ISSN:0018-019X    

DOI:10.1002/hlca.19920750402

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractA fast electrochemical technique for the discrimination of one‐ and two‐electron mechanisms in the oxidative addition of alkylating agents (RX) to corrinato‐ and porphyrinatocobalt(I) ([CoIL]) is described. It is based on single‐scan voltammograms of [CoIIL] in the presence of RX and variable amounts of the radical trap acrylonitrile. In the first part of the voltammogram, [CoIIL] is reduced, and fast oxidative addition of RX to [CoIL] is triggered. If the reaction proceedsviaa two‐electron mechanism, [RCoIIIL] is formed independently of acrylonitrile concentration, but if a transient free radical R is involved, R is competitively trapped by acrylonitrile and [CoIIL] to yield, at high enough acrylonitrile concentration, exclusively the olefin‐inserted [RCNCoIIIL]. [RCNCoIIIL] is reducible in the intermediate potential range, [RCoIIIL] at the negative end of the single‐scan voltammogram. Hence, from the appearance of the reduction waves due to [RCNCoIIIL] and [RCoIIIL], the mechanism of oxidative addition of RX to [CoIL] is easily deduced. The method is applied to the study of the mechanistic borderline of oxidative addition using a series of 15 RX and 4 [CoL]'s,i.e.cobalamin (Cbl), heptamethyl cobyrinate (‘Cby’), (tetraphenylporphyrinato) cobalt ([Co(tpp])), and (octaethylporphyrinato) cobalt ([Co(oep)]). All non‐activated primary alkyl iodides and bromides exhibit, at room temperature, pure two‐electron mechanisms with all [CoIL]'s, except neopentyliodide with Cb1Iand ‘Cby’I. All secondary alkyl iodides involve free radicals with Cb1Iand ‘Cby’I, but a pure two‐electron mechanism or a mixed one‐electron two‐electron mechanism with [CoI(tpp)] and [CoI(oep)]. The mechanistic switch from a two‐electron to a one‐electron mechanism for increasingly sterically demanding RX's occurs earlier with the supernucleophilic

ISSN:0018-019X    

DOI:10.1002/hlca.19920750403

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractUnder the influence of an electric field, trialkyltin compounds of the type R3SnX behave as electrically neutral carriers for anions in poly(vinyl chloride) liquid membranes. The interaction of tinorganic compounds with oxoanions was studied in organic phase by means of119Sn‐NMR‐monitored titrations. In the case of tributyltin chloride, no appreciable amount of complex was formed with hydrogensulfate, whereas dihydrogenphosphate gave rise to a new species. Dioctyltin dichloride and dioctyltin dioctyltin diacetate formed a 1:2 (salt/ligand) complex with hydrogenphosph

ISSN:0018-019X    

DOI:10.1002/hlca.19920750404

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

Abstract(R)‐ and (S)‐α‐ionone ((R)‐ and (S)‐1, resp.) were prepared from (R)‐ and (S)‐α‐damascone ((R)‐ and (S)‐3, resp.) without racemization in 48% yield employing a new enone transposition. The described transposition is complementary to existing methods whose application is often prohibited by the structural requirements of the substrate. The now easily accessible α‐ionones of desired absolute configuration are useful as chiral building blocks

ISSN:0018-019X    

DOI:10.1002/hlca.19920750405

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractWe studied the stability of theSchiffbases formed between pyridoxal 5′‐phosphate (PLP) and leucine in the presence of (hexadecyl)trimethylammonium bromide (CTAB) over a wide pH range by determining the kinetic constants of formation and hydrolysis of these compounds. The results show that the stability of theSchiffbases is increased by the presence of CTAB as a result of increased rates of formation and decreased hydrolysis rate constants. The ionic head groups of CTAB favour the formation of the bases, while its hydrophobic rests protect the imine double bond from hydrolysis. This model system permits one to obtain partially hydrophobic media with no need for any non‐aqueous sol

