Distribution of substituents in O-(2-hydroxypropyl) derivatives of cyclomalto-oligosaccharides (cyclodextrins: influence of increasing substitution of the base used in the preparation, and of macrocyclic size.

Samples of O-(2-hydroxypropyl)derivatives of cyclomaltoheptaose (beta-cyclodextrin) with increasing substitution were prepared by withdrawing aliquots at different times from a reaction mixture containing cyclomaltoheptaose and an excess of (S)-propylene oxide in 0.39 M aqueous sodium hydroxide. The distributions of substituents between the different molecules and between the different alpha-D-glucopyranosyl residues in these samples were determined by mass spectrometry and methylation analysis, respectively. The solubilities of the samples and their association constants with phenolphthalein were also determined. The relative reactivities at O-2 and O-3 versus O-6, calculated using Spurlin's equations, decreased with increasing degree of substitution, probably because of steric hindrance. No significant differences were observed when different strong bases were used as promoters of the hydroxypropylation of cyclomaltoheptaose. The hydroxypropylation of cyclomaltohexaose (alpha-cyclodextrin) and cyclomalto-octaose (gamma-cyclodextrin) was briefly investigated.