Synthese und Photoisomerisierung von sterisch gehinderten 2,6-Dialkylpyridin-N-oxiden / Synthesis and Photoisomerization of Sterically Hindered 2,6-Dialkylpyridine-N-oxides

Abstract

Synthesis of the new sterically hindered pyridine-1-oxides 2c and 6a–c is described. Copper(II)-catalyzed photolysis of 2 and 6a in aqueous medium provides only small amounts of 2-acylpyrroles 8 and 10a together with by-products. Exclusive formation and better yields of 10 can be received upon irradiation of 6 in none protic solvents at low temperature. Introduction of bulky substituents as in 6 failed to stabilize any of the postulated intermediates in order to be identified during photoreaction. It is assumed that the high rate of polymerization is caused by unstable 1,3-oxazepines like 13.