title: Synthesis and crystal structures of C24-epimeric 20(R)-ocotillol-type saponins.
abstract: 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.
authors.affiliation: School of Pharmacy, Key Laboratory of Molecular Pharmacology and Drug Evaluation (Yantai University), Ministry of Education, Collaborative Innovation Center of Advanced Drug Delivery System and Biotech Drugs in Universities of Shandong, Yantai University, Yantai 264005, People's Republic of China.
authors.forename: Yang Rong
authors.initials: YR
authors.lastname: Xu
authors.affiliation: School of Pharmacy, Key Laboratory of Molecular Pharmacology and Drug Evaluation (Yantai University), Ministry of Education, Collaborative Innovation Center of Advanced Drug Delivery System and Biotech Drugs in Universities of Shandong, Yantai University, Yantai 264005, People's Republic of China.
authors.forename: Jing Jing
authors.initials: JJ
authors.lastname: Yang
authors.affiliation: School of Pharmacy, Key Laboratory of Molecular Pharmacology and Drug Evaluation (Yantai University), Ministry of Education, Collaborative Innovation Center of Advanced Drug Delivery System and Biotech Drugs in Universities of Shandong, Yantai University, Yantai 264005, People's Republic of China.
authors.forename: Juan
authors.initials: J
authors.lastname: Liu
authors.affiliation: School of Pharmacy, Binzhou Medical University, Yantai 264003, People's Republic of China.
authors.forename: Gui Ge
authors.initials: GG
authors.lastname: Hou
authors.affiliation: School of Pharmacy, Key Laboratory of Molecular Pharmacology and Drug Evaluation (Yantai University), Ministry of Education, Collaborative Innovation Center of Advanced Drug Delivery System and Biotech Drugs in Universities of Shandong, Yantai University, Yantai 264005, People's Republic of China.
authors.forename: Qing Guo
authors.initials: QG
authors.lastname: Meng
mesh_terms: D018360:Crystallography, X-Ray; D036145:Ginsenosides / Q000138:chemical synthesis / Q000737:chemistry*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D012503:Saponins / Q000138:chemical synthesis / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D036145:Ginsenosides; D012503:Saponins; C524540:ocotillol
keywords: crystal structure; dammaranetriol; epimer; hydrogen bonding; ocotillol-type saponin; synthesis