Conjugates of polyhedral boron compounds with carbohydrates. 4. hydrolytic stability of carborane–lactose conjugates depends on the structure of a spacer between the carborane cage and sugar moiety

Abstract A novel 1,2‐dicarba‐ closo ‐dodecaborane–lactose conjugate ( 4a ) with an N ‐glycosidic linkage was synthesized. This conjugate was found to be much more stable against hydrolytic deboronation ( closo to nido tranformation of the carborane cage) under neutral conditions than a related carborane–lactose conjugate ( 1a ) with an O ‐glycosidic linkage. This result demonstrates that the hydrolytic stability of carborane–carbohydrate conjugates in neutral aqueous solutions may depend dramatically on the chemical nature of the spacer that links the carbohydrate moiety with the boron cage, the rate of hydrolysis varying by orders of magnitude. We relate a significant decrease in the deboronation rate to the formation of more strongly bound supramolecular aggregates, in which the boron cage is less accessible to nucleophilic attack by solvent molecules, in the solution of the carborane– N ‐lactoside conjugate 4a . Copyright © 2006 John Wiley & Sons, Ltd.