摘要:

The Sommelet rearrangement of (+)-ethylmethyl-p-nitrobenzylsulfonium perchlorate, (+)-1, and (+)-ethylmethyl-p-chlorobenzylsulfonium perchlorate, (+)-2, are described. Elution of (+)-1through an hydroxide exchange resin generated ethylmethylsulfoniump-nitro-benzylide (+)-3which decomposed in methanol at room temperature to ethyl 2-methyl-5-nitro-benzyl sulfide,6, and (+)-methyl α-(2-methyl-5-nitrophenyl)ethyl sulfide, (+)-7, with 18 to 20.3% asymmetric induction. Decomposition of (+)-2in sodium methoxide solution at 70 °C for 2 h produced ethyl 2-methyl-5-chlorobenzyl sulfide,8, and (+)-methyl α-(2-methyl-5-nitro-phenyl)ethyl sulfide, (+)-9, with 21 to 25.5% asymmetric induction. The lower estimates of asymmetric induction for each sulfide were made by comparison with specific rotations of authentic samples obtained by synthesis and resolution. The higher estimates were obtained by the use of a chiral lanthanide shift reagent Eu(hfbc)3with the sulfone derivatives of these chiral sulfides. The ylide (+)-3reacted with aldehydes in high yield to produce oxiranes with no induction of asymmetry.

ISSN:0008-4042    

DOI:10.1139/v76-031

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

The equilibrium constant for the addition of sodium methoxide to methyl trifluoroacetate, in methanol as solvent, has been measured by19F nmr, and is 7 M−1. From this was calculated an equilibrium constant, 2 × 10−4 M−1, for addition of methanol to the ester. The equilibrium constant for formation of methyl trifluoroacetate in aqueous solutions is 0.06 M−1. These results, with literature data, permit calculation of the free energies of formation in aqueous solution of orthotrifluoroacetic acid and its mono-, di-, and trimethyl esters. These in turn permit calculation of the standard free energy changes for addition of water and methanol to trifluoroacetic acid and its methyl ester. These combined with the analogous values for formic and acetic acids permit evaluation of ρ* values for these addition reactions. Linear plots are obtained if correction is made for steric effects, and the ρ* values are somewhat larger, 2.1–2.9, than was observed for the analogous carbonyl addition reactions.

ISSN:0008-4042    

DOI:10.1139/v76-032

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

The 1,2,3-triketone PhC(Me)2COCOCOPh has been synthesized from mandelic acid in seven steps. Its proton magnetic resonance spectrum features a sharp six-proton methyl singlet at room temperature which broadens considerably at low temperatures but is not resolved into two peaks at 173 K. Similar dynamic behaviour is observed for the monoketone PhC(Me)2COPh. It is concluded that the barrier to interconversion of enantiomeric tricarbonyl helical conformations is significantly less than 7 kcal/mol. The rotation–rotation surface of the ketodialdehyde HCO—CO—CHO has been computed by the CNDO/2 method; the barrier to stereomutation of enantiomeric helical conformations of this tricarbonyl system is found to be 3.9 kcal/mol. The dynamics of this process are discussed in terms of a recently proposed definition of symmetric and asymmetric reactions.

ISSN:0008-4042    

DOI:10.1139/v76-033

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

Thermal condensation of 2-carbomethoxy and 2-cyano-aziridines with perfluorobutene and perfluoropropene produces polyfluorinated pyrrolidines. These compounds, treated with sodium methoxide, lead to 3,4-fluorinated pyrroles. In the case of perfluoropropene, the (2 + 3) cycloaddition is not completely regiospecific.

ISSN:0008-4042    

DOI:10.1139/v76-034

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

The minimum energy conformations of some mono- and di-substituted benzophenones were determined from contour maps of potential energy calculated by a semiempirical approach. The electrostatic interactions play a relevant role in the relative stabilities when different minima are possible. Comparison with the available experimental data reveals a good agreement with geometrical data, and a poorer agreement with energies in highly strained conformations.

