In the present work, aged cotton linters have been analyzed for their chromophore content according to the CRI (''chromophore release & identification'') method. Despite the very low contents in the ppb range, nine chromophores have been unambiguously identified, which makes this account the first one on defined chromophoric structures isolated from cotton. A common feature of the chromophores are 2-hydroxy-[1,4]benzoquinone, 2-hydroxyacetophenone and 5,8-dihydroxynaphthoquinone moieties, which resemble chromophoric structures found in other cellulosic substrates, such as bleached pulps or fibers. The finding of these compounds in lignin-free cotton linters confirms the previous hypothesis that those chromophores are formed from (oxidized) carbohydrate structures rather than from lignin fragments.

Hexeneuronic acids (HexA) have long been known as triggers for discoloration processes in glucuronoxylan-containing cellulosic pulps. They are formed under the conditions of pulping from 4-Omethylglucuronic acid residues, and are removed in an ''A stage'' along the bleaching sequences, which mainly comprises acidic washing treatments. The chemical structures of HexA-derived chromophoric compounds 4-8, which make up 90% of the HexAderived chromophores, are reported here for the first time. The compounds are ladder-type, mixed quinoidaromatic oligomers of the bis(furano)-[1,4]benzoquinone and bis(benzofurano)-[1,4]benzoquinone type. The same chromophoric compounds are generated independently of the starting material, which can be either a) HexA in pulp, b) the HexA model compound methyl 1-13 C-4-deoxy-b-L-threo-hex-4-enopyranosiduronic acid (1) or c) a mixture of the primary degradation intermediates of 1, namely 5-formyl-furancarboxylic acid (2) and 2-furancarboxylic acid (3). Isotopic labeling ( 13 C) in combination with NMR spectroscopy and mass spectrometry served for structure elucidation, and final confirmation was provided by X-ray structure analysis.13 C-Isotopic labeling was also used to establish the formation mechanisms, showing all the compounds to be composed of condensed, but otherwise largely intact, 2-carbonylfuran and 2-carbonylfuran-5-carboxylic acid moieties. These results disprove the frequent assumption that HexA-derived or furfural-derived chromophores are linear furanoid polymers, and might have a direct bearing on structure elucidation studies Chromophores in cellulosic materials. Part XVI.

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In the present study, three representative residual chromophores recently isolated from highly bleached and aged pulps, 2,5-dihydroxy-w1,4x-benzoquinone (1), 5,8-dihydroxy-w1,4x-naphthoquinone (2), and 2,6-dihydroxyacetophenone (3), were selected to study their effects with regard to cellulose degradation under bleaching conditions, similar to those typically used in the pulp and paper industries.

The mechanism of a novel reaction type, the “furan endwise peeling” reaction, has been studied. Making extensive use of experiments on model compounds, the structural prerequisites to the reaction were found to be a 2‐hydroxpyran β‐glycosidically linked to a 2‐(2‐furyl)ethyl moiety. Proceeding from the reducing end of the celluloses, the reaction causes the one‐by‐one loss of anhydroglucose units (AGUs) which are concomitantly converted into 2‐(tetrahydroxybutyl)furans (4) by co‐reacting with 1,3‐dicarbonyl compounds such as ethyl acetoacetate (1). The key step of the reaction is the formation of a furan at the reducing end followed by the attack of the 2‐OH group of the last‐but‐one AGU at this terminal furan. The intermediate, unstable dioxepane fragments by cleavage of the glycosidic bond. The terminal AGU is released as a substituted furan, the “new” reducing end reacting immediately with the 1,3‐dicarbonyl component to form a new terminal furan which, in turn, undergoes the same cycle. The “furan endwise peeling” reaction represents an interesting way to convert cellulosic biomass by an easy procedure into substituted furans that are valuable fine chemicals with multiple uses.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)