Conformation of ring A of 19β,28-epoxy-18α-oleanan-3-one in solution and in the solid state. Crystal structure and carbon-deuterium stretching frequencies

Abstract

The crystal structure of allobetulone (I) has been determined. The compound is orthorhombic, P212121, a = 14.824(4), b = 25.701(7), c = 6.545(2) *A, Z = 4, R = 0.050 for 2 303 observed reflections. The rings B, C, D and E assume chair conformation, the chair of the ring E being considerably deformed by the ether bridge. The five-membered ether ring exists in an envelope form. The ring A assumes a boat form of an unusual type: one half has a typical classical boat geometry with coplanar atoms C(10), C(1), C(2) and C(3) whereas the other half resembles a twist boat. This conformation does not correspond to the published physical data found in solution. The conformation of ring A in crystal and in solution was further studied using IR spectra of 2α- and 2β-deuterio derivatives of allobetulone (II and III, respectively) in the carbon-deuterium stretching vibration region. The solid phase spectra are compatible with the boat form of ring A. Comparison of the solid phase spectra with solution spectra clearly indicates a chair-boat equilibrium in solution of the 3-oxo derivatives I-III. The carbon-deuterium stretching frequencies of other deuterium-labelled triterpenoid ketones (IV-VI) and alcohols (VII-X) are also discussed.