Face selectivity of the Diels‐Alder additions and cheletropic additions of sulfur dioxide to 2‐ vinyl‐7‐oxabicyclo[2.2.1]hept‐2‐ene derivatives

Racemic 6‐ethenyl‐7‐oxabicyclo[2.2.1]hept‐5‐en‐2‐one ( 23 ), 5‐ethenyl‐7‐oxabicyclo[2.2.1]hept‐5‐en‐2‐one ( 25 ) and their ethylene acetals 24 and 26 , respectively, were derived from the Diels‐Alder adduct of furan to 1‐cyanovinyl acetate ( 27 ). The Diels‐Alder additions of 26 to dimethyl acetylenedicarboxylate, to methyl propynoate, to N ‐phenylmaleimide, and to methyl acrylate were highly exo ‐face selective, as were the cycloadditions of methyl propynoate to dienones 23 and 25 and of dimethyl acetylenedicarboxylate to ethylenedioxy‐diene 24 . The cheletropic additions of SO 2 to 23 – 26 gave exclusively the corresponding sulfolenes 57 – 60 resulting from the exo ‐face attack of the semicyclic dienes under conditions of kinetic and thermodynamic control.