Mechanism of the formation of a dehydrated ion by an unusual loss of oxygen at the 4'-carbonyl group of abscisic acid methyl ester in electron ionization mass spectrometry

Methyl ester of abscisic acid (ABA), a plant hormone, gives a dehydrated ion at m/z 260 in electron ionization mass spectrometry (EI-MS). This dehydrated ion had been considered to be derived only from the elimination of the tertiary hydroxyl group at C-1'. We found that 34% of the dehydrated ion was formed by elimination of the oxygen atom at the 4'-carbonyl group, and the remaining 66% by elimination of the 1'-hydroxyl group. This unusual elimination of the carbonyl oxygen was shown with [4'-(18)O]ABA methyl ester. Involvement of the 4'-carbonyl oxygen in dehydration was observed in methyl ester of phaseic acid (PA), a natural metabolite of ABA, but not in 1'-deoxy-ABA methyl ester or isophorone. This suggested that the 1'-hydroxyl group was necessary for the elimination of the 4'-carbonyl oxygen. ABA methyl esters labeled with stable isotopes showed that hydrogen atoms at the 1'-hydroxyl group and at C-4 or -5 or -3' or - 5' or -7' were eliminated with the 4'-carbonyl oxygen. These results allow us to propose a formation mechanism of the dehydrated ion derived from the elimination of 4'-carbonyl oxygen and hydrogen atoms at C-4 and 1'-oxygen in ABA methyl ester as follows: first, ionization at the 1'-hydroxyl group occurs to give an ion radical, and the proton at the 1'-oxygen migrates to the 4'-carbonyl oxygen after the bond fission between C-1'-C-6'; second, migration of the proton at C-4 to the 1'-oxygen is followed by migration of the protons at C-5 and C-7' to C-4 and C-5, respectively; finally, the proton at the 1'-oxygen migrates to the 4'-hydroxyl group, and H(2)O at C-4' is eliminated to give the dehydrated ion. Our findings point out that a dehydrated ion is not always derived from the elimination of a hydroxyl group.     

Clorobiocin biosynthesis in Streptomyces: identification of the halogenase and generation of structural analogs

Clorobiocin (clo) and novobiocin (nov) are potent inhibitors of bacterial DNA gyrase. The two substances differ in the substitution pattern at C-8' of the aminocoumarin ring, carrying a chlorine atom or a methyl group, respectively. By gene inactivation, clo-hal was identified as the gene of the halogenase responsible for the introduction of the chlorine atom of clorobiocin. Inactivation of cloZ did not affect clorobiocin formation, showing that this ORF is not essential for clorobiocin biosynthesis. Expression of the methyltransferase gene novO in the clo-hal(-) mutant led to the very efficient formation of a hybrid antibiotic containing a methyl group instead of a chlorine atom at C-8'. Comparison of the antibacterial activity of clorobiocin analogs with -Cl, -H, or -CH(3) at C-8' showed that chlorine leads to 8-fold higher activity than hydrogen and to 2-fold higher activity than a methyl group.     

Extended pi-conjugated dendrimers based on truxene

The largest pi-conjugated dendrimers containing up to nine 10,15-dihydro-5H-diindeno[1,2-alpha;1',2';-c]fluorene (truxene) moieties have been prepared with good yields by repetitive Friedel-Crafts acetylation and acid-promoted cyclotrimerization reactions. An alternative approach to the convergent synthesis of desired dendrimers has been developed, in which the core is generated "in-situ" by acid-promoted cyclotrimerizations of aryl methyl ketones. This proves valuable to afford large-size and precisely well-defined dendrimers in an accelerated dendrimer-growth strategy utilizing enlarged repeat units. The increasing amount of SiCl4 dramatically improves the yield of cyclotrimerization reactions. The introduction of hexahexyl groups onto C-5, C-10, C-15 positions of the truxene moiety greatly enhances the solubility of our compounds. Further investigation indicates that the torsion angle between the truxene segment and the benzene ring might play a key role in determining the photo properties of pi-conjugated dendrimers.     

Isolation and structure determination of a benzofuran and a bis-nor-isoprenoid from Aspergillus niger grown on the water soluble fraction of Morinda citrifolia Linn. leaves

The leaves of Morinda citrifolia, Linn. afforded a new benzofuran and a bis-nor-isoprenoid, blumenol C, hitherto unreported from this source. The structures of these have been elucidated as 5-benzofuran carboxylic acid-6-formyl methyl ester (1) and 4-(3'(R)-hydroxybutyl)-3,5,5, trimethyl-cyclohex-2-en-1-one (2) respectively through spectroscopic studies. The NMR data (including 1D, 2D techniques) and stereochemistry at C-3' of Compound 2 is also being reported for the first time.     

