Iridoids from the green leaves of Eucommia ulmoides

The bark of Eucommia ulmoides is a well-known crude drug in oriental medicine, and its leaves have been consumed as a beverage. From the green leaves of this plant, three new iridoids (1-3) were isolated, together with 12 known compounds. Compound 1 is the first iridoid possessing a saturated bond between C-3 and C-4 and having an ether linkage between C-3 and C-2 of the glucose unit. Furthermore, 2 and 3 may be regarded as the first naturally occurring conjugates of an iridoid and an amino acid.     

Resin glycosides from the herbal drug jalap (Ipomoea purga)

Three new resin glycosides, purginosides I and II (1 and 2) and purgin I (3), were isolated from the aerial parts of Ipomoea purga and purified by preparative-scale recycling HPLC from a chloroform-soluble extract. Their structures were established through NMR spectroscopy and mass spectrometry. Purginosides I and II (1 and 2) are partially acylated branched pentasaccharides derived from operculinic acid A, which is composed of one D-fucose, one D-glucose, and three l-rhamnose units. The site of the aglycon macrolactonization is at C-2 of the second saccharide (rhamnose). In both compounds 1 and 2, three different esterifying residues were located at C-2 of the second rhamnose unit and at C-2 (or C-3) and C-4 on the third rhamnose moiety. The acylating residues were characterized as trans-cinnamic, n-decanoic, and either (+)-(2S)-2-methylbutanoic or n-hexanoic acid. Purgin I (3) was found to be an ester-type dimer of operculinic acid A, acylated by n-dodecanoic, (+)-(2S)-2-methylbutanoic, and trans-cinnamic acids at the same oligosaccharide core positions found in compounds 1 and 2. The site of lactonization by the aglycon in unit A was placed at C-2 of the second saccharide. The position for the ester linkage for the monomeric unit B on the macrocyclic unit A was identified as C-4 of the terminal glucose. This is the first report on the isolation, purification, and structure elucidation of intact individual resin glycoside constituents from the herbal drug jalap.     

New minor taxane derivatives from the needles of Taxus canadensis

Seven minor taxane derivatives were isolated for the first time from the needles of Taxus canadensis. Four of these natural taxanes are O-glycosylated (1-4), with 4 being the only reported taxane with a glucose on ring C, and two have a dimethylamino-C-5 side chain (5 and 6). An unusual double bond at C-5(6) has been identified in taxane 7.     

Triterpenoid saponins from the fruits of Caryocar glabrum

Twenty-one new triterpenoid saponins, named caryocarosides (1-21), glycosides of 2beta-hydroxyoleanolic acid, hederagenin, bayogenin, and gypsogenic acid, have been isolated from the fruits of Caryocar glabrum along with nine known triterpenoid saponins (22-30) that are described for the first time from a plant in the Caryocaraceae. Their structures were established by 1D and 2D NMR techniques ((13)C, COSY, TOCSY, HSQC, HMBC, and ROESY experiments), ESIMS, and acid hydrolysis. The isolated compounds could be classified into two series: glucosides (1-8, 22, 27, and 30) derived from the 3-O-monoglucoside and glucuronides (9-21, 23-26, 28, and 29) derived from the 3-O-monoglucuronide. In 22 of the saponins (1-8, 12-22, and 24-26), a galactose moiety was linked to C-3 of a glucuronic acid or a glucose moiety. The galactose was substituted in position 3 by a second galactose unit (6, 7, 20, and 21) or by a xylose unit (8). Seven saponins (4, 5, 16-19, and 26) were found to be bidesmosides with one glucose unit linked to C-28 of the aglycon. The hemolytic activity of the major saponins (2, 3, 5, 12-15, 17, 24, and 28) was measured on sheep erythrocytes in order to establish structure-activity relationships based on the type of sugar attached to the aglycon and on the structure of this aglycon.     

Paeonianins A-E,new dimeric and monomeric ellagitannins from the fruits of Paeonia lactiflora

Four new dimeric ellagitannins, paeonianins A-D (2-5), were isolated from the fruits of Paeonia lactiflora, together with a new ellagitannin monomer, paeonianin E (1). Their structures were determined by spectroscopic methods. Paeonianins A-D (2-5) are positional isomers formed by condensation of pentagalloyl-beta-D-glucose (8) with 5-desgalloylstachyurin (6) or casuariin (7). Paeonianin E is a C-glycosidic ellagitannin having a gallic acid methyl ester moiety at the glucose C-1 position. This is the first report of the isolation of dimeric ellagitannins from a plant in the family Paeoniaceae.     

Application of lipase-catalyzed regioselective esterification in the preparation of digitonin derivatives.

