87,315 research outputs found
Il giovane cittadino : traduzione e adattamento del testo che si pubblica ogni anno nella Svizzera Romanda per uso delle scuole complementari : anno scolastico 1910-1911
No full textfatti per incarico del Dipartimento della pubblica educazione dai dottor F. Chiesa, ispettore G. Marioni e maestro G. Grand
The detection of ionic intermediates during the bromination of 5H-dibenz[b,f]azepine-5 carboxamide in 1,2-dichloroethane.
No full textThe bromination of 5H-dibenz[b,f]azepine-5-carboxamide (carbamazepine) (1) in 1,2-dichloroethane gives only the trans-dibromide 2. The reaction, followed spectrophotometrically, obeys a third-order rate law (second-order in Br2), but the value of k3 changes considerably depending on the monitoring wavelength. It is shown that this is due to the presence of reversibly formed Br3- salt intermediates, that make a more important contribution to the absorbance in the lower wavelength range of the monitored 360-540 nm region. A significant conductivity is also measured during the course of the bromination. Both spectrophotometric and conductimetric measurements are consistent with the presence of Br3- salt intermediates at a maximum concentration amounting to ca. 2% of that of the initial reagents. The structure of dibromide 2, determined by X-ray diffraction, shows a considerable angle strain at carbons bearing bromine. This strain is responsible for an easy spontaneous debromination of 2, as well as for a relatively high barrier for the formation of 2 from the bromonium-tribromide intermediate, that makes possible the accumulation of the intermediate itself during the bromination of 1
Factors affecting the reversibility of the electrophilic step in olefin bromination. The case of 5H-Dibenzo[a,d]cycloheptene.
No full textThe tendency of ion-pair intermediates generated by treatment of trans-10-bromo-10,11-dihydro-11-hydroxy-5H-dibenzo[a,d]cycloheptene (5) with HBr to release Br2 giving 5H-dibenzo[a,d]cycloheptene (1) has been evaluated. The observed 9:1 ratio between ion-pair collapse to the trans-dibromide (7) and Br2 release to olefin 1, as compared with the 3:7 ratio found with the 5H-dibenz[b,f]azepine-5-carbonyl chloride system, has been interpreted as indicating a much lower propensity for reversibility of ion-pair formation in the bromination of 1. The structural parameters obtained by x-ray diffraction of the dibromide 7 show for this compound an angle strain at C-10 and C-11 analogous to that found in trans-10,11-dibromo-10,11-dihydro-5H-dibenz[b,f]azepine-5-carbonyl chloride. Olefin 1 adds Br2 in 1,2-dichloroethane at 25-degrees-C according to a third-order rate law, with k3 = 30(3) M-2 s-1. About 10% of 5-bromo-5H-dibenzo[a,d]cycloheptene, arising from collapse of a dibenzo[a,d]tropylium ion intermediate (9), is formed in this reaction besides the expected trans-dibromide 7. Spectral evidence for the formation of 9, arising by rearrangement of a first formed weakly bridged ionic intermediate in the reaction of 1 with Br2, is presented. the results obtained with the 5H-dibenzo[a,d]cycloheptene system have been rationalized on the basis of the formation of very weakly bridged intermediates having a much lower tendency to revert back to olefin and Br2 with respect to the corresponding symmetrically bridged bromonium ions
ENANTIOSELECTIVITY OF THE ENZYMATIC-HYDROLYSIS OF CYCLOHEXENE OXIDE AND (+/-)-1-METHYLCYCLOHEXENE OXIDE - A COMPARISON BETWEEN MICROSOMAL AND CYTOSOLIC EPOXIDE HYDROLASES
No full textENANTIOSELECTIVE HYDROLYSIS OF SUBSTITUTED PHENYLOXIRANES BY RABBIT LIVER MICROSOMAL EPOXIDE HYDROLASE
No full textThe rabbit liver epoxide hydrolase contained in the microsomal fraction (mEH) has been checked for its enantioselectivity towards several racemic ortho-, meta-, and para-substituted phenyloxiranes, benzyloxirane, and phenoxymethyloxirane. While for the last two substrates non-selective hydrolysis is observed, the orientation of substituents on the benzene ring is found to affect significantly the selective ring opening of phenyloxiranes
REVERSIBILITY OF BROMONIUM ION FORMATION AND ITS EFFECT ON OLEFIN REACTIVITY IN ELECTROPHILIC BROMINATION - NEW EVIDENCE FROM THE 5H-DIBENZ[B,F]AZEPINE SYSTEM
No full textEVIDENCE FOR A REVERSIBLE ELECTROPHILIC STEP IN OLEFIN BROMINATION - THE CASE OF STILBENES
No full textDeterminazione della melatonina in Achillea millefolium fresca pretrattata con triptofano
No full textPRODUCT ENANTIOSELECTIVITY IN THE MICROSOMAL EPOXIDE HYDROLASE CATALYZED-HYDROLYSIS OF 10,11-DIHYDRO-10,11-EPOXY-5H-DIBENZO[A,D]-CYCLOHEPTENE
No full textEnantioselective 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
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