摘要:

AbstractN‐Acetyl‐S‐(alkylsulfanyl)cysteine benzyl esters were synthesized as models forN‐acylatedS‐(alkylsulfanyl)cysteine residues linked via an ester bond to a solid phase. The S‐protections involved were: the ethylsulfanyl, the well‐establishedtert‐butylsulfanyl and the newly developed tritylsulfanyl group. We investigated the chemical behaviour of the disulfides in reagents commonly applied in solid‐phase peptide synthesis using the Fmoc strategy (Fmoc SPPS). It was found that the tritylsulfanyl group as a thiol protection is comparable with thetert‐butylsulfanyl group in these respects. It is stable in trifluoroacetic acid and is rapidly reduced by thiols and phosphines. For all three cysteine esters rapid racemization was observed in piperidine (25%) in DMF, the amides being chirally stable. The demonstrated chiral instability of cysteine esters has consequences for the solid‐phase synthesis of peptides us

ISSN:0165-0513    

DOI:10.1002/recl.19951140102

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe mechanism of exciplex formation of two donor‐bridge–acceptor compounds, both containing a strong donor ‐ acceptor pair interconnected by a piperidine ring, was studied in nonpolar solvents as a function of temperature. Fluorescence spectra at low temperature in methylcyclohexane show that in both compounds the exciplex is formed via the harpooning mechanism,i.e.long‐range charge separation leading to an extended charge‐transfer state, followed by Coulomb‐induced folding to the exciplex state. The kinetic link between these two emissive states could be established by studying their transient fluorescence curves. Furthermore, the activation energy of the folding process was estimated from the decay rates of the extended charge‐transfer state in various solvents. An interpretation of the observed activation energies in terms ofKramerstheory is presented, which gives an indication of the relative importance of internal steric effects and solvent viscosity on the dynamics of the fo

ISSN:0165-0513    

DOI:10.1002/recl.19951140103

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

Abstract1,3‐Dinitrobenzene (1a) and some simple monosubstituted derivatives (1b–g) were aminated with a liquid methylamine solution of potassium permanganate (LMA/PP) to give the corresponding mono‐ and bis(methylamino)‐substituted compounds (3a‐eand4a). 1,2‐Dinitrobenzene (1i) was aminated with LMA/PP to give 2‐(methylamino)‐1‐nitrobenzene (3f) in high yield. The intermediacy of 4‐(methylamino) σ‐adducts of 1,3‐dinitrobenzene (2a), 2‐chloro‐ (2b) and 2‐amino‐1,3‐dinitrobenzene (2c) was established by1H‐NMR spectroscopy. Quantum‐chemical calculations for the dinitrobenzenes suggest that, in general, the experimentally observed regioselectivity of the methylamination is satisfactorily described by frontier molecular orb

ISSN:0165-0513    

DOI:10.1002/recl.19951140104

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractMetallation ofN‐vinylpyrrole under presumed kinetic conditions, usingn‐butyllithium in THF/hexane mixtures,n‐butyllithium/N,N,N′,N′‐tetramethylethanediamine in hexane, orn‐butyllithium/potassiumtert‐butoxide (1:1 molar ratio) in THF/hexane mixtures gives almost equal amounts of derivatives metallated at the α‐vinyl or 2‐pyrrole position. In the presence of 7–10 mol% diisopropylamine the α‐vinyl‐lithiated compound is transformed relatively slowly into the pyrrole‐lithiated derivative at ∼ +30°c. The combined use of catalytic amounts of diisopropylamine and equivalent amounts ofn‐butyllithium and potassiumtert‐butoxide gives rise to a fast and almost complete conversion into the pyrrole‐metallated derivative at temperatures in the range ‐60 to –80°c. A number of 2‐substitutedN‐vinylpyrroles have been obtained in good yields by reaction with various electrophilic reagents, in most cases preceded by the addition of anhydrous lithium bromide.N‐Deprotection is successful on

ISSN:0165-0513    

DOI:10.1002/recl.19951140105

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractNew methods were used to prepare a nearly stoichiometric Mn3O4catalyst. The redox behavior of two kinds of Mn3O4(i.e.the non‐ and nearly stoichiometric Mn3O4) were compared. The relations between the IR parameters, Mn/O stoichiometry and redox properties of Mn3O4were established. It was found that the Mn/O stoichiometry but not IR parameters was closely related to the redox behavior of the compound

ISSN:0165-0513    

DOI:10.1002/recl.19951140106

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractMitsunobu reaction mediated introduction of nitrogen nucleophiles intotrans1‐hydroxy eudistomins afforded in high yields rearranged products instead of the anticipatedciseudistomins due to neighboring group participations. Transannular interaction in the 6,5,6,7‐membered tetracyclic ring system of the sulfur atom in the 1,6,2‐oxathiazepine ring, to give a thiiranium ion containing intermediate, was followed by intermolecular attack of the present nucleophiles to give a 6,5,6,6‐membered tetracyclic ring system, now with a 1,5,2‐oxathiazine ring. Substitution of the hydroxyl group in atrans1‐hydroxy dethia carba eudistomin derivative (i.e.with a 1,2‐oxazepine ring) also gave rearranged products due to transannular interaction from the bridgehead nitrogen atom, to give via an aziridinium ion intermediate, products with a 6,5,7,6‐membered tetracyclic ring system containing an 1

ISSN:0165-0513    

DOI:10.1002/recl.19951140107

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractLiquid ammonia directly drawn from a cylinder can be made super‐dry by alternating, portion‐wise, addition of potassium andt‐butyl chloride, using trace amounts of triphenylmethane as an indi

ISSN:0165-0513    

DOI:10.1002/recl.19951140108

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractAlcohol3, an intermediate in indole alkaloid synthesis, was converted to the ox‐azepinoindoloquinolizine 5 by oxidation with DMSO activated by the sulfur trioxide/pyridine complex. The ring closure proceeds through the common oxidation side‐product, methylthiomethyl ethe

ISSN:0165-0513    

DOI:10.1002/recl.19951140109

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

ISSN:0165-0513    

DOI:10.1002/recl.19951140110

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

ISSN:0165-0513    

DOI:10.1002/recl.19951140112

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY