D-nor steroids: anomalous conformation and reactivity of C-D-cis D-norandrostanes

The C/D-cisD-norandrostanes 1a, 1b, 1c, 2a, 2b, and 2c were prepared and their reactivity studied. Whereas carbonium ion reactions of 1b and 1c are readily interpreted on the basis of their assigned stereochemistry, reactions of 2b and 2c suggest that these compounds react via conformations in which the 16beta-substituents are pseudoequatorial, requiring a ring C boat conformation as shown in formula 3b. X-ray crystallographic studies on 1a and 2a establish that while 1a has the expected conformation, 2a exists preferentially with ring C in the boat form. Molecular models suggest that this is the result of severe nonbonded interactions in the alternative conformation (3a) in which a chair C ring is retained.