Introduction
Cyclodextrins (CD) are doughnut-shaped, macrocyclic oligosacharides constructed of glucose units linked by α- (1→4) bonds. The compound with seven glucose units is known as β-cyclodextrin (β-CD)[
1]. Hidroxypropyl-β-cyclodextrin is a derivative was several of the HO
- groups are substituted by hydroxypropyl groups and is more soluble in water than β-CD. Organic compounds included in CD form complexes with different properties from those of the free substrate [
2]. Meassuring the changes occurring in some of these properties, it is possible to determine the association constant, K
as.
The mechanism of hydrolysis of monoamides and monoesters of phthalic acid [
3] maleic acid [
4] is known to involve intramolecular catalysis by the neighboring carboxyl group. The formation of an inclusion complex with cyclodextrins may affect the efficiency of the catalysis by favoring or hindering the interaction of the hydroxyl group with the reactive center. This effect should result in modification of the hydrolysis rate. We report here results regarding the effect of HPCD on the hydrolysis of compounds
1-
3 Experimental
The substrates 1-3 were prepared by reaction of phthalic or maleic anhydride (sublimated) with aniline, 4-nitro-aniline, adamanthylamine and phenol. All products were purified and characterized by MS and 1H and 13 C NMR.
The rate constants were determined by measuring the absorption changes with time at 380 nm for 1a, 276 nm for 1b and 2b, 225 nm for 1c, and 300 nm for 3b. The reaction conditions were: pH 2, 40°C for 1a,b,c 2b and 25°C for 3b, ionic strength 0.5 M using NaCl as compensating electrolyte and water as solvent with 4% of dioxane (for 1) or acetonitrile (for 1b,c 2b and 3b).
Results and Discussion
The addition of HPCD decreased the hydrolysis rate of all the substrates. The Figure representative plot of the observed rateconstant as a function of the HPCD concentration (1a (triangules), 1b (squares) and 2b (cicles))
These results are interpreted in terms of the formation of 1:1 complexes as shown in the following scheme: