Synthesis and crystal structures of a 3-acetylated (20S,24S)-ocotillol-type saponin and its C-24 epimer

摘要

In order to study the in vivo protective effect on myocardial ischemia, (20S, 24R)-epoxydammarane-12 beta 25-diol, (V), and (20S, 24S)-epoxydammarane-12 beta 25-diol, (VI), were synthesized through a novel synthetic route. Two key intermediates, namely (20S, 24R)-3-acetyl-20,24-epoxydammarane-3 beta,12 beta,25-triol, (III) [obtained as the hemihydrate, C32H54O5 center dot 0.5H(2)O, (IIIa), and the ethanol hemisolvate, C32H54O5 center dot 0.5C(2)H(5)OH, (IIIb), with identical conformations but different crystal packings], and (20S, 24S)-3-acetyl-20,24-epoxydammarane-3 beta 12 beta,25-triol, C32H54O5, (IV), were obtained during the synthesis. The structures were confirmed by H-1 NMR, C-13 NMR and HRMS analyses, and single-crystal X-ray diffraction. Molecules of (IIIa) are extended into a two-dimensional network constructed with water molecules linked alternately through intermolecular O-H center dot center dot center dot O hydrogen bonds, which are further stacked into a three-dimensional network. Compound (IIIb) contains two completely asymmetric molecules, which are linked in a disordered manner through intermolecular C-H center dot center dot center dot O hydrogen bonds. While the crystal stacks in compound (IV) are linked via weak C-H center dot center dot center dot O hydrogen bonds, the hydrogen-bonded chains extend helically along the crystallographic b axis.