Studies on the Stereoselectivity of Osmylation of cis-Bicyclo(3.3.0) oct-6-enes.

Treatment of cis-2-oxabicyclo[3.3.0]oct-6-en-3-one 1 with osmium tetraoxide under a variety of conditions led predominantly to attack from the more hindered face of the alkene to give, after protection of the diol, the endo-acetonide 8 as the major product. It is apparent that the stereochemical outcome of this reaction is due to a directing effect from the lactone carbonyl at C-3 since osmylation of the analogous lactols 3, acetals 4 and cyclic ethers 5 gave attack mainly from the less hindered exo-face. The exo-acetonide 7 was prepared in a 3-stage procedure from cis-bicyclo[3.2.0]heptenone 6 in 54% yield. In this case the stereoselectivity of the osmylation reaction was governed, as expected, by the conformation of the bicyclic skeleton, no directing effect from the ketone carbonyl was apparent.