Neighboring general acid participation in thiolester aminolysis in an aprotic solvent

The kinetics of n-butylaminolysis of acyl-substituted phenyl tiolbenzoates in acetonitrile at 25°C was studied. From the second order rate constants a Hammett rho value of 1.45 was obtained. The results indicate a proinounced neighboring group effect in the reaction of phenyl thiolsalicylate, providing the first kinetic evidence for intramolecular general acid participation by a hydroxy function in an aprotic solvent.

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- - 25 °C. , 1,45. -, .

     

Crown ether salt catalysis of ester aminolysis in an aprotic solvent

The RCO ₂ ^– catalyzed n-butylaminolysis of 4-nitrophenyl acetate in chlorobenzene has been studied. Logarithms of the catalytic rate constants can be linearly correlated with Taft polar substituent constants for the R groups. Product analysis as well as the lack of steric hindrance by bulky R substituents indicate that the carboxylates act as general base catalysts.

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- 4- . R. , R , .

     

Neighboring group participation by sulfonamido nitrogen

The ability of sulfonamido nitrogen to enter into neighboring group participation was established in two different reaction settings. The first was uncovered during the bromination of benzodiazocine 8 in dichloromethane at 0 degrees C, and the second during the base treatment of 15a en route to allene 18.     

Neighboring group participation by the pyrrole nucleus

Using proton and carbon-13 NMR spectroscopy, the transformation of 2-hydroxyethylpyrroles (5,15) into 2-haloethylpyrroles by treatment with thionyl chloride or carbon tetrabromide/triphenylphosphine is shown to proceed by randomization of the two side-chain carbon atoms. A spirocyclopropylpyrrolium ion (19) is postulated as an intermediate in this unexpected process.     

Neighboring group participation in cyclodehydration. A regiospecific isoxazole synthesis

The regiospecific synthesis of isoxazoles 2, 11, 14 , and 15 is explained in terms of neighboring group participation of the ortho nitrogen of the heterocyclic ring in the cyclodehydration step.     

Amic acid complexes with aprotic polar solvents V. Complexes with amide solvents and isomerism of trimellite-dianilic acid

Complexes of trimellite-dianilic acid (TMA) with aprotic amide solvents have been sythetized, separated and investigated by TG and MTA methods. It was shown that the composition of these complexes is [TMA]₁ · [Sol]₁. The PMR spectra were used to show the ability of TMA to undergo isomeric transitions under mild conditions in an aprotic solvent.     

Neighboring group participation of 9-anthracenylmethyl group in glycosylation: preparation of unusual C-glycosides

[reaction: see text] A coupling of 2-O-arylmethylated D-glucose-derived thioglycosides with various alcohols in the presence of DMTST as an activator is described. The requisite glycosyl donors are efficiently prepared by one-pot procedures. When the aryl groups are phenyl, p-methoxyphenyl, 1-naphthyl, and 2-naphthyl groups, a mixture of alpha- and beta-anomeric O-glycosides is obtained under the conditions, whereas when the aryl group is the 9-anthracenyl group, a highly stereoselective formation of the unusual C-glycosides in good yields via neighboring group participation of the 9-anthracenylmethyl group followed by coupling with a variety of alcohols is observed. Three new chiral centers including a quaternary carbon are created in one single step.     

Neighboring pyrrolidine amide participation in thioether oxidation. Methionine as a "hopping" site

Methionine residues have been shown to function as efficient "hopping" sites in long-range electron transfer in model polyprolyl peptides. We suggest that a key to this ability of methionine is stabilization of the transient sulfur radical cation by neighboring proline amide participation. That is, in a model system a neighboring pyrrolidine amide lowers the oxidation potential of the thioether by over 0.5 V by formation of a two-center three-electron SO bond.     

Cellulose solutions in dipolar aprotic solvents

Preparation conditions and properties of cellulose solutions in DMAA containing 7–8% lithium chloride are considered. Investigation of the optical anisotropy and structure of cellulose solutions in this solvent confirms that the LS state is attained with an increase in the concentration of cellulose. On the basis of these cellulose solutions, fibers not ranking below viscose fibers in mechanical properties are prepared. The use of a mixed solution of 95% cellulose and 5% poly(amidobenzimidazole) in DMAA containing 7% lithium chloride makes it possible to prepare films and fibers whose strength is more than two times greater than the strength of cellulose hydrate fibers. The deformation-driven orientation and supramolecular structure of fibers and films are studied by spectroscopy and small-angle scattering of polarized light.     

Mechanism for acetic acid-catalyzed ester aminolysis

A computational study is described to elucidate the mechanism for acetic acid-catalyzed ester aminolysis to produce amides. A concerted acyl substitution mechanism is proposed to involve concurrent acyl-O bond cleavage and acyl-N bond formation where acetic acid acts as a bifunctional catalyst connecting to both the alkoxide and the amino moieties. This mechanism does not involve the intermediacy of a tetraheral adduct nor the rehybridization of acyl carbon, in sharp contrast to classic stepwise acyl substitution mechanism.