Studies on the enzyme immunoassay of bio-active constituents contained in oriental medicinal drugs. V. Preparation of bovine serum albumin conjugate and beta-galactosidase labelled antigen for enzyme immunoassay of 3 beta-(monoglucuron-1' beta-yl)-18 beta-glycyrrhetic acid

In order to prepare an immunogen for enzyme immunoassay of 3 beta-(monoglucuron-1'-beta-yl)-18 beta-glycyrrhetic acid (3MGA), which was isolated from a patient with glycyrrhizin-induced pseudoaldosteronisms, benzyl glycyrrhetate (3) was allowed to react with an acetobromosugar (2) in the presence of silver carbonate to give benzyl 3 beta-(methyl 2',3',4'-triacetyl-glucuron-1' beta-yl)-glycyrrhetate (5) and methyl 3',4'-diacetyl-alpha-1',2'-O-[1-(benzyl glycyrrhet-3 beta-yl)- ethylidene]-D-glucuronate (4). On the other hand, this reaction was carried out in the presence of mercuric cyanide in nitromethane to give compound 5, benzyl 3 beta-acetyl glycyrrhetate (6) and benzyl 11-oxo-A-neooleana-3(5),12-dien-3-oate (7). 4-Aminomethylcyclohexanecarboxylic acid and glycine were introduced as chemical bridges at C-30 of 3 beta-(tert-butylglucuron-1' beta-yl)-glycyrrhetic acid (11) derived from compound 5. The former bridge was used to prepare an immunogenic conjugate with bovine serum albumin, and the latter bridge was used for antigen labelled with beta-galactosidase.     

Solid-state photodimerization of cholest-4-en-3-one

Crystalline cholest-4-en-3-one undergoes solid-state dimerization by UV radiation to give two ring A - ring A connected dimers. No dimerization occurs in solution. The first dimer, characterized by a cyclobutane ring, is formed by connection of C-2 and C-3 of a moiety with C-5' and C-6' of another moiety, respectively. The latter dimer has a six-membered ketal ring formed by connection of C-2 with C-5' and of O, linked to C-3, with C-3'. The structures have been determined by spectroscopic means. X-ray analysis of title compound evidences the proximity of the axial H-2 of a molecule to the C-4' of a molecule in the upper layer. The transfer of the hydrogen and the connection between C-2 and C-5' might be the driving force of dimerization.     

New chromane derivatives isolated from the brown alga, Sargassum micracanthum

Two new chromane type meroterpenoids were isolated from the methanolic extract of the brown alga, Sargassum micracanthum. Their structures were elucidated based on spectroscopic analysis and chemical conversion. The absolute stereochemistry of the methyl group at C-8' in 1 and related compounds were determined by modified Mosher's method.     

Diastereofacial Selective Addition of Ethynylcerium Reagent and Barton-McCombie Reaction as the Key Steps for the Synthesis of C-3'-Ethynylribonucleosides and of C-3'-Ethynyl-2'-deoxyribonucleosides

We describe the preparation of 3'-alkynyluridine 4a and -adenosine 4b and of 3'-alkynyl-2'-deoxyuridine 16a and -adenosine 16b starting from the corresponding nucleosides. The desired stereochemistry of the C-3' tertiary alcohol was obtained by reaction of an ethynylcerium-lithium reagent on a 3'-ketonucleoside with the hydroxyl group at C-5' unprotected. The 2'-deoxygenation was performed by a Barton-McCombie reaction under appropriate conditions where the addition of tin hydride to the triple bond was suppressed. Evaluation of the anti-HIV activity of the C-3' modified nucleosides is reported.     

Assignment of the liposidomycin diazepanone stereochemistry

The liposidomycins comprise a family of complex nucleoside antibiotics that inhibit bacterial peptidoglycan synthesis. Their structures (1, 2) feature nucleoside, ribofuranoside, diazepanone, and lipid regions. Several stereogenic centers remain unassigned, including three within the diazepanone region: C-6', C-2'", and C-3'". An intramolecular reductive amination reaction has been used to prepare model diazepanones. Analysis of 40 and two of its diastereomers by NMR spectroscopy, X-ray crystallography, and molecular modeling indicates a close relative configurational and conformational match between 40 and the liposidomycin diazepanone degradation product 43 and allows the assignment of stereochemistry of the natural products as either [C-6'(R), C-2'"(R), C-3'"(R)] or [C-6'(S), C-2'"(S), C-3'"(S)].     

