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 1a 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 1 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 .