Tetracyclic triterpene synthesis. Part 2. Deduction of ring fusion configurations in cis- and trans-7-methoxy-3a,9b-dimethyl-1,3,3a,4,5,9b-hexahydrobenz[e]inden-2-ones by nuclear magnetic resonance spectroscopy

Abstract

Detailed analyses of the ¹H and ¹³C n.m.r. spectra of the cis- and trans-3a,9b-dimethyl derivatives of 7-methoxy-1,3,3a,4,5,9b-hexahydrobenz[e]inden-2-one enable a clear distinction to be made between the two ketones. The ¹H spectra show that the cis-ketone exhibits rapid inversion of the cyclohexene ring whereas the trans-ketone is conformationally rigid. The spectral parameters for these two compounds, and their 3a-CD₃ and 3a-CD₂H analogues, are discussed.