摘要:

The geometric isomers of 1-amino-1,2-cyclopentanedicarboxylic acid were isolated as the hydrolysis products of 2-cyclopentanecarboxylic acid-5,5′-hydantoin. The hydantoin ester was formed from ethyl -2-oxocyclopentanecarboxylate via a Bucherer–Bergs reaction. Hydrolysis of the hydantoin at elevated temperatures gave a mixture oftrans- andcis-1 -amino-1,2-cyclopentanedicarboxylic acids indicating epimerization during hydrolysis. Thetransproduct has not been described previously and thecisnot extensively characterized. X-ray crystallography reveals that both isomers are zwitterionic in the solid state and thecisisomer molecules are linked by unusually strong hydrogen bonding. Structures were confirmed by13C NMR, X-ray crystallography, and elemental analysis. Physical data are also presented.

ISSN:0008-4042    

DOI:10.1139/v93-011

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Cobalt(II) chloride catalyzed oxidation of 2-substituted 1,3-dioxolanes in 1,2-dimethoxyethane afforded formate esters and acids in high yields. It was found that the presence of catalytic amounts of ZnCl2increased the rate of oxidation. A free-radical mechanism is proposed, involving participation of superoxocobalt, and the esterification of the alcohol and acid.

ISSN:0008-4042    

DOI:10.1139/v93-012

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Practical and simple techniques are described for using nonmodified domestic microwave ovens as safe and convenient laboratory devices to obtain numerous esters. High pressures are avoided by conducting reactions with reactants impregnated on solid mineral supports in "dry media" or by phase transfer catalysis (PTC) in the absence of organic solvents. Two kinds of microwave effects are involved: (1) displacement of the equilibrium by evaporation of volatile polar molecules (water or alcohols) in esterifications and transesterifications; (2) acceleration of ionic reactions in carboxylate alkylations. As solvents are avoided, there is no need for sealed vessels and water separators.

ISSN:0008-4042    

DOI:10.1139/v93-013

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

[9](N6,9)-6-Aminopurinophane,9, was synthesized by Mitsunobu coupling of 9-azidononanol,5, and6-chloropurine,6. Reduction of the azide allowed for intramolecular nucleophilic displacement of chloride by the resultant amine and cyclization to the cyclophane. Variable temperature proton NMR showed the presence of two conformers below −25 °C separated by an activation barrier having a ΔGc≠ = 50.2 ± 2.5 kJ mol−1. The conformers arose from partial rotation about the C6—N6 bond, with theanticonformation assigned to the major isomer,9a, and thesynconformation to the minor,9b. The crystal structure of the [9](N6,9)-6-aminopurinophane, C14H21N5, was determined at 173 K. The crystals are monoclinic, of space groupP21/n, witha = 16.941(3) Å,b = 8.512(2) Å,c = 19.300(2) Å, β = 95.90(1)°,V = 2769(1) Å3,Dc = 1.24 g cm−3Dm = 1.27 g cm−3, forZ = 8, λ(CuKα) = 1.540598 Å.R1 = 0.0364,R2 = 0.0395 for 2846 reflections, ((R1) = 0.0319,R2 = 0.0365 for 2556 reflections with1 > 2.5σ(I)). The purine ring shows small but significant distortions from planarity. The N(1)—C(6)—N(6)—C(1′) fragment adopts ananticonformation, which is twisted by approximately 35° from the plane of the purine ring, and the exocyclic 6-nitrogen is partially pyramidalized. Such pyramidalization is consistent with the Bader–Wiberg theory of barriers to rotation about bonds.

ISSN:0008-4042    

DOI:10.1139/v93-014

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Due to their important activity and selectivity, nickel catalysts are often used to realize the methanation reaction; however, the usual process based on these catalysts is not fitted for the present economic situation. To raise their stability, three catalysts with similar nickel content but calcinated at different temperatures have been studied for the reaction of methanation. The results obtained show that the catalyst calcinated at the highest temperature (973 K) shows the greatest promise because a great part of its activity and of its stability is preserved even when the reaction conditions are extreme (temperature of 773 K).

ISSN:0008-4042    

DOI:10.1139/v93-015

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

AbinitioCI studies have been performed to determine the geometry of the ground and first electronically excited states of the chlorothioformyl radical, ClCS, and the vertical excitation energies of its ten lowest doublet states and two lowest quartet states. The results are used to construct a more complete energy level correlation diagram for the photofragmentation of Cl2CS. The lowest excited electronic state of ClCS lies only 0.79 eV (adiabatic) above the ground state. Its discovery indicates that the results of previous photofragmentation experiments may need to be reinterpreted.

