摘要:

Condensations of stabilized phosphorane1with 3-substituted phthalic anhydrides were investigated. The importance of various effects influencing regio- and stereoselectivity of these reactions is discussed. It is proposed that the oxygen atom on the substituents in position 3 can act as a Lewis base toward the electron-deficient phosphorus of the ylid. The resulting complexation stabilizes the transition state for the reaction at theorthocarbonyl group, thus offsetting the usual steric and "push" effects, which favour attack at themetacarbonyl function.Keywords: Wittig condensations, phthalic anhydrides, regioselectivity, stereoselectivity.

ISSN:0008-4042    

DOI:10.1139/v89-086

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

The cycloaddition of α-hydroxy orthoquinodimethane, generated photochemically from 2-methylbenzaldehyde, to the fumarate and acrylate ofS-methyl lactate has been found to give a single diastereomer with high asymmetric induction (>95% de). This reaction provides a new and versatile synthetic route to substituted tetralins of high optical purity. Atransstereochemistry between the vicinal hydroxyl and carboxylactyl groups has been established for these cycloadducts. This is in contrast to previous work wherecisstereochemistry has always been found for major cycloadducts of α-hydroxyo-QDMs. The high asymmetric induction, unusual diastereoselectivity, and the potential use of these reactions in asymmetric synthesis are discussed.Keywords:o-quinodimethanes, Diels–Alder, asymmetric, cycloaddition, induction, diastereoselective.

ISSN:0008-4042    

DOI:10.1139/v89-087

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

The thujone-derived chiral synthon1is converted, via selenium chmistry, to the important cross-conjugated dienone2and the latter is then convertible to the unsaturated cardenolide analogue3, which through known methodology (3 → 26 → 27 → 4), completes a formal synthesis of digitoxigenin (4). Extensive studies to afford2from androstenedione (5) are also described and a new approach to elaborate the essential butenolide ring system, characteristic of the cardiac active steroids, is developed.Keywords: thujone, steroid synthesis, digitoxigenin.

ISSN:0008-4042    

DOI:10.1139/v89-088

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

A series of 30 phenyl substituted derivatives of 2-phenyl-1,1,3,3-tetramethylguanidine was synthesized and the pKavalue of each was measured in acetonitrile using a conventional, general purpose glass electrode. The slope of the Hammett relationship, for theparaandmetasubstituted compounds, ρ = 2.18 ± 0.16, is significantly lower than ρ = 2.85 ± 0.06 found for anilines in water, even though the phenyl ring is attached directly to the basic centre in both cases. The influence of electronic and steric effects on the basicity of the compounds is discussed. The solvent effect on the basicity is also discussed, by comparison of the pKavalues in acetonitrile and water. The mean value ofis slightly higher than the value previously observed for ammonia and aliphatic amines,. Additionally, the pKa values were measured for seven other cyclic bases, including 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN).Keywords: 2-phenyl-1,1,3,3-tetramethylguanidines, pKameasurements.

ISSN:0008-4042    

DOI:10.1139/v89-089

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

Peptides consisting of portions of the sequence Arg-(Lys)2-(Ser)2-Lys-(Ala)3have been immobilized onto a polyacrylamide support by solid phase peptide synthesis. Isotherms at 0 °C for the adsorption of bilirubin from phosphate buffer by these peptide resins are highly dependent on pH, in the range 7.80–11.00, demonstrating the importance of ionic interactions. The adsorption behaviour of bilirubin-related tetrapyrroles shows that structural factors also influence the process. Peptide resins containing two or more residues of lysine have two binding sites on each pendant and are characterized by positive cooperativity, to a degree that is highly dependent on the number of residues. It is proposed that bilirubin molecules bound by lysine-containing pendants are stacked, the dimer state being stabilized by π-π electron interactions, hydrogen bonds, and hydrophobic interactions. Site binding constants show that arginine-containing pendants manifest a stronger affinity for bilirubin.Keywords: bilirubin, adsorption, lysine, oligopeptides, cooperativity.

ISSN:0008-4042    

DOI:10.1139/v89-090

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

The electron affinities of 14 substituted nitrobenzenes including nitrobiphenyls were determined by measurement of electron transfer equilibria [1] in the gas phase with a pulsed high pressure mass spectrometer: A− + B = A + B−[1]. These data, when combined with previous determinations from this laboratory, lead to electron affinities for 35 substituted nitrobenzenes and provide a comprehensive data set for the examination of substituent effects. The data are used to derive Taft gas-phase substituent parameters. A qualitative discussion based on frontier orbital molecular theory examines the substituent effect on the benzene and nitrobenzene LUMOs. The lifetimes for electron autodetachment from excited nitrobenzene negative ions, (A−)*, studied earlier by Christophorou, are examined in light of the present electron affinity data.Keywords: electron affinities, substituent effects, frontier orbital treatment, electron autodetachment from nitrobenzene radical anions.

