An expeditious total synthetic route to naturally occurring tricarbocycIic ring ring-C-aromatic diter penoids: mechanisms of cyclialkylations

Abstract

The critical role of the nature of aromatic ring substituents in the open chain substrates in the acid-catalyzed cyclialkylation reaction through 2-2(arylethyl)-l,3,3-trimethylcyclohexyl cation is exemplified with a large number of easily accessible mono- and disubstituted 2-(2-arylethyl)-l,3,3-trimethylcyclohexanols and a few 2-(2- arylethyl) -3,3-dimethyl-l-methylenecyclohexanes i n the distributions of the respective trans- and cis-podocarpatrienes. The cyclohexanol or the cyclohexylidene precursors having unactivated aromatic ring proceeds with high stereoselectivity leading to the respective trans- products, while the substrates with an electron donating substituent with respect to the site of electrophilic attack result in the corresponding cis- and the trans-product mixtures. Consistent mechanisms for these stereochemical results have been advanced. Based on these results simple syntheses of a few diterpenes have been realized.