A study on the scope of the photochemical ring contraction of substituted 2-amino-3-cyano-4H-pyrans to cyclobutenes: crystal structure of 3-carbamoyl-3-cyano-1-ethoxycarbonyl-4-isopropyl-2-phenylcyclobutene

Abstract

A study on the influence of substitution on the photochemical reactivity of a series of differently substituted 2-amino-4H-pyrans 10a–h has been carried out. All of the compounds undergo ring contraction to the corresponding cyclobutenes 11a–h on direct irradiation. Variable amounts of the enamides 12 and the alkynes 13 were also obtained, as secondary photoproducts. The results obtained show that the cyclization takes place in reasonable yields with alkyl, phenyl and hydrogen substitution at C-4 and at C-6. Cyano, ethoxycarbonyl and benzoyl substitution at C-5 also promotes the reaction. A high degree of stereochemical control was observed in most cases. The molecular geometry of cyclobutene 11a₁ has been established by X-ray diffraction analysis. This study also shows that the crystal packing is formed by a system of chains linked by strong hydrogen bonds.