ISSN:0018-019X    

DOI:10.1002/hlca.19920750406

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

Heterocycles Starting from Bis(alkoxycarbonyl)ketene Ethylene Acetals ( = Dialkyl 2‐(1,3‐Dioxolan‐2‐ylidene)propane‐1,3‐dioate). Synthesis and Properties of a New Class of Pyrazolium BetainesThe readily available bis(alkoxycarbonyl)ketene ethylene acetals1react with bifunctional nucleophiles to give heterocycles2–5(Scheme 1). Their reactions withN,N‐dialkylhydrazines lead to the pyrazolium betaines7a–f(Scheme 4). CyclicN,N‐dialkylhydrazines give spiro compounds7d–f. The reaction of thioketene acetal12and of the derivative15of methanetricarboxylic acid withN,N‐dimethylhydrazine results in the formation of 3‐(methylthio)‐ and 3‐methoxypyrazolium betaine7gand7h, respectively (Scheme 4). The chemical reactivity of the synthesized pyrazolium betaines7was tested. The structure of the 3‐(methylthio) derivative7gw

ISSN:0018-019X    

DOI:10.1002/hlca.19920750407

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractCatalytic double osmylation is described for a series of cyclohexadienes in acetone/H2O in the presence of the co‐oxidantN‐methylmorpholineN‐oxide (NMO). The formation of polyols occurred stereospecifically with cyclohexadienes3,7, and11a, leading thereby to tetrols5a, and9aand toallo‐inositol (14a), respectively. To the contrary,trans‐cyclohexadiene‐diol15agave a mixture of the stereoisomeric inositols18a(epi),19a(neo), and20a(chiro). High‐field NMR let to clearcut conformational analyses of the polyhydroxylate

ISSN:0018-019X    

DOI:10.1002/hlca.19920750408

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractEsculetin (1) and the novel compounds 5‐chloroesculetin (5) and 5‐bromoesculetin (6) were obtained from a light‐induced cyclization oftrans‐caffeic acid (3) catalyzed by [FeNa(edta)] and/or H2SO4, HCI, or HBr (Scheme 1). The experimental conditions fortrans‐cis‐isomerization of the cinnamic‐acid derivative3and subsequent non‐enzymatic cyclization were described. The photoperiod and the presence of air and iron‐chelate catalyst are shown to be important parameters that markedly affect yields. The reactions probably occur by a free‐radical mechanism involving a photo‐initiated one‐electron r

ISSN:0018-019X    

DOI:10.1002/hlca.19920750409

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractThe tris‐carotenoid macrobicycles1and2were obtained in good yields in a one‐step macrobicyclisation condensation between the tripode N(CH2CH2NH2)3and the polyolefinic dialdehydes5and6. They form dinuclear cryptates by complexation of two CuIions. The crystal structure of the tris‐carotenoid compound2confirms that it contains three parallel polyolefinic strands. These substances may formally be considered as prototypes of molecular ‘cables’ formed by three electron‐conducting mole

ISSN:0018-019X    

DOI:10.1002/hlca.19920750410

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractCoprine (1), a toxine of the mushroomCoprinus atramentarius, was synthesized starting from the 2‐amino and 1‐carboxy‐protectedL‐glutamic acids4and12. Compound4was first decarboxylated by a radical chain reaction to bromide5which underwent ring closure to cyclopropanecarboxylate6on treatment with NaH (Scheme 1). Subsequent oxidative electrolysis of7to formtert‐butylN‐(1‐ethoxycyclopropyl)carbamate (8) and acidic hydrolysis yielded the 1‐aminocyclopropanol hydrochloride (9). Selective cleavage of the amino‐protecting group of8(→10or11), coupling of the corresponding amine13withL‐glutamic acid12, and acidic hydrolysis of the resultingL‐glutamine derivative17yieldedO‐ethylco

ISSN:0018-019X    

DOI:10.1002/hlca.19920750411

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY