Structure and reactivity of the dammarenyl cation: configurational transmission in triterpene synthesis

[reaction: see text] The dammarenyl cation (13) is the last common intermediate in the cyclization of oxidosqualene to a diverse array of secondary triterpene metabolites in plants. We studied the structure and reactivity of 13 to understand the factors governing the regio- and stereospecificity of triterpene synthesis. First, we demonstrated that 13 has a 17beta side chain in Arabidopsis thaliana lupeol synthase (LUP1) by incubating the substrate analogue (18E)-22,23-dihydro-20-oxaoxidosqualene (21) with LUP1 from a recombinant yeast strain devoid of other cyclases and showing that the sole product of 21 was 3beta-hydroxy-22,23,24,25,26,27-hexanor-17beta-dammaran-20-one. Quantum mechanical calculations were carried out on gas-phase models to show that the 20-oxa substitution has negligible effect on substrate binding and on the activation energies of reactions leading to either C17 epimer of 13. Further molecular modeling indicated that, because of limited rotational freedom in the cyclase active site cavity, the C17 configuration of the tetracyclic intermediate 13 can be deduced from the angular methyl configuration of the pentacyclic or 6-6-6-6 tetracyclic product. This rule of configurational transmission aided in elucidating the mechanistic pathway accessed by individual cyclases. Grouping of cyclases according to mechanistic and taxonomic criteria suggested that the transition between pathways involving 17alpha and 17beta intermediates occurred rarely in evolutionary history. Two other mechanistic changes were also rare, whereas variations on cation rearrangements evolved readily. This perspective furnished insights into the phylogenetic relationships of triterpene synthases.     

Enzymatic synthesis of an indole diterpene by an oxidosqualene cyclase: mechanistic, biosynthetic, and phylogenetic implications

Petromindole (1) is an unusual indole diterpene that bears a triterpene-like carbon skeleton, suggesting biogenesis from 3-(omega-oxido-geranylgeranyl)indole (4). We found that lupeol synthase (LUP1) from Arabidopsis thaliana cyclizes 4 to 1. Chiral HPLC comparisons of racemic 1 (from biomimetic cyclization of N-pivaloyl-4) with the LUP1 product and authentic 1 established the absolute stereochemistry of petromindole (3S) as that of cyclic triterpenes. Quantum mechanical calculations and conformational analysis of intermediates in the cyclization of 4 to 1 indicated that petromindole biosynthesis differs fundamentally from that of other indole diterpenes. This analysis revealed that radarins also originate from cyclization of 4 but undergo a backbone rearrangement rather than annulation to indole. The combined results support our hypothesis that native fungal petromindole synthase evolved from a pentacyclic triterpene synthase distant from most other indole diterpene synthases.     

Mechanistic insights into oxidosqualene cyclizations through homology modeling

2,3-Oxidosqualene cyclases (OSC) are key enzymes in sterol biosynthesis. They catalyze the stereoselective cyclization and skeletal rearrangement of (3S)-2,3-oxidosqualene to lanosterol in mammals and fungi and to cycloartenol in algae and higher plants. Sequence information and proposed mechanism of 2,3-oxidosqualene cyclases are closely related to those of squalene-hopene cyclases (SHC), which represent functional analogs of OSCs in bacteria. SHCs catalyze the cationic cyclization cascade converting the linear triterpene squalene to fused ring compounds called hopanoids. High stereoselectivity and precision of the skeletal rearrangements has aroused the interest of researchers for nearly half a century, and valuable data on studying mechanistic details in the complex enzyme-catalyzed cyclization cascade has been collected. Today, interest in cyclases is still unbroken, because OSCs became targets for the development of antifungal and hypocholesterolemic drugs. However, due to the large size and membrane-bound nature of OSCs, three-dimensional structural information is still not available, thus preventing a complete understanding of the atomic details of the catalytic mechanism. In this work, we discuss results gained from homology modeling of human OSC based on structural information of SHC from Alicyclobacillus acidocaldarius and propose a structural model of human OSC. The model is in accordance with previously performed experimental studies with mechanism-based suicide inhibitors and mutagenesis experiments with altered activity and product specificity. Structural insight should strongly stimulate structure-based design of antifungal or cholesterol-lowering drugs.     

