Batatins I and II, Ester-Type Dimers of Acylated Pentasaccharides from the Resin Glycosides of Sweet Potato

Abstract

Batatins I (1) and II (2), two ester-type dimers of acylated pentasaccharides, have been isolated by recycling HPLC from the hexane-soluble extract of sweet potato (Ipomoea batatas var. batatas). The structures were elucidated by a combination of high-resolution NMR spectroscopy and mass spectrometry. Complete analysis and unambiguous assignments of the 1H and 13C NMR spectra of 1 and 2 were achieved by 2D shift correlation measurements. The glycosidic acid forming each branched pentasaccharide monomeric unit was confirmed as simonic acid B. Three different fatty acids esterify this core at the same positions in both batatins: C-2 on the second rhamnose unit and C-4 and C-2 (or C-3) on the third rhamnose moiety. The acylating residues were identified as (+)-(2S)-methylbutanoic, dodecanoic (lauric), and cinnamic acids. The site of lactonization by the aglycon in unit A was placed at C-3 of the second saccharide. The position for the ester linkage for monomeric unit B on the macrocylic unit A was identified as C-3 of the terminal rhamnose' ''. Through spectroscopic simulation of these complex oligosaccharides, the chemical shifts and coupling constants were deduced for the overlapped proton resonances with a non-first-order resolution. The experimental NMR spectroscopic values registered for batatinoside I (3), a new polyacylated macroyclic pentasaccharide also isolated from this plant, were used as the starting point for spectral simulation of 1 and 2. Both polymers 1 and 2 represent dimers of compound 3.