Envelope-flattened chair conformation

An interesting result was obtained for compound 255 bearing an endo-1 hydroxy group. In the solid state, it adopts an envelope-flattened chair conformation (from X-ray data) (194). 

The flattening of ring a in a 4,4-dimethyl-5a-androstan-3-one, inferred from spectroscopic data, is confirmed by X-ray crystallography. Distortion arises mainly from rotation about the C(4)-C(5) bond, and is markedly less in the corresponding 19-nor-compound. The reality of conformational transmission is evident from different conformations of ring d in the 10)3-H and 10/ -methyl compounds. Ring d approximates to a half-chair, distorted either towards a C-14 envelope, or towards a C-13 envelope, respectively. 

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