Resin glycosides from Convolvulaceae plants

Resin glycosides are well known as purgative ingredients, which are characteristic of certain crude drugs such as Mexican Scammony Radix, Orizabae Tuber, and Jalapae Tuber, all of which originate from Convolvulaceae plants. Depending on their solubility in ether, these are roughly classified into two groups—jalapin (soluble) and convolvulin (insoluble). Almost all jalapins hitherto isolated and characterized had common intramolecular macrocyclic ester structures. These are composed of 1 mol of oligoglycoside of hydroxyl fatty acid (glycosidic acid) partially acylated by some organic acids at the sugar moiety, some examples of which are ester-type dimers. On the other hand, convolvulin is regarded as an oligomer of a variety of acylated glycosidic acids. This review describes the isolation and structural elucidation of resin glycosides from some Convolvulaceae plants, including Ipomoea operculata, Pharbitis nil, Quamoclit pennata, Calystegia soldanella, and I. muricata.


Introduction
Resin glycosides are well known as purgative ingredients, which are characteristic of crude drugs such as Mexican Scammoniae Radix (the root of Convolvulus scammonia L.), Orizabae Tuber [the tuber of Ipomoea orizabensis (Pellet) Ledanois], and Jalapae Tuber (the tuber of I. purga Hayne). They are commonly found in plants belonging to the Convolvulaceae family [1]. Chemical investigations on these resin glycosides were initiated in the middle of the nineteenth century. The obtained resin glycosides were roughly classified into an ether-soluble group called jalapin or an ether-insoluble group denoted convolvulin [2]. Upon alkaline hydrolysis, both jalapins and convolvulins afforded organic acids such as isobutyric, 2-methylbutyric, (E)-2-methylbut-2-enoic (tiglic), 2-methyl-3-hydroxybutyric (nilic) acids and oligoglycosides of hydroxyl fatty acid (glycosidic acid). Furthermore, the glycosidic acid provided several kinds of monosaccharides (e.g., glucose, rhamnose, and fucose) and a hydroxyl fatty acid (e.g., 11-hydroxyhexadecanoic (jalapinolic), 11-hydroxytetradecanoic (convolvulinolic), and 3,11-dihydroxytetradecanoic (ipurolic) acids) on acidic hydrolysis. Considering this behavior toward acids and bases, Mannich and Schumann speculated that convolvulins from I. purga were oligomers of a glycosidic acid partially acylated by some organic acids at the sugar moiety [3]. However, until our research group isolated pure genuine jalapins, chemical studies had been confined only to the characterization of the component organic acids and glycosidic acids afforded by alkaline hydrolysis of a crude resin glycoside. In 1987, our research group succeeded, for the first time, in the isolation and structural elucidation of four jalapins from Orizabae Tuber [4]. Hitherto, almost all isolated and characterized jalapins have had intramolecular macrocyclic ester The original version of this article was revised due to a retrospective Open Access order. structures composed of 1 mol of oligoglycoside of hydroxyl fatty acids partially acylated by some organic acids at the sugar moiety, while some examples were ester-type dimers [5][6][7][8][9][10][11][12][13].
In this review, our studies on the isolation and structural elucidation of resin glycosides from some Convolvulaceae plants, including I. operculata (Gomes) Mart., Pharbitis nil, Quamoclit pennata, and I. muricata, are summarized.

I. operculata
Rhizoma Jalapae Braziliensis (Brazilian Jalap), the root of I. operculata (Gomes) Mart., is known to be a substitute for Jalapae Tuber (Vera Cruz Jalap, I. purga), and its ethanol extract, Brazilian resin, is a potent laxative. Brazilian Jalap contains both jalapins and convolvulins as active ingredients.

Separation of crude jalapin and crude convolvulin fractions
The methanol (MeOH) extract of the root of I. operculata was partitioned between ether and H 2 O. The ether-soluble fraction was subjected to chromatography over MCI gel CHP 20P and Sephadex LH-20 columns to afford the crude jalapin fraction. The H 2 O-soluble fraction was partitioned between n-butanol and H 2 O. The n-butanol-soluble fraction was subjected to an MCI gel CHP 20P column to give the crude convolvulin fraction [17].

Jalapin
Component organic acids and hydroxyl fatty acid The crude jalapin fraction was subjected to alkaline hydrolysis to furnish organic acid and glycosidic acid fractions. The former fraction was methylated with diazomethane-ether and then examined by GC-MS, exhibiting two peaks identical to those of methyl n-decanoate and n-dodecanoate. The latter fraction furnished an aglycone fraction and a monosaccharide fraction upon acidic hydrolysis. The aglycone fraction was methylated with diazomethane-ether to furnish methyl ester (1) ([a] D ? 0.9°) of jalapinolic acid (2) [17].
Determination of the absolute configurations of hydroxyl fatty acids including jalapinolic acid Mosher's method [18] was used to determine the absolute configuration of 1. The 1 H-NMR spectrum of (?)-2-methoxy-2-trifluoromethylphenylacetate (MTPA ester) of the racemate of 1 as well as the 1 H-NMR spectra of the (-)-and (?)-MTPA esters (3, 4; 5, 6; 7, 8) of commercial 2S-pentanol (9), 2Shexanol (10) and 2S-heptanol (11) as model compounds, indicated that Mosher's method is applicable for the determination of the configuration of 1 (Fig. 1). This determination was accomplished using the chemical shift of the terminal methyl group, which is separated by four methylene groups from the asymmetric carbon. The chemical shift difference (Dd) [d(-)-MTPA ester-d(?)-MTPA ester] of the H 3 -16 signals in the 1 H-NMR spectra of the (-)-and (?)-MTPA esters (12, 13) of 1 indicated the configuration at C-11 of 1 to be S [19,20]. At the same time, Shibuya et al. determined the configuration of (?)jalapinolic acid, which was obtained from the tuber of Merremia mammosa, to be S by syntheses of S-and Rjalapinolic acid via Sharpless asymmetric epoxidation [21].
Ipurolic acid (18), the common aglycone of pharbitic acids C and D, has hydroxyl groups at C-3 as well as at C-11. The 1 H-NMR spectral data of the (-)-and (?)-MTPA esters (19,20) of commercial methyl 3S-hydroxybutyrate (21), used as model compounds, indicated that the configuration at C-3 could be determined by the Dd values of the signals originating from the 2-methylene protons and ester methyl protons. Comparison of the 1 H-NMR spectrum of (-)-MTPA ester (22) of methyl ipurolate (23) ([a] D ? 1.2°) with that of (?)-MTPA ester (24) suggested that the configurations at C-3 and C-11 of 18 are both S. The influence on their Dd values by interactions between two MTPA residues at C-3 and C-11 in 22 and 24 was ruled out as follows. Methyl 11-hydroxytetradec-2-enoate, which was prepared from crude pharbitic acid according to Okabe et al. [22], was hydrogenated to furnish 17. In addition, the 1 H-NMR spectra of (-)-and (?)-MTPA esters (25,26) of methyl 3-hydroxy-11-oxo-tetradecanoate (27), which was obtained by the partial oxidation of 23, showed similar Dd values to those of 22 and 24 for signals due to 2-methylene protons and ester methyl protons. Thus, the configurations at C-3 and C-11 of 18 were both determined to be S [20].
Mosher's method was thus found to be useful for the determination of the absolute configuration of a minute amount of hydroxyl fatty acids.
Isolation of resin glycosides The crude jalapin fraction was subjected to silica gel and ODS column chromatography and HPLC on silica, ODS, and octyl silica columns to yield operculins I-XVIII (43-60) [19,[28][29][30]. Preparative recycling HPLC was a useful method for the isolation of two constitutional isomers, in which the n-decanoyl group and n-dodecanoyl group were mutually interchanged between the same positions (49 and 50; 51 and 52) [28,29].
Structures of resin glycosides Operculin I (43) is composed of 2 mol of n-dodecanoic acid and 1 mol of 36 and is intramolecularly linked with a hydroxyl group of the sugar moiety to form a macrolactone structure, which was elucidated by using 1 H-and 13 C-NMR spectra, negative-ion FAB-MS, as well as analyses of the hydrolysates produced by alkaline hydrolysis. The site of each ester linkage in jalapinolic and n-dodecanoic acids were fixed by analyses of the acylation shifts observed in the 1 H-NMR spectrum of 43 as well as peak assignments in the EI-MS of the peracetate of 43 and in the negative ion FAB-MS of 43. From these data, the structure of 43 was defined as 11S- 1, 2 00 -ester, as shown in Fig. 3. [19,28].
Compounds 43-60 are the first known resin glycosides to feature fatty acids, n-dodecanoic and/or n-decanoic acid, as component organic acids.

