Microbial hydroxylation of steroids. 10. Rearrangement during epoxidation and hydroxylation, and the stepwise nature of these enzymic reactions

1985 ◽  
Vol 63 (5) ◽  
pp. 1121-1126 ◽  
Author(s):  
Herbert L. Holland ◽  
Elly Riemland
Keyword(s):  
Double Bond ◽  
Product Formation ◽  
Enzymatic Oxidation ◽  
Allylic Rearrangement ◽  
Stepwise Mechanism ◽  
Microbial Hydroxylation ◽  
A Site

A series of unsaturated steroids has been incubated with fungi which are known to hydroxylate at a site corresponding to the allylic position in the analogous saturated steroids. In some cases, the anticipated hydroxylation occurred without rearrangement of the double bond. In a number of instances, however, products were obtained whose structures implied that allylic rearrangement had occurred during the reaction. The formation of these products is consistent with a stepwise mechanism of enzymatic oxidation. Possible routes for product formation are presented which incorporate this proposal.

Evidence for a cytochrome P-450-catalyzed allylic rearrangement with double-bond topomerization

1988 ◽  
Vol 110 (6) ◽  
pp. 1979-1981 ◽  
Author(s):  
Robert H. McClanahan ◽  
Alain C. Huitric ◽  
Paul G. Pearson ◽  
John C. Desper ◽  
Sidney D. Nelson
Keyword(s):  
Double Bond ◽  
Allylic Rearrangement ◽  
Cytochrome P 450

Oxidation of Benzyl Alcohols by the Laccase-Mediator System (LMS) a Comprehensive Kinetic Description

Holzforschung ◽  
2001 ◽  
Vol 55 (1) ◽  
pp. 47-56 ◽  
Author(s):  
A. Potthast ◽  
T. Rosenau ◽  
K. Fischer
Keyword(s):  
Benzyl Alcohol ◽  
Cation Radical ◽  
Product Formation ◽  
Zero Order ◽  
Enzymatic Oxidation ◽  
Second Phase ◽  
Rate Law ◽  
Zero Order Kinetics ◽  
Mediator System ◽  
Laccase Mediator System

SummaryInvestigations into the reaction kinetics of the laccase-mediator system (LMS) have been carried out. Two widely used mediators, 2,2′-azino-bis(3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS,3) and 1-hydroxybenzotriazole (HOBT,4), were compared by means of a model reaction, the oxidation of 2,4-dimethoxybenzyl alcohol (DMBA,1) to 2,4-dimethoxybenzaldehyde (DMA,2). The consumption of dioxygen was recorded electrochemically, substrate consumption and product formation were monitored by GLC.With ABTS as the mediator, the LMS reaction proceeded in two clearly distinguishable stages. The first phase is characterized by a fast decrease in oxygen with zero-order kinetics and no detectable formation of 2,4-dimethoxybenzaldehyde (2). ABTS is converted into oxidized species, the cation radical6and the dication7, respectively. In the second phase, oxygen consumption was considerably slower and followed a second-order kinetics, while the benzaldehyde was produced according to a zero-order rate law. According to the kinetic studies, the ABTS dication, but not the enzyme itself, is acting as the actual oxidant. The rate of oxidation product formation increased with increasing mediator / benzyl alcohol ratio. Less oxygen than the equivalent amount was consumed in the second reaction stage indicating that the oxidized ABTS formed in the first stage acts as an oxidant reservoir, being reduced to ABTS in turn.The LMS reaction with HOBT (4) as the mediator did not exhibit distinguishable phases, and was characterized by a comparatively slow oxygen uptake with zero-order kinetics throughout. Enzymatic oxidation of HOBT to the HOBT radical (5), which acts as the actual oxidant towards the benzyl alcohol, was the rate-determining step. The production of 2,4-dimethoxybenzaldehyde thus followed a zeroorder rate law as well. The reaction rate increased with increasing HOBT / benzyl alcohol ratios. Increasing concentrations of4caused less oxygen to be consumed per equivalent of benzaldehyde formed, indicating the occurrence of another reaction pathway at high mediator charges. At low HOBT / benzyl alcohol ratios the HOBT radical (5) acts as one-electron oxidant and is reduced to HOBT in a reversible process. In contrast, at higher HOBT / benzyl alcohol ratios5acts as a three-electron oxidant, being irreversibly reduced to benzotriazole. At commonly employed mediator concentrations, a superposition of both mechanisms results. The pure borderline cases can only be observed at HOBT / benzyl alcohol ratios below 1 and above 6, respectively.

