Ursethoxy acid,a new triterpene from Lantana camara

A new pentacyclic triterpene, named ursethoxy acid (1) was isolated from the aerial parts of Lantana camara Linn. Its structure has been elucidated as 3,25-epoxy-3alpha-ethoxy-urs-12-en-28-oic acid through extensive 1D- and 2D-NMR (COSY-45, NOESY, J-resolved, HMQC and HMBC) studies.     

Petatrichol B： A Pentacyclic Triterpenoid with Unusual Skeleton from Petasites tricholobus

A novel pentacyclic triterpenoid, petatrichol B, was isolated from the rhizome of Petasites tricholobus. Its structure was established by means of spectroscopic analysis （EIMS, HRSIMS, IR, 1D NMR and 2D NMR）.     

3β-Methoxyolean-18-ene (miliacin) from the marine fungus Chaetomium olivaceum

A pentacyclic tritepenoid, 3-methoxyolean-18-ene (miliacin), was isolated for the first time from the marine fungus Chaetomium olivaceum. Its structure was determined on the basis of 2D NMR spectroscopy and X-ray diffraction data.     

On the ionization state of the substrate in the active site of glutamate racemase. A QM/MM study about the importance of being zwitterionic

Computer simulations on a QM/MM potential energy surface have been carried out to gain insights into the catalytic mechanism of glutamate racemase (MurI). Understanding such a mechanism is a challenging task from the chemical point of view because it involves the deprotonation of a low acidic proton by a relatively weak base to give a carbanionic intermediate. First, we have examined the dependency of the kinetics and thermodynamics of the racemization process catalyzed by MurI on the ionization state of the substrate (glutamate) main chain. Second, we have employed an energy decomposition procedure to study the medium effect on the enzyme-substrate electrostatic and polarization interactions along the reaction. Importantly, the present theoretical results quantitatively support the mechanistic proposal by Rios et al. [J. Am. Chem. Soc. 2000, 122, 9373-9385] for the PLP-independent amino acid racemases.     

Triterpenoids from Eugenia grandis: structure elucidation by NMR spectroscopy

A new pentacyclic triterpenoid, 2alpha,3beta-dihydroxylup-12-en-28-oic acid (1) and a rarely encountered pentacyclic triterpenoid, 3beta-hydroxylup-12-en-28-oic acid (2), together with five known compounds, friedelin (3), 3beta-friedelinol (4), betulinic acid (5), oleanolic acid (6) and beta-sitosterol (7) were isolated from the chloroform extract of stem bark of Eugenia grandis (Syn: Syzygium grande). The structure and stereochemistry of the new compound (1) and the rarely encountered compound (2) were established by 1D and 2D NMR spectroscopic techniques. All the above isolated compounds from this plant are reported for the first time.     

Structural determination of 3beta-stearyloxy-urs-12-ene from Maytenus salicifolia by 1D and 2D NMR and quantitative 13C NMR spectroscopy

Six pentacyclic triterpenoids, 3beta-stearyloxy-urs-12-ene (1), friedelin (2), 3beta-friedelinol (3), alpha-amyrin (4), beta-amyrin (5), and lupeol (6), have been isolated from the hexane extract of Maytenus salicifolia Reissek (Celastraceae) leaves. The molecular and structural formula as well as the stereochemistry of a new pentacyclic triterpene (1) were determined using data obtained from 1H and 13C NMR spectra, DEPT135 and by 2D HSQC, HMBC, COSY and NOESY experiments. The molecular formula C48H84O2 was established using quantitative 13C NMR, and the molecular weight (692 Da) was confirmed by elemental analysis and mass spectrometry (GC-MS).     

Intramolecular Diels-Alder reactions: Construction of aza- and diaza-steroid type skeletons

The preparation of 2-(4-phenyl)butadienyl-piperidine 5 is described. An intramolecular Diels-Alder reaction of the intermediately formed fumaramide thereof produces stereoselectively the tricyclic lactam 6 . Its structure, as well as the configurational relationship of its 5 asymmetric centers, is corroborated on the basis of the NMR.-data. Cycloacylation of the thermodynamically stable precursors 13 and 20 leads to pentacyclic aza- or diaza-steroid type skeletons. Their structures ( 14, 16 , and 21 ) and in particular their relative configurations are elaborated. A few qualitative kinetic aspects of this intramolecular (4+2)-cycloaddition are presented.     

