Syntheses and Comparison of 2,6-Di-O-methyl Celluloses from Natural and Synthetic Celluloses

Debenzylation of Benzyl-Protected Methylcellulose

Methyl cellulose and its derivatives are widely used in the food industry, cosmetics, and as construction materials. The properties of methyl celluloses (MC) strongly depend on their degrees and positions of substitution. In order to generate MCs with uncommon blocky substitution, we apply fully protected O-benzyl-O-methyl celluloses (BnMC). Such complex polysaccharide derivatives could not be deprotected completely and without shift of the composition by methods usually applied to mono- and oligosaccharides. Therefore, a facile debenzylation method was developed based on photo-initiated free-radical bromination in the presence of hydrobromic acid scavengers followed by alkaline treatment. The reaction proceeds under homogeneous conditions and without the aid of any catalyst. There is no need for expensive equipment, materials, anhydrous reagents, or running the reaction under anhydrous conditions. Reaction parameters were investigated and optimized for successful debenzylation of completely protected BnMC with degrees of methyl substitution (DSMe) around 1.9 (and DSBn around 1.1). Side-product-free and almost complete debenzylation was achieved when 1,2-epoxybutane (0.5 eq./eq. N-bromosuccinimide) and 2,6-di-tert-butylpyridine (0.5 eq./eq. N-bromosuccinimide) were used in the reaction. Furthermore, ATR-IR and 1H NMR spectroscopy confirmed the successful removal of benzyl ether groups. The method was developed to monitor the transglycosylation reaction of the BnMC with permethylated cellulose, for which the deprotection of many small samples in parallel is required. This comprises the determination of the methyl pattern in the glucosyl units by gas-liquid chromatography (GLC), as well as oligosaccharide analysis by liquid chromatography mass spectrometry (LC-MS) after perdeuteromethylation and partial hydrolysis to determine the methyl pattern in the chains. The unavoidable partial chain degradation during debenzylation does not interfere with this analytical application, but, most importantly, the DS and the methyl pattern were almost congruent for the debenzylated product and the original MC, indicating the full success of this approach The presented method provides an unprecedented opportunity for high throughput and parallel debenzylation of complicated glucans, such as BnMC (as a model compound), for analytical purposes. For comparison, debenzylation using Na/NH3 was applied to BnMC and resulted in a completely debenzylated product with a remarkably high recovery yield of 99 mol% and is, thus, the method of choice for synthetic applications, e.g., for the transglycosylation product prepared under the selected conditions in a preparative scale.

Analysis of the Heterogeneities of First and Second Order of Cellulose Derivatives: A Complex Challenge

The complexity of the substituent distribution in polysaccharide derivatives is discussed and defined. The challenges regarding analytical characterization that results from various interrelated categories of distributions, including molecular weight, chemical composition, and microstructure, are outlined. Due to these convoluted levels of complexity, results should always be interpreted with carefulness. Various analytical approaches which have been applied to starch and cellulose derivatives are recapped, including enzymatic, mass spectrometric, and chromatographic methods. The relation of heterogeneities of first and second order among and along the polysaccharide chains is addressed. Finally, examples of own analytical work on cellulose ethers are presented, including the MS analysis of methyl cellulose (MC) blends and fractionation studies of fully esterified MC, especially its 4-methoxybenzoates by gradient HPLC on normal phase. Preparative fractionation according to the degree of substitution (DS) allows follow-up analysis in order to get more detailed information on the substituent distribution in such sub-fractions.

Chemistry and applications of polysaccharide solutions in strong electrolytes/dipolar aprotic solvents: an overview

Biopolymers and their derivatives are being actively investigated as substitutes for petroleum-based polymers. This has generated an intense interest in investigating new solvents, in particular for cellulose, chitin/chitosan, and starch. This overview focuses on recent advances in the dissolution and derivatization of these polysaccharides in solutions of strong electrolytes in dipolar aprotic solvents. A brief description of the molecular structures of these biopolymers is given, with emphases on the properties that are relevant to derivatization, namely crystallinity and accessibility. The mechanism of cellulose dissolution is then discussed, followed by a description of the strategies employed for the synthesis of cellulose derivatives (carboxylic acid esters, and ethers) under homogeneous reaction conditions. The same sequence of presentation has been followed for chitin/chitosan and starch. Future perspectives for this subject are summarized, in particular with regard to compliance with the principles of green chemistry.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2007-2008

This review is the fifth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2008. The first section of the review covers fundamental studies, fragmentation of carbohydrate ions, use of derivatives and new software developments for analysis of carbohydrate spectra. Among newer areas of method development are glycan arrays, MALDI imaging and the use of ion mobility spectrometry. The second section of the review discusses applications of MALDI MS to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, biopharmaceuticals, glycated proteins, glycolipids, glycosides and various other natural products. There is a short section on the use of MALDI mass spectrometry for the study of enzymes involved in glycan processing and a section on the use of MALDI MS to monitor products of the chemical synthesis of carbohydrates with emphasis on carbohydrate-protein complexes and glycodendrimers. Corresponding analyses by electrospray ionization now appear to outnumber those performed by MALDI and the amount of literature makes a comprehensive review on this technique impractical. However, most of the work relating to sample preparation and glycan synthesis is equally relevant to electrospray and, consequently, those proposing analyses by electrospray should also find material in this review of interest.

Regioselective esterification and etherification of cellulose: a review

Deep understanding of the structure-property relationships of polysaccharide derivatives depends on the ability to control the position of the substituents around the monosaccharide ring and along the chain. Equally important is the ability to analyze position of substitution. Historically, both synthetic control and analysis of regiochemistry have been very difficult for cellulose derivatives, as for most other polysaccharide derivatives. With the advent of cellulose solvents that are suitable for chemical transformations, it has become possible to carry out cellulose derivatization under conditions sufficiently mild to permit increasingly complete regiochemical control, particularly with regard to the position of the substituents around the anhydroglucose ring. In addition, new techniques for forming cellulose and its derivatives from monomers, either by enzyme-catalyzed processes or chemical polymerization, permit us to address new frontiers in regiochemical control. We review these exciting developments in regiocontrolled synthesis of cellulose derivatives and their implications for in-depth structure-property studies.

Chemical structure analysis of starch and cellulose derivatives

Starch and cellulose are the most abundant and important representatives of renewable biomass. Since the mid-19th century their properties have been changed by chemical modification for commercial and scientific purposes, and there substituted polymers have found a wide range of applications. However, the inherent polydispersity and supramolecular organization of starch and cellulose cause the products resulting from their modification to display high complexity. Chemical composition analysis of these mixtures is therefore a challenging task. Detailed knowledge on substitution patterns is fundamental for understanding structure-property relationships in modified cellulose and starch, and thus also for the improvement of reproducibility and rational design of properties. Substitution patterns resulting from kinetically or thermodynamically controlled reactions show certain preferences for the three available hydroxyl functions in (1→4)-linked glucans. Spurlin, seventy years ago, was the first to describe this in an idealized model, and nowadays this model has been extended and related to the next hierarchical levels, namely, the substituent distribution in and over the polymer chains. This structural complexity, with its implications for data interpretation, and the analytical approaches developed for its investigation are outlined in this article. Strategies and methods for the determination of the average degree of substitution (DS), monomer composition, and substitution patterns at the polymer level are presented and discussed with respect to their limitations and interpretability. Nuclear magnetic resonance spectroscopy, chromatography, capillary electrophoresis, and modern mass spectrometry (MS), including tandem MS, are the main instrumental techniques employed, in combination with appropriate sample preparation by chemical and enzymatic methods.