1,720,976 research outputs found
Enantioselective hydrolysis of epoxides: the employment of the soluble fraction from Vicia sativa seedlings
No full textBiocatalytic hydrolysis of meso and racemic aryl- and alkyl-oxiranes was accomplished by employing the epoxide hydrolase activity of the soluble fraction of Vicia sativa seedlings. Whereas meso epoxides were not hydrolyzed by this fraction, racemic compounds were transformed into the corresponding diols by formal anti-stereoselective water attack. Both substrate and product enantioselectivity were strongly influenced by the chains length and the presence of a hydroxyl group. (C) 2001 Elsevier Science B.V. All rights reserved
The anomalous course of the microsomal transformation of the exo-2,3-epoxides of norbornene and norbornadiene. The possible involvement of a general acid activation during the enzymatic hydrolysis of these oxides
No full textThe enzymatic hydrolysis of exo-2,3-epoxy-norbornane (1) with a crude rabbit liver microsomal preparation occurred with a rearrangement and gave selectively (2 R,7S)-bicyclo[2.2.1]heptane-2,7-diol (3), enantiomeric excess (ee) 30 +/- 2%. The analogous exo-2,3-epoxy-5-norbornene (2) gave, under the same conditions, exclusively endo-6-hydroxymethylbicyclo-[3.1.0]hex-2-ene (4), arising from the microsomal catalyzed reduction of the first formed endo-6-formylbicyclo-[3.1.0]hex-2-ene (5). A mechanistic explanation for the observed products is proposed. (C) 2000 Elsevier Science B.V. All rights reserved
Stereochemistry of the biotransformation of 1-hexene and 2-methyl-1-hexene with rat liver microsomes and purified P450s of rats and humans
No full textThe epoxidation of 1-hexene (1a) and 2-methyl-1-hexene (1b), two hydrocarbons present in the ambient air as: pollutants, is catalyzed by some human and rat P450 enzymes. The enantioselectivities of these processes, when the reactions were carried out using rat and human liver microsomal preparations, were modest and dependent on both P450 composition and substrate concentrations. Various P450 isoforms (rat P450 2B1 and human P450 2C10 and 2A6) catalyzed the double bond oxidation of 1a and 1b with different product enantioselectivities. In the case of 1a, a moderately enantioselective hydroxylation at the allylic C(3) with the formation of 1-hexen-3-ol (4a) by microsomes from control or preinduced rats was also observed. The oxidation of this metabolite was, in turn, catalyzed by rat liver microsomes and mainly by rat P450 2C11, leading exclusively to the formation of 1-hexen-3-one, with no double bond epoxidation being observed. The stereochemical course of the microsomal epoxide hydrolase-catalyzed hydrolysis of the epoxy alcohols, threo-(+/-)- and erythro-(+/-)-1,2-epoxyhexan-3-ol, theoretically expected to be formed from 4a, has been investigated
Steric strain and reactivity: Electrophilic bromination of trans-(1-Methyl-2-adamantylidene)-1-methyladamantane
No full texttrans-(1-Methyl-2-adamantylidene)-1-methyladamantane (DMAD, 1b) reacts with Br-2 in chlorinated hydrocarbon solvents to give either a bromonium polybromide ion pair or a substitution product, depending on bromine concentration. The first intermediate is a 1:1 pi-complex having K-f = 1.85-(0.19) x 10(3) M-1 at 25 degrees C, which rapidly evolves to the bromonium tribromide ion pair. At high bromine concentration, which shifts all equilibria involving the counteranion of the ion pair intermediate toward the pentabromide species, this bromonium ion is stable and unable to further evolve into products. Temperature-dependent NMR spectra indicate chemical exchange of Br+ between the sides of the plane containing the two carbons of the bromonium ion. At very low bromine concentration, no ionic intermediate is detected and the reaction rapidly yields a rearranged substitution product, identified as 10. Under these conditions the disappearance of the pi-complex follows a first-order rate law, and the observed rate constant increases with increasing olefin concentration, showing that product formation implies Br- as counteranion of the ionic intermediate, whose formation is a reversible process. A comparison of the results reported here for the bromination of 1b with those previously found for the parent olefin, adamantylideneadamantane (1a), shows that steric strain markedly affects the reactivity of the double bond
Substituent dependence of the diastereofacial selectivity in iodination and bromination of glycals and related cyclic enol ethers
