Biomimetic total synthesis of steroids VIII. Stereospecific synthesis of B‐homo‐19‐norsteroids

Cationic cyclization of 2‐[7‐(3‐methoxyphenyl)‐(E)‐3‐heptenyl]‐3‐methyl‐2‐cyclopentenol (7) gave mainly tetracyclic products via Ar1‐6 and/or Ar2‐7 participation. The major product, 3‐methoxy‐17‐methyl‐B‐homo‐1,3,5(10),13(17)‐gonatetraene (9), was converted in two steps into B‐homoestrone methyl ether 20. High‐resolution 1H NMR spectra indicated that the seven‐membered ring in 9 occurs mainly in a boat‐like conformation whereas in its 1‐methoxy isomer 8 it occurs in an unsymmetrical twist‐boat conformation. In 20, the B‐ring probably occurs both in chair and boat conformations.