Carbohydrate Polymers 86 (2011) 1451–1460 Contents lists available at ScienceDirect Carbohydrate Polymers jo u rn al hom epa ge: www.elsevier.com/locate/carbpol Synthesis of N,N,N-trimethyl chitosan homopolymer and highly substituted N-alkyl-N,N-dimethyl chitosan derivatives with the aid of di-tert-butyldimethylsilyl chitosan Berglind E. Benediktsdóttir a , Vivek S. Gaware a , Ögmundur V. Rúnarsson a,b , Sigríður Jónsdóttir c , Knud J. Jensen d , Már Másson a,∗ a Faculty of Pharmaceutical Sciences, School of Health Sciences, University of Iceland, Hofsvallagata 53, IS-107 Reykjavík, Iceland b Center for Analysis and Synthesis, Department of Chemistry, Lund University, P.O. Box 124, SE-22100 Lund, Sweden c Science Institute, University of Iceland, IS-107 Reykjavik, Iceland d Faculty of Life Sciences, Department of Basic Sciences and Environment, University of Copenhagen, Copenhagen, Denmark a r t i c l e i n f o Article history: Received 12 May 2011 Received in revised form 30 May 2011 Accepted 4 June 2011 Available online 13 June 2011 Keywords: Chitosan tert-Butyldimethylsilyl Deprotection Stepwise reductive alkylation Quaternary chitosan Trimethyl chitosan a b s t r a c t A highly chemoselective strategy for the synthesis of N,N,N-trimethyl chitosan (TMC) homopolymer and highly substituted N-alkyl-N,N-dimethyl chitosan derivatives was achieved using di-tert- butyldimethylsilyl-3,6-O-chitosan (di-TBDMS chitosan) as a precursor. The influence of different solvents, reagents and other reaction conditions on the reduction, trimethylation and quaternization of these di- TBDMS chitosan derivatives was studied. Products were characterized by NMR after each step. Di-TBDMS chitosan was reacted with methyl iodide in NMP, giving a 100% substituted TMC with the trimethyl group appearing at 3.35 ppm in 1 H NMR spectrum. N-Propyl-, N-butyl- and N-hexyl-N,N-dimethyl chitosan derivatives were synthesized by stepwise reductive alkylation of di-TBDMS chitosan, followed by quat- ernization with dimethyl sulfate in dichloromethane, giving 65–72% substituted N-alkyl-N,N-dimethyl chitosan derivatives under optimized conditions. Analysis of these water-soluble chitosan derivatives by FT-IR, 1 H NMR, 13 C NMR, 1 H– 1 H COSY and 1 H– 13 C HSQC NMR enabled detailed structural characteriza- tion. All peaks could be assigned to N-modification, showing the selectivity of the di-TBDMS protection. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Chitosan is a natural -(1 → 4) linked glucosamine polymer, usually derived from chitin by deacetylation. Chitosan has many interesting properties which are useful in biological applications, including antimicrobial activity (Kong, Chen, Xing, & Park, 2010), and it has been studied as gene delivery agent (de la Fuente, Csaba, Garcia-Fuentes, & Alonso, 2008) and permeation enhancer for macromolecular drugs (Mourya & Inamdar, 2009). Furthermore, it is biodegradable and considered to be safe in non-parental dosage formulations (Baldrick, 2010). However, one of the main disad- vantages of chitosan for many potential applications is its poor aqueous solubility at physiological pH. Therefore, synthetic strate- gies have been developed to modify the polymer backbone, in order to enhance aqueous solubility and biological activity (Alves & Mano, 2008). This can be achieved for example by trimethylating the free amino group (Runarsson, Holappa, Jonsdottir, Steinsson, & Masson, ∗ Corresponding author. Tel.: +354 525 4463; fax: +354 525 4071. E-mail address: mmasson@hi.is (M. Másson). 2008), producing a derivative with permanent positive charge that is highly soluble independent of pH. N,N,N-Trimethyl chitosan (TMC) is usually synthesized by dis- persing chitosan in NMP with methyl iodide (CH 3 I) in the presence of NaOH, either directly (Domard, Rinaudo, & Terrassin, 1986; Sieval et al., 1998) or by first forming N,N-dimethyl chitosan by reductive alkylation, followed by additional methylation using CH 3 I (Muzzarelli & Tanfani, 1985). These reactions are not selective and produce a heterogenous mixture of N-monomethyl-, N,N- dimethyl-, N,N,N-trimethyl- and O-methyl chitosan (Domard et al., 1986; Ledung, Milas, Rinaudo, & Desbrieres, 1994; Muzzarelli & Tanfani, 1985; Sieval et al., 1998). Although modifications of this reaction have given higher degrees of trimethylation without O- methylation (Runarsson, Holappa, et al., 2008; Verheul et al., 2008), the resulting material is only partially trimethylated. Therefore, there is still a need for a selective approach to produce fully homogenous TMC materials. Protecting groups have been used in chitosan chemistry to enable selective N-modifications and increase their solubility in organic solvents (Kurita, 2006). Although the most common strat- egy is to use a triphenylmethyl (trityl) group for O-protection 0144-8617/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbpol.2011.06.007