Tosylation of the secondary side of S-cyclodextrin, which has previously been incorrectly reported, can be achieved by tosyl transfer from 3-nitrophenyl toluenesulfonate to the C-2 hydroxyl of the cyclodextrin.
Cyclodextrins are toroidal cyclic oligosaccharides with the secondary hydroxyls of glucose C-2 and C-3 on their more open face and the primary C-6 hydroxyl on the other face. Their ability to bind good-sised organic molecules in the central cavity has provided a basis for the construction of models for receptor binding and for enzymatic catalysis.
2 Selective sulfonation of primary hydroxyl groups of a-cyclodextrin (cyclohexaamylose) and of g-cyclodextrin (cycloheptaamylose) have permitted the attachment of functional groups for modified binding' or catalysis. 4 S t elective functionalieation of the secondary side has normally been possible only by acyl transfer reactions of bound substrates. 5 For this reason, the reports6 that a-and B-cyclodextrin could be selectively sulfonated on the secondary side with tosyl chloride in aqueous medium have excited considerable attention. We have examined the reported process carefully, and unfortunately we find7 that the reported "secondary" monotosylate of B-cyclodextrin ( 2) is identical in all respects with the known primary 6-cyclodextrin-6-tosylate (l_). However, we have devised an authentic procedure for the preparation of the secondary g-cyclodextrin-2-tosylate (2). The properties of this material are very different from those of l_, or reported6 for "g'.
Tosylate 3 was prepared by reaction of 12 g 6-cyclodextrin with one equiv. of 3-nitrophenyl tosylate (3) in 120 ml DMF by adding 72 ml of 0.2 y carbonate buffer (pH 9.9) and stirring the reaction mixture at 60' C for 1 hour. Then the mixture was neutralised with 1 N HCl, and one liter of acetone was added to precipitate cyclodextrin derivatives other than 2. The filtrate was concentrated almost to dryness; addition of one liter of acetone produced solid 3 in ca. 10% yield. It was further purified by Sephadex chromatography, to furnish the pure pentahydrate.
Found (Calc'd. for C49H76037s