Ring-D aromatic phytosteroids; a model for biogenesis by way of carbon radical rearrangement

Abstract

A mechanism is postulated for the biogenesis of the unique ring-D aromatic phytosteroids from Nicandra physaloides, which involves rearrangement, ring expansion, and aromatisation of a carbon radical generated by cytochrome P450 (Scheme 1). In support, model hydrindene acids 15b and 18b have been synthesized and subjected to homolytic decarboxylation; the latter acid yielded 6-methyltetralin 24, in biomimetic fashion. The isomeric acid afforded not only 6-methyltetralin 24 but also 5-methyltetralin 26; mechanisms for this unusual rearrangement are discussed.