ABSTRACT
Utilization of 17-keto-androstanes as starting materials for the synthesis of alpha- or beta-oriented steroidal 20-->16-gamma-carbolactones has been explored following two different strategies. A highly efficient, stereospecific protocol has been developed for the beta-oriented cis-gamma-lactone. A different approach, involving prior attachment of a 3-carbon side chain on C-17 of a 17-oxo-16beta-acetoxyandrostane led to the epimeric, alpha-oriented lactone. The mechanism of the rearrangement of epimeric 16beta- or 16alpha-hydroxy-17-keto-androstanes to 17beta-hydroxy-16-keto-androstanes was studied by 13C NMR spectroscopy. The former occurs through a 1,2-sigmatropic H-shift, while the latter is likely to take place by simple enolization-reprotonation.