Reversible reaction via a carbanion intermediate in the elimination of ammonia from L-histidine catalysed by histidine ammonia-lyase

L-[5'-2H2]Histidine was used as a substrate to investigate the enzymatic reaction mechanism with histidine ammonia-lyase from Pseudomonas fluorescens. The study was performed to determine the exchange rate of deuterium at C-5' of the imidazole ring with solvent hydrogen relative to the net urocanic acid production. The extent of hydrogen exchange at C-5' of histidine or urocanic acid was measured by gas chromatography-mass spectrometry-selected ion monitoring, monitoring the molecular ion intensities of the respective gas chromatographic derivatives, at m/z 380 and 379 for histidine and at m/z 267 and 266 for urocanic acid. The observed hydrogen exchange at C-5' suggested a reversible mechanism via a carbanion intermediate in the reaction with histidine ammonia-lyase.     

Synthesis and structural characterization of pyrimidine bi- and tricyclic nucleosides with sugar puckers conformationally locked into the eastern region of the pseudorotational cycle

Reaction of 5'-O-tosyl TSAO-m(3)T (1) with amines has led to the synthesis of new classes of bi- and tricyclic nucleosides. Full details about the synthesis of these compounds and a plausible mechanism to explain their obtention are reported. In addition, we describe the development of a second, more efficient, and higher yielding synthetic route as a general approach for the synthesis of some of these bicyclic nucleosides. To study the conformational behavior of the bi- and tricyclic nucleosides described in this paper, intensive NMR investigations and molecular modeling studies were performed. Conformational analysis indicates that the furanose ring of the compounds described here prefers the eastern side of the pseudorotation cycle with the base substituents preferentially in the anti range. The torsion angle gamma describing the C-4'[bond]C-5' is found to prefer the +sc range. These compounds represent a novel class of E-type conformationally restricted bicyclic ribo-nucleosides containing a [3.3.0]-fused carbohydrate moiety. The bicyclic nucleosides described herein present an interesting potential for diverse and selective chemical treatments on the 2'-hydroxyl and/or the functionalities incorporated at the bridge between C-3' and C-5'.     

Synthesis of C-3' methyl taxotere (docetaxel)

Protected (3R,4S)-N-Boc-3-hydroxy-4-methyl-4-phenylazetidin-2-one has been synthesized stereoselectively and used to esterify protected 10-desacetylbaccatin III to give, following removal of the protecting groups, novel C-3' methyl taxotere (docetaxel).     

Synthesis of Sialic Acid Analogues with the Oxime Group at C-4 Or C-5 of KDN 1

Abstract

The readily available methyl (methyl 3-deoxy-5,8:7,9-di-O-isopropylidene-β-D-glycero-D-galacto-2-nonulopyranosid)onate (7) was converted in five synthetic steps into methyl (methyl 4-acetamido-3,4-dideoxy-β-D-glycero-D-talo-2-nonulopyranosid)onate (11). Selective protection of the C-4, C-7, C-8 and C-9 hydroxy groups of methyl (methyl 3-deoxy-8,9-O-isopropylidene-β-D-glycero-D-galacto-2-nonulpyranosid)onate (2) followed by oxidation of the C-5 hydroxy group and then its oximination gave 5-hydroxyimino derivatives (15 and 16).

     

The mechanism of the reaction catalyzed by ADP-beta-L-glycero-D-manno-heptose 6-epimerase

ADP-l-glycero-d-manno-heptose 6-epimerase (AGME, RfaD) is a bacterial enzyme that is involved in lipopolysaccharide biosynthesis and interconverts ADP-beta-l-glycero-d-manno-heptose (ADP-l,d-Hep) with ADP-beta-d-glycero-d-manno-heptose (ADP-d,d-Hep). AGME is known to require a tightly bound NADP+ cofactor for activity and presumably employs a mechanism involving transient oxidation of the substrate. Four mechanistic possibilities are considered that involve transient oxidation at either C-7' ', C-6' ', or C-4' ' of the heptose nucleotide. In this contribution, the use of solvent isotope incorporation studies and alternate substrates provides strong evidence for a mechanism involving nonstereospecific oxidation/reduction directly at C-6' '. It was found that the epimerization proceeds without any detectable incorporation of solvent-derived deuterium or 18O-isotope into the product. This argues against mechanisms involving either proton transfers at carbon or dehydration/rehydration events. In addition, the deoxygenated analogues, 7' '-deoxy-ADP-l,d-Hep and 4' '-deoxy-ADP-l,d-Hep, were both found to serve as substrates for the enzyme, indicating that oxidation at either C-7' ' or C-4' ' is not required for catalysis.     

Acid-catalysed rearrangements of steroid alkenes-III1. backbone rearrangement of de-a-steroid alkenes and preparation of an aromatic de-a-steroid.

