Selective transformations of secologanin: dihydroxylation

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Abstract
Sweroside tetraacetate (2b) has been converted in high yield by osmylation to the epimeric glycols 3 and 4 (R = H) which have been separated. This reaction proceeds with high chemoselectivity for reaction at the vinyl side chain but with low stereoselectivity, leading to a small excess of the 3-R isomer. Secologanin derivatives appear to show a much lower chemoselectivity in this reaction, but the stereoselectivity is considerably higher, and opposite to that in the sweroside example. Conditions have been found for oxidizing the dimethylacetal 5b of secologanin tetraacetate in useful yields to the epimeric glycols 6 and 7 (R = H); in this reaction a large excess of the 3-S isomer is formed. The reasons for these differences in selectivity appear to be related to differences between the two series in conformational preference.

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