Synthesis of B-Ring-Modified Steroids through BF₃-Promoted Rearrangement/Substitution of 6β-Hydroxy-5,19-cyclosteroids

Abstract

The BF₃·Et₂O-promoted reaction of 3β-acetoxy-5,19-cyclo-pregnan-6β-ol-20-one with different nucleophiles was investigated. B-<i>homo</i> steroids (3β-acetoxy-B-homo-6a-β-alkoxy-pregna-5(10)-en-20-ones) were obtained with primary and secondary alcohols, while the reaction with common carboxylic acids selectively afforded the corresponding 3β-acetoxy-6β-(acyloxymethyl)-pregna-5(10)-en-20-ones. The transformations are supposed to proceed via the rearrangement of a cyclopropyl-methyl cation (bicyclobutonium) intermediate, which is regioselectively opened in dependence on the nucleophile employed. The method represents an efficient, diversity-oriented entry to new B-ring-modified steroids, which are of potential pharmaceutical relevance