Concepts for improved regioselective placement of O-sulfo, N-sulfo, N-acetyl, and N-carboxymethyl groups in chitosan derivatives

In the present paper a new strategy has been studied to introduce solely or in combination N-sulfo, O-sulfo, N-acetyl, and N-carboxymethyl groups into chitosan with highest possible regioselectivity and completeness and defin ed distribution along the polymer chain. The aim was to generate compounds having lowest toxicity for determining the pharmacological structure-functi on relationships among different backbone structures and differently arrang ed functional groups compared to those of heparin and heparan sulfate. The water-soluble starting material, chitosan, with a degree of acetylation (DA ) of 0.14 and a molecular weight of 29 kD, allows one to apply most of the known reactions of chitosan as well as some reactions of heparin chemistry successfully and with improved regioselectivity and completeness. On the ot her hand, a number of these reactions were not successful by application to water-soluble high-molecular-weight chitosan (DA 0.45 and 150 kD). The sta rting material showed statistical N-acetyl (N-Ac) distribution along the po lymer chain according to the rules of Bernoulli, with highest abundance of the GlcNAc-GlcNAc diad along with a lower abundance of triads, tetrads, and pentads. The space between the N-Ac groups was filled up in homogeneous re actions by N-sulfo and/or N-carboxymethyl groups, which also resulted in a Bernoulli statistical distribution. The N-substitution reaction showed high est regioselectivity and completeness with up to three combined different f unctional groups. The regioselectivity of the 3-O-sulfo groups was improved by regioselective 6-desulfation of nearly completely sulfated 3,6-di-O-sul fochitosan. By means of desulfation reactions, all of the possible intermed iate sulfated products are possible. 6-O-Sulfo groups can also be introduce d with highest regioselectivity and completeness, and a number of partially 6-desulfated products are possible. (C) 2001 Elsevier Science Ltd. All rig hts reserved.