An oxidosqualene cyclase makes numerous products by diverse mechanisms: a challenge to prevailing concepts of triterpene biosynthesis

The genome of the model plant Arabidopsis thaliana encodes 13 oxidosqualene cyclases, 9 of which have been characterized by heterologous expression in yeast. Here we describe another cyclase, baruol synthase (BARS1), which makes baruol (90%) and 22 minor products (0.02-3% each). This represents as many triterpenes as have been reported for all other Arabidopsis cyclases combined. By accessing an extraordinary repertoire of mechanistic pathways, BARS1 makes numerous skeletal types and deprotonates the carbocation intermediates at 14 different sites around rings A, B, C, D, and E. This undercurrent of structural and mechanistic diversity in a superficially accurate enzyme is incompatible with prevailing concepts of triterpene biosynthesis, which posit tight control over the mechanistic pathway through cation-pi interactions, with a single proton acceptor in a hydrophobic active site. Our findings suggest that mechanistic diversity is the default for triterpene biosynthesis and that product accuracy results from exclusion of alternative pathways.     

Triterpene saponins with XOD inhibitory activity from the roots of Ilex pubescens

A new triterpene saponin with 28-nor-urs-12(13),18(17)-dien-3β-ol as aglycone,named ilexsaponin C(1) was isolated from the roots of Ilex pubescens,together with three known saponins 2,3 and 4.The structure of 1 was elucidated on the basis of spectral analysis including 1D and 2D NMR and HR-ESI-MS.Saponins 1 and 4 exhibited significant XOD inhibitory activity in the test.     

A new triterpene synthase from Arabidopsis thaliana produces a tricyclic triterpene with two hydroxyl groups

[structure: see text] Thirteen oxidosqualene cyclase homologues exist in the genome of Arabidopsis thaliana. One of these genes, At4g15340, was amplified by PCR and expressed in yeast. The yeast transformant accumulated tricyclic triterpene, (3S,13R)-malabarica-17,21-dien-3,14-diol (arabidiol), whose structure was determined by NMR and MS analyses. Its epoxide analogue, (3S,13R,21S)-malabarica-17-en-20,21-epoxy-3,14-diol (arabidiol 20,21-epoxide), was also isolated from the transformed yeast. This is the first example of a triterpene synthase that yields a tricyclic triterpene with two hydroxyl groups.     

Triterpenes and triterpene saponins from the stems of Akebia trifoliata

To characterize the stems of Akebia trifoliata chemically, a detailed phytochemical examination was carried out on A. trifoliata stems, with particular attention to the triterpene and triterpene saponin constituents, and resulted in the isolation of three new triterpenes (1-3) and three new triterpene saponins (11-13), together with seven known triterpenes (4-10) and 12 known triterpene saponins (14-25). The structures of the new compounds were determined on the basis of spectroscopic analysis, including two-dimensional NMR spectroscopic data, and the results of hydrolysis. Four saponins (22-25), which were obtained in good yields and were not isolated from Akebia quinata stems, are concluded to be applicable as marker compounds in chemically distinguishing between A. trifoliata and A. quinata by conventional TLC examination. To the best our knowledge, the current work is the first chemical investigation of A. trifoliata.     

Synthesis of the hexacyclic triterpene core of the jujuboside saponins via tandem Wolff rearrangement–intramolecular ketene hetero-Diels–Alder reaction

The jujubosides are saponin natural products reported to have immunoadjuvant, anticancer, antibacterial, antifungal, and antisweet activities. The triterpene component, jujubogenin contains a unique tricyclic ketal motif comprising the DEF ring system. Herein, we describe our efforts toward the total synthesis of jujubogenin, using a sterically-demanding intermolecular Diels–Alder reaction to assemble the C-ring and a tandem Wolff rearrangement–intramolecular ketene hetero-Diels–Alder reaction to form the DF-ring system. Acid-catalyzed cyclization of the resulting bicyclic enol ether then closes the E-ring to provide the hexacyclic core of jujubogenin.     

