Versatile use of hindered oxalates for the stereoselective preparation of novel 11-modified androst-5-ene derivatives

The 11alpha- and 11beta-modified androst-5-ene derivatives 3a,b as well as the exo- and endocyclic dehydrated compounds 4a-c and 5b-c were produced using the oxalate derivatives of the highly hindered 11beta-hydroxyandrost-5-enes 1a-c. The 11-tetrahydrofuran derivative 6 was produced for the first time with good diastereoselectivity by an intramolecular 5-exo cyclization under radical conditions from the corresponding oxalate as precursor.