Reaction of vinylcarbenoids with benzaldehydes: Formation of vinylcarbonyl ylides followed by ring closure to oxiranes and dihydrofurans

Rh-2(OAc)(4)-catalyzed reaction of vinyldiazo compound la in the presence of p-methoxybenz- 2a, mesit- 2b, and p-chlorobenzaldehyde 2c gave a mixture of isomeric vinyloxiranes 3a-c and 4a-c, and sterically unstable (E)-dihydrofurans 5a-c, but not stable (Z)-dihydrofurans 6. However, the reactions with p-nitro- 2d and 2,4-dinitrobenzaldehyde 2e gave (Z)-dihydrofurans 6d,e along with 3d, 4d, and 5d,e. The reaction in the presence of maleic anhydride and dimethyl fumarate gave single 1,3-dipolar cycloadducts 11 and 13, respectively, indicating that a single conformer, the sterically unstable endo-aryl-endo-cyanostyryl carbonyl ylide 14 (15), is initially formed in the reaction of I with 2. It was concluded that the endo-vinyl-exo-aryl vinylcarbonyl ylides 17a-c arising from 2a-c undergo disrotatory cylization to exlusively produce 5, whereas the ylides 17d,e arising from 2d,e undergo partly symmetry-forbidden conrotaory cyclization to sterically stable trans-diaryldihydrofurans 6d,e as well as the symmetry-allowed process to 5d,e.