A new method of solvent free O- and N-glycosylation using activated carbon fiber (ACF) as a promoter. Application to the synthesis of saponin and nucleoside analogues

By using activated carbon fiber (ACF) as solid acid promoter, we display a new system for solvent free O- and N-glycosylation, with a simple process which takes place in a drying oven; this method has been applied to the synthesis of sterol and triterpene O-glycosides (saponin analogues), as well as nucleoside analogues.     

An entry to the azocino[4,3-b]indole framework through a dehydrogenative activation of 1,2,3,4-tetrahydrocarbazoles mediated by DDQ: formal synthesis of (±)-uleine

It is presented that hexahydro-1,5-methano[4,3-b]indoles were efficiently synthesized in high yields (up to 89% yield) through the cyclization reaction of starting tetrahydrocarbazoles bearing a monoalkylaminocarbonylmethyl moiety at the C-2 position mediated by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). A mechanistic proposal is also given that mainly includes two cascade reactions: (i) formation of a vinylogous iminium cation via DDQ-mediated dehydrogenation of tetrahydrocarbazole functionality and (ii) intra-molecular and syn-selective addition of the amide functionality as the nucleophile to the vinylogous iminium cation. Furthermore, this cyclization reaction was successfully utilized in the formal total synthesis of (±)-uleine, an Aspidospermatan skeletal type alkaloid.     

Steric fixation of bromovinyluracil: Synthesis of furo[2,3-d]pyrimidine nucleosides

A new synthetic procedure for the preparation of 5,6-dihydrofuro[2,3-d]pyrimidin-2(3H)-one ( 3 ) and its deoxyriboside 8 is reported. Compound 3 undergoes nucleophilic reactions with various agents to yield 5-substituted uracil derivatives. The dehydro derivative of 3 , furo[2,3-d]pyrimidin-2(3H)-one ( 18 ) was synthesized by cyclization of BVU 15 , which made us develop a reproducible and high yield method for the synthesis of BV(D)U. Starting from 18 , the α-deoxyriboside 20 and the β-riboside 22 were prepared.     

Medicinal Flowers. XXXI. Acylated oleanane-type triterpene saponins, Sasanquasaponins I-V, with antiallergic activity from the flower buds of Camellia sasanqua

The methanolic extract and its 1-butanol-soluble fraction from the flower buds of Camellia sasanqua THUNB. were found to show inhibitory activities on the release of β-hexosaminidase from rat basophile leukemia (RBL-2H3) cells. From the 1-butanol-soluble fraction, five new acylated oleanane-type triterpene saponins, sasanquasaponins I-V, were isolated together with a known saponin and their chemical structures were elucidated on the basis of chemical and physicochemical evidence. The principal saponin constituents, sasanquasaponins I-III, with an acyl group at the 22-position of the aglycon part showed the inhibitory effects on the release of β-hexosaminidase and some structure-activity relationships were reported.     

Progress toward a biomimetic synthesis of phomoidride B

[reaction: see text]. An intramolecular cyclization strategy for effecting a biomimetic synthesis of the core structure of the fungal secondary metabolites phomoidrides A and B is described. The cyclization substrate 20 is prepared in eight steps from dibromide 10. Treatment of 20 with triethylamine in acetonitrile results in a rapid cyclization to give 21 and 22 in good yield.     

Stereochemical course in water addition during LUP1-catalyzed triterpene cyclization

Arabidopsis thaliana LUP1 (At1g78970) catalyzes the cyclization of oxidosqualene into lupeol and 3beta,20-dihydroxylupane (lupanediol). The stereochemical course of water addition to the lupanyl cation was studied. The X-ray crystal structure of lupanylepoxide 3,5-dinitrobenzoate established the configuration of epoxide as 20S. LiAlD4 reduction of the epoxide enabled the chemical shift assignment of prochiral methyl groups at C20 of lupanediol. Correlation of these methyl groups with biosynthetic lupanediol from [1,2-(13)C(2)] acetate established the stereochemical course of water addition. [reaction: see text].     

Synthesis of functionalized chromans by PnBu3-catalyzed reactions of salicylaldimines and salicylaldehydes with allenic ester

P(n)Bu(3)-catalyzed cyclization reactions of salicylaldimines and salicylaldehydes with ethyl 2,3-butadienoate gave the corresponding functionalized chromans in moderate to good yields in THF under mild conditions. The new reaction provides a new method for the synthesis of biologically active chroman products.     