Backbone rearrangement of 10a(methyl)-de-A-cholest-5-ene (3c), 6-ene (3d), 9-ene (3s) and 5(10)-ene (3b) affords products isomeric at C-20 and with the C-10 methyl group in the more stable equatorial position (6a. and 6b). 5-Methylene-10a(methyl)-de-A-cholestane (5) affords similar C-20 isomeric products with both the C-5 and C-10 methyls in the more stable equatorial positions (9a and 9b). The de-A-alkenes (3) provided a convenient starting point for preparation of de-A- cholesta-5,7,9-triene (7). Components (6a, 6b, 7, 9a and 9b) have been used to confirm the widespread occurrence of homologous series of de-A-steroids in marine shales with a mild thermal history.     

Proton magnetic resonance studies of compounds with bridgehead nitrogen atoms. XIV .: The influence of an axial 5-methyl group on the chemical shifts of C2 methylene protons in 1,3-heterosystems

The shielding of a C-2 axial proton by an axial C-5 methyl group in 1,3-heterocyclic systems is described and explained in terms of conformational distortion.     

Comparative reactivity of 3-methyl-5-phenylisoxazole and 3-methyl-5-phenylisothiazole against electrophilic compounds

A comparative study of reactions of 3-methyl-5-phenylisoxazole and 3-methyl-5-phenylisothiazole with electrophilic compounds in the presence of n-BuLi, LICA or LICA-TMEDA is reported. By using LICA-TMEDA, regioselective reactions of the heterocyclic compounds at the C-3 methyl group are obtained. With n-BuLi or LICA and the isoxazole derivative a product mixture at the C-4 position and the C-3 methyl group is found. In the case of isothiazole compound, only with methyl iodide and n-BuLi, the dialkylated product at both positions is formed.     

Biosynthesis of vitamin B1 in yeast. Derivation of the pyrimidine unit from pyridoxine and histidine. Intermediacy of urocanic acid

Incorporation studies with 13C-, 15N-, and 2H-labeled substrates, followed by NMR analysis, show that the pyrimidine unit of thiamin (Vitamin B1) originates from a C5N fragment, derived from C-2',2,N,C-6,5,5' of pyridoxol (Vitamin B6) and an N-C-N fragment derived from L-histidine. Urocanic acid serves as an intermediate on the route of the N-C-N fragment of histidine into the thiamin pyrimidine.     

Cal-B-catalyzed alkoxycarbonylation of a-ring stereoisomeric synthons of 1alpha,25-dihydroxyvitamin D3 and 1alpha,25-dihydroxy-19-nor-previtamin D3: a comparative study. first regioselective chemoenzymatic synthesis of 19-nor-A-ring carbonates

A comparative study of alkoxycarbonylation processes of both 19-nor-A-ring and A-ring stereoisomers of 1alpha,25-dihydroxyvitamin D3 analogues catalyzed by Candida antarctica lipase B (CAL-B) has been described. The presence of the methyl group in the A-ring at C-2, as in 3-6, has a determining role in the regioselectivity of the biocatalysis, mainly allowing the hydroxyl group at C-5 position to react. For the 19-nor-A-ring stereoisomers 7-10, which lack the C-2 methyl group, the configurations at C-3 and C-5 have a high influence in the selectivity exhibited by CAL-B. Thus, each couple of enantiomers showed opposing regioselectivities depending on the C-3 configuration. When C-3 possesses an (S)-configuration, enzymatic alkoxycarbonylations took place at the C-5-(R) or C-5-(S) hydroxyl groups. However, if the chiral centers at C-3 are (R), CAL-B alkoxycarbonylated the C-3-(R) hydroxyl group independently of the configuration at C-5. The corresponding carbonates are useful A-ring precursors of 1alpha,25-dihydroxyvitamin D3 analogues, selectively modified at the C-1 or C-3 positions. In addition, an improved synthesis of cis A-ring synthons 5 and 6 is described using a Mitsunobu methodology.     

Synthesis and PPAR-gamma ligand-binding activity of the new series of 2'-hydroxychalcone and thiazolidinedione derivatives

Fifteen chalcones and three thiazolidinedione (TZD) chalcones were prepared to evaluate their peroxisome proliferator-activated receptor-gamma (PPAR-gamma) ligand-binding activities. Among the three TZDs, one compound possessed PPAR-gamma transactivation potential, while the others showed antagonistic activity against PPAR-gamma transactivation. Among the chalcones, compound 5 was the most potent, and structure-activity relationship studies indicated that a methoxyl group in position C-4 and hydroxyl group in position C-4' or 5' in chalcone plays a key role in determining the potency of PPAR-gamma activation.