No full textThe stereochemical course of the electrophilic iodination and bromination of tri-O-benzyl-D-glucal under various conditions has been compared to that of substituted dihydropyrans 2-5. IN3 addition in acetonitrile affords trans-alpha -iodoazides (80-87%), besides small amounts of trans-beta -adducts, in the presence or the absence of benzyloxy substituents at C-3 or C-4, and in agreement with bridged iodonium ion intermediates. In contrast, the diastereofacial selectivity of bromine addition in dichloroethane going through open bromo oxacarbenium ions depends strongly on the substituents. Whereas the trans-alpha -dibromides are the main (85-95%) adducts in the absence of C-4 and C-5 substituents, in their presence a moderate to exclusive selectivity for cis-alpha -addition (60-99%) is observed. The predominance of trans-alpha -addition is again observed whatever the substituents when the bromination is carried out in the same solvent but with a tribromide ion salt, supporting a concerted addition of the two bromine atoms under these conditions. Finally, bromine addition in methanol exhibits a completely different behavior with the nonselective formation of trans-alpha- and trans-beta -methoxybromides and a small dependence on the substituents. In agreement with the absence of azide trapping of any cationic intermediate, it is concluded that these brominations which do not go through an ionic intermediate are concerted additions of bromine and methanol with very loose rate- and product-determining transition states. Finally, the substituent conformation at C-4 influences drastically the stereoselectivity in all these brominations. Evidence for alpha -anomeric control of the nucleophile approach at C-4 is given by the highly predominant formation of alpha -adducts, except in the methanolic bromination. The factors determining the versatile selectivity of the electrophile approach are discussed in terms of PPFMO theory and of the special mechanisms of glycal reactions
Stereochemical course of the biotransformation of isoprene monoepoxides and of the corresponding diols with liver microsomes from control and induced rats
No full textThe stereochemical course of the biotransformation of isoprene by liver enzymes from control and induced rats has been determined. Between the two primarily formed metabolites, 2-methyl-2-vinyloxirane (2) and isopropenyloxirane (3), epoxide 2 is rapidly transformed into the corresponding vicinal racemic diol 4, predominantly through a nonenzymatic hydrolysis reaction. At variance, epoxide 3 is mainly biotransformed into the diol 5 by microsomal epoxide hydrolase (mEH) to give, before 50% conversion, selectively (R)-3-methyl-3-butene-1,2-diol, 5. The hydrolysis competes with the oxidation of the monoepoxide 3 to the corresponding diepoxides 6. Epoxidation of 3 catalyzed by P450 is characterized by a moderate stereoselectivity which, however, was strongly dependent on P450 induction. Treatment of rats with phenobarbital (PB) (an inducer of P450 2B1 and 3A) leads to threo-(2R,2'R)-6 with a high selectivity, while with pyrazole (Pyr) (an inducer of P450 2E1), the formation of both erythro-(2S,2'R)- and threo-(2R,2'R)-6 is favored. The mEH-catalyzed hydrolysis of diepoxides 6 proceeds, although with a moderate turnover rate, with substrate and product diastereo- and enantioselection by nucleophilic attack on the more substituted oxirane ring to give selectively (2R,3S)-3,4-epoxy-2-methyl-1,2-diol (7). Both diols 4 and 5 may be further oxidized on their double bond by P450. These reactions, which occur at a slow rate and are dependent on P450 induction with PB and Pyr, may be negligible in the overall isoprene biotransformation. On the other hand, the epoxydiol 7, which is formed by hydrolysis of diepoxides 6 but it is itself not hydrolyzable, may play an important role in the isoprene toxicity
Stereochemical aspects in the 4-vinylcyclohexene biotransformation with rat liver microsomes and purified P450s. Monoepoxides and diols
No full textThe stereochemical course of the biotransformation of 4-vinylcyclohexene (VCH, 1) by liver microsomes from male and female control and induced rats and purified rat F450 2B1 and 2E1 has been determined. The epoxidation of 1, catalyzed by male microsomes, occurs on both the endo- and exocyclic double bond to give four isomeric epoxides, cis-4-vinylcyclohexene 1,2-epoxide (2), trans-4-vinylcyclohexene 1,2-epoxide (3), (4R*,7S*)-4-vinylcyclohexene 7,8-epoxide (4), and (4R*,7R*)-4-vinylcyclohexene 7,8-epoxide (5). On the other hand, microsomes from female rats catalyzed primarily the endocyclic epoxidation. The stereoselectivity of this process was strongly dependent on gender and P450 induction. Only the phenobarbital and pyrazole, at lower levels, were able to enhance the epoxidation of 1 and mostly on the endocyclic double bond. Also, P450 2E1 and 2B1 in a reconstituted system were able to perform the epoxidation of 1 primarily on its endocyclic double bond. The metabolites, cis- and trans-4-vinylcyclohexene 1,2-epoxide (2 and 3, respectively) and the isomeric 4-vinylcyclohexene 7,8-epoxides (4 and 5), were rapidly biotransformed into the corresponding vicinal diols by mEH-catalyzed hydrolysis. The reaction of the endocyclic epoxides occurred with good substrate diastereo- and enantioselectivity favoring the hydrolysis of epoxides (1S,2R,4S)-3 and (1R,2S,4S)-2 to give, before 50% conversion, selectively (1R,2R,4S)-diol (6). At variance, the hydrolysis of the exocyclic epoxides was characterized by a high level of substrate enantioselection associated with a very low, if any, level of substrate diastereoselection, the two epoxides, (4R,7S)-4 and (4R,7R)-5, being hydrolyzed practically with the same rate. On the basis of the major resistance to mEH hydrolysis, the endocyclic epoxides, (1R,2S,4R)-3 and (1S,2R,4R)-2, are expected to be further oxidized, in a stereochemical manner, to the specific mutagenic diepoxides which are thought to play a crucial role in VCH ovotoxicity. Thus, VCH ovotoxicity may be markedly affected by the reactivity of the diepoxidic stereoisomers formed and detoxicated