ISSN:0008-4042    

DOI:10.1139/v93-016

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Model calculations were made on the hydrated electron by using theabinitioMO method combined with the MR-SD-Cl method and the coupled cluster theory. The models used in the calculations were water clusters denoted by [e−(H2O)n(H2O)m], wheren = 2,3,4, and 6 for the first solvation shell andm = 0–28 for the second and third solvation shells. In these model calculations, the interactions between the excess electron and the water molecules in the first solvation shell are explicitly calculated byabinitioMO methods and the water molecules in the second and third solvation shells were represented by the fractional charges obtained at the MP2/D95V** level. The stabilization energies and the solvation radiusr(e−–O), in terms of the distance between the center of the cavity and an oxygen atom of the surrounding water molecules, increased monotonically with the number of water molecules in the first solvation shell. On the other hand, the first excitation energy was not dependent on the number of water molecules in solvation shells, but constant, with the value of ca. 2.0 eV. On the basis of the present calculations, we suggest that (1) the energetic stability of excess electrons depends on both short-range interaction and long-range interaction, (2) the first excitation energy is critically affected by only the short-range interactions, and the excitation is theoretically attributed to the1s→2ptransition of the excess electron.

ISSN:0008-4042    

DOI:10.1139/v93-017

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

A series of di- and trisaccharide glycosides based on the α-L-Rha(1 → 3)β-D-GlcNAc and α-L-Rha(1 → 3)α-L-Rha(1 → 3)β-D-GlcNAc elements have been synthesized to locate the minimal oligosaccharide determinant of theShigellaflexneriO-polysaccharide, which is built from a tetrasaccharide repeating unit, [ → 2) α-L-Rhap(1 → 2)α-L-Rhap(1 → 3)α-L-Rhap(1 → 3)β-D-GlcNAcp(1-]n. These compounds also serve to identify the carbohydrate surface of theShigellaantigen that interacts with a monoclonal antibody, currently the subject of crystallographic studies. Two strategies utilizing suitably protected glycals1and19were employed to obtain analogs bearing either terminal or glycosylated 2,6-dideoxy-α-L-arabino-hexopyranosyl (2-deoxy-α-L-rhamnopyranosyl) residues.N-Iodosuccinimide activation of the glycals in the presence of selectively protected mono- and disaccharide alcohols afforded 2-deoxy-2-iodo-α-L-rhamnopyranosides and these were ultimately reduced during deprotection stages to afford the desired functionality. Di-O-acetylL-rhamnal1reacted with monosaccharides2and7, and with disaccharide11, to yield disaccharides4and8, and trisaccharide12, each bearing a terminal 2-deoxy-α-L-rhamnopyranosyl residue. The selectively protected 3-O-benzoyl-4-O-benzyl-L-rhamnal19was synthesized fromL-rhamnal and used to prepared trisaccharide22, which contained an internal 2-deoxy-2-iodo-α-L-rhamnopyranosyl unit. Removal of protecting groups gave the oligosaccharides6,10,14, and23. Oligosaccharides that contained a 2-deoxy-α-L-rhamnopyranosyl residue showed enhanced inhibitory power: in the case of trisaccharide23a 1.8 kcal mol−1relative increase in free energy of binding compared to a larger pentasaccharide epitope, α-L-Rhap(1 → 2)α-L-Rhap(1 → 3)α-L-Rhap(1 → 3)β-D-GlcNAcp(1 → 2)α-L-Rhap-1 → OMe. These data suggest that the rhamnose O–2 hydroxyl of residue C points toward and has important interactions with binding site amino acids.

ISSN:0008-4042    

DOI:10.1139/v93-018

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Theoretical barriers and reaction energetics were determined usingabinitiocalculations at the PMP4/6-311 + +G** level with UMP2/6-311G** optimized geometries for hydrogen abstraction reactions of HFCO and HClCO by fluorine and chlorine atoms. The largest barriers are predicted for fluorine and chlorine atom abstractions of hydrogen from HFCO. Fluorine and chlorine atom abstractions of hydrogen from HClCO are predicted to be barrierless. The predicted barrier for fluorine atom abstraction of hydrogen from HFCO is calculated to be 1.3 kcal mol−1and is in good agreement with the experimental value of 1.8 ± 0.4 kcal mol−1.

ISSN:0008-4042    

DOI:10.1139/v93-019

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

SCF and Møller–Plesset calculations were done for four chalcogen hexahalide systems (SeCl6, SeCl62−, TeCl6, and TeCl62−) to study their geometric structures and relative stabilities in their ground electronic states. Flexible all-electron and valence-electron polarized basis sets were used. For some of the basis sets used, the stationary points located on the SCF potential energy surface of the ions inOhsymmetry were found to possess negative eigenvalues of the Hessian matrix; the structural parameters of the corresponding stationary points inC3vsymmetry were determined. At the MP2 level, all ions were found to restore the octahedral symmetry.

ISSN:0008-4042    

DOI:10.1139/v93-020

出版商:NRC Research Press    

年代:1993

数据来源: NRC