摘要:

This paper describes the chemistry presented during the Merck Frosst Centre for Therapeutic Research Lecture Award given at the 79th Chemistry in Canada Conference held in St. John's, Newfoundland in June 1996. The first section describes the synthesis of (+)-xestoquinone using an asymmetric palladium-catalyzed polyene cyclization as the key step that creates the C and D rings and the stereogenic centre (68% ee) in one step. Extensions of the work involving an in situ Suzuki reaction are presented. The synthesis ofC2-symmetric biaryls and the synthesis of a recently isolated binaphthyl natural product is described using this new method. A new one-pot desilylation–oxidation procedure of silyl ethers is described in detail for the preparation of aldehydes and ketones directly without the need for the isolation of the alcohol intermediate. Finally, a highly diastereoselective (>97%) Diels–Alder reaction is presented using (+)-cis,cis-spiro[4.4]nonane-1,6-diol as a new chiral auxiliary. One of the alcohols is attached to a pivalate, the other to an acrylate, and the Diels–Alder reaction with cyclopentadiene provides only one adduct (by1H NMR and HPLC) with theendostereochemistry.Keywords: (+)-xestoquinone, asymmetric palladium-catalyzed polyene cyclization, in situ Suzuki reaction, desilylation–oxidation reaction, spirodiols, chiral auxiliaries.

ISSN:0008-4042    

DOI:10.1139/v97-139

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

N-Methyl-L-4-nitrotryptophan methyl ester (2) andt-Boc-L-phenylalanine (3) were condensed to form the dipeptide4, which was then cyclized to give 12-N-methylcyclo-(L-4-nitrotryptophyl-L-phenylalanyl) (1), a.k.a. thaxtomin C, the principle phytotoxin associated withStreptomycesipomoeae.Keywords:Streptomycesipomoeae, phytotoxin, thaxtomin C.

ISSN:0008-4042    

DOI:10.1139/v97-140

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Hartree–Fock/6-31G** calculations on the benzenoid hydrocarbons benzene, naphthalene, phenanthrene, anthracene, pyrene, tetracene, triphenylene, chrysene, perylene, and coronene are used to investigate the link between aromaticity and the electron distribution. Topological charge density analysis is used, concentrating on the electron distribution ρ (and its Hessian) at bond and ring critical points. With regard to the bond critical point data, it is shown that ρc,ρc, and the bond "ellipticity" ϵ are closely correlated with the bond lengths so, as aromaticity indicators, they have little to add over and above existing indices based on structure. However, the same properties evaluated at the ring critical points in the total density, and also at the equivalent stationary points in the π and σ densities, correlate closely with two different aromaticity indices (one based on structure, the other on magnetic properties), the curvature of ρ perpendicular to the ring plane giving (marginally) the best results. Hence a ring critical point (RCP) index is proposed as a way of quantifying aromaticity, based directly on the electron distribution.Keywords: quantum chemistry, electron density, aromaticity, aromaticity index, HOMA, NICS.

ISSN:0008-4042    

DOI:10.1139/v97-141

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Pd3(dppm)3CO2+exhibits a two-electron reduction process at ~ −0.5 V vs. SCE in 0.2 M Bu4NPF6solutions of organic solvents such as dry acetonitrile,N,N-dimethylformamide (DMF), tetrahydrofuran (THF), and dichloromethane to reversibly produce the zero valent Pd3(dppm)3CO, which exhibits a ν(CO) band in the 1761–1766 cm−1range in the IR spectra. The mechanistic aspects of this multistep two-electron process are discussed experimentally on the basis of r.d.e. (rotating disk electrode) voltammetry, cyclic voltammetry, and electrolysis results. Geometry optimizations using DFT (Density Functional Theory) predict that the Pd—Pd and C=O bond lengths are 3.008 and 1.182 Å, respectively, which compare favorably with the experimental data for other zero valent polynuclear complexes.Keywords: palladium, cluster, reduction, electrochemistry, density functional theory.

ISSN:0008-4042    

DOI:10.1139/v97-142

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The reaction ofZ,E,Z-PhSN=C(Ar)N=NC(Ar)=NSPh (Ar = 4-CH3C6H4) (1a) with MCPBA results in ring closure via an intramolecular redox process to give the 1,2,3,5-thiatriazole(4). An X-ray structural determination revealed that4contains a planar five-membered CNS(VI)NN ring withd(N—N) = 1.402(6) Å. The reaction of ArCN2(SiMe3)3(Ar = 4-CH3C6H4) with one equivalent of MeSO2Cl, followed by two equivalents of PhSeCl, produces the sulfonyl-containing diazene MeSO2N=C(Ar)N=N(Ar)C=NSO2Me (11), which was shown by X-ray crystallography to adopt aZ,E,Zgeometry. By contrast, the reaction of ArCN2(SiMe3)3with one equivalent of ArS(O)Cl, followed by two equivalents of PhSeCl, yielded the sulfur(II)-containing diazeneZ,E,Z-ArSN=C(Ar)N=NC(Ar)=NSAr (Ar = 4-CH3C6H4) (1b).Keywords: thiatriazoles, sulfonyl diazenes, intramolecular redox cyclization.

