Photoinduced transformations. Part 38. Photoreactions of 17-ethoxycarbonylmethylene-etiojerva-5,13(17)- and -5,16-diene-3β,11β,20ξ-triol 3,20-diacetate 11-nitrites

Abstract

Photolysis of one of the title nitrites (5) in benzene with a Pyrex-filtered light afforded a complex mixture including as major products 17-ethoxycarbonylmethylene-etiojerva-5,13(17)-diene-3β,11β,20ξ-triol 3,20-diacetate (4), 17-ethoxycarbonylmethylene-11a-aza-C-homoetiojerva-5,8,11,12(14), 13(17)-pentaene-3β,20ξ-diol 3,20-diacetate 11a-oxide (6), and 17-ethoxycarbonylmethylene-11a-aza-C-homoetiojerva-5,11a, 13(17)-triene-3β,11α,20ξ-triol 3,20-diacetate 11a-oxide (7); in contrast, photolysis of N-acetyl-(22S,25S)-5α-veratr-13(17)-enine-3β,11β,23β-triol 3,23-diacetate 11-nitrite (1) having a 12α-hydrogen is known to give (22S,25S)-N-acetyl-11a-aza-C-homo-5α-veratr-11a, 13(17)-diene-3β,11α,23β-triol 11a-oxide (2) as the sole product. The formation of the nitrone (7) having an α-oriented hydroxy-group supports the mechanism of the rearrangement previously proposed. An estimation by an empirical force-field method with a model hydrocarbon indicates that the five-membered ring of the nitrite (5) is more strained than that of the nitrite (1) by ca. 6 kcal mol^–1. Thus, the difference in the products obtained from the nitrites (1) and (5) may be attributable to a difference in the polar effects of the C-17-substituents which influence the degree of stabilization of the polar transition state.

The result of the photolysis of 17-ethoxycarbonylmethylene-etiojerva-5,16-diene-3β,11β,20ξ-triol 3,20-di-acetate 11-nitrite (10) is also described.