Convolvulin
Component organic acids Alkaline hydrolysis of the crude convolvulin fraction afforded organic acid and glycosidic acid fractions. The organic acid fraction was esterified with p-bromophenacyl bromide, and the product was subjected to chromatographic separation to afford pbromophenacyl isovalerate (61), p-bromophenacyl tiglate (62), and two compounds 63 and 64 (Fig. 4). Both 63 and 64 were identified as the p-bromophenacyl esters of the exogonic acid reported by Graf and Dahlke [14] using NMR and FD-MS data. Furthermore, it was elucidated that 63 and 64 are isomers and that interconversion between them occurred because of inversion at the C-6 spiro-center. The absolute configurations at C-3 and C-9 of exogonic acid were identified as S and R, respectively, by applying Mosher's method to methyl 3-hydroxy-6,9-epoxydecanoate (65), methyl 9-hydroxy-3,6-epoxydecanoate (66), and methyl 3,9-dihydroxydecanoate (67), which were provided by hydrogenation of methyl exogonate. It was also found that this acid existed as a mixture of epimers that only differ in the spirocyclic carbon configuration at C-6 [23]. These results were consistent with those of the enantioselective syntheses performed by Lawson et al. [31]. Component glycosidic acids The glycosidic acid fraction was treated with diazomethane-ether and subsequently subjected to chromatography to isolate the methyl ester (68), the alkaline hydrolysis of which afforded a glycosidic acid, named operculinic acid H (69). Acidic hydrolysis of 68 afforded an aglycone and a mixture of monosaccharides, which were composed of D-glucose and L-rhamnose. Methylation of the aglycone with diazomethane-ether yielded a methyl 3S,12S-dihydroxyhexadecanoate (30) [23].
The structure of operculinic acid H (69) was characterized as shown in Fig. 5 using the NMR data of 68 and its peracetate, negative-ion FAB-MS of 69, HR-EI-MS of the permethylate of 68, and structural analyses of the five hydrolysates produced by partial methanolysis of 68 [23]. Despite the large difference between the structures of the sugar moieties in 68 and Wagner's operculinic acid [16], both compounds were considered to be identical. The organic acid and glycosidic acid components of the convolvulin fraction were different from those of the jalapin fraction obtained from the crude drug.
Structures of acylated glycosidic acid methyl esters Despite numerous attempts, the isolation of pure resin glycosides from the crude convolvulin fraction has been unsuccessful. In consideration of previous results, it was evident that the component resin glycosides of the crude convolvulin fraction possessed at least one carboxyl group. Hence, the fraction was treated with indium(III) chloride in MeOH, which was reported to be a catalyst for mild methyl esterification of carboxylic acids by Mineno and Kansui [32]. The treated fraction exhibited a number of separate spots by TLC on silica gel and was successively separated by Sephadex LH-20 and silica gel column chromatography and HPLC using an ODS column, affording four compounds, referred to as IOM-1 (70)-IOM-4 (73) [33]. The 1 H-NMR spectra of 70-73 showed that they consisted of mixtures of epimers that differed in the spirocyclic carbon configuration only at C-6 of the exogonoyl residue (Exg). Since HPLC using a naphthylethyl group-bonded silica (p-nap) column of 70-73 indicated that interconversion of the epimers of Exg in 70-73 was facile, structural analyses were carried out using a mixture of the epimers.
The structures of 70-73 were determined as shown in Fig. 5 using their NMR spectra, including HMBC and 1 H-1 H TOCSY spectra, negative-ion FAB-MS, and the In addition, no distinct peak was detected in the region of m/z 1700-4000 in the negative-ion FAB-MS of the crude convolvulin fraction. Therefore, a part of the crude convolvulin fraction might be a mixture of monomers composed of free carboxylic acid forms corresponding to 72 and 73 [33].
Thus, we consider that methyl esterification of carboxylic acids via indium(III) chloride catalysis is a useful tool for structural investigations involving convolvulins.

