Introduction of a nitrogen heterocycle into sulphated chitosan oligomers

Chitin is a linear polysaccharide consisting of [-4) 2-acetamido-2-deoxy-p-Dglucopyranose (1- and -4) 2-amino-2-deoxy P-D-glucopyranose (l-]n which occurs widely in nature and which after further de-//-acetylation yields chitosan. Chitin, chitosan and many of their derivatives have found applications in industry, medicine, pharmacology and food technology.1 Many recent studies have reported modifications of chitosan in attempts to bestow upon it a variety of properties.2'6 This article reports the preparation of an unusual derivative of chitosan oligomers containing a nitrogen heterocycle which was obtained by depolymensation of chitosan in sulphuric acid followed by a number of sulphation steps to obtain persulphated chitosan fragments 1-3, treatment of the latter under strong basic conditions resulted in an unusual ring closure reaction to give an //-sulphonatoaziridine substituted product 4 (Scheme). The ring closure reaction involves internal nucleophilic attack at C-3 of the glucosamine ring by the nitrogen atom attached at C-2 to cause inversion of configuration at C-3 giving a D-allo configuration product with formation of an N-sulphonatoaziridine ring. The nitrogen at C-2 requires an iV-sulphoamino group for the reaction to proceed under these conditions.7 The aziridine group in 4 is characterised by signals at high field for A-2 and A-3 (Table) in both *H and 13C spectra and large UCH coupling constants at these positions.