the unusual 17 beta side-chain orientation of 2 with a c16-c17-c20-o20 torsion angle of +13 degrees compared to -7 degrees for progesterone would seem to preclude hydrogen bonding with the progestin receptor binding site and provides strong supporting evidence for the contention that this site is located above the beta face of the molecule .
C0033308
progesterone
C0033306
progestin