Reevaluation of the stereochemical courses of the allylic rearrangement and the double-bond reduction catalyzed by Brevibacterium ammoniagenes fatty acid synthase

1991 ◽  
Vol 113 (10) ◽  
pp. 3997-3998 ◽  
Author(s):  
Mary C. O'Sullivan ◽  
John M. Schwab ◽  
T. Mark Zabriskie ◽  
Gregory L. Helms ◽  
John C. Vederas
Keyword(s):  
Fatty Acid ◽  
Double Bond ◽  
Fatty Acid Synthase ◽  
Allylic Rearrangement

scholarly journals Sequential Diels–Alder/[3,3]-sigmatropic rearrangement reactions of β-nitrostyrene with 3-methyl-1,3-pentadiene

2013 ◽  
Vol 9 ◽  
pp. 2137-2146 ◽  
Author(s):  
Peter A Wade ◽  
Alma Pipic ◽  
Matthias Zeller ◽  
Panagiota Tsetsakos
Keyword(s):  
Double Bond ◽  
Substituent Effects ◽  
Product Formation ◽  
Sigmatropic Rearrangement ◽  
Diels Alder ◽  
Initial Assessment ◽  
Concerted Mechanism ◽  
Rearrangement Process ◽  
Zwitterionic Intermediate ◽  
Nitronic Ester

The tin(IV)-catalyzed reaction of β-nitrostyrene with (E)-3-methyl-1,3-pentadiene in toluene afforded two major nitronic ester cycloadducts in 27% and 29% yield that arise from the reaction at the less substituted diene double bond. Also present were four cycloadducts from the reaction at the higher substituted diene double bond, two of which were the formal cycloadducts of (Z)-3-methyl-1,3-pentadiene. A Friedel–Crafts alkylation product from the reaction of the diene, β-nitrostyrene, and toluene was also obtained in 10% yield. The tin(IV)-catalyzed reaction of β-nitrostyrene with (Z)-3-methyl-1,3-pentadiene in dichloromethane afforded four nitronic ester cycloadducts all derived from the reaction at the higher substituted double bond. One cycloadduct was isolated in 45% yield and two others are formal adducts of the E-isomer of the diene. The product formation in these reactions is consistent with a stepwise mechanism involving a zwitterionic intermediate. The initially isolated nitronic ester cycloadducts underwent tin(IV)-catalyzed interconversion, presumably via zwitterion intermediates. Cycloadducts derived from the reaction at the less substituted double bond of (E)-3-methyl-1,3-pentadiene underwent a [3,3]-sigmatropic rearrangement on heating to afford 4-nitrocyclohexenes. Cycloadducts derived from the reaction at the higher substituted diene double bond of either diene failed to undergo a thermal rearrangement. Rates and success of the rearrangement are consistent with a concerted mechanism possessing a dipolar transition state. An initial assessment of substituent effects on the rearrangement process is presented.

