摘要:

Cyclopropanegem-dicarboxylates are prepared through cyclocondensation of chloro or bromomalonate carbanion and electrophilic alkenes. The carbanion can be formed in THF by deprotonation of the monohalomalonates with powdered potassium carbonate; the chloromalonate always leads to better yields. The influence of the nature and the number of substituents on the alkene reactivity has been investigated. If side reactions (for instance protonation of anionic intermediates by halomalonate) can take place, better yields are obtained when the carbanion is generated by electroreduction of dihalomalonate in the presence of the Michael acceptor. Key words: halomalonates, cyclopropanegem-dicarboxylates, electroreduction.

ISSN:0008-4042    

DOI:10.1139/v91-113

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Two novel diterpenoids, chromodorolides A (1) and B (2), have been isolated from skin extracts of the tropical dorid nudibranchChromodoriscavae. Complete NMR assignments are presented for chromodorolide A (1). The structure of chromodorolide B (2) has been solved by spectroscopic analysis. Chromodorolides A (1) and B (2) are the first examples of diterpenoids with the highly rearranged chromodorane carbon skeleton.Keywords: nudibranch, diterpene, chromodorane,Chromodoris.

ISSN:0008-4042    

DOI:10.1139/v91-114

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The antifungal antibiotic pisolithin B (p-hydroxymandelic acid, 2-(4′-hydroxyphenyl)-2-hydroxyethanoic acid,1a) was shown to have the absolute (R) configuration. The stereochemistry was established via comparison of its optical rotation to that of its synthetic (R) and (S) enantiomers. The synthetic samples were prepared by the stereospecific reduction of the prochiral α-keto acid,p-hydroxybenzoylformic acid (2-(4′-hydroxyphenyl)-2-oxoethanoic acid,2a), with (R) or (S)-2,2′-dihydroxy-1,1′-binaphthyl lithium aluminum hydride (BINAL-H). The absolute configuration and enantiomeric purity of both products were determined using the1H NMR of their isobutyl esters in the presence of the chiral solvating agent (R)-(−)-2,2,2-trifluoro-1-(9-anthryl)ethanol.Keywords: pisolithin B,p-hydroxymandelic acid, antifungal, absolute configuration.

ISSN:0008-4042    

DOI:10.1139/v91-115

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

A general procedure for the synthesis of diverse cyclopentanols via free-radical cyclization of unsaturated 1,3-oxathiolanes and 1,3-oxathiolan-5-ones is described. The scope of these reactions, including preliminary rate studies that indicate that the 1,3-oxathiolan-5-ones cyclize at a useful rate (1.4 × 106 M−1s−1at 80°C), has been examined. This permits the rapid assembly, by intramolecular annulation, of various additional ring systems, including bicyclo[3.3.0]octanols, bicyclo[4.3.0]nonanols, and bicyclo[3.2.1]octanols for use in total synthesis.Keywords: radical cyclization, oxathiolanes, oxathiolanones.

ISSN:0008-4042    

DOI:10.1139/v91-116

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

A-Nor-5α-cholestan-2-one (1) was oxidized with BSA under various conditions. The stoichiometric reaction afforded diketones (3and4) and α-phenylseleno-ketone (2) while catalytic oxidation with the BSA/PhIO2/p-TsOH system resulted in lactones (7aand7b) and anhydrides (5and6). The mechanism of these reactions is discussed.Keywords: benzeneseleninic anhydride,A-nor-5α-cholestan-2-one, catalytic oxidation.

ISSN:0008-4042    

DOI:10.1139/v91-117

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Cross-homo- and cross-heterotactic enthalpic coefficients,respectively, were determined at 25 °C, measuring the enthalpies of dilution of ternary aqueous solutions containing two different α-amino acids of the same or different chirality. Differences of about 200–300 J mol−2kg between cross-homo- and cross-heterotactic coefficients were found, well beyond the experimental uncertainty. The role of the zwitterionic interaction, already proposed to explain the nature of chiral recognition, was strengthened.Keywords: α-amino acids, excess enthalpy, chiral recognition.

