Acetyls production process with esterification

The ability of enzymes to achieve the selective esterification of one enantiomer of an alcohol over the other has been exploited by coupling this process with the in situ metal-catalysed racemisation of the unreactive enantiomer. Marr and co-workers have used the rhodium and iridium NHC complexes 44 and 45 to racemise the unreacted enantiomer of substrate 7 [17]. In combination with a lipase enzyme (Novozyme 435), excellent enantioselectivities were obtained in the acetylation of alcohol 7 to give the ester product 43 (Scheme 11.11). A related dynamic kinetic resolution has been reported by Corberdn and Peris [18]. hi their chemistry, the aldehyde 46 is readily racemised and the iridium NHC catalyst 35 catalyses the reversible reduction of aldehyde 46 to give an alcohol which is acylated by an enzyme to give the ester 47 in reasonable enantiomeric excess. 

Pretreatment/activation involves swelling mostly chemical pulp fibers with glacial acetic acid and 4-7% moisture (on cellulose basis). Activation ensures uniform acetylation. Esterification involves the treatment of preactivated cellulose with a mixture of acetic acid and acetic anhydride at a consistency of about 10%. The acetic anhydride content of the mixture is approximately 25%, and this amounts to a 10-40% excess over stoichiometric requirement. The reaction proceeds at 50 °C. Hydrolysis refers to a process step in which the DS of the esterification product is reduced from its level of >2.9 to a level between 2.3 to 2.7 by the addition of 5 to 10% water. The temperature during hydrolysis is maintained at 40 to 80 °C. The consequence of hydrolysis is a uniform release of acetyl groups from the cellulose backbone so as to assure uniform solubility. Precipitation involves either the addition of the acetylation mixture into water or dilute acetic acid to produce a flake product, or the addition of water... 

It is well known that the nucleophilic displacement reactions at tosylated polysaccharides are limited or at least mainly directed towards the primary positions . Therefore, our interest was focused on 6-0-tosyl starch samples with DStos 1- One suitable synthesis path is the protection of 0-2 and the subsequent tosylation. A useful protecting group may be the acetyl ester function. It was recently found that in contrast to conventional esterification processes of starch with acetic anhydride, which leads to a statistic distribution of the ester groups, an acetylation of starch dissolved in DMSO with acetic acid vinyl ester in the presence of sodium chloride yields 2-0-acetyl starch of varying DSac from 0.1 to 1.0. The functionalisation patterns of these new starch products were unambiguously proved by means of various NMR measurements including two dimensional methods . 

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