摘要:

There are several different types of adenosine receptors found in the brain and molecules capable of interacting with the adenine ring system may have an effect on brain activity. Phenobarbital and diphenylhydantoin, two commonly used antiepileptic drugs have been shown to form hydrogen bonded complexes with adenine derivatives. Although 5,5-diphenylbarbituric acid has anticonvulsant properties equivalent to phenobarbital, neither its structure nor possible interactions with adenine have been investigated. 5,5-Diphenylbarbituric acid and 9-ethyladenine were, therefore, cocrystallized and the three-dimensional 1:1 hydrogen bonded complex was determined by X-ray diffraction. The crystals are monoclinic,C2/c,a= 28.457(9),b= 10.633(3),c= 13.995(4) Å, &bgr; = 100.20(2)°,Z= 8,R= 0.067 for 1680 reflections withI> 3&sgr;(I). The molecules propagate along the b crystallographic axis in an alternating fashion in which adjacent adenine and barbiturate molecules interact through a pair of hydrogen bonds. Two-fold symmetry related barbiturate carbonyl groups in neighboring planes of hydrogen bonded ribbons are involved in dipole–dipole attractive interactions. This hydrogen bonding pattern is compared to patterns found in other barbiturate–adenine complexes.Key words: hydrogen bonding, crystal structure, barbiturates, adenine, anticonvulsa

ISSN:0008-4042    

DOI:10.1139/v00-102

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Stable free radicals derived from 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS·+) have been extensively employed to monitor the antioxidant capacity of biological fluids and beverages. Besides reacting with typical antioxidants (such as phenols or thiols) these radicals react with a variety of hydrogen or electron donors. The present work reports on the kinetics and mechanism of these radical reactions with several amino acids. Reaction rates notably increase when the pH of the media increases and, when measured under similar conditions, follows the ordercysteine > > tryptophan > tyrosine > histidine > cystineThe kinetics of the process is interpreted in terms of a mechanism comprising an initial pH dependent reversible step, followed by secondary reactions of the substrate derived radical with itself or with another ABTS·+; this simple three-step mechanism leads to very complex kinetic expressions. The specific rate constants of several of the elementary steps were determined by working under a wide range of substrate, radical, and ABTS concentrations. The values obtained for the initial interaction between the ABTS derived radical and the substrate range from 0.5 M–1s–1to 1.9 × 106M–1s–1for histidine and cysteine, respectively.Key words: ABTS radical cation, 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid), amino acids, kine

ISSN:0008-4042    

DOI:10.1139/v00-099

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

A total synthesis of the marine alkaloid manzamine C has been accomplished. A Ramberg-Bäcklund reaction was used as a key step to construct the required azacycloundecene ring.Key words: alkaloid, macrocycle, Ramberg–Bäcklund, manzamine

ISSN:0008-4042    

DOI:10.1139/v00-103

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Theoretical calculations at the HF/6-31+G* level suggest the secondary &agr;-deuterium kinetic isotope effects for hydride ion SN2 reactions are much larger than expected for the structure of the transition state. The secondary &agr;-deuterium kinetic isotope effects for the SN2 reactions between sodium borohydride (hydride ion) andpara-methyl- andpara-chlorobenzyl chlorides are much larger than expected as the theoretical calculations suggest. It appears the secondary &agr;-deuterium isotope effects are larger than expected for the structure of the SN2 transition state because the hydride ion is too small to affect the C&agr;—H(D) out-of-plane bending vibrations in the transition state.Key words: secondary &agr;-deuterium kinetic isotope effects, SN2, nucleophilic substitution, transition state, substituent effects

