The cleavage of 17,20-oxygenated pregnanes

A variety of oxidative reagents can be used to convert 17-hydroxypregnanes containing an additional oxygen function at C-20 into 17-ketoandrostanes. Among the most widely used have been chromium trioxide, lead tetraacetate, periodic acid and sodium bismuthate. 

Chromium trioxide (usually in acetic acid) is the least selective of all the 

The 17,20-glycol grouping (with or without an additional C-21 alcohol) can be cleaved to the 17-ketone by any of the four reagents. However, the presence of a 20-ketone has a marked influence on reactivity. Thus, 17a-hydroxy-20-ketones lacking additional oxygenation at C-21 are readily cleaved only by chromium trioxide. A recent report,however, claims that 17a-hydroxypregnenolone 3-acetate can be quantitatively cleaved to dehydroepiandrosterone acetate using lead tetraacetate in an aprotic solvent 

The mechanisms of oxidation using bismuthate, periodate or lead tetraacetate, while still not completely understood, are probably similar, involving some type of cyclic ester formation as the first step.  

Sodium Bismuthate Degradation To a solution of 9a,llj5-dichloro-17a,21-dihydroxypregna-l,4-diene-3,20-dione (1 g) in 50% aqueous acetic acid (800 ml) is added sodium bismuthate (18 g) and the suspension is stirred at room temperature for 20 hr. The reaction mixture is then filtered 

---