Ultrasound assisted oxidations and reductions in steroid synthesis

Abstract

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-3β-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-Δ 5-steroids. Application of ultrasound to aluminium amalgam reductive opening of α,β-epoxy-ketones improved the reaction rates and yields of β-hydroxy ketones, in particular for α′-oxygenated counterparts. Zinc reduction of α,β-unsaturated oxosteroids has been also enhanced by ultrasound. Fast quantitative reactions led to olefins or allylic alcohols, according to the α-enone system used. Regio- and stereoselectivities are marked. A new Clemmensen-type reduction promoted on the last conditions gave a mild and effcient method to reduce selectively 3-oxo groups in polihydroxysteroids.