Guianolactones A and B,Two Rearranged Pentacyclic Limonoids from the Seeds of Carapa guianensis

Two novel rearranged limonoids, guianolactones A ( 1 ) and B ( 2 ), were isolated from Carapa guianensis Aubl. (Meliaceae) seeds. The structures of 1 and 2 with their absolute configurations were elucidated in detailed examinations using single‐crystal X‐ray diffraction analyses and 2D NMR spectra. Guianolactone A ( 1 ) has a novel 5/6/6/6/6 pentacyclic core including two δ ‐lactone and a tetrahydropyran ring, while guianolactone B ( 2 ) is a novel limonoid with a 6/6/5/6/6 pentacyclic core featuring a δ ‐lactone and a tetrahydrofuran ring.     

New insights into the reaction mechanism catalyzed by the glutamate racemase enzyme: pH titration curves and classical molecular dynamics simulations

The mechanism of the reactions catalyzed by the pyridoxal-phosphate-independent amino acid racemases and epimerases faces the difficult task of deprotonating a relatively low acidicity proton, the amino acid's alpha-hydrogen, with a relatively poor base, a cysteine. In this work, we propose a mechanism for one of these enzymes, glutamate racemase (MurI), about which many controversies exist, and the roles that its active site residues may play. The titration curves and the pK1/2 values of all of the ionizable residues for different structures leading from reactants to products have been analyzed. From these results a concerted mechanism has been proposed in which the Cys70 residue would deprotonate the alpha-hydrogen of the substrate while, at the same time, being deprotonated by the Asp7 residue. To study the consistency of this mechanism classical molecular dynamics (MD) simulations have been carried out along with pK1/2 calculations on the MD-generated structures.     

Über den pentacylischen Chrom (III)-Komplex aus 1,6-Di-(2-amino-benzolsulfonamido)-hexan ⇉β-Naphtol

A five- or six-membered ring in pentacyclic azo-dyestuff-chromium complexes with the sandwich arrangement, having sulphonamido groups as complex formers, was extended into a nine-membered ring. This altered not only the electron spectrum but also the stability of the complex. Whilst the five- and six-membered pentacycles are very stable, the corresponding nine-membered ring complex is considerably less stable. Its stability approaches that of the corresponding 1:2-Cr complexes which, because of deformed valency angles, are very strained and, therefore, are also very unstable. This tension is counteracted in pentacycles having a lower number of members (5 and 6), and so the system is stabilised. If the number of ring members is increased, this counteraction is reduced and, therefore, the stabilising effect is lessened.     

Absolute stereochemistry of citrinadins a and B from marine-derived fungus

[Structure: see text]. Citrinadin A (2) is a pentacyclic indolinone alkaloid isolated from the cultured broth of a fungus, Penicillium citrinum, which was separated from a marine red alga. The absolute stereochemistry of the pentacyclic core in 2 and its new congener, citrinadin B (1), was elucidated by analysis of the ROESY spectrum for the chlorohydrin derivative (3) of 1 as well as comparison of the electronic circular dichroism (ECD) spectra for 1 and 2 with those of known spirooxiindole alkaloids. On the other hand, the absolute configuration at C-21 bearing an epoxide ring was assigned as S by comparison of the vibrational circular dichroism (VCD) spectra of 1 with those of model compounds 2S- and 2R-2,3-epoxy-3,3-dimethyl-1-phenylpropan-1-one (4a and 4b, respectively).     

New pyrano[3,4-b]indoles from 2-hydroxymethylindole and l-dehydroascorbic acid

2-Hydroxymethylindole reacts with l-dehydroascorbic acid under mild conditions to give (3R,3aR,10cS)-3-[(1S)-1,2-dihydroxyethyl]-3a,10c-dihydroxy-3a,5,6,10c-tetrahydrofuro[3′,4′:5,6]pyrano[3,4-b]indol-1(3H)-one. Its tosyl derivative undergoes cyclization to form a pentacyclic ketal derivative.     

A novel formal total synthesis of cephalotaxine

A formal total synthesis of cephalotaxine (CET), the parent structure of antileukemia Cephalotaxus alkaloids, was achieved through a novel synthesis of the pentacyclic amino enone 4 by a rapid annulation of readily available beta-(3,4-methylenedioxy)phenethylamine (2), delta-valerolactone, and bromoacetone.     

A new ethylene glycol triterpenoid from the leaves of Psidium guajava

One new pentacyclic triterpenoid psidiumoic acid (5) along with four known compounds beta-sitosterol (1), obtusol (2), oleanolic acid (3), and ursolic acid (4) have been isolated from the leaves of Psidium guajava. The new constituent 5 has been characterized as 2 alpha-glycolyl-3beta-hydroxyolean-12-en-28-oic acid through 2D NMR techniques. This is the first report of isolation of compound 2 from the genus Psidium.     