人参中达玛烷型皂苷的化学转化产物结构和转化途径研究

利用高效液相色谱-电喷雾-多级串联质谱(HPLC-ESI-MSn)技术分析人参中3种达玛烷型皂苷(三七皂苷R1,人参皂苷Rd、20(S)-Rg3)在12-磷钨酸环境中转化的产物结构和转化途径。由原人参三醇型皂苷R1转化获得9种产物:20(S)-25-OH-R2、20(R)-25-OH-R2、25-OH-T5、20(S)-R2、20(R)-R2、20(S)-25-epoxy-R2、20(R)-25-epoxy-R2、T5、3β,12β-二羟基-6α-(2-O-β-D-吡喃木糖基-β-D-吡喃葡糖氧基)达玛烷-20(22),24-二烯。由原人参二醇型皂苷Rd和20(S)-Rg3转化得到10种产物:20(S)-25-OH-Rg3、20(R)-25-OH-Rg3、25-OH-Rk1、25-OH-Rg5、20(S)-Rg3、20(R)-Rg3、(20S,25)-epoxy-Rg3、(20R,25)-epoxy-Rg3、Rk1、Rg5。通过分析转化产物结构,并考察主要产物含量随转化时间的变化趋势,总结了人参中达玛烷型皂苷在酸性水溶液环境中的转化途径,即通过C20位去糖基化和差向异构化反应,以及烯烃链的水合、脱水、环合反应转化为稀有皂苷。     

A new dammarane-type triterpene saponin from Gynostemma pentaphyllum

<正>One new dammarane-type triterpene saponin,named(20S)-3β,20,21-trihydroxydammar-24-ene 3-O-[α-L-rhamnopyranosyl- (1→2)][β-D-xylopyranosyl(1→3)]-β-D-glucopyranoside(1),was isolated from the aerial parts of Gynostemma pentaphyllum (Thunb.) Makino.Its structural elucidation was accomplished mainly on the basis of the interrelation of spectroscopic methods, such as IR,HR-TOF-MS,NMR.     

Electrophile-induced domino cyclization reaction for the synthesis of 2,2a,10,11-tetrahydrofuro[2′,4′:4,6]pyrano[2,3-b]quinolines

A new and efficient synthesis of furo[2′,4′:4,6]pyrano[2,3-b]quinolines, via a domino cyclization approach, has been achieved by iodine and mercuric oxide-catalyzed intramolecular cyclization of 3-homoallyl-2-quinolones in acetic acid.     

Deciphering the mechanistic dichotomy in the cyclization of 1-(2-ethynylphenyl)-3,3-dialkyltriazenes: competition between pericyclic and pseudocoarctate pathways

The mechanistic aspects of the cyclization of (2-ethynylphenyl)triazenes under both thermal and copper-mediated conditions are reported. For cyclization to an isoindazole, a carbene mechanistic pathway is proposed. The carbene intermediate can react with oxygen, dimerize to give an alkene, or be trapped either intermolecularly (using 2,3-dimethyl-2-butene to generate a cyclopropane) or intramolecularly (using a biphenyl moiety at the terminus of the acetylene to form a fluorene). Density-functional theory (DFT) calculations support a pseudocoarctate pathway for this type of cyclization. Thermal cyclization to give a cinnoline from (2-ethynylphenyl)triazenes is proposed to occur through a pericyclic pathway. DFT calculations predict a zwitterionic dehydrocinnolinium intermediate that is supported by deuterium trapping studies as well as cyclizations performed using a 2,2,6,6-tetramethylpiperidine moiety at the 3-position of the triazene.     

A new friedelane triterpenoid and saponin with moderate antimicrobial activity from the stems of Drypetes laciniata

A new friedelane-type triterpene named 3β-hydroxyfriedelane-7,12,22-trione,as well as nine known compounds were isolated from the whole stems of Drypetes laciniata Hutch.(Euphorbiaceae).Their structures were established on the basis of spectroscopic methods.The new triterpene derivative and a known saponin were tested for antimicrobial and antifungal activities and they appeared to be moderate active.