Novel Bioactive Cubé Insecticide Constituents: Isolation and Preparation of 13-homo-13-Oxa-6a,12a-dehydrorotenoids

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Abstract
Two novel rotenoid-like compounds isolated from cubé resin, used as an insecticide and piscicide, have an unprecedented skeleton consisting of a 1,5-benzodioxepin (A and B rings) and a chromone (C and D rings). Their structures are assigned as (−)-13-homo-13-oxa-6a,12a-dehydrorotenone (5) and 13-homo-13-oxa-6a,12a-dehydrodeguelin (6) based on 1H and 13C NMR, MS, UV, and optical rotation. X-ray structure determination confirmed the assignment of 5 and revealed that the 1-methylethenyl group is disordered into two sets of atoms. Compound 5 from cubé resin was identical with a product previously reported as being formed by reaction of rotenone (1) with acetyl chloride and DMF and misassigned as “6a,12a-epoxyrotenone” (7). A modified procedure was developed for preparation of the new oxarotenoid component of the commercial insecticide. The cubé resin from which 5 and 6 were isolated in equal amounts contains 1 and deguelin (2) in a >10:1 ratio; this large difference in ratio indicates that 5 and 6 are not artifacts formed from 1 and 2, respectively. Rotenoid 5 acts as a respiratory inhibitor with 50% inhibition of NADH:ubiquinone oxidoreductase activity at 0.11 μM, of goldfish survival at 1 ppm, and of the viability of three cell lines at 4−8 μM.