ISSN:0008-4042    

DOI:10.1139/v89-091

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

Measurements of the gas phase proton transfer equilibrium [1]: AH + B− = A− + BH with a pulsed electron high pressure mass spectrometer (PHPMS) lead toanddata. These relative acidities are converted to the absolute acidities [2]: AH = A− + H+bycalibration of the relativeandscale to the known values forandof a reference compound(HCl). Earlier determinations that included 16 aliphatic AH are extended in the present work and provide data for 47 aliphatic carboxylic acids. These include halogen, OH, CH3O, C2H5O, PhO, and NH2substituted acetic acids, and halogen substituted, unsaturated, and cyclic RCO2H of higher carbon number. The effects of the various substituents on the gas phase acidity are briefly examined.Keywords: acidities in gas phase, proton transfer equilibria, ion-molecule equilibria, mass spectrometry.

ISSN:0008-4042    

DOI:10.1139/v89-092

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

A new semiconducting ternary compound of stoichiometry TEDA•I2.5•TCNQ is reported. The crystal is made up of linear chains of TCNQ−1/2, nonlinear chains of I3−, and monodides of doubly charged triethylenediamines (TEDA). Half the TEDA's are hydrogen bonded to the TCNQ's and I3−. The conductivity measurements indicate semiconducting behaviour with two discontinuities in activation energy, one at high (330 K) and the other at low (160 K) temperatures. Resonance Raman spectra of the crystals manifest strong resonance enhancement of the iodide moieties. In particular, selective resonance excitation at 647 nm of nonsymmetric I3−is observed, thus indicating an increasing asymmetry of the I3−in the low temperature phase and suggesting coupling between the three species in this phase.Keywords: semiconductor, ternary compound, resonance Raman spectra, tetracyanoquinodimethane (TCNQ), tri-iodide (I3).

ISSN:0008-4042    

DOI:10.1139/v89-093

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

Abinitiocalculations (Hartree–Fock) at the level of 3-21G and 6-31G* as well as MP2/6-31G* have been performed on the carcinogen dimethylnitrosamine and its products of oxidative metabolismE- (andZ-) hydroxymethylmethylnitrosamines and certain ester conjugates. TheE- andZ-hydroxymethylmethylnitrosamines are found to be close in energy and to be configurationally stable as is found experimentally. Alternative pathways are examined for the spontaneous decomposition of the primary metabolite in aqueous solution. Direct cleavage ofE-hydroxymethylmethylnitrosamine toZ-methanediazohydroxide and formaldehyde is highly endothermic. Protonation of the NO group of the hydroxylated metabolite strongly stabilizes this species in contrast to nitrosoureas but in accord with experiment. Incorporation of water together with consideration of entropy effects satisfactorily accounts for the spontaneous exothermic decomposition ofE-hydroxymethylmethylnitrosamine in aqueous media, as well as its relative stability in aprotic media.Abinitiocalculations were also performed on selected nitrosoiminium ions which participate in the enzymatic activation and metabolism of certain dialkylnitrosamine carcinogens.Keywords: carcinogenesis, dimethylnitrosamine,abinitiocalculations, nitrosoiminium ions.

ISSN:0008-4042    

DOI:10.1139/v89-094

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

The kinetics of oxidation of proline by periodate has been studied at pH 1.40–8.83 and 30.0 °C. The reaction rate is first order in both periodate and amino acid, and the overall reaction follows second-order kinetics. There was no evidence for the formation of an appreciable amount of intermediate. The reaction rate is highest at pH 4–7 and the oxidation is catalysed byions. The pH dependence of the reaction rate can be explained in terms of reaction of periodate monoanion and the protonated and dipolar forms of the amino acid. The mechanism proposed and the derived rate law are consistent with the observed kinetics. The rate constants obtained from the derived rate law are in agreement with the observed rate constants, thus justifying the rate law and the proposed mechanistic scheme.Keywords: oxidation of proline, oxidation by periodate.

ISSN:0008-4042    

DOI:10.1139/v89-095

出版商:NRC Research Press    

年代:1989

数据来源: NRC