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

Abstract

In order to study the in vivo protective effect on myocardial ischemia, (20S,24R)-epoxydammarane-12β,25-diol, (V), and (20S,24S)-epoxydammarane-12β,25-diol, (VI), were synthesized through a novel synthetic route. Two key intermediates, namely (20S,24R)-3-acetyl-20,24-epoxydammarane-3β,12β,25-triol, (III) [obtained as the hemihydrate, C32H54O5·0.5H2O, (IIIa), and the ethanol hemisolvate, C32H54O5·0.5C2H5OH, (IIIb), with identical conformations but different crystal packings], and (20S,24S)-3-acetyl-20,24-epoxydammarane-3β,12β,25-triol, C32H54O5, (IV), were obtained during the synthesis. The structures were confirmed by 1H NMR, 13C 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...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...O hydrogen bonds. While the crystal stacks in compound (IV) are linked via weak C—H...O hydrogen bonds, the hydrogen-bonded chains extend helically along the crystallographic b axis.