ISSN:0008-4042    

DOI:10.1139/v91-118

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The rates of methyl transfer between benzohydroxamates and sulfonates show large βnucvalues (ca. 0.8) indicating much charge transfer to the C atom, similar to the results ofN,N-dialkylaminofluorene anions. A small α effect shows that even in methyl substrates, with certain leaving groups, the effect is displayed. Investigation of the β1gshows that from the leaving group side of the reaction it is a normal SN2 reaction. Correlation of the reactivity with computed gas phase ionization potentials shows that desolvation of anions is probably not important. These results suggest an α-effect model that explains the larger size of the effect at carbonyl groups in terms of greater solvation of the charge transferred to the O atom.Keywords: α effect, Brønsted plot, ionization potentials, cyclic voltammetry, β1g, βnuc, AMl,

ISSN:0008-4042    

DOI:10.1139/v91-119

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

FiveN-(2-chloroethyl)phosphorotriamidates, (RNH)2P(O)NHCH2CH2Cl, have been synthesized, and their behavior under strongly basic conditions was studied. ForN-alkyl derivatives (R = Me, PhCH2), base-promoted intramolecular displacement of chloride yielded theN-phosphorylated aziridine, (RNH)2P(O)NC2H4, as the exclusive cyclization product. ForN-aryl derivatives (R = Ar), both the aziridine and the 1,3,2-diazaphospholidine,, products could be obtained in comparable yields. TheN-aromatic cyclic products are mutually interconvertible: 1,3,2-diazaphospholidines rearrange to the corresponding aziridines upon treatment with base, while bromide ions catalyze the reverse isomerization.Keywords: phosphoramidates acidity,N-phosphorylated aziridines – 1,3,2-diazaphospholidines isomerization, 1,3 vs. 1,5 cyclization.

ISSN:0008-4042    

DOI:10.1139/v91-120

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Starting from chloride1, a series ofpara-substituted aryl 2-acetamido-2-deoxy-β-D-glucopyranosides were prepared using phase transfer catalysis conditions with tetrabutylammonium hydrogen sulfate in 1 M sodium hydroxide and methylene chloride at room temperature. Zemplén de-O-acetylation afforded the unprotected glycosides. Optimization of reaction conditions was evaluated. Several functional group manipulations were effected to widen the number and nature of thepara-substituents.Keywords: phase transfer catalysis, aryl 2-acetamido-2-deoxy-β-D-glucopyranosides.

ISSN:0008-4042    

DOI:10.1139/v91-121

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Information concerning the solubilization of hydrocarbons in ionic surfactant micelles was obtained from2H NMR relaxation,1H NMR chemical shifts, and1H NMR paramagnetic relaxation measurements. The rotational motion of deuterated hydrocarbons, which is related to the micellar microviscosity at the location of the hydrocarbons, was probed by2H NMR relaxation. The relaxation data are interpreted using both the two-step and the single-step models, and the results are discussed in terms of the micellar microviscosity and the location of the hydrocarbons in micelles. The location of the hydrocarbons in micelles was further investigated by determining the aromatic ring current-induced1H chemical shifts along the surfactant alkyl chain and by comparing the1H spin-lattice relaxation enhancement of the hydrocarbons and the surfactant alkyl chain, induced by Mn2+on the micellar surface. The hydrocarbons used include benzene, naphthalene, acenaphthalene, triphenylene, cyclohexane, cyclododecane, andtert-butylcyclohexane and the surfactants studied are hexadecyl-, tetradecyl-, and dodecyltrimethylammonium bromide; hexadecyl-, tetradecyl-, and dodecylpyridinium halide; and sodium dodecyl sulfate. The results indicate that the micellar microviscosity at the location of saturated hydrocarbons is approximately 5 cP for both the cationic and anionic micelles, whereas the micellar microviscosity at the location of unsaturated hydrocarbons is much higher. The unsaturated hydrocarbons are found to reside primarily near the surfactant headgroup in the cationic micelles, but are distributed evenly throughout the anionic SDS micelles. The saturated hydrocarbons appear to be located in the interior of the micelles. Key words: NMR, relaxation, solubilization, surfactant, micelle.

ISSN:0008-4042    

DOI:10.1139/v91-122

出版商:NRC Research Press    

年代:1991

数据来源: NRC