ISSN:0008-4042    

DOI:10.1139/v00-100

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

The complexes PdCl2(PPh2vinyl)2and PdI2(PPh2vinyl)2have been prepared and crystallographically characterized as theirtranssquare planar isomers.Trans-PdCl2(PPh2vinyl)2exists in a centrosymmetric structure with a planar PdCl2P2core whereastrans-PdI2(PPh2vinyl)2shows no local Pd-centred symmetry and significant distortion of the PdI2P2core from planarity. Crystallographic data is in accord with the31P{1H} CP/MAS NMR spectra of the two complexes, which display a single resonance for the chloro-complex and two resonances for the iodo-complex. UV irradiation of CH2Cl2solutions of PdCl2(PPh2vinyl)2followed by re-dissolution in CDCl3indicates no permanent chemical change. However, post-irradiation CP/MAS31P{1H} NMR spectroscopy demonstrates the presence oftrans-PdCl2(PPh2vinyl)2in two solid state structures, pluscis-PdCl2(PPh2vinyl)2. PdI2(PPh2vinyl)2exists only in itstransform in both solid state and solution. Irradiation results in phosphine displacement and the formation ofsym-[PdI2(PPh2vinyl)]2and free phosphine, the latter being characterized as Ph2P(O)vinyl following aerobic oxidation. PdCl2(PPh2vinyl)2reacts in the presence of Et2O·BF3to afford [Pd(µ-Cl)(PPh2vinyl)2]2[BF4]2whereas PdI2(PPh2vinyl)2is recovered in 95% yield, with the remaining material undergoing phosphine abstraction to formsym-[PdI2(PPh2vinyl)]2.Key words: palladium, phosphine, alkene, isomerization, CP/MAS.

ISSN:0008-4042    

DOI:10.1139/v00-101

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

A group of 4-[1-(2-deoxy-&bgr;-D-ribofuranosyl)]-derivatives of 5-fluoroaniline possessing a variety of aryl C-2 substituents (6aR = H,6bR = F,6cR = Me) were synthesized. Accordingly, a Heck-type coupling reaction of the 4-iodoaniline derivatives (13a–c) with the bis(tert-butyldimethylsilyl)glycal (11) in the presence of Pd(OAc)2and Ph3As, followed by removal of thetert-butyldimethylsilyl protection groups usingn-Bu4N+F-, yielded the corresponding 4-(&bgr;-D-glycero-pentofuran-3-ulos-1-yl)aniline derivatives (14a–c) having a C-3 C=O in the sugar ring. Reduction of the C-3 C=O compounds (14a–c) using NaB(OAc)3H afforded the target 4-[1-(2-deoxy-&bgr;-D-ribofuranosyl)]-derivatives of the respective 2-substituted-5-fluoroaniline (6a–c). The deoxycytidine mimic, 3-fluoro-4-[1-(2-deoxy-&bgr;-D-ribofuranosyl)]aniline (6a), in which the cytosine ring of deoxycytidine is replaced by a 4-(3-fluoroaniline) ring system, was inactive as an anticancer agent against a variety of tumor cell lines, and as an antihuman immunodeficiency virus (HIV-1, HIV-2) agent. The failure of this unnatural deoxycytidine mimic (6a) to exhibit anticancer-antiviral activity may be due to its inability to undergo phosphorylation by host cell- and virus-induced kinases.Key words:fluoroanilines, deoxycytidine mimics, anticancer-antihuman immunodeficiency virus (HIV) evaluat

