Abstract. Two crystallographically independent molecules with differing half-chair conformations of ring A were found in the analysis of Org OD 14. These results are compared with published X-ray structure analyses and Westheimer-type calculations of related steroids. It is concluded that the energy differences between the half-chair and envelope conformations of ring A of 3-0x0 5( 10)-unsaturated steroids are small. lntroductionThe conformation of the A-ring of 3-0x0 5(10)-unsaturated steroids has been the subject of a number of studies. A Westheimer-type calculation carried out by Cohen' indicated that estr-5( 1 O)-en-3-one 1 should exist in the half--chair form A, which has 0.9 kcal/mol less energy than the alternative half-chair B (Fig. I).In estra-5( l0),9( 1 I)-dien-3-one 2, however, calculations showed that the half-chair conformation B had 0.3 kcal/ mol less energy than conformation A. Half-chair conformations of ring A in 3-0x0* Aiithor to whom correspondence should be addressed.X-ray structure determination of 17P-(iodoacetoxy)estr--5( IO)-en-3-one 3' and of 17P-hydroxyestr-5( lO)-en-3-one 43 showed that for these two compounds the conformation of the A-ring, in the crystalline state, is an envelop C with C2 displaced to the a-side (Fig. 2). OH---C E C H rl4 5 R = H 6 R = CH, Two crystallographically independent molecules were found in an X-ray structure analysis of 17-hydroxy-1 %nor--I7m-pregn-5( lO)-en-20-yn-3-one 54. The A-rings in both 0 c -c=c+c

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Abstract.norprogesterone derivatives show interesting progestational activities.The syntheses of the title compound and of some ester derivatives are described. These

Problems, specific for QSAR studies of steroids, are reviewed. The attention is drawn to the extensive use of X‐ray structure determinations to assist in weeding‐out chance correlations which arise from the need to use too small series of derivatives and the tendency in older studies to underestimate the flexibility of steroids. A series of papers on the QSAR of the cardiac glycosides illustrate some problems associated with the use of molecular modelling.

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