Acylated-Oxypregnane Glycosides from the Roots of Asclepias syriaca

Abstract

177Asclepias syriaca L. is a plant indigenous to North America and distributed widely. The monarch butterfly (Danaus plexippus L.) feeds on the Asclepias genus for protection from vertebrate predators. 1) Asclepias species are known to contain many kinds of cardenolides and their glycosides. 2-9)A. syriaca has also been reported to contain these compounds. [10][11][12] We have previously studied cardenolides and their glycosides.13) But, until now, there have been no reports about pregnane glycosides from A. syriaca. Pregnane glycosides are considered characteristic of Asclepias species.14-18) Accordingly, we started an investigation of pregnane glycosides and found twenty new 12-O-acylated-oxypregnane glycosides in the roots of A. syriaca. The present paper describes the isolation and structural determination of these new pregnane glycosides.A MeOH extract from the dried roots of A. syriaca was suspended in water. The suspension was extracted with diethyl ether and partitioned into an ether-soluble fraction, a water-soluble fraction and an ether-water emulsified fraction. The residues of these fractions were chromatographed on a silica gel column to give fractions of acylated-oxypregnane glycosides from which twenty new compounds were obtained along with seven known compounds. The structural determination of the known compounds 7, 10, 12, 13, 18, 23, and 27 was made based on comparisons of NMR spectral data with data in the literature. 16,17,19) In order to acquire the component aglycones and sugars, the fraction containing pregnane glycosides from the silica gel column chromatography was subjected to acid hydrolysis. The afforded aglycones were identified as ikemagenin (1a), 19,20) 5a,6-dihydroikemagenin (20a), 17) and 12-Otigloylisolineolon (21a), 17) in view of the 1 H-and 13 C-NMR spectral data.The acquired sugar mixtures were fractionated to cymarose, oleandrose, digitoxose, and canarose using silica gel column chromatography. The absolute configurations of these sugars were believed to be D-forms on the basis of the optical rotation values.Compound 1 was suggested to have the molecular formula Thus, 1 was glycosylated at the C-3 position, and was considered to be ikemagenin 3-O-diglycoside. Moreover, acid hydrolysis of 1 showed that the sugar moiety consisted of digitoxose and oleandrose, and these sugars were identified as b-D-digitoxopyranose and b-D-oleandropyranose, as judged from the J values of each anomeric proton signal (Jϭ9.5, 2.5 Hz). The sequence of the sugar moiety was determined from measurements of the rotating frame nuclear Overhauser effect (ROE) difference spectra irradiating at the anomeric proton of each sugar in 1. ROEs were found between Acylated-Oxypregnane Glycosides from the Roots of Asclepias syriacaTsutomu WARASHINA* and Tadataka NORO Institute for Environmental Sciences, University of Shizuoka; 52-1 Yada, Suruga-ku, Shizuoka 422-8526, Japan. Received October 29, 2008; accepted November 25, 2008; published online November 26, 2008 Twenty new pregnane glycosides were obtained...

8,12;8,20-Diepoxy-8,14-secopregnane Glycosides from the Aerial Parts of Asclepias tuberosa

“…[1][2][3][4][5][6][7][8] In the preceding paper, we reported on 8,12;8,20-diepoxy-8,14-secopregnane glycosides from the aerial parts of Asclepias tuberosa L.9) A. tuberosa L. is a plant indigenous to North America and distributed widely. Its roots are known as "pleurisy root" and used to treat pleurisy and bronchitis.…”

Calotroposides H–N, new cytotoxic oxypregnane oligoglycosides from the root bark of Calotropis procera

“…In addition, the benzoylisolineolon-related oxypregnane glyco- [22][23][24]. It was also reported that the C-1 of the inner cymaropyranose unit is characteristically resonating up-field around 95.0-95.9 ppm when linked to C-3 of the aglycone in contrast to C-1 of a terminal cymaropyranose (97.8-100.4 ppm) when linked to a sugar unit [14].…”

“…5-5.8 Hz) together with C-17 chemical shift at 56. 2-59.5 ppm [13,[17][18][19][20], while the b-configured H-17 resonates at d H 3.42-3.55 (J = 9.5 Hz) with 60.1-60.7 ppm for C-17 [16,[21][22][23].…”

“…The identification of the monosaccharides in the hydrolysates of compounds 1-7 was confirmed by co-TLC with authentic sugars as well as comparison of the GCMS retention times with standards monosaccharides. The absolute configuration of the deoxy sugars was determined to be D-forms by comparison of 13 C NMR data and optical rotation with those of the corresponding sugars[16,[21][22][23][24]. A survey of closely related glycosides revealed that all the b-linked 2,6-dideoxy sugars have the D-configuration, whereas the a-linked sugars are mostly L-con-figured.…”

Acid epimerization of 20-keto pregnane glycosides is determined by 2D-NMR spectroscopy

“…The 17α configuration was also confirmed by the observation that the carbonyl carbon of an α-linked methyl ketone at C-17 appears at δ 209.2, 209.3 and 214.5 ppm for the aglycones 6 , 7 and 8 , respectively, and at 214.4 ppm for the steroidal glycoside 3 , when compared with δ 217 ppm for the β configuration (Table 1). 17-Epimers having an α-oriented side chain were isolated from the twigs of Pergularia pallida (Khare et al 1984), the roots of Cynanchum wilfordii Hemsley (Tsukamoto et al 1985), the roots of Calotropis gigantea Dryand (Shibuya et al 1992), the roots of Cynanchum caudatum M. (Warashina and Noro 1995), the aerial part of Asclepias incarnata L. (Warashina and Noro 2000), the roots of Cynanchum auriculatum Royle ex Wight (Xhang et al 2000), the aerial parts of Cynanchum aphyllum L. (Kanchanapoom et al 2002), the stem of Marsdenia tenacissima (Deng et al 2005), and from the pericarps, hairy seeds and leaves of Solenostemma argel Hayne (Perrone et al 2008), the roots of Asclepias curassavica L. (Warashina and Noro 2008), the roots of Asclepias syriaca L. (Warashina and Noro 2009) (all of which species belong to the Asclepiadaceae family), and from Nerium oleander (Bai et al 2007) . …”