Dammarane triterpenes of Trevoa trinervis: structure and absolute stereochemistry of trevoagenins A, B, and C

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Abstract
Trevoagenins A, B, and C, extracted from Trevoa trinervis Miers, are shown, by chemical and spectral means, to be isomeric dammarane triterpenes possessing the general 3β,25,30-trihydroxy-(20,24)-epoxydammaran-16-one structure with stereoisomeric side-chains of the ocotillol type. Trevoagenin A (20R,24R)-(1), whose stereochemistry has been established by chemical methods and confirmed by X-ray analysis, was transformed into (20)R,24ξ–ocotillone (26) and the C-24 stereochemistry was assigned as R. As a consequence, the C-24 stereochemistry for ocotillol-related compounds of the (20R)-series, unestablished so far, has been determined. The stereochemistry of trevoagenin B, (20S,24R)-(13), was established by chemical correlation with trevoagenin A. Moreover, trevoagenin B was transformed into ocotillol (20S,24R)-(21) and its recently questioned C-24 stereochemistry has been reaffirmed. Trevoagenin C, (20S,24S)-(17), is the C-24 isomer of trevoagenin B as shown by degradation of both compounds to the lactone (16).

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