OXIDATIVE CYCLIZATION OF IODO KETONES - SYNTHESIS OF 6-OXA-5-ALPHA-PREGNANE-3,20-DIONE

The steroidal delta- and gamma-iodo ketones 1 and 9 were converted to the cyclic hemiketals 3 and 10, by oxidation to the iodoso derivatives with m-CPBA. Spontaneous cyclization of the latter intermediates to t he corresponding oxocarbenium ions, followed by stereoselective additi on of water, rendered the hemiketals. Depending on the reaction condit ions, the five-membered oxocarbenium ion derived from the gamma-iodo k etone 9 may add H2O or m-CPBA to give either the hemiketal or a Baeyer -Villiger type product 12, while the oxocarbenium derived from 1 gives exclusively the hemiketal. When the reaction was carried out in dry m ethanol, methyl ketals were formed. Use of this methodology allowed us to synthesize 6-oxa-5 alpha-pregnanes with and without functionalizat ion at C-19.