Selektive doppelbindungsisomerisierungen an a-nor-pregnadienen

The reaction sequence 9→11→ 20→23 was carried out as model investigation for the total synthesis of 20-methyl-pregn-4-en-3-on (23) via 9 obtainable by cyclization of tetraen-ol 1. A-nor-steroid-3(5),8(14) diene 9 together with isomers 10–12 was prepared by HBr induced rearrangement of the cyclosteroids 6/7, obtained from ergosterin. 10 and 12 were isomerized on palladium charcoal to 9 or 11 respectively; 9 could be isomerized by HBr to 11. Regioselective hydrogenation of 11 yielded the mono-en 20 which was converted to 23.