The stereochemistry and conformation of (±)-6a,12a-dihydrorotoxen-12(6H)-one and natural rotenoids from spin-coupling considerations

Abstract

The thermodynamically stable form of 6a,12a-dihydrorotoxen-12(6H)-one is shown to be the cis-form by analysis of the spin-coupling of the protons attached to C-6, C-6a, and C-12a. Assignments are confirmed by deuteration and by examination of the 6,6-dimethyl derivative and enable the probable ring-B conformation in deuteriochloroform solution to be determined. Masking of the multiplets by other groupings occurs in rotenone and some natural rotenoids, but in suitable solvents elliptone can be analysed, confirming the cis-B/C fusion allotted to natural rotenoids on other grounds. The α-orientation of the hydroxyl grouping resulting from borohydride reduction of a cis-B/C rotenoid is also supported by n.m.r. results.