摘要:

In the absence of a source of acidic protons, methyl (triphenylphosphoranylidene)acetate1exists as a mixture of thecisandtransisomers even at room temperature. Acidic protons catalyze the interconversion of the two forms and facilitate the reaction of the ylide with active carbonyl compounds such as benzaldehyde or phthalic anhydride.Keywords: stabilized phosphoylide, structure, reactivity.

ISSN:0008-4042    

DOI:10.1139/v90-325

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Rates of hydrolysis of divinyl ether (CH2=CHOCH=CH2), measured in dilute H2O and D2O solutions of perchloric acid at 25 °C, provide the catalytic coefficientskH+ = 0.0084 M−1 s−1andkD+ = 0.0028 M−1 s−1, and these lead to the isotope effectkH/kD = 3.0. The magnitude of this isotope effect indicates that this reaction occurs by rate-determining hydron transfer from catalyst to substrate and thus follows the conventional mechanism for vinyl ether hydrolysis.Keywords: divinyl ether, vinyl ether hydrolysis, solvent isotope effect.

ISSN:0008-4042    

DOI:10.1139/v90-326

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Ultrasonic absorption studies over the frequency range 0.6–210 MHz were conducted in aqueous solutions of the following modified β-cyclodextrins (β-CD): 2,6-O-dimethyl-β-CD (DMβCD), 2,3,6-O-trimethyl-β-CD(TMβCD), and hydroxypropyl-β-CD (HPβCD). Anomalous ultrasonic absorption found in the three systems was attributed to three relaxations in the DMβCD and HPβCD systems and two in the TMβCD system. These relaxations were assigned to hydrogen bond "wig-wagging", to CD cavity water exchange, and to bond rotation within the CD units. The 1:1 DMβCD/decyltrimethylammonium bromide system was also investigated ultrasonically in solution.Keywords: ultrasonic absorption, modified β-cyclodextrins, surfactant inclusion, relaxation frequency.

ISSN:0008-4042    

DOI:10.1139/v90-327

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

The syntheses of the acyclic dienynes14a(cis-trans-acetylene) and14b(trans-trans-acetylene) are described. The tandem macrocyclization and transannular Diels–Alder reaction of the allylic chloride14bwas conducted in the presence of Cs2CO3at 85 °C in one-pot to provide tricyclic product16, with the two hydrogens in ring B beingcis. On the other hand, treatment of the allylic chloride14aunder the same conditions afforded 14-membered macrocycle17, which could be transformed at 250 °C into a similar tricyclic product18with the two hydrogens in ring B beingtrans, along with an interesting by-product19. The mechanism for the formation of19is also discussed.Keywords: stereocontrolled synthesis, macrocycle, tricyclic compound, transannular Diels–Alder reaction.

ISSN:0008-4042    

DOI:10.1139/v90-328

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

The synthesis of the four acyclic trienes11a,band20a,bis reported. The tandem macrocyclization and stereoselective transannular Diels–Alder reaction of acyclic trienes11b(trans-trans-cis) and20b(trans-trans-trans) were observed in the presence of Cs2CO3at 85 °C to give tricycles30(TST) and32(CAT) respectively. However, treatment of acyclic trienes11a(cis-trans-cis) and20a(cis-trans-trans) under the same conditions yielded the 14-membered macrocycles21and23, which were stereoselectively transformed at 250 °C into tricycles22(CST) and24(CAT) respectively in excellent yield.Keywords: stereocontrolled synthesis, macrocycle, tricyclic compound, transannular Diels–Alder reaction.

ISSN:0008-4042    

DOI:10.1139/v90-329

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Silylation of quinoline with Me3SiCl/Li/THF reagent has been studied. We obtained new tri-, tetra-, and hexasilylated derivatives whose stereochemistry has been established using NMR data. Comparison of these results to those previously obtained from naphthalene shows the effect of nitrogen on the silylation process.Keywords: silylation, quinoline, polysilyl-hydro-quinolines.

ISSN:0008-4042    

DOI:10.1139/v90-330

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

The kinetics of oxidation of eight monosubstituted anilines catalyzed by horseradish peroxidase compoundIIhas been studied at pH 7.00 and 7.60. Withp-toluidine the rates of oxidation by compoundIIhave been measured at 21 pH values between 3.60 and 10.25. The rate–pH profile indicates that an acidic form of compoundIIand the electrically neutral, unprotonated form ofp-toluidine are reactive. The correlation of rate constants for the substituted anilines with the substituent constant σ in the Hammett equation suggests that the aromatic amine donates an electron to compoundIIin the rate-controlling step and loses a proton simultaneously. The value of ρ, the susceptibility factor in the Hammett equation, is −6.0 ± 0.7. The reactivity of anilines with HRP-II observed in this study is lower than that of anilines with HRP-I observed previously, although the value of ρ is the same within experimental error (D. Job and H. B. Dunford. Eur. J. Biochem.66, 607 (1976)). The difference in reactivity is explained by the relative complexities of the reactions of compoundsIandII.Keywords: horseradish peroxidase, peroxidase compoundII, aniline oxidation, Hammett correlation, enzymatic oxidation.

ISSN:0008-4042    

DOI:10.1139/v90-331

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

The absorption spectra, fluorescence spectra, fluorescence lifetimes, fluorescence quenching, phosphorescence spectra, phosphorescence lifetimes, and picosecond transient absorption spectra of several purinyl-pyridinium salts have been measured in several solvents at room temperature and in ethanol glasses at 77 K. It is concluded that the previously observed photochemical transformation ofN-[9-(2′,3′,5′-tri-O-acetyl-β-D-ribofuranosyl)purin-6-yl]pyridinium chloride1into the highly fluorescent tri-O-acetylluminarosine6occurs via an excited triplet state. It is also shown that intersystem crossing in purinyl-pyridinium salts is induced by an intermolecular charge-transfer interaction with the counter ions.Keywords: purinyl-pyridinium salts, fluorescence, phosphorescence.

ISSN:0008-4042    

DOI:10.1139/v90-332

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Proton transient nuclear Overhauser enhancement (TnOe) and spin-lattice relaxation times (T1) have been used to evaluate the conformations of several monosaccharides and disaccharides containing (13C) and (2H) substitution. Absolute1H–1H internuclear distances were determined by TnOe and DESERT (deuterium substitution effects on relaxation times) experiments on conformationally rigid methyl β-D-galactopyranoside and α- and β-D-xyloses, respectively. The DESERT method was extended to examineO-glycoside conformation in two blood-group disaccharides that were prepared enzymically with (13C) and (or) (2H) substitution. Preferred disaccharide conformations deduced from these distance measurements are compared to those determined from13C–13C and13C–1H spin coupling constants, theoretical calculations, and crystallographic studies.Keywords: TnOe, DESERT, carbohydrate conformation.

ISSN:0008-4042    

DOI:10.1139/v90-333

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

A stoichiometric complex of formula maleic acid•2H2O•18-crown-6 has been obtained from maleic acid and the macrocyclic polyether 18-crown-6. Crystals of this complex have been shown by X-ray diffraction crystallography to belong to theCcspace group of the monoclinic system. The acid molecules in the adduct are linked to each other through a water molecule, giving infinite [-acid-H2O-]nchains. They are also linked to the crown ether via water molecules. The infrared and Raman spectra of the complex are presented and compared to those of crystalline maleic acid.Keywords: maleic acid/18-crown-6, structure, X-ray, spectra.

ISSN:0008-4042    

DOI:10.1139/v90-334

出版商:NRC Research Press    

年代:1990

数据来源: NRC