摘要:

The tricyclic intermediate4has been converted into2, a compound that possesses the ring system and stereochemistry of the cyathins. The transformation was accomplished by the preparation of11, which underwent efficient base catalysed epimerization to give12. Functional group manipulation and selective oxidation provided keto-aldehyde16. Upon treatment with acid,16undergoes stereoselective aldol cyclization, transacylation, and hemiacetal formation to give18. Methylation under basic conditions provides4.

ISSN:0008-4042    

DOI:10.1139/v87-397

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

Trifluoroethyl benzyl sulfide was prepared (in 2-propanol) and subsequently chlorinated as a further test of substituent electronegativity based regiochemical predictions. The initial attempt to prepare that sulfide by reaction of 2,2,2-trifluoroethyl methanesulfonate and benzyl mercaptide anions (in methanol) furnished benzyl methyl sulfide. Mechanistic possibilities are discussed in detail and some synthetic consequences of this novel reaction are presented.

ISSN:0008-4042    

DOI:10.1139/v87-398

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

Further investigation of the South African treeCombretumcaffrum(Combretaceae) for murine P388 lymphocytic leukemia (PS) cell-growth inhibitory substances has led to discovery of three new active constituents designated combretastatins A-2 (5a, PS ED500.027 μg/mL), A-3 (5b, PS ED500.026 μg/mL), and B-2 (3b, PS ED500.32 μg/mL). Both combretastatins A-2 and A-3 were found to markedly inhibit tubulin polymerization. The structure of each combretastatin was firmly established by a combination of high resolution (400 MHz)1H and13C nuclear magnetic resonance and mass spectral analyses followed by total syntheses. The conversion of methyl gallate (7b) to combretastatin A-2 via intermediates7c → 7d → 7e → 7aand6a → 5aillustrates the practical synthetic route utilized for obtaining these substances. The Wittig reaction employed as the penultimate step in obtaining combretastatins A-3, afforded predominantly the naturalZisomer.

ISSN:0008-4042    

DOI:10.1139/v87-399

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

The preparations of ten decalindiones for investigation as substrates of horse liver alcohol dehydrogenase are reported. The structure characterizations include clarifications of some ambiguities in the decalin literature.

ISSN:0008-4042    

DOI:10.1139/v87-400

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

The solution conformation of adriamycin was studied by1H nuclear magnetic resonance spectroscopy. The hydrochloride was found to be rigid on the nmr time scale in each of CDCl3, DMF-d7, and DMSO-d6. The sugar and D rings were found to have the same conformations as were found previously by X-ray crystallography. Similar results were found for the free base in DMF-d7and CDCl3solutions although the free base was found to be conformationally mobile in acetone-d6and CD3OD solutions.

ISSN:0008-4042    

DOI:10.1139/v87-401

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

The glycosaminoglycans are naturally occurring anionic polysaccharides whose polyelectrolyte properties and counterion interactions may underlie a number of biological activities. We have investigated the ability of the sulfated glycosaminoglycans to accelerate the reaction of Br(NH3)5Co2+with Hg2+. Chondroitin sulfates A, B, and C, heparin, and heparan sulfates all accelerate this reaction to nearly the same extent, but do so at different optimal concentrations, above which inhibition is observed. This phenomenon of rate acceleration, sometimes called "polyelectrolyte catalysis", has previously been explained by a two-phase model, in which the reactant counterions are thought to equilibrate rapidly between the polymer domain and the bulk phase. Rate effects would then be due to higher local concentrations in the neighbourhood of the polymer. Our results show that this is an oversimplified view for the glycosaminoglycans. Rather, they demonstrate similar rate accelerations independent of net structural charge density. It is argued that counterion condensation, as described by Manning, reduces the surface potential of all glycosaminoglycans to nearly constant values. Different optimal concentrations of the glycosaminoglycans may arise in part from the unreactivity of a subpopulation of bound ions.

ISSN:0008-4042    

DOI:10.1139/v87-402

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

The kinetics of cooxidation of several substituted phenyl methyl sulfoxides and oxalic acid with Cr(VI) have been carried out in the presence of perchloric acid. The reaction is first order each in sulfoxide, oxalic acid, Cr(VI), and H+. The products of oxidation are sulfones and carbon dioxide. Electron-releasing groups in the phenyl ring accelerate the rate while electron-withdrawing groups retard it. The Hammett correlation yields a reaction constant of −0.927 ± 0.08 (r = 0.994) at 313 K. Addition of aluminium nitrate prevents the occurrence of cooxidation. Based on the kinetic information a suitable mechanism has been proposed. Diaryl sulfoxides behave in an analogous manner in the cooxidation.

ISSN:0008-4042    

DOI:10.1139/v87-403

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

The association of methylviologen, 1,1′-dimethyl-4,4′-bipyridynium (PQ2+), with halide and pseudo-halide ions was studied in water at 25 °C by ultraviolet spectrophotometry. A charge transfer band could be observed in the region around 300 nm. From difference absorption measurements the first association constant (K1,) was obtained as 1.7 (Cl−), 3.3 (Br−), 3.9 (I−), 0.7 (CN−), and 2.0(SCN−) M−1. Similar results were obtained for the dication, 6,7-dihydrodipyrido[1,2-a:2′,1′-c]pyrazinium (DQ2+), with first association constants 3.3 (Cl−), 4.1 (Br−) and 4.6 (I−) M−1. First and second (K2) association constants were also determined by a potentiometric technique using ion selective electrodes:K1 = 2.0 M−1;K2 = 0.4 M−1for the system PQ2+–Cl−andK1 = 5.3 M−1;K2 = 0.3 M−1for DQ2+Br−.

ISSN:0008-4042    

DOI:10.1139/v87-404

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

A method for preparing alkyl derivatives of cage-structure compounds is proposed. It relies on the use of Grignard reactions with a high boiling point solvent. The reactions take place at atmospheric pressure. Methylation of adamantane, diamantane, and homoadamantane occurs with quantitative yield. With other primary alkyl groups, yields are better than 60%. Competition between alkylation and secondary reactions is discussed on the basis of a free radical mechanism.

ISSN:0008-4042    

DOI:10.1139/v87-405

出版商:NRC Research Press    

年代:1987

数据来源: NRC

摘要:

A proton, carbon-13, and vanadium-51 nuclear magnetic resonance study is reported on the number, stoichiometry, geometry, and relative stability of the complexes that form when vanadate(V) solutions are mixed with each one of the following organic α-hydroxyacids in the pH range ~2.5 – ~7: glycolic, lactic, chloro-3- and phenyl-3-lactic, mandelic, glyceric, and malic acids. The predominant complexes have 1:1 composition (almost certainly in a polymeric structure) in contrast with the 1:2 (metal:ligand) stoichiometry of the corresponding Mo(VI) and W(VI) complexes.

ISSN:0008-4042    

DOI:10.1139/v87-406

出版商:NRC Research Press    

年代:1987

数据来源: NRC