Temperature effects upon aqueous micellar-assisted decarboxylation of 6-nitrobenzisoxazole-3-carboxylate and its 5-methyl derivative.
An investigation of temperature effects upon first-order rate constants in the micellar pseudophase for decarboxylation of 6-nitrobenzisoxazole-3-carboxylate (6-NBIC) and its 5-methyl derivative (6-NBIC-5-Me) was carried out. Surfactants used were cationic cetyltrialkylammonium bromide with alkyl = methyl (CTABr), ethyl (CTEABr), n-propyl (CTPABr), and n-butyl (CTBABr). The investigation shows that micelles speed up reactions by decreasing enthalpies of activation. Increase in head group bulk further speeds reactions still by a small decrease in the enthalpies, for both substrates. Values of DeltaH# and DeltaS# for 6-NBIC in the various surfactants give linear isokinetic plot, with CTBABr as outlier.
Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion in surfactant self assemblies
Decarboxylation of the 6-nitrobenzisoxazole-3-carboxylate ion is speeded more by small assemblies of didodecyl(dimethyl)ammonium chloride (DDDACl) than by fully formed assemblies or by normal cationic micelles. The first-order rate constants, k′M, of reaction of fully micelle-bound substrate increase with decreasing surface charge density of normal cationic micelles with a change from hydrophilic to less hydrophilic counter-ions, e.g., in going from CTAOH to CTAOTos (CTA = C16H33NMe3 and Tos = toluene-p-suIphonate), or from cationic to zwitterionic micelles. These changes are ascribed to changes in transfer free energies of the initial-state carboxylate ion and the charge-delocalized transition state so that small assemblies of cationic amphiphiles, e.g., of DDDA or (C8H17)3N+R, are better catalysts than cationic micelles because of less initial-state stabilization. A similar explanation can be applied to catalysis of decarboxylation by synthetic cationic vesicles.
Synthesis of 2,4-dideoxy-.beta.-D-erythro-hexopyranosyl nucleosides
The synthesis of 1-(2,4-dideoxy-β-D-erythro-hexopyranosyl)thymine (12) 1 was accomplished using two different synthetic routes. It was obtained starting either from 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose or from tri-O-acetyl-D-glucal. The other modified nucleosides, with either a cytosine, guanine, or adenine moiety, were synthesized using the second reaction scheme. Deoxygenation reactions were accomplished with the (2,4-dichlorophenoxy)thiocarbonyl derivatives generated in situ