ISSN:0008-4042    

DOI:10.1139/v76-035

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

13C nmr chemical shifts and13C–31P couplings through one to five bonds are reported for seven dimethylphosphono compounds of known geometry. Vicinal couplings are maximal for a dihedral angle of 180° and are severely attenuated by OH substitution, particularly when the OH istrans-coplanar to the carbon terminus of the coupling path. When a cyclopropyl system is part of the C–C–C–P path theJvalues are much less than those predicted on the basis of dinedral angle. A highly asymmetric dihedral dependence of vicinalJ's is suggested. Some 'non-W' long range C–P couplings through saturated networks are found. The 'gauche-γ' shift of the —P(O)(OCH3)2group is about 2 ppm.

ISSN:0008-4042    

DOI:10.1139/v76-036

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

The synthesis of 2,3,12,13-tetraselena[4,4]metacyclophane (1) by alkaline hydrolysis ofmeta-xylylene selenocyanate (3a) in 8% yield is described and is shown to have different properties to those described for1by Günther and Salzmann. The variable temperature pmr spectra of1is described as is its conversion to 2,11-diselena[3,3]metacyclophane (4) by tris(diethylamino)phosphine.

ISSN:0008-4042    

DOI:10.1139/v76-037

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

The inactivation of the proteolytic enzyme papain by hydrogen peroxide produces a sulfenic acid by oxidation of the essential SH of cysteine 25 at the enzyme active site:The kinetics of repair of this entity by cysteine were consistent with the two reactions:Reaction 4 was the faster withk4 ≥ 800 M−1s−1, andk5 = 11.3 ± 0.5 M−1s−1. A computer program was developed to evaluate the contributions of peroxide-inactivation and cysteine-repair when they occur simultaneously in N2O-saturated solutions in the absence of catalase. The yields predicted by this program agreed well with the inactivation caused by peroxide in irradiated systems.The effect of cysteine on the inactivation of papain by OH radicals produced by radiolysis of N2O-saturated solutions containing catalase was also investigated. Protection against permanent inactivation was much more efficient than expected on the basis of a simple competition between cysteine and papain for OH radicals, but there was a marked increase in the yield of repairable damage which was not due to hydrogen peroxide. These observations can be qualitatively accounted for by the reactions:The same rate constant was obtained for the repair of PapainCys25SSCys from this source as from the peroxide inactivation and treatment with cysteine. However, there was also evidence for additional cysteine-sensitized production of mixed disulfide and this probably occurs through reactions of CysS• radicals:

ISSN:0008-4042    

DOI:10.1139/v76-038

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

Based on the Onsager reaction field theory of the dielectric polarization of a continuum, a model has been developed to calculate the Van der Waals contribution σwto the gas-to-liquid medium shifts of nonpolar solutes. The addition of a site factor correction, to account for the off-center location of the measured nucleus in the solute molecule, provides a marked improvement. The agreement between experimental and calculated σw's is good (±0.05 ppm), but a separate scale factor is required for each solute. This nonuniversality is shown to be related to the Onsager approximation for the solute cavity radius;a3 = 3V1/4πNA. It is shown that Van der Waals shifts can be used for the determination of the Onsager cavity radii; for a number of molecules these cavity radii are given. It is claimed that this method is as precise, but much easier to apply, than the Böttcher–Onsager optical method.No evidence could be found for the existence of acontribution to the medium shifts due to permanent electric dipoles of solvent molecules. Combination of experimental medium shifts in anisotropic solvents with calculated σwcontributions allows the evaluation of the neighbor anisotropy contribution σa. The σaeffect is found to be solute dependent; there is aV1−1/6proportionality related to the molar volumeV1of the solute and in addition there are site effects with site factors both greater and less than unity, depending on molecular shape. The solvent dependence of σaincludes not only the magnetic susceptibility anisotropy Δχ2and the molar volume (V2−1/3) but also a term corresponding to the geometric anisotropy of the solvent molecule.The solvent CS2is found to behave as an isotropic molecule.

ISSN:0008-4042    

DOI:10.1139/v76-039

出版商:NRC Research Press    

年代:1976

数据来源: NRC

摘要:

A novel condensation reaction of 4-hydroxy-6-methyl-2-pyridone with aldehydes giving4has been explored. A number of by-products are accounted for. The mass spectra of the major condensation products are discussed.

ISSN:0008-4042    

DOI:10.1139/v76-040

出版商:NRC Research Press    

年代:1976

数据来源: NRC