Photoinduced transformations. Part XXX. Photorearrangement of (22S,25S)-N-acetylveratra-5,8,13(17)-trienine-3β,11β,23β-triol 3,23-diacetate 11-nitrite, a fused cyclopentenyl nitrite, to two spiroisoxazolines

Abstract

Irradiation of the title nitrite (3) in protic or aprotic solvents with Pyrex-filtered light induced an extensive rearrangement to afford two products with novel isoxazoline structures, designated photo-TVTTN and isophoto-TVTTN, in good yields. The structure and absolute configuration of photo-TVTTN (4) were determined by chemical and spectroscopic studies and by an X-ray crystallographic analysis of an iodine-containing derivative (14). The structure and absolute configuration of isophoto-TVTTN (5) were deduced by spectroscopic studies.

A photoreaction carried out with monochromatic light (362 ± 4 nm) confirmed that light was only involved in homolysis of the O–NO bond, and that photo- and isophoto-TVTTN were formed via an unstable hydroxyimino-aldehyde (19), which was isolated. The hydroxyimino-aldehyde was transformed smoothly into photo- and isophoto-TVTTN on silica gel during t.l.c. Whereas the silica gel-induced transformation was non-stereoselective, leading to both photo-(4) and isophoto-TVTTN (5), the transformation of the hydroxyimino-aldehyde with acetic acid or [²H]acid was stereoselective, giving mainly isophoto-TVTTN (5) or [9β-²H]isophoto-TVTTN (21), respectively. Deuterium is incorporated into the TVTTNs in the course of intramolecular Michael reaction catalysed by acetic [²H]acid. This unusually ready intramolecular reaction seems to be the first reported example of a Michael addition of an oxime hydroxy-group to an αβ-unsaturated carbonyl system.