2,5-Anhydro-d-Mannose End-Functionalized Chitin Oligomers Activated by Dioxyamines or Dihydrazides as Precursors of Diblock Oligosaccharides.

Diblock oligosaccharides based on renewable resources allow for a range of new but, so far, little explored biomaterials. Coupling of blocks through their reducing ends ensures retention of many of their intrinsic properties that otherwise are perturbed in classical lateral modifications. Chitin is an abundant, biodegradable, bioactive, and self-assembling polysaccharide. However, most coupling protocols relevant for chitin blocks have shortcomings. Here we exploit the highly reactive 2,5-anhydro-d-mannose residue at the reducing end of chitin oligomers obtained by nitrous acid depolymerization. Subsequent activation by dihydrazides or dioxyamines provides precursors for chitin-based diblock oligosaccharides. These reactions are much faster than for other carbohydrates, and only acyclic imines (hydrazones or oximes) are formed (no cyclic N-glycosides). α-Picoline borane and cyanoborohydride are effective reductants of imines, but in contrast to most other carbohydrates, they are not selective for the imines in the present case. This could be circumvented by a simple two-step procedure. Attachment of a second block to hydrazide- or aminooxy-functionalized chitin oligomers turned out to be even faster than the attachment of the first block. The study provides simple protocols for the preparation of chitin-b-chitin and chitin-b-dextran diblock oligosaccharides without involving protection/deprotection strategies.