摘要:

Reactions of 3,4-dihydro-2H-pyran-2-carboxaldehyde (acrolein dimer) with ethylmetallics based on lithium, titanium, and copper were examined for stereoselectivity. High selectivity (92:8) was found for non-chelation controlled addition yielding theerythroproduct when ethyllithium was employed in the presence of BF3•OEt2. High selectivity (89:11) for products of chelation controlled addition (threo) was observed for ethylcopper reagents in the presence of magnesium salts. Titanium reagents yielded mixtures rich in products of either chelation or non-chelation controlled addition depending on the titanium substituents. In no cases were stereoselectivities obtained with titanium reagents better than those obtained by use of the aforementioned lithium and copper reagents.

ISSN:0008-4042    

DOI:10.1139/v88-035

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

Photolyses of 5-hydroxy-5α- and 5β-cholestan-6-one and their 3β-acetoxy- and 3β-benzyloxy derivatives in benzene or ethanol proceed stereospecifically with retention of configuration at C-5 to give the corresponding lactones, 6-oxa-B-homocholestan-7-ones. Photolyses of 3β-acetoxy-7α-deutero-5-hydroxy-5α- and 5β-cholestan-6-ones also proceed stereospecifically to give the corresponding 5-deutero lactones. 3β-Acetoxy-5-deuteroxy-5α- and 5β-cholestan-6-one give on irradiation 1:3 and 7:1 mixtures, respectively, of the corresponding 7aα- and 7aβ-deutero lactones. Irradiation of 3β-acetoxy-5-methoxy-5α-cholestan-6-one in ethanol leads to the stereoselective formation of ethyl 3β-acetoxy-5-methoxy-5,6-seco-5α-cholestan-6-oate, while that of 3β-acetoxy-5α-cholestan-6-one gives mainly photoreduction products. These observations are interpreted in terms of α-cleavage of the C-5—C-6 bond of the ketols to give alkyl acyl diradicals that undergo hydrogen transfer to give hydroxy ketenes, which then form the lactones. It is proposed that retention of configuration at C-5 results from two major factors—the nonplanar geometry of the hydroxyalkyl radical center, and fast hydrogen transfer in the diradical, the latter resulting from restricted rotation about the C-9—C-10 bond. The specific transfer of the 7α-deuterium atom in the 7α-deutero ketols is attributed to these factors and to the preferred direction of opening to the diradical on α-cleavage. TheO-deuterium labelling results are interpreted in terms of product development control in the conversion of the hydroxy ketenes to the lactones and are in accord with restricted rotation about the C-9—C-10 bond. The photolysis of 3β-acetoxy-5-amino-5α-cholestan-6-one proceeds stereoselectively to give the 5β-lactam analogue of the 5β-lactone formed from the analogous 5α-ketol.

ISSN:0008-4042    

DOI:10.1139/v88-036

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

A synthesis of maytenoquinone (11) from natural podocarpic acid (1) is described.

ISSN:0008-4042    

DOI:10.1139/v88-037

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

In an acidic solution, containing initially chlorous acid and bromate in excess, two consecutive reactions are observed, the oxidation of chlorous acid to chlorine dioxide followed by the oxidation of chlorine dioxide: BrO3− + 4ClO2 + 2H2O = 4ClO3− + 3H+ + HOBr. The kinetics of this reaction has been studied and a mechanism proposed. It agrees with the one accepted for the related reaction between bromate and Ce(III). Extended by reactions of chlorous acid, the mechanism reproduces the experimental curves of chlorine dioxide evolution in chlorous acid – bromate solutions. The values of the equilibrium and kinetic constantes are discussed.

ISSN:0008-4042    

DOI:10.1139/v88-038

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

The formation of fluoride and chloride complexes in water – hydrogen fluoride mixtures has been studied using polarography. The reduction of the complexation by the fluoride ions from CdF3−down to CdF+is in good agreement with the reduction of the fluoride ion activity,R0(F) with increasing HF concentration. At higher HF concentration than 60%, the half-wave potential for the reduction of cadmium is shifted in a pattern that cannot be explained by fluoride in complexation. The formation of CdCl3−and CdCl42−has been observed in H2O–HF–KCl mixtures when the HF concentration was increased to 60%.

