ULTRASOUND ASSISTED OXIDATIONS AND REDUCTIONS IN STEROID-SYNTHESIS

Ultrasonic irradiation made feasible some important oxidative and reductive reactions in steroid synthesis. These unusual conditions have improved existing methods and promoted new ones. The very common 5-en-3 beta-ol homoallylic system in steroids was oxidized to 4-ene-3,6-diones by a new one-step process, using tetra-n-propylammonium perruthenate with a co-oxidant, under sonochemical conditions. Ultrasonic irradiation has been also successfully used to promote the generation of radicals in the hypoiodite reaction, in order to prepare tetrahydrofurans as precursors of 19-hydroxy-Delta(5)-steroids. Application of ultrasound to aluminium amalgam reductive opening of alpha,beta-epoxy-ketones improved the reaction rates and yields of beta-hydroxy ketones, in particular for alpha'-oxygenated counterparts. Zinc reduction of alpha,beta-unsaturated oxosteroids has been also enhanced by ultrasound. Fast quantitative reactions led to olefins or allylic alcohols, according to the alpha-enone system used. Regio- and stereoselectivities are marked. A new Clemmensen-type reduction promoted on the last conditions gave a mild and efficient method to reduce selectively 3-oxo groups in polihydroxysteroids.