Mechanistic Studies of Deoxygenation of Steroidal Ring-D 16,17-Ketols with Trimethylsilyl Iodide

Deoxygenation reaction of steroidal 16,17-ketols 1, 2 and 6 as well as their silyl ethers 3 and 7 and 16- and 17-iodoketone analogs 11, 12, and 14 with trimethylsilyl iodide (TMSI) or HI under various conditions was examined. The results indicate that the deoxygenation producing 16- and 17-ketones 9 and 8 proceeds through multiple reaction pathways; a direct iodination of a siloxy group of the ketol silyl ethers by iodide ion to give the iodoketones (path b), addition of TMSI to a carbonyl group of the ketol silyl ethers to yield diiodo derivatives 22 and 23 through iodo-bis-TMS compounds 20 and 21 (path a), and cleavage of ether bond of dimers 15-18 initially produced are, at least, involved. In these sequences, rearrangement of the 16-ketols 1 and 2 to the 17β-ketol 6 also plays a significant role. The yields of the ketones 9 and 8 and their relative amounts would be dependent on the relative importance of each pathway in the reaction.