Component organic acids
Alkaline hydrolysis of pharbitin furnished organic and glycosidic acid fractions. The GC of the organic acid fraction revealed the presence of 2-methylbutyric, tiglic, and nilic acid in a molar ratio of approximately 17:1:11. Furthermore, the organic acid fraction was acylated with pbromophenacyl bromide followed by chromatographic separation to yield p-bromophenacyl 2S-methylbutyrate and p-bromophenacyl nilate, which was found to be a mixture of the 2R, 3R-and the 2S, 3S-forms with a ratio of approximately 6:5 based on the ratio of H 3 -5 signal intensities obtained from the 1 H-NMR spectrum of its (-)-MTPA ester [40,41].
Component glycosidic acids The glycosidic acid fraction was converted into p-phenylphenacyl ester, and chromatography yielded three glycosidic acid esters. The alkaline hydrolysis of each ester furnished a new glycosidic acid, named pharbitic acid B (76), along with pharbitic acids C (74) and D (75), whose structures were corrected by NMR and negative-ion FAB-MS data as well as the EI-MS data for the permethylates of 74 and 75 [40] (Fig. 6). Using NMR and negative-ion FAB-MS, 76 was concluded to be a homolog of Structures of acylated glycosidic acid methyl esters Pharbitin was treated with indium(III) chloride in MeOH, in a similar manner to the treatment of the crude convolvulin fraction from I. operculata. This treated pharbitin was successively subjected to Diaion HP20, Sephadex LH-20, and silica gel column chromatography and HPLC using an ODS column to afford seven compounds, temporarily named PM-1 (77)-PM-7 (83) [42]. The structures of 77-83 were determined as indicated in Fig. 6 using similar methods as those used for 70-73 [33]. Compounds 77-83 were all monomers of methyl esters of glycosidic acids esterified with several organic acids at the sugar moiety. In addition, negative-ion FAB-MS of pharbitin exhibited ion peaks at m/z 1551, 1451, and 1305, which corresponded to the values of [M -H]ion peaks of the demethylated derivatives of 80, 78, and 82, respectively. However, for pharbitin, no intense ion peaks were observed in the m/z 1580-3500 region. Therefore, a part of the pharbitin was considered to be a mixture of monomers composed of free carboxylic acid forms corresponding to 77-83. Furthermore, it should be noted that 80 and 81 had 2R,3R-nilic acid and its enantiomer as the component organic acids in each molecule [42].

Quamoclit pennata
Quamoclit pennata is native to tropical regions of South America, and is cultivated primarily as an ornamental plant. This seed contains both jalapins and convolvulins.

Jalapin
Component organic acids and glycosidic acid The component organic acids were elucidated to be 2S-methylbutyric, n-decanoic, and n-dodecanoic acids. On the other hand, the glycosidic acid fraction yielded a new glycosidic acid, named quamoclinic acid A (84), whose structure was determined to be 11S-convolvulinolic acid 11 2)-b-D-fucopyranoside on the basis of spectroscopic data and chemical evidence (Fig. 7). It is noteworthy that the aglycone (11S-convolvulinolic acid) of 84 differs from jalapinolic acid [43].
Structures of resin glycosides Six resin glycosides with macrolactone structures, named quamoclins I (85)-VI (90) and one acylated glycosidic acid methyl ester, named quamoclin VII (91), were isolated. Their structures were assigned using similar methods as those employed for operculins. Compounds 85-90 were the first examples of jalapins with an organic acid (2S-methylbutyric acid) and fatty acid components (n-decanoic or n-dodecanoic acids) [43,44] (Fig. 7). Compound 91 was considered to be an artifact produced from 89 and/or 90 during the extraction and isolation procedures.
Structures of glycosidic acids The glycosidic acid fraction furnished five new glycosidic acids, named quamoclinic acids B (92)-H (98). These compounds were characterized on the basis of spectroscopic data as well as chemical evidence (Fig. 8). Compounds 95-97 were the first examples of heptaglycosides of glycosidic acid, and 98 was the first octaglycoside of glycosidic acid. Furthermore, 97 and 98 were the first known glycosidic acids to feature bisdesmosides with sugar linkages at the C-3 position of ipurolic acid as well as at its C-11 position [41,45].
Since two acylated trisaccharides closely related to the resin glycosides were previously reported as natural constituents of the seeds of Cuscuta chinensis (Convolvulaceae) [49], 108-112 are presumably formed from the corresponding acylated saccharides with a reducing end during treatment with indium(III) chloride in MeOH. It was therefore presumed that one of the reasons for the difficulty in isolating convolvulin from the crude convolvulin fraction of Q. pennata was the co-existence of the acylated saccharides with a reducing end, because the aand banomers easily reach equilibrium.