Performance Evaluation of end-to-end security protocols in an Internet of Things
Full text linkWireless Sensor Networks are destined to play a fundamental role in the next-generation Internet, which will be characterized by the Machine-to-Machine paradigm, according to which, embedded devices will actively exchange information, thus enabling the development of innovative applications. It will contribute to assert the concept of Internet of Things, where end-to-end security represents a key issue. In such context, it is very important to understand which protocols are able to provide the right level of security without burdening the limited resources of constrained networks. This paper presents a performance comparison between two of the most widely used security protocols: IPSec and DTLS. We provide the analysis of their impact on the resources of embedded devices. For this purpose, we have modified existing implementations of both protocols to make them properly run on our hardware platforms, and we have performed an extensive experimental evaluation study. The achieved results are not a consequence of a classical simulation campaign, but they have been obtained in a real scenario that uses software and hardware typical of the current technological developments. Therefore, they can help network designers to identify the most appropriate secure mechanism for end-to-end IP communications involving constrained devices
Strain and reactivity: Electrophilic addition of bromine and tribromide salts to cyclic allenes
No full textThe kinetics and the products of the bromination of several cyclic allenes, from C-9 to C-13 (1a-e), with tetrabutylammonium tribromide (TBAT) and Br-2 have been investigated in 1,2-dichloroethane (DCE) and methanol. The first product of the interaction between the allene and Br-2 is a 1:1 pi complex. The stability constant of this complex, determined at 25degreesC for allene 1a, is 7.4m(-1). The comparison of this value with those reported for several alkenes and alkynes further support the hypothesis of the existence of sizeable structural effects on the stability of these complexes. The negative values of the apparent activation energy for the reaction of allenes 1a-e with Br-2 in DCE demonstrate the involvement of these complexes as essential intermediates along the reaction coordinate. Different stereochemical behavior was observed in the bromine addition on going from the strained 1,2-cyclononadiene to the larger compounds. Furthermore, a solvent-dependent stereochemistry has been observed for each compound. The kinetic and product distribution data have been interpreted in terms of the influence of the strain on the nature of the intermediate and by considering the competition between pre-association and ion-pair pathways on going from aprotic to nuclophilic solvents or when nucleophilic bromide ions are added. Ab initio (MP2/6-311 + G**) and density functional (B3LYP/6-311 + G**) computations of 1:1 Br-2 complexes showed that the association energies of allene Br-2 and ethene Br-2 complexes are nearly the same but are greater than that of acetylene Br-2 complexes. Allene 2Br(2) complexes are more stable than their ethene 2Br(2) counterparts. Br-2 allene Br-2 structures, in which the bromine molecules interact either with a single allene double bond or individually with both double bonds, are not preferred significantly over alternatives with (Br2Br2)-Br-... interactions. As a result of the entropy, the association of bromine with unsaturated hydrocarbons is usually unfavorable in the gas phase (except at extremely low temperatures); complexes are observed in solution (under ambient conditions), since the entropy loss is reduced as a result of restricted translation and rotation and possible association to the solvent. The 1,2-cycloheptadiene Br-2 > 1,2-cyclononadiene Br-2 > 1,3-dimethylallene Br-2 association energies increase with ring strain
Reactivity of homoallylic substituted adamantylideneadamantanes with bromine. Substituent effects on the stability of the ionic and nonionic intermediates RID A-2996-2011 RID E-4986-2010
No full textSterically congested adamantylideneadamantanes (1b-g) (X = Br, Cl, F, OH, OEt, OCOCH3), homoallylically substituted with equatorial groups (X), react with bromine in 1,2-dichloroethane to give a stable bromonium ion intermediate or a substitution product depending on the nature of the substituent and on the bromine concentration. The nature of the substituent markedly affects the formation constant of the 1:1 pi-complexes, as well as of the formation constant and reactivity of bromonium ion intermediates. The different reactivity of the ionic intermediates, which depends on the nature of substituents, is attributed to bromonium or bromocarbenium character of the intermediate, with the support of theoretical investigations. Ab initio calculations on 1:1 adamatyl-ideneadamantane-Br-2 complexes (2a-f) show that the substituent affects the stability of these species through electrostatic and dispersion effects. Solvent effects may also contribute to modulate the relative stability of these species
- …