The mechanism of the reaction of lead tetraacetate–hydrogen fluoride with 1-octene

1976 ◽  
Vol 54 (15) ◽  
pp. 2417-2425 ◽  
Author(s):  
Dennis D. Tanner ◽  
Peter Van Bostelen
Keyword(s):  
Double Bond ◽  
Hydrogen Fluoride ◽  
Product Formation ◽  
Lead Tetraacetate ◽  
Olefinic Double Bond ◽  
Markovnikov Addition ◽  
Mechanism Of The Reaction

The reaction of 1-octene with the lead tetraacetate–hydrogen fluoride reagent led to products resulting from the Markovnikov addition of lead and its ligand (fluoro or acetoxy) to the olefinic double bond. Displacement of lead from the organometalic intermediate via three separate pathways could be distinguished; displacement concomitant with hydride migration alkyl migration, or direct displacement by an external ligand. The pathways for the formation of the product difluorides, fluoroacetates, and diacetate could be followed by an analysis of the products obtained from carrying out the reaction using 1-octene-1-13C as substrate. The major pathways to product formation resulting from either hydride or alkyl migration could be rationalized as proceeding through the intermediacy of a resonance stabilized α-fluorocarbonium ion.

Degradation of rutin by Aspergillus flavus. Studies on specificity, inhibition, and possible reaction mechanism of quercetinase

1972 ◽  
Vol 18 (4) ◽  
pp. 493-508 ◽  
Author(s):  
T. Oka ◽  
F. J. Simpson ◽  
H. G. Krishnamurty
Keyword(s):  
Reaction Mechanism ◽  
Double Bond ◽  
Aspergillus Flavus ◽  
Chelating Agents ◽  
Enzymatic Oxidation ◽  
Maximal Rate ◽  
Carbonyl Groups ◽  
Sulfhydryl Reagents ◽  
Nitrogen Bases ◽  
Chelating Ability

The dioxygenase produced by Aspergillus flavus that oxidizes quercetin contains 2 moles of copper per mole of enzyme and binds at least 2 moles of substrate. The enzyme is not inhibited by sulfhydryl reagents or affected by H2O2, but is inhibited by copper-chelating agents and reducing agents. Nitrogen bases with chelating ability are more effective inhibitors than those that do not form metal chelates.The dioxygenase oxidizes flavones that possess a double bond between carbons-2 and -3 and a hydroxyl on carbon-3. Hydroxyls at other positions affect both the relative rates of enzymatic oxidation and the concentration of substrate required to attain half maximal rate (Km). The evidence indicates that the substrate binds to the copper of the enzyme via the 3-hydroxyl and 4-carbonyl groups of the substrate. A possible mechanism for the reaction is presented.

Cu(I)-Catalyzed Coupling of (9-Benzylpurin-6-yl)magnesium Chloride with Allyl Halides: An Approach to 6-Allylpurine Derivatives

2006 ◽  
Vol 71 (8) ◽  
pp. 1221-1228 ◽  
Author(s):  
Martin Klečka ◽  
Tomáš Tobrman ◽  
Dalimil Dvořák
Keyword(s):  
Double Bond ◽  
Magnesium Chloride ◽  
Allylic Rearrangement ◽  
Allyl Halides ◽  
Purine Ring

6-Allylpurine derivatives are formed by Cu(I)-catalyzed coupling of (9-benzyl-9H-purin-6-yl)- magnesium chloride with allyl halides. The reaction is accompanied by allylic rearrangement in some cases. Under acid conditions the double bond of the allyl group rearranges to the conjugation with purine ring.

Eels Under Standing Wave Conditions

1982 ◽  
Vol 40 ◽  
pp. 492-495
Author(s):  
O.L. Krivanek ◽  
J. TaftØ
Keyword(s):  
Standing Wave ◽  
Wave Conditions ◽  
Set Up ◽  
A Site ◽  
Complex Crystal

It is well known that a standing electron wavefield can be set up in a crystal such that its intensity peaks at the atomic sites or between the sites or in the case of more complex crystal, at one or another type of a site. The effect is usually referred to as channelling but this term is not entirely appropriate; by analogy with the more established particle channelling, electrons would have to be described as channelling either through the channels or through the channel walls, depending on the diffraction conditions.

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