Abstract
The oxidation of tetraacetylsecologanin dimethyl acetal (7) by m-chloroperbenzoic acid has been studied and the results have been compared with those obtained when tetraacetylsweroside was oxidized by the same reagent. Complete site selectivity for the vinyl side chain and a relatively low stereoselectivity leading to the formation of a small excess (7:2) of the epoxide with the 3-R configuration was observed with the sweroside derivative. These results parallel those in the osmium tetraoxide oxidation, and the configurations of the glycols produced have been correlated chemically with those of the epoxides. In contrast, there is little site selectivity in the oxidation of the secologanin derivative; in fact, it is the β-alkoxyacrylate site that appears to be oxidized faster to produce an epoxide (9) that then undergoes a second epoxidation at the vinyl side chain. Stereoselectivity is complete in the first epoxidation and is high in the epoxidation of the vinyl side chain, leading to the isomer with the 3-R configuration as the major product. The site selectivity appears to parallel that observed in dihydroxylation of the secologanin derivative, but the stereoselectivity of oxidations at the vinyl side chain is opposite in the two series; the configurations of the glycols and epoxides have been correlated chemically. A rational explanation can be advanced for the observed stereoselectivity in every case, but no convincing argument to explain the observed differences in site selectivity has been found.
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