Stereostructure, conformation and reactivity of beta- and alpha-gardiol from Burchellia bubalina

From the leaves of Burchellia bubalina (Rubiaceae) pure beta- and alpha-gardiol have been isolated, the latter in crystalline form for the first time. X-ray analysis of beta-gardiol establishes its relative configuration. Treatment of the tosyl derivative of beta-gardiol with base affords a crystalline elimination product amenable to X-ray analysis. Evidence is presented for the partial conversion of alpha-gardiol to the epimeric beta-gardiol at room temperature. Molecular dynamic studies at 1000 K on the two gardiols provide useful conformational information. In the case of beta-gardiol, the analysis shows interesting correlations between conformations adopted at 1000 K and those present in the crystalline state at room temperature.