Previous studies from our laboratory using 17 alpha-E- and 17 alpha-Z-halovinyl estradiols demonstrated a marked enhancement of receptor binding by the Z-isomers. This suggested tolerance at the 17 alpha-position was not previously observed by investigations using 16 alpha and 17 alpha-substituted estradiols. Because of the synthetic access provided by vinyl tin chemistry, we prepared the 17 alpha-E and Z-phenylthiovinyl and phenylselenovinyl estradiols and compared their binding characteristics to those of the previously reported 16 alpha/17 alpha-phenylseleno and methylseleno estradiols. The results, in addition to demonstrating a facile preparation of the target compounds, indicated that significant receptor affinity was retained by these compounds (relative binding affinity = 24.5-117). The highest affinity was demonstrated by the 17 alpha-Z-phenylthiovinyl estradiol 5a, which, by molecular modeling, exhibited a significantly different molecular conformation from the corresponding 17 alpha-E-phenylthiovinyl isomer or the 17 alpha-phenyl-thioethynyl analog. The current series possessed better binding characteristics than the phenylseleno and methylseleno estradiols but somewhat poorer binding than the 17 alpha-E/Z-halovinyl series. The observations suggest that some steric limitations exist in a portion of the 17 alpha-region, and that the region is better accessed by compounds possessing Z-vinyl stereochemistry.