ISSN:0008-4042    

DOI:10.1139/v00-105

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

A series of [n.2]metacyclophanenes (3) and (6) were prepared in good yields by a McMurry cyclization of 1,n-bis(3-acetyl-4-methoxyphenyl)alkanes (2) and 1,3-bis(3-formyl-4-methoxyphenyl)propane (5), respectively. Compounds3b,3cexist in thesyn-conformation due to the steric repulsion between the methyl groups at the ethano bridge and the methoxy groups at the aromatic rings while compound6prefers theanti-conformation typical of [3.2]metacyclophanes. The assignment ofsyn-conformations has been confirmed by1H NMR analyses and X-ray diffraction studies. Photoinduced transannular cyclization of [n.2]metacyclophanenes (3) and (6) in the presence of iodine as an oxidant afforded phenanthrene-anellated polycyclic aromatic hydrocarbons. Apparently, the rate of the photocyclization ofanti-6was found to be much faster than that ofsyn-3band almost completed within 1 h. Thus, the different reactivities for the irradiation ofsyn- andanti-conformer were observed. The reason for the present preference for the formation oftrans-dihydrophenanthrene rather thancis-dihydrophenanthrene as the intermediate might be attributable to the more stable chair form transition state than boat one and the conformational fixation to the chair form in the ground and transition state is possible in theanti-conformer.Key words: cyclophanes, strained molecule, McMurry reaction, C—C coupling, conformation analysis, cyclizations, photolysis, transannular reactions, transition states

ISSN:0008-4042    

DOI:10.1139/v00-110

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Thiolysis ofp-nitrophenyl esters (acetate to decanoate) by the anion of 2-mercaptoethanol (ME) is catalyzed by micelles of cetyltrimethylammonium bromide (CTAB) in aqueous solution. At fixed [ME], the observed rate constants (kobs) show saturation with respect to added [CTAB], consistent with ester binding in the micelles. Plots ofkobsvs. [ME] are linear in the absence and in the presence of the CTAB, and analysis of the slopes of the plots afford rates constants for thiolate ion attack on the esters in the aqueous phase (kN) and in the micellar phase (kcN). The strengths of substrate binding and transition state binding to the micelles are strongly correlated, with a slope ofunity, because they have the same dependence on the ester chain. Consequently, the catalytic ratios (kcN/kN) are independent of the length of the ester. Similar behaviour is found for thiolysis by the dianions of mercaptoacetic acid, 3-mercaptopropionic acid, and cysteine, and also for ester cleavage by the anions of glycine and 2,2,2-trifluoroethanol, as earlier for cleavage by hydroxide ion. The results are consistent with Kirby's dissection of transition state binding into "passive" and "dynamic" components. The passive component involves hydrophobic binding of the ester chain which is more or less the same as in the substrate binding. The dynamic component is associated with reaction in the Stern layer of the micelle, and its magnitude varies with the nucleophiles because of differences in their ease of exchange between the aqueous medium and the Stern layer.Key words: catalysis, esters, thiolysis, micelles.

ISSN:0008-4042    

DOI:10.1139/v00-113

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

The reaction between aryl or heteroarylhydrazines with fluorinated &bgr;-diketones (CF3COCH2COR) yields a variety of 3-, 5-, and 3,5-trifluoromethylpyrazoles and 5-trifluoromethyl-5-hydroxy-&Dgr;2-pyrazolines. Twenty-one of such compounds have been isolated and identified by13C and19F NMR. Together with the results from the literature they provide a comprehensive overview of the reaction. Semi-empirical calculations at the PM3 level have been used to rationalize these results. The outcome that emerges seems to be that the dehydration of a pair of 3,5-dihydroxypyrazolidines kinetically controls the isomer formed.Key words: hydrazines, 1,3-diketones, pyrazolines, pyrazoles, PM3 calculations.

ISSN:0008-4042    

DOI:10.1139/v00-104

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Excess molar volumes and excess refractive indices of then-octanol + 1,4-dioxane + 2-butanol system and the three corresponding binary mixtures have been determined at 298.15 K by measuring densities and refractive indices. Different expressions exist in the literature to predict these excess properties from binary data. An estimation of excess molar volumes is also evaluated using a modified Heller equation which depends on the refractive indices of the mixtures. A comparison of the predictions of different methods with the experimental values of the physical properties has been made.Key words: excess molar volumes, densities, refractive indices,n-octanol, 1,4-dioxane, 2-butanol, ternary system.

ISSN:0008-4042    

DOI:10.1139/v00-111

出版商:NRC Research Press    

年代:2000

数据来源: NRC