Addition reactions at the 16(17) double bond of 3-methoxy-13alpha-estra-1,3,5(10),16-tetraene.

The epoxidation, the addition of hypobromous acid, and the hydroboration of 3-methoxy-13α-estra-1,3,5(10),16-tetraene 1 with diborane, catecholborane, and 9-BBN were investigated in order to determine the stereochemical outcome and to synthesize new 13α-estra-1,3,5(10)-trienes for biological and conformational investigations. It was shown that the sterically demanding reagent 9-BBN participated in a preferred β attack (53% 16βOH 10, 34% 17βOH 8, 13% 16αOH 11). This stereochemical result is in agreement with that from another cis addition reaction, the recently described OsO4 dihydroxylation of 1 [Steroids 68 (2003) 113]. With smaller reagents such as B2H6, catecholborane, or magnesium monoperoxyphthalate, a diminished stereoselectivity was observed with only a slight excess of β attack. The ionic trans addition of hypobromous acid gave two 17-bromo-16-alcohols with 16β,17α (4, 76%) and 16α,17β configuration (5, 24%) formed by trans cleavage of the 16,17α- and β-bromonium ion at position 16. The same regioselective and stereoselective course was found for the cleavage of the 16α,17α- and 16β,17β-epoxides (3 and 2) with hydrazoic acid (3→16βN3,17αOH 7, 2→16αN3,17βOH 6). The stereochemistry of the addition reactions to 1 can be explained in terms of a twist-boat conformation involving the C ring of compound 1. From a synthetic viewpoint the synthesis of the β-epoxide 2 from the bromohydrin 4, the cleavage of this epoxide to 16α-substituted-17β-hydroxy compounds, such as 6, and hydroboration/oxidation with 9-BBN to the hitherto unknown 16β-hydroxy compound 10 are useful procedures. The bromohydrin 5 is the first 13α-steroid with a 17β-bromo substituent. X-ray analysis revealed twist-boat and 16β-envelope conformations for rings C and D, respectively.