ISSN:0008-4042    

DOI:10.1139/v88-039

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

Density and electrical conductance measurements of 0.35[XNaNO3 + (1 − X)KNO3] + 0.65Na2S2O3•5H2O melt were made as functions of temperature andX. Molar volume,V, is found to be additive. The percent deviation ofVext(extrapolatedVof the pure solute from the plot ofVvs. total added alkali ion fraction) fromVcal(calculatedVof the pure solute from its high temperature density data) increases monotonically as the amount of NaNO3in the hydrate melt increases, thereby manifesting a "structure-forming" tendency of NaNO3. The non-Arrhenius temperature dependence of molar conductance, Λ is analyzed in terms of the Vogel–Tammann–Fulcher (VTF) equation. Mixed alkali effect (MAE) has been observed on Λ andT0(ideal glass transition temperature). A competitive polarization model has been used to explain the MAE on Λ.

ISSN:0008-4042    

DOI:10.1139/v88-040

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

The stereochemistry of3a, one of the 2-acetyl-3,5,6-trihydroxy-5,6-dimethylcyclohexenones, obtained by rearrangement of 2,3-diacetyl-4-hydroxy-4-methylcyclopentenone,2a, under basic conditions, was determined by an X-ray crystal structure analysis. An ORTEP plot shows the configuration of (5RS,6RS)-2-acetyl-3,5,6-trihydroxy-5,6-dimethylcyclohexenone and the presence of a conjugated chelated system involving the H-bonding between O(3)… H(31) and H(31)… O(2).Crystals of3aare triclinic,a = 10.979(4),b = 7.766(3),c = 6.382(3) Å, α = 86.23(2), β = 72.86(1), γ = 88.23(2)°,Z = 2, space group. The structure was solved by direct methods and was refined by full-matrix least-squares procedures toR = 0.036 andRw = 0.039 for 1324 reflections withI > 3σ(I).The structure of3aconsists of centrosymmetric dimers which contain a nearly planar bicyclic system of a cyclohexenone moiety and a chelated system (Scheme 2).The pathway of the reaction allows one to put forward some hypothesis on the stereochemistry of some analogues of the cyclohexenones3aand3b.Faisant appel à la diffraction des rayons-X, on a déterminé la stéréochimie du composé3a, une des acétyl-2 trihydroxy-3,5,6 diméthyl-5,6 cyclohexénones obtenues par une transposition de la diacétyl-2,3 hydroxy-4 méthyl-4 cyclopenténone,2a, en milieu alcalin. Une courbe ORTEP démontre que la configuration est (5RS,6RS) pour l'acétyl-2 trihydroxy-3,5,6 diméthyl-5,6 cyclohexénone et qu'il existe un système de chélation conjugué impliquant des liaisons hydrogènes entre O(3)… H(31) et H(31)… O(2).

ISSN:0008-4042    

DOI:10.1139/v88-041

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

Interaction of Pd(II), Pt(II), Ag(I), and Au(III) with 4,6-diamino-2-thiopyrimidine (DATP) and 4,6-diamino-2-methyl-thiopyrimidine (DAMTP) in aqueous medium resulted in the formation of new complexes which have been characterized by elemental analysis, ir,1H-nmr spectroscopic techniques, magnetic susceptibility measurements in the solid state, and conductivity measurements in DMF solution. The ir and1H-nmr spectra were consistent with coordination involving the non-protonated ring nitrogen atom, the exocyclic sulphur atom, orN,S-chelation.

ISSN:0008-4042    

DOI:10.1139/v88-042

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

The catalytic conversion of cyclohexene was studied on steam-activated carbon using microcatalytic technique. Three sets of experiments were carried out to investigate the conversion of cyclohexene, depending on reaction temperature. The first set was conducted at a constant temperature while the other two sets were performed either by increasing the temperature from 300 to 400 °C or by decreasing it from 400 to 300 °C. In all cases, the activity of carbon in cyclohexene conversion was influenced by the nature of condensation products formed on the surface during conversion.

ISSN:0008-4042    

DOI:10.1139/v88-043

出版商:NRC Research Press    

年代:1988

数据来源: NRC

摘要:

The anodic oxidation of UO2has been studied in aqueous phosphate solutions over the pH range 4 to 11, using a combination of electrochemical and X-ray photoelectron spectroscopic techniques. The early stages of oxidation, leading to the formation of a film of composition UO2.33, are unaffected by the presence of phosphate in the solutions. Phosphate concentrationsprevent the formation of higher-oxide films that are present in phosphate-free solutions at higher oxidation potentials. Dissolution under steady-state conditions proceeds via a surface film of uranyl phosphate. For potentials(vs. SCE), the rate of dissolution to yieldis controlled by charge-transfer kinetics. For potentials, the rate-determining step is the chemical dissolution of the uranyl phosphate layer.

ISSN:0008-4042    

DOI:10.1139/v88-044

出版商:NRC Research Press    

年代:1988

数据来源: NRC