The oligosaccharide chain of the monodesmosidic haemolytic saponin digitonin (1) undergoes an efficient and regioselective acylation in organic solvent by use of Novozym 435 (lipase B from Candida antarctica supported on acrylic resin) in the presence of an activated ester. With vinyl acetate, acetylation occurs at C-6 OH of glucose(II) and C-4 OH of xylose to afford the previously unreported diacetyl derivative 2 and the monoacetyl derivatives 3 and 4. With vinyl laurate only the monolauryl derivative 5 is formed. The structures of these acylated digitonins have been established using modern 2D NMR techniques, which allowed complete assignments of all proton resonances. The hemolytic activity of derivatives 2-5 is significantly reduced compared to that of digitonin.     

Four dimeric aporphine-containing alkaloids from Thalictrum fauriei.

Four dimeric alkaloids (1-4) containing an aporphine unit and representing unique structural features were obtained from Thalictrum fauriei and were characterized by spectral and chemical methods. Fauripavine (1), with a munitagine pavine unit, is the first such dimer with an aporphine C-1 diphenyl ether connection; although fauridine (2), also the first of its class, has codamine, a benzyl tetrahydroisoquinoline, linked at the same position. Faurithaline (3) and its 3-methoxy analogue 4, both with reticuline, a benzyl tetrahydroisoquinoline as the second unit, are the first dimers with the diphenyl ether linkage at C-8 of an aporphine oxygenated at C-10 and C-11. Oconovine (8) was converted to 8-hydroxy-O-methyloconovine (12) to determine the effect of oxygenation at C-8 on the proton chemical shift of the C-7 hydrogens in a study to support the C-8 ether linkage of alkaloids 3 and 4. All the alkaloids have the S-configuration at their asymmetric centers as established from their CD spectra when compared with those of model compounds.     

Synaptosides A and A1, triterpene glycosides from the sea cucumber Synapta maculata containing 3-O-methylglucuronic acid and their cytotoxic activity against tumor cells

Two novel triterpene holostane glycosides, synaptosides A ( 1) and A 1 ( 2), have been isolated from the Vietnamese sea cucumber Synapta maculata (Synaptida, Apodida). Their structures were elucidated by spectroscopic methods (NMR and MS) and chemical transformations. Glycosides 1 and 2 have rare branched pentasaccharide carbohydrate chains featuring a 3- O-methylglucuronic acid residue not previously reported in glycosides from sea cucumbers and a 6- O-sulfated glucose. Glycoside 2 has an oxo group at C-7 and a 8(9)-double bond. All these structural features are unknown in glycosides from sea cucumbers. Glycoside 1 has moderate cytotoxic activity (IC 50 8.6 microg/mL) and glycoside 2 is inactive against HeLa tumor cells.     

Inhibitory effects of bufadienolides on interleukin-6 in MH-60 cells

Derivatives of bufogenin isolated from the skin of the Chinese toad, Bufo bufo gargarizans Cantor ("Ch'an Su"), and several semisynthetic derivatives of 20beta,21beta-epoxy-resibufogenin (13) have been evaluated for interleukin-6 (IL-6) antagonistic activity due to their growth-inhibitory activities on IL-6-dependent MH-60 cells. Among the naturally derived compounds (1-17), 20S,21-epoxy-resibufogenin formate (1) showed potent inhibitory activity on the IL-6-dependent growth of MH-60 cells. Epoxide groups at both the C-14, C-15 and C-20, C-21 positions are required to exhibit this type of activity. Compounds acetylated at the C-16 position (7 and 9-11) showed a loss of activity. An oxo group at the C-3 position (8, 14, and 15) resulted in cytotoxicity for both cell lines. Stereochemistry is important for selectivity on suppression of IL-6 activity. Among the semisynthetic derivatives (18-25) of 13, compound 19, with an acetyl group introduced at the C-3 position in comparison to 13, demonstrated considerable growth inhibition of IL-6-dependent MH-60 cells.     

Phytoecdysteroids from Ajuga macrosperma var. breviflora roots

Three new phytoecdysteroids, ajugacetalsterones C (1) and D (3) and breviflorasterone (2), were isolated from the roots of Ajuga macrosperma var. breviflora along with five known compounds, namely, 20-hydroxyecdysone, cyasterone, makisterone A, 20-hydroxyecdysone 3-acetate, and 20-hydroxyecdysone 2-acetate. The structures of 1-3 were elucidated on the basis of extensive 1D and 2D NMR spectroscopic studies. The new compounds possess acetal oxygen bridges between C-26 and C-20/C-22, or C-26/C-23, or a lactone bridge between C-26 and C-23.     

