Preparation of 2-methyl-3-oxo triterpenoids of 18α-oleanane series and the conformation of ring A

2-Methyl-3-hydroxy derivatives X, XIII, XV, XVI and 2-methyl-3-oxo derivatives III, VIII and IX of 19β,28-epoxy-18α-oleanane were prepared from ketones I and XVII. From spectral data it was deduced that compounds III, VIII and XV containing a 2β-methyl group have their A ring in boat conformation. Both isomeric 2-methyl-3-oxo derivatives VIII and IX are approximately equally stable; 52 ± 4% of the 2β-isomer VII were found in their equilibrium mixture.