Steric Effects on Oxymercuration : Oxymercuration-Demercuration of Taraxasteryl, Pseudo-Taraxasteryl and 19-Epi-Taraxasteryl Acetatest

Department of Chemistry, University College of Science, Calcutta-700 009

Contrary to usual expectation, taraxasteryl acetate (1b) and pseudo-taraxasteryl acetate (2b) were found to be totally insensitive to normal oxymercuration. This has been rationalised as being due to the steric effects of the 19- and 17-methyl groups on the formation of intermediate mercurium ions, and on the nucleophile attacking these species. Conformational analysis of lb shows that its ring E is not a normal chair, but assumes a twisted conformation due to severe nonbonded interaction between the 12-methylene and the 19-methyl group. Both 1b and 2b, however, underwent oxymer­curation-demercuration with allylic rearrangement to give the same compound 2c, when heated with Hg(OAc)₂ in glacial HOAc. A partial synthesis of 19-epi-taraxa^­steryl acetate (1d) has been achieved by Witting reaction on the 19-epi-norketone (4b) formed by base-catalysed epimerisation of 4a which in turn was derived from lb. The steric effects providing the driving force for this unconventional epimerisation of 4a to 4b have been explained and supported by comparative ¹³C nmr spectral analysis of 4a and 4b. Unlike 1b and 2b, 19-epi-taraxasteryl acetate (1d) underwent normal oxymer­curation. The stereochemical aspect of this reaction has been discussed.