Total synthesis of the cytotoxic threo, trans, threo, trans, threo annonaceous acetogenin asimin and its C-10 epimer: unambiguous confirmation of absolute stereochemistry.

A convergent synthesis of asimin (1) and its C-10 epimer 33 is reported. The essential features of this synthesis include (a) the addition of an enantioenriched gamma-OMOM allylic indium reagent to a core C-23 aldehyde precursor to install the C-24-C-34 segment with concomitant introduction of the C-24 and C-23 stereocenters; (b) the addition of an enantioenriched gamma-OMOM allylic indium reagent to a core C-16 aldehyde to install the C-10-C-15 segment with formation of the C-15 and C-16 stereocenters, (c) the addition of a dialkyl zinc reagent, catalyzed by a chiral triflamide-Ti(O-i-Pr)(4) complex, to introduce the C-1-C-9 segment with creation of either the 10(R) or 10(S) stereocenters; and (d) aldol condensation of the foregoing C-1-C-34 segment with OTBS-protected lactic aldehyde to incorporate the C-35-C-37 butenolide segment. Removal of the three MOM protecting groups was achieved with aqueous HCl in THF. The 10(R) diastereomer was found to correspond to natural asimin.     

Isolation and biosynthesis of aurachin P and 5-nitroresorcinol from Stigmatella erecta

The isolation of aurachin P (2) from Stigmatella erecta strain Pd e32 is described. Spectroscopic data, in particular NMR data, indicate it is 1'-hydroxyaurachin A with a 1'R,2'S,3'R relative configuration. In addition, a further compound, 5-nitroresorcinol (4a), was isolated and identified as a novel natural product. Feeding of (13)C- and (15)N-labeled precursors indicated this was synthesized solely from glucose and ammonia. To account for the labeling pattern, phloroglucinol (8) is postulated as an intermediate branching off from 3-dehydroquinate (7) in the shikimate pathway.     

Hypoglycemic diterpenoids from Tinospora crispa

Three new diterpenoids, 2-O-lactoylborapetoside B (1), 6'-O-lactoylborapetoside B (2), and tinocrispol A (3), and nine known diterpenoids (4-12) were isolated from an EtOH extract of Tinospora crispa vines. Their structures were elucidated by spectroscopic analyses. The C-6 glucosyloxy group in borapetoside C (6) was revised to be α-oriented. The in vivo hypoglycemic activities of the major components, borapetosides A-C (4-6), were examined. Intraperitoneal injection of 4 and 6 (5 mg/kg) showed significant lowering of plasma glucose levels in normal and streptozotocin-induced type 1 diabetic mice. Borapetoside C increased glucose utilization in peripheral tissues and reduced hepatic gluconeogenesis, accounting for the hypoglycemic effect.     

Biosynthesis of elsamicin A, a novel antitumor antibiotic

The 1H noise-decoupled 13C-nmr spectrum of elsamicin A [1] prepared from the cultures of an unknown actinomycete species (ATCC 39417) supplemented with [1-13C]acetate and [2-13C]acetate showed enrichment of all 19-carbons in the aglycone. In addition, L-[methyl-13C]methionine- and D-[1-13C]glucose-supplemented cultures enriched the carbons of the two methyl groups on the disaccharide moiety and the C-1 carbons of the disaccharide, respectively. The results demonstrated that the aglycone of elsamicin A is derived entirely from acetate and the disaccharide portion is biosynthesized from two units of glucose and methionine.     

Lepadins F-H, new cis-decahydroquinoline alkaloids from the Australian ascidian Aplidium tabascum

Chemical investigation of a Great Barrier Reef ascidian, Aplidium tabascum, has resulted in the isolation of three new cis-decahydroquinoline alkaloids, lepadins F-H (4-6). The three new compounds differ from the previously isolated lepadins A-C (1-3) in that they contain a fully saturated 5-hydroxyoctyl side chain attached at C-5, an unsaturated eight-carbon ester moiety attached to C-3, and opposite stereochemistry at C-5 and C-3. Lepadins G (5) and H (6) are epimers at C-2. NMR and molecular modeling studies indicated that the three new compounds adopt a chair-chair conformation in which the nitrogen equatorially substitutes the cyclohexyl ring. This contrasts with lepadins A-C (1-3), which adopt a chair-chair conformation in which the nitrogen axially substitutes the cyclohexyl ring.     

