摘要:

AbstractTreatment of vinylogous urethanes9aand9bwith tri‐n‐butyl in hydride generated a tree radical that underwent cyclization‐fragmentation to afford oxindole‐ketones10and11in high yield. Vinylogous urethane13, gave only cyclization‐reduction products15and16upon treatment with tri‐n‐butyltin hydride at 80 °C, but also afforded a low yield of cyclization‐fragmentation product14upon treatment with tris(trimethyl‐sily

ISSN:0009-4536    

DOI:10.1002/jccs.199500118

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe 1,3‐dipolar cycloaddition reactions of nitrile oxides and α,β‐unsaturated 1,3‐dioxolanes were effectively accelerated by ultrasound irradiation to give the A2‐isoxazolines with yields and regioselectivities surpassing those of the corresponding thermal

ISSN:0009-4536    

DOI:10.1002/jccs.199500119

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractIn Raman magnetic resonance aninitially prepared multiplequantum coherence (MQC) in a spin system introduces all the possible single quantum (SQ) and multiple quantum (MQ) NMR signals when an irradiation is applied during the detection period. If the irradiation is weak, the induced signals include those MQs up to the very order of this given MQC, while in the strong irradiation case, those MQs beyond the very order of the given MQC will also appear. An analytical approach based on the product formalism is developed to predict the intensities and frequencies of these induced signals. The analysis provides a complete treatment to cover both cases of weak and strong irradiation. It is also demonstrated with an AX spin system. Experimental results are in qualitative agreement with the analysis.

ISSN:0009-4536    

DOI:10.1002/jccs.199500120

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractIn a heteronuclear spin system, with initially prepared single quantum (SQ) coherence of X‐spin, the irradiation of the proton spins will induce all the possible transitions including those SQs and multiple quanta (MQs) available in thesystem to be studied. The MQs appear in a rather weak irradiation while a strong irradiation results in a complete decoupling situation. Theoretical analysis is made to explain this phenomenon and is agreed qualitatively with experiments in the CH2group. This phenomenon of MQ induction may happen in adecouplingexperiment, when the effective irradiation strength is rather weak and it may also provide a convenient approach to create and then to manipulate MQ coherences in heteronuclear spin system

ISSN:0009-4536    

DOI:10.1002/jccs.199500121

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe Brønsted acidity of the various Si(nAl) sites present in zeolites is evaluated from proton binding energy and LUMO energy calculated by the semiempiricalMNDOquantum chemical method. The two calculated energy values both exhibit a linear correlation with the existing29Si NMR chemical shift and the IR hydroxyl stretching frequency data. The inter‐convertibility between different Si(nAl) sites during an alumination or dealumination process is also evaluated based on the calculated substitution energy. The results indicate that alumination processes are less favorable to occur in zeolites than dealumination processes and the latter is more likely to occur for Si(nAl) clusters that contain the maximum number of aluminum nearest neighbo

ISSN:0009-4536    

DOI:10.1002/jccs.199500122

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractMolecular dynamic simulations of the 3′‐cytidine monophosphate (3′‐CMP)/RNase A complex were carried out at three temperatures, 273 K, 300 K and 323 K. The trajectories obtained allowed us to calculate the dynamics of 3′‐CMP in protein complex. The O‐P bond was found to exert angular fluctuations with an average magnitude of 20° 26°, and 30° at 273 K, 300 K, and 323 K, respectively. These values compare quite favorably with those obtained from lineshape simulation of the31P NMR powder patterns. The magnitude of the translational fluctuation of the center of mass of the ligand was found to be in the range of 0.17 A to 0.21 Å. On the other hand, the phosphate atom was found to fluctuate with amplitude ranging from 0.22 Å to 0.42 Å, depending on orientation. Fluctuation of the dihedral angle, defined by P‐O3‐C3‐C4′ bonds of the ligand, was more restricted in the protein complex as compared to that in free form. The average number of hydrogen bonds formed between the phosphate group and the protein moiety was 1.

ISSN:0009-4536    

DOI:10.1002/jccs.199500123

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractA resonance‐complex set was developed to investigate the process of combination of hydrogen atoms with a modified resonance‐complex theory and Monte‐Carlo trajectory method over the temperature range 40‐300 K. The obtained ratios of ortho to para at 300 K and at 77 K agree satisfactorily with experimental results. Unlike previous results, a maximum of the rate coefficient as a function of temperature is found nea

ISSN:0009-4536    

DOI:10.1002/jccs.199500124

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractCrystals of two alkali‐metal tin(IV) arsenates, KSnOAsO4and RbSnOAsO4, were grown from a flux and structurally characterized by single‐crystal X‐ray diffraction. Crystal data: RSnOAsO4, orthorhombic, Pna21(No. 33), a = 13.406(3) Å, b = 6.678(1) Å, c = 10.921(2) Å, Z = 8, R = 0.037 for 1173 independent reflections with I>2.5ó(l); RbSnOAsO4, as above except a = 13.567(3) Å, b = 6.791( 1) Å, c = 10.891(2) Å, R = 0.035 for 1958 independent reflections. The two compounds are isostructural with thenonlinear optical materialKTiOPO4. The oxygen frameworks of both tin compounds approximate closely toa centrosymmetricarrangement, as indicated from the analysis of atomic coordinates. Second harmonics weregenerated in powderedRbSnOAsO4to confirm the absence of a center of symmetry in t

ISSN:0009-4536    

DOI:10.1002/jccs.199500125

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractAn efficient and highly chemoselective desilylating method is described. Trimethylsilyl ethers (0.25 M) in a CH3OH/CCl4(1:1) solvent mixture are deprotected to their corresponding alcohols with ultrasound in a commercial ultrasonic cleaning bath. Selective deprotection oftert‐butyldimethylsilyl ethers of benzylalcohols and phenols is achieved under ultrasonic conditions. We deprotected alsotert‐butyldimethylsilyl ethers of primary alcohols, whereastert‐butyldimethylsilyl ethers of secondary and tertiary alcohols are stable under these condi

ISSN:0009-4536    

DOI:10.1002/jccs.199500126

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractPure hexaphenylcyclotrisiloxane (P3) and octaphenylcyclotetrasiloxane (P4) were synthesized with high yields via proper choice of preparation conditions, such as catalysts, solvents and reaction temperatures, from diphenylsilanediol. Up to 98% yield of this pure starting material can be obtained from diphenyldi‐chlorosilane through modifying a known procedure on carefully adjusting the temperature of hydrolysis and washing procedure. This method produced pure P3exclusively with high yield (approximately 90%) in ethanol in the presence of concentrated sulfuric acid as the catalyst at various reaction temperatures. P4was produced exclusively in high yield (approximately 90%) in alcohols in the presence of sodium hydroxide. A method applying HPLC was used to analyze the reaction products quantitativel

ISSN:0009-4536    

DOI:10.1002/jccs.199500127

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY