Preparation of isomeric 2-methyl-1-oxotriterpenoids of the 18α-oleanane series; Conformation of ring A

Abstract

2β-Methyl-1α-hydroxy (XV) and two isomeric 2-methyl-1-oxo derivatives (XVI and XVII) of 19β,28-epoxy-18α-oleanane were prepared from 19β,28-epoxy-2-methyl-18α-olean-1-en-3-one (XI) via 3β-chloro derivative XIII and unsaturated 1α-hydroxy derivative XIV. Allylic oxidation of 2-methyl-2-ene VI was studied as an alternative approach to compound XIV. Oxidation with selenium dioxide led to diol VII, aldehyde VIII and alcohol IV, oxidation with tert-butyl chromate gave epoxy ketone X. According to 1H NMR and CD data, ring A in the 2β-methyl derivatives XV and XVI exists in a boat conformation. The 2α-methyl ketone XVII (chair form) is more stable than its 2β-epimer XVI (boat form): only 9 ± 3% of XVI exists at equilibrium. Conformation of ring A in the unsubstituted 1-ketone I and differences in stability of the boat form in 1,2- and 2,3-disubstituted triterpenoids are discussed.