Eudesmanolides (1, 17) was subjected to a novel skeletal-cyclization rearrangement to trimethylethanoadamantanone (2) using hot phosphoryl chloride. When the reaction product was treated with hot aqueous sodium hydroxide, new epoxytricycloeudesmanone (5) and tricycloeudesmendiones (6, 7) were isolated in addition to the epoxy ketone (4). The absolute stereostructures of 5, 6 and 7 were determined by spectrometric and X-ray crystallographic methods. Tricycloeudesmenones (8, 9) as the main product in the aforementioned reaction were neatly separated through the reoxidation of their alcohol derivatives (10, 11), and well characterized by correlating 8 and 9 with 4, 5, 6, and 7. A unique structure of the byproduct (12) obtained besides 10 and 11 on LiAlH_4 reduction of 8 and 9 was also elucidated. The proposed reaction pathway from 1 to 2 through carboxyeudesmene intermediates and the key intermediate tricycloeudesmenones (8, 9) was rationalized on the basis of acid catalized rearrangement experiments and the force field calculation for possible candidate intermediates (8, 9 to 2) created by using a computer technique.