Configurational assignment of cyclic peroxy metabolites provides an insight into their biosynthesis: isolation of plakortolides, seco-plakortolides, and plakortones from the Australian marine sponge Plakinastrella clathrata

Sixteen new compounds, comprising nine new plakortolides K-S (1-9), four seco-plakortolides (10-13), and three plakortones (14-16), were isolated from the Australian sponge Plakinastrella clathrata. Structural elucidation, including relative configurational assignment, was based on extensive spectroscopic analysis, while the absolute configurations of 1-4 were deduced from (1)H NMR analyses on MPA esters derived from Zn/AcOH reduction products. Diastereomeric sets of plakortolides, e.g., K and L, or M and N, differed in configuration at C-3/C-4 rather than at C-6, a stereochemical result that compromises a biosynthetic pathway involving Diels-Alder cycloaddition of molecular oxygen to a Δ(3,5)-diunsaturated fatty acid precursor. The biosynthesis may plausibly involve cyclization of a 6-hydroperoxydienoic acid intermediate following stereospecific introduction of the hydroperoxy group into a polyketide-derived precursor. Isolated seco-plakortolides converted under mild conditions into plakortones with full retention of configuration, suggesting C-6 hydroxy attack on an α,β-unsaturated lactone intermediate. The NMR data reported for the compound named plakortolide E are inconsistent with the current literature structure and are those of the corresponding seco-plakortolide (19). The reported conversion of the metabolite into a plakortone ether on storage is consistent with this structural revision.     

Biotransformation of the fungistatic sesquiterpenoid patchoulol by botrytis cinerea

Biotransformation of the fungistatic sesquiterpenoid patchoulol (1) by the fungus Botrytis cinerea affords the 5-, 7- and (8R)-hydroxy (2, 3, and 5) derivatives as the major metabolites, together with a number of minor metabolites (4, 6-9) arising from hydroxylation at C-2, C-3, C-5, C-9, C-13, and C-14.     

Antidiabetic complications and anti-Alzheimer activities of sophoflavescenol, a prenylated flavonol from Sophora flavescens, and its structure-activity relationship

It was previously reported that prenylated flavonols from Sophora flavescens are inhibitors of rat lens aldose reductase (RLAR), human recombinant aldose reductase (HRAR), advanced glycation endproducts (AGE), β-secretase (BACE1) and cholinesterases (ChE). Based upon structure-activity relationships, 3,4'-dihydroxy flavonols with a prenyl or lavandulyl group substitution at the C-8 position, and a hydroxy group at the C-5, are important for such inhibition. In our ongoing study to isolate active principles from S. flavescens by an activity-guided isolation procedure, further detailed phytochemical investigations of the CH(2)Cl(2) fraction were conducted via repeated chromatography over silica gel and Sephadex LH-20 columns. This ultimately resulted in the isolation of a promising active sophoflavescenol with higher inhibitory activities among the current prenylated flavonols isolated from S. flavescens against RLAR, HRAR, AGE, BACE1 and ChEs. The results further support that 3,4'-dihydroxy flavonols with a prenyl or lavandulyl substitution at the C-8 position and a methoxy group at C-5 represent a new class of RLAR, HRAR and AGE inhibitors. Nevertheless, the C-5 hydroxyl group of prenylated flavonoids is important for inhibition of BACE1 and ChEs, indicating that the hydroxyl group at C-5 might be the main contributor to the augmentation and/or modification of prenylated flavonol activity.     

Characterization of an abeo-taxane: brevifoliol and derivatives

Brevifoliol is a natural diterpene isolated from Taxus baccata Nutt. A series of brevifoliol 1 derivatives, 2-8 and 10, were prepared for characterization and semisynthesis purposes and included the introduction of acetyl, Troc, and TES groups at C-5 and C-13. Derivatives 16-20 of 5-acetylbrevifoliol 2 were obtained via esterification with cinnamic acid, with both 2S-(-)- and 2R-(+)-3-phenyllactic acid, and with N-benzoyl-(2'R,3'S)-3'-phenylisoserine at C-13. Brevifoliol compounds 12, 13, and 15 with either 2S-(-)-phenyllactate moieties at C-5 and C-13 or an N-benzoyl-(2'R,3'S)-3'-phenylisoserinyl at C-13 were also prepared. An abeo-taxane structure for 1 was clearly defined from the (13)C NMR analysis of the 5-acetyl-13-oxo derivative 8 and from the conversion of 1 into 10, a conformationally restrained compound having a C-13, C-15 oxygen bridge. The biological activity of each of these derivatives is being studied.     

Iridoids from Rothmannia macrophylla.

A new iridoid glucoside with an ether linkage between C-3 and C-10 and a novel nonglycosidic iridoid with an ether linkage between C-3 and C-6 and a lactonic linkage at C-1, named macrophylloside (1) and macrophyllide (2), respectively, were isolated from the leaves of Rothmannia macrophylla, along with six known iridoids. Their structures were established by NMR and MS spectroscopies.