Analysis of pentacyclic triterpenes by LC-MS. A comparative study between APCI and APPI

The analytical performances of three atmospheric-pressure sources, electrospray (ESI), atmospheric-pressure chemical ionization (APCI), and atmospheric-pressure photoionization (APPI), were evaluated for the analysis of pentacyclic triterpenes in liquid chromatography-mass spectrometry (LC-MS). Among these sources, APPI and APCI are particularly well adapted to sensitive analyses of pentacyclic triterpenes by LC-MS. Detection parameters were optimized for both the sources, and the effects of three dopants (toluene, acetone and anisole) on the detection (sensitivity and ion fingerprints in MS spectra) were studied in detail for APPI-MS.The limits of quantification were measured under selected ion monitoring conditions, in the range of 0.005-0.015 mg l(-1) and 0.002-0.84 mg l(-1) in APPI and APCI, respectively, depending on the studied pentacyclic triterpene. Overall, APPI was found more sensitive than APCI in positive ion mode, whereas APCI shows the greatest sensitivity for acidic triterpenes in negative ion mode.Following this study, the developed LC-MS method was used for the characterization of pentacyclic triterpenes in three plant extracts. High amounts of betulinic acid, betulinic aldehyde and betulinic aldehyde acetate were observed in plane bark. The main component of birch bark is betulin and extracts of okoume resin exhibit high amounts of alpha- and beta-amyrin.     

Total synthesis and properties of the crambescidin core zwitterionic acid and crambescidin 359

The total synthesis of the crambescidin core acid 9, crambescidins 359 (8) and 431 (7), and the properties of the crambescidin core are described. A key step of the synthetic route to guanidinium carboxylate 9 is Pd(0) catalyzed cleavage of the ester side chain of pentacyclic cinnamyl ester 15. This ester is also employed to prepare a small library of crambescidin alkaloid analogues that differ in their C14 side chain. The zwitterionic guanidinium carboxylate 9 was shown to readily decarboxylate to form crambescidin 359 (8). Decarboxylation of crambescidin core acid 9 was fastest under basic conditions. In the presence of base, up to eight deuterium atoms can be incorporated into the pentacyclic crambescidin core. Both deuterium incorporation and decarboxylation of crambescidin core acid 9 are the result of facile ring opening of the spirocyclic ether rings of the pentacyclic guanidinium moiety.     

Methylierung von 4-Hydroxy-4-(2-piperidyl)-4H-pyrazolo [1,5-a]indol mit Formaldehyd und Ameisensäure. 11. Mitteilung über metallorganische Reaktionen und Folgeprodukte

In the 10^th communication of this series [1] the synthesis of 4-hydroxy-4-(2-piperidyl)-4H-pyrazolo[1,5-a]indole ( 4 ) was described (Scheme). Surprisingly enough, methylation of this compound with formaldehyde and formic acid led via ring closure and a subsequent rearrangement to a pentacyclic ketone. By means of ¹³C-NMR.-spectroscopy and mass spectroscopy, this ketone could be identified as a indolizino-pyrazolo-indole ( 9 ). Its structure and configuration were determined by X-ray structure analysis.     

Brief total synthesis of the cell cycle inhibitor tryprostatin B and related preparation of its alanine analogue

Tryprostatin B was synthesized in 32% overall yield from the readily available dipeptide anhydride cyclo-(l-Trp-l-Pro). Its tandem C-3 prenylation/cyclization gave the corresponding pentacyclic pyrroloindole systems bearing a prenyl group at the indole C-3 position. These compounds were then submitted to acid-catalyzed opening of the newly formed ring, with concomitant migration of the prenyl group to the indole C-2 position. The alanine analogue of tryprostatin B was also prepared using a similar sequence. The successful implementation of this strategy strengthens the case for a biosynthetic route for the tryprostatins along similar lines.     

Chemical constituents from the leaves of Psidium guajava

Five constituents including one new pentacyclic triterpenoid guajanoic acid (1) and four known compounds beta-sitosterol (2), uvaol (3), oleanolic acid (4), and ursolic acid (5) have been isolated from the leaves of Psidium guajava. The new constituent 1 has been characterized as 3beta-p-E-coumaroyloxy-2alpha-methoxyurs-12-en-28-oic acid through 2D NMR techniques and chemical transformations. This is the first report of isolation of compound 3 from the genus Psidium.     

Synthesis and 20π-electrocyclisation of heptahendecafulvadiene - An unusual sequence of pericyclic processes

Upon thermal activation the newly synthesised heptahendecafulvadiene E ( 3) is isomerised into the pentacyclic hydrocarbons 9/11 (xylene,t_1/2(150°C) ca.5min), the formation of which is explained by initial conrotatory 20-electron electrocyclisation followed by a cascade of 10π/6π-pericyclic processes.