The bromination of 17-alpha-cyanomethyl-17-beta-hydroxy-13-beta-methylgona-5(10)-ene-3-one (I) with polyvinylpyridinium bromide perbromide yields dibromide (II), which immediately debrominates to the final product.
1,4-Dihydroestronemethylether (I) was reacted with dimethylsulfonium-methylide (A) in a highly stereoselective manner to give the oxirane (II), which formed the cyanomethyl compound (III) by the addition of sodium cyanide. (III) was hydrolyzed with oxalic acid (B) in methanol and the product (IV) treated with pyridine perbromide hydrobromide to the dibromoketone, which yielded STS-557 after dehydrobromination.