NMR and X-ray studies of isomeric 22,23-dihydroxy stigmastanes

A comparative conformational study of steroidal side chain of (22R,23R)- and (22S,23S)-dihydroxy stigmastane derivatives was performed using single crystal X-ray diffraction and NMR spectroscopy. The preferred conformation in solution was shown to be close to that in the crystal. (22R,23R)-Isomers typical for natural plant steroid hormones brassinosteroids adopt a conformation in which both hydroxyl groups are pointed toward unhindered α-side of the steroidal plane and can thus participate in biochemical processes. Unnatural (22S,23S)-counterparts exhibit a conformation with the two hydroxyl groups oriented in the opposite direction and sterically hindered by 21-methyl group and terminal side chain fragment.