摘要:

AbstractN‐Tosyl‐ andN‐methyl‐substituted sulfonimidoyl fluorides (2) have been synthesized from the corresponding chlorides by displacement with fluoride ion.S‐Phenyl‐N‐tosylsulfonimidoyl fluoride (2a) is a stable (persistent) shelf compound, which has been used for the synthesis of (S‐phenyl‐N‐tosylsulfonimidoyl)methyl isocyanides (4). The latter compounds form a new class of derivatives of methyl isocyanides, which, much like tosylmethyl isocyanides, form azoles by baseinduced cycloadditions to unsaturated substrates. Fluoride2ahas been partially resolved i

ISSN:0165-0513    

DOI:10.1002/recl.19841030201

出版商:WILEY‐VCH Verlag    

年代:1984

数据来源: WILEY

摘要:

AbstractFour imidazole‐containing isocyanides having differentN(Im)‐protecting groups at theteleas well as at theprosposition have been synthesized. These isocyanides includeNτ(Im)‐tosyl‐Nα‐carbylhistamine (4a),Nτ(Im)‐benzyl‐Nα‐carbylhistamine (4b),Nπ(Im)‐benzyl‐Nα‐carbylhistamine (4c) andNτ,Nπ(Im)‐dibenzyl‐Nα‐carbylhistamine bromide (4d). Compounds4aand4dpolymerize in the presence of nickel(Il) ions, whereas compounds4band4cdo not. This result is explained by the type of nickel complex that is formed,viz.a tetrahedral or square‐planar complex with general formula Ni(CNR)42+in the case of4aand4d, and an octahedral complex with general formula Ni(CNR)32+in the case of4band4c. In the latter complexes, not only the isocyano groups but also the imidazole groups coordinate to the nickel. Thus, the imidazole groups block the coordination sites which are required for po

ISSN:0165-0513    

DOI:10.1002/recl.19841030202

出版商:WILEY‐VCH Verlag    

年代:1984

数据来源: WILEY

摘要:

AbstractNon‐covalent semisynthetic ribonuclease analogues are used to study the role of histidine‐119 in the active site of RNase A. The solid‐phase synthesis of three RNase 111‐124 peptides, in which histidine‐119 is replaced by 3‐(3‐pyrazolyl)‐L‐alanine,Nπ‐methyl‐L‐histidine andNτ‐methyl‐L‐histidine, respectively, is described. The binding of these peptides to RNase 1‐118 protein is examined. The enzymatic activities of the resulting complexes towards first and second step substrates and their binding to the inhibitor 3′‐cytidine monophosphate are determined.TheNπ‐methylhistidine‐ and the 3‐(3‐pyrazolyl)alanine RNase analogues are devoid of catalytic activity. TheNτ‐methylhistidine analogue is found to be enzymatically active, using yeast ribonucleic acid and 2′,3′‐CMP as substrates. With the latter substrate, kinetic parameters at pH 6.0 have been determined. The results provide strong evidence to suggest that thepros‐nitrogen atom of His‐119 plays an essential role as acid/base catalyst in the enzymatic reaction of RNase. There are indications that thetele‐nitrogen atom of histidine‐119 in the natural enzyme – besides effecting the right pKvalue at thepros‐nitrogen centre ‐ may also contribute by favouring the optimal steric orien

ISSN:0165-0513    

DOI:10.1002/recl.19841030203

出版商:WILEY‐VCH Verlag    

年代:1984

数据来源: WILEY

摘要:

AbstractThe catalytic constants of six acids (pkaranging from 2. 6 to 8) as well as those of hydroxonium ion and water have been measured in the hydrolysis of six ethyl‐substituted phenylbenzaldehyde acetals.A good Brønsted correlation is obtained for all compounds, the α coefficient decreasing regularly from 0.7 to 0.5 with the basicity of the leaving group. On the contrary theHammettplots obtained for each acid are bad, excepted those corresponding to hydronium ion and water.A model of concerted proton transfer is presented which extends and puts in a mathematical form those previously proposed byJencks, KreevoyandGuthrie. With this model we can demonstrate that: (i) the hydrolysis of a given compound can be either a specific‐acid‐or a general‐acid‐catalysed reaction depending upon the strength of the catalyst, (ii) the α Brønsted coefficient is not a measure of the “degree of proton transfer in the transition state” but is rather a measure of the easiness of cleavage

ISSN:0165-0513    

DOI:10.1002/recl.19841030204

出版商:WILEY‐VCH Verlag    

年代:1984

数据来源: WILEY

摘要:

AbstractThe synthesis is described of several fragments of the neurohypophyseal nonapeptide hormones arginine vasopressin and oxytocin. One group of fragments is characterized by the presence of an asymmetrical disulfide (a cystine residue in position 6), a second group consists of dimeric (symmetrical disulfides) fragments.As common intermediates, peptides containing anS‐tritylcysteine residue were synthesized by the fragment condensation approach. Treatment with methoxycarbonylsulfenyl chloride followed by reaction with the free thiol function of cysteine gave the asymmetrical disulfides, while treatment of theS‐tritylcysteine containing peptide with iodine resulted in the corresponding dimers. Peptides with anN‐terminal glutamine residue (position 4) were found to convert spontaneously into the corresponding pyroglutamic acid pro

ISSN:0165-0513    

DOI:10.1002/recl.19841030205

出版商:WILEY‐VCH Verlag    

年代:1984

数据来源: WILEY

摘要:

AbstractThe secondary kinetic deuterium isotope effect has been measured for the general acid‐catalysed hydrolysis of benzaldehyde ethyl phenyl acetal (1). It has been found that this α isotope effect increases from 1.06 (H3O+) to 1.26 (H2O), when the strength of the acid catalyst decreases. The significance of this finding is discuss

ISSN:0165-0513    

DOI:10.1002/recl.19841030206

出版商:WILEY‐VCH Verlag    

年代:1984

数据来源: WILEY

摘要:

AbstractThe synthesis and complex formation with copper(II) of a new symmetric octadentate N6O2ligand is described. The ligand 1,1,10,10‐tetrakis(1‐methylbenzimidazol‐2‐ylmethyl)‐1,10‐diaza‐4,7‐dioxadecane (megtb) is prepared by condensation ofo‐phenylenediamine and [ethylene‐bis(oxyethylenenitrilo)]tetraacetic acid, followed by methylation. This ligand appears to yield dinuclear compounds with copper(II) salts with different kinds of bridging anions. Some spectroscopic and magnetic properties of these compounds, as well as the structure of a representative compound Cu2(megtb)F2(BF4)2(H2O)2, are

ISSN:0165-0513    

DOI:10.1002/recl.19841030207

出版商:WILEY‐VCH Verlag    

年代:1984

数据来源: WILEY

ISSN:0165-0513    

DOI:10.1002/recl.19841030208

出版商:WILEY‐VCH Verlag    

年代:1984

数据来源: WILEY