Mechanistic studies of the rearrangements of steroidal 16,17-ketols and syntheses of 20→16-cis-γ-carbolactones
摘要:
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-16 beta-acetoxyandrostane led to the epimeric, cc-oriented lactone. The mechanism of the rearrangement of epimeric 16 beta- or 16 alpha-hydroxy-17-ketoandrostanes to 17 beta-hydroxy-16-keto-androstanes was studied by C-13 NMR spectroscopy. The former occurs through a 1,2-sigmatropic H-shift, while the latter is likely to take place by simple enolization-reprotonation. (C) 1999 Elsevier Science Ltd. All rights reserved.