Calystegia soldanella
Calystegia soldanella Roem. et Schult. (Convolvulaceae) is distributed widely on the sandy beaches of seas and lakes in temperate regions of the world. The roots of this plant are used for the treatment of arthritis. Gasper reported the isolation and structural elucidation of two resin glycosides, soldanellines A and B, as chemical constituents of the root [50,51].

Component organic acids and glycosidic acids
Alkaline hydrolysis of the crude resin glycoside fraction obtained from the MeOH extract of the leaves, stems, and roots gave four new glycosidic acids, named calysolic acids A (113)-D (116), along with one known glycosidic acid, soldanellic acid B (117) [51], and three organic acids, 2Smethylbutyric, tiglic, and 2S,3S-nilic acids. The structures

I. muricata
The seeds of I. muricata (L.) Jacq. are used as a laxative and carminative in the folk medicine of India. In previous papers, Noda et al. reported that alkaline hydrolysis of the crude resin glycoside fraction of the seeds of I. muricata furnished three organic acids, isobutyric, 2S-methylbutyric, and 2R,3R-nilic acid, along with a glycosidic acid fraction composed of Lrhamnose, D-fucose, D-quinovose, and 11S-jalapinolic acid (2). They also discussed the isolation and structural elucidation of three glycosidic acids including muricatic acids A-C [20,57,58]. Furthermore, eight genuine resin glycosides, muricatins I-VIII, which possessed characteristic macrolactone structures, were reported [58,59].

Structure of resin glycoside
A new resin glycoside, named muricatin IX (136), was isolated from the seeds of I. muricata. The structure of 136 was determined on the basis of spectroscopic data as well as chemical evidence. It should be noted that 136 is different from all the resin glycosides isolated so far, in that the carboxyl group of its aglycone moiety is linked with a hydroxyl group of the organic acid, which was attached to a sugar moiety by ester linkage, to form a macrocyclic ester ring [60] (Fig. 11).

Conclusions
Resin glycosides were classified into two groups by Mayer in 1855, according to their solubility in ether. These groups consisted of jalapins, which are ether-soluble, and Therefore, a part of the crude ether-insoluble resin glycoside fraction might be a mixture of acylated glycosidic acid monomers with free carboxylic acid forms. On the other hand, calysolins IV, XI, and XIV, which have a macrolactone structure, hardly dissolve in ether. In addition, muricatin IX is different from all the resin glycosides isolated so far, in that the carboxyl group of its aglycone moiety linked with a hydroxyl group of the organic acid, which is attached to a sugar moiety by an ester linkage, to form a macrocyclic ester ring. Furthermore, the root of I. operculata and the seeds of Q. pennata contain both ethersoluble resin glycosides and ether-insoluble resin glycosides. The component organic and glycosidic acids of ether-soluble resin glycoside fractions of these materials were different from those of the corresponding ether-insoluble resin glycoside fraction, respectively. Therefore, classification of resin glycosides into two groups is considered to be meaningful. As mentioned above, many findings concerning the structures of resin glycosides have been obtained so far. Taking those findings into account, it would be preferable that the terms 'jalapin' and 'convolvulin' should not be used based on their solubility in ether as classified by Mayer, but rather to describe structural groups, i.e., the resin glycosides having intramolecular cyclic structures and the acylated glycosidic acid monomers and polymers with free carboxylic acid forms, respectively [61].
Sciences, Setsunan University, and Prof. Toshihiro Nohara of Pharmaceutical Sciences, Kumamoto University for their helpful suggestions and collaboration. Thanks are also due to Prof. Junei Kinjo and Dr. Masafumi Okawa of Pharmaceutical Sciences, Fukuoka University, Prof. Hitoshi Yosimitsu and Prof. Kazumi Yokomizo of Pharmaceutical Sciences, Sojo University, and Dr. Tomoko Mineno of Faculty of Pharmacy, Takasaki University of Health and Welfare for collaboration. I also sincerely thank the researchers and students who collaborated on this project. This research was supported in part by a Grant-in-Aid for Scientific Research (C) (No. 16K08306) from the Japan Society for the Promotion of Science and Research and by the Study Program/Project of Tokai University Educational System General Research Organization (Kanagawa, Japan).
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