ISSN:0008-4042    

DOI:10.1139/v97-143

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

High-level ab initio molecular orbital and density functional theory calculations predict the existence of a very short-strong hydrogen bond in the monoanion of maleic acid (hydrogen maleate). At all levels of theory (HF, MP2, BLYP, and B3LYP) except B3PW91 the potential energy surface is predicted to contain two minima, and hence resembles a double well. The barrier to proton transfer via a symmetrical transition state is calculated to be very small at all levels of theory. In all cases the calculated zero point vibrational energy available to the system is larger than the calculated barrier for proton transfer, thus the resulting hydrogen bond formed in hydrogen maleate is predicted to be symmetrical. Using the B3PW91 functional and the 6-31 + G(d,p) basis set results in a single-well potential and a symmetrically positioned hydrogen. All correlated methods predict the gas phase hydrogen bond energy to be approximately 27 kcal/mol. Effects due to solvents were estimated using solvent cavity methods. Approximating the solvent as a dielectric continuum reduces the calculated hydrogen bond energy by roughly 6 kcal/mol at all levels of theory.Keywords: low-barrier hydrogen bonds, short-strong hydrogen bonds, hydrogen maleate, ab initio, density functional theory.

ISSN:0008-4042    

DOI:10.1139/v97-144

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The one-pot reaction of equimolar amounts of salicylaldehyde, diphenylborinic acid anhydride, and a tertiary amine in the presence of an alkanol (R′OH) led to the addition of R′OH to the aldehyde group and the formation of anO,O-acetal moiety within the chelate anion with the charge balanced by an ammonium cation arising from the tertiary amine. Exchanging the diphenylborinic acid in the three-component reaction system for phenylboronic acid did not give the analogous adduct or chelate but one additional mole equivalent of phenylboronic acid (anhydride) was incorporated, leading to a polycyclic anion containing pyroboronate and acetal boronate functions with an associated ammonium cation. Crystals of 1-methylpiperidinium 4-ethoxy-2,2-diphenyl-1,3-dioxa-2-borata-1,2,3,4-tetrahydronaphthalenate,6d, are monoclinic,a = 9.522(1),b = 17.703(1),c = 14.974(1) Å, β = 98.275(10)°,Z = 4, space groupP21/n, and those of 1-methylpiperidinium 6,10-epoxy-4-methoxy-6,8-diphenyl-5,7,9-trioxa-8-bora-6-borata-5,6,7,8,9,10-hexahydrobenzocyclooctenate,10e, are monoclinic,a = 7.968(1),b = 19.3707(4),c = 15.9400(5) Å, β = 100.186(5)°,Z = 4, space groupP21/n, The structures were solved by direct methods and refined by full-matrix least-squares procedures toR = 0.050 and 0.033 (Rw = 0.040 and 0.032) for 2815 and 3529 reflections withI ≥ 3σ(F2), respectively. Compounds6dand10eare both representative of new classes of organoboron chelates.Keywords: salicylaldehyde semiacetal diphenylboron chelate, salicylaldehyde bisboronate, organoboron compound, crystal structure.

ISSN:0008-4042    

DOI:10.1139/v97-145

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The syntheses of a series of oxygenated cyclohexene ring A synthohs20,22–27,34,36, and37that possess suitable functionality for further elaboration for taxoid synthesis are described. These compounds have been prepared from β-ionone by a series of oxidative and addition reactions or alternatively by Lewis acid catalyzed Diels–Alder cycloadditions with the oxygen-containing trimethyl substituted dienes31and33.Keywords: taxoid, paclitaxel, dienes, Diels–Alder, β-ionone.

ISSN:0008-4042    

DOI:10.1139/v97-146

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Barriers to rotation have been calculated by molecular mechanics, using the force field MM+, for a fragment of PEEK and its methyl, ethyl, isopropyl,tert-butyl and di-tert-butyl, substituted derivatives. The barrier for the nearer C—O bond increases from 2.0 kcal/mol in KEEK to 16 kcal/mol in dibutyl substituted KEEK as the bulkiness of the substituent increases. The rotation barrier of the further C—O bond is not affected by the substituent.Keywords: rotation barriers, PEEK, molecular mechanics.

ISSN:0008-4042    

DOI:10.1139/v97-147

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

A theoretical study on the reaction of proton transfer from ethanol to benzene anions was performed at the ab initio level. It was found that in the gas phase the reaction proceeds without potential energy barrier with respect to reactants held at infinite distance, even though a potential energy barrier exists when measured from the prebarrier complex formed by the reactant molecules. When restrictions on orientation of the reactant molecules in the solid phase are taken into account, the potential energy barrier found exceeds 40 kcal/mol. A theoretical estimate of the kinetic isotope effect due to the tunneling effect, κH/κD, of 9.0 × 103is found, in good agreement with available experimental data.Keywords: isotope effects, proton transfer, tunneling.

ISSN:0008-4042    

DOI:10.1139/v97-148

出版商:NRC Research Press    

年代:1997

数据来源: NRC