Synthesis and crystal structures of C24-epimeric 20(R)-ocotillol-type saponins.

Ocotillol-type saponins have a wide spectrum of biological activities. Previous studies indicated that the configuration at the C24 position may be responsible for their stereoselectivity in pharmacological action and pharmacokinetics. Natural ocotillol-type saponins share a 20(S)-form but it has been found that the 20(R)-stereoisomers have different pharmacological effects. The semisynthesis of 20(R)-ocotillol-type saponins has not been reported and it is therefore worthwhile clarifying their crystal structures. Two C24 epimeric 20(R)-ocotillol-type saponins, namely (20R,24S)-20,24-epoxydammarane-3β,12β,25-triol, C30H52O4, (III), and (20R,24R)-20,24-epoxydammarane-3β,12β,25-triol monohydrate, C30H52O4·H2O, (IV), were synthesized, and their structures were elucidated by spectral studies and finally confirmed by single-crystal X-ray diffraction. The (Me)C-O-C-C(OH) torsion angle of (III) is 146.41 (14)°, whereas the corresponding torsion angle of (IV) is -146.4 (7)°, indicating a different conformation at the C24 position. The crystal stacking in (III) generates an R4(4)(8) motif, through which the molecules are linked into a one-dimensional double chain. The chains are linked via nonclassical C-H...O hydrogen bonds into a two-dimensional network, and further stacked into a three-dimensional structure. In contrast to (III), epimer (IV) crystallizes as a hydrate, in which the water molecules act as hydrogen-bond donors linking one-dimensional chains into a two-dimensional network through intermolecular O-H...O hydrogen bonds. The hydrogen-bonded chains extend helically along the crystallographic a axis and generate a C4(4)(8) motif.