Methyl 2-aryl-2H-azirine-3-carboxylates as dienophiles

Abstract

Diels–Alder reactions of the methyl 2-aryl-2H-azirine-3-carboxylates 1a and 1b have been investigated in order to determine the stereoselectivity and regioselectivity of the reaction. The azirines react with a range of electron rich dienes at room temperature to give Diels–Alder adducts. The reaction leads to the formation of endo-cycloadducts with both cyclic and acyclic dienes; furan is exceptional in giving an exo-adduct 7. X-Ray crystallography has established the structure 9 of the Diels–Alder adduct formed from the azirine 1a and 1-acetoxybutadiene. The same regioselectivity, in which the more nucleophilic terminus of the diene combines with the carbon atom of the CN bond, is found in the adducts from several other 1- and 2-substituted dienes. Only isoprene, among the dienes examined, gives a mixture containing the opposite regioisomer 14 as a minor component.