Novel derivatives of ascomycin through photochemistry

Abstract

Irradiation of ascomycin <b>1</b> leads to three reaction pathways: (a) [1,3]-sigmatropic shift of the allylic carbonyl group leading to the ring-contracted products <b>3</b> and <b>4</b>, (b) electron-transfer from the amide nitrogen to the carbonyl group followed by reorganisation leading to a zwitterionic intermediate <b>Z</b> which undergoes either intramolecular cyclisation (leading to. 6 in CH₃CN) or addition of methanol (leading to 2 in MeOH), and, (c) decarbonylative fragmentation resulting in the acyclic lactone <b>5</b>. The effects of solvent, wavelength, and the substrate-srtucture on the product distribution have been examined. Simple transformations of the photoproducts leading to novel derivatives are presented.