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Summary. The crystal and molecular structures of 17-ethylenedioxy-3-methoxy-6,7,8-methylidyne-l,3,5(10)-estratrienc (1) ancl of 17~-bromo-acetoxy-3-nicthoxy-8wmethyl-l, 3,5(10), 6-estratetracnc (3) havc bcen detcrniined by X-ray analysis. A short discussion of the structures is given, and a new picture of the structure-activity relation of rstrogenics is proposed.The geminal hydrogenolysis of the steroidal bicyclobutane 1 leading to the 8wmethylsteroid 2 has been described in an earlier communication [l]. The X-ray analysis of 1 and 3, the 17-bromoacetyl derivative obtained from 2, now confirms the predictions [I] concerning the structures of 1 and 2 and supports the proposed reaction mechanism. Structures of 8 a-estra-derivatives are of interest in connection with estrogenic activity. Indeed, a number of 8 a-estra-derivatives show activities comparable to that of estradiol itself [5] in spite of the considerable difference in molecular shape between the 8cr-and the natural 8B-steroids. A structural correlation of 3 with estradiol and 84-methyl-estradiol suggests a general specification of the geometrical requirements for estrogenic activity.Structure analysis of 17-ethylenedioxy-3-methoxy-6,7,8-methylidyne-l,3,5 (lO)-estratriene (1). -The compound crystallizes from 2-propanol in the form of diamond-shaped translucent platelets. Cell dimensions and space group were determined from precession photographs ; intensity measurements were made with a linear diffractometer [6] using graphite-monochromatized MoKcr radiation. Crystallographic

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