Abstract：
:The mechanistic aspects of the alkali-catalyzed rearrangement of 16alpha-hydroxy-17-keto steroid 1 to 17beta-hydroxy-16-keto steroid 2 are elucidated by use of (18)O- and deuterium-labeling experiments. The (18)O-labeling experiments refute the gem-hydration-quasi-diaxial dehydration mechanism for the rearrangement previously proposed and support the conventional enolization mechanism. Moreover, equilibrium by gem-hydration-dehydration occurs at the C-17 carbonyl more efficiently than at the C-16 carbonyl. Enolization rate of a carbonyl group at C-16 of 17beta-ketol 2 toward the C-17 position (k(16,17)) was about 8-10 times higher than those of 16alpha-ketol 1 toward the C-16 position (k(17,16)) and ketol 2 toward the C-15 position (k(16,15)). The marked deuterium-isotope effect on each enolization was observed with k(H)/k(D) ranging between 5.4 and 8.8. The present findings reveal that the initial hydration-dehydration equilibration at the C-17 carbonyl of ketol 1 followed by enolization of the carbonyl gives the ene-diol intermediate that isomerizes quantitatively to the 16-keto isomer of which the 16-carbonyl moiety enolizes preferentially toward the C-17 position rather than the C-15 position, yielding the ene-diol. Computational calculations of ground state energies of ketols 1-M and 2-M, trans-cyclohexane/cyclopentane structures, and their activation energies in the rearrangement support the dynamic aspects of the rearrangement as well as the kinetics data of the enolization.
journal_name
Steroidsjournal_title
Steroidsauthors
Numazawa M,Nagaoka M,Matsuzaki H,Yamashita K,Komatsu S,Osawa Ydoi
10.1016/j.steroids.2008.03.001subject
Has Abstractpub_date
2008-09-01 00:00:00pages
798-805issue
8eissn
0039-128Xissn
1878-5867pii
S0039-128X(08)00097-4journal_volume
73pub_type
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