the presence of maleic anhydride gave single 1,3-dipolar cycloadducts 15, indicating that onyl endo-aryl-endo-cyanostyryl carbonyl ylide 7 (8) is initially formed in the reaction of 1 with 2.