Acetyl chloride, reaction with carbohydrates

Alcohol Compounds.—That the carbohydrates are, in fact, alcohol compounds is shown, both by their relation to poly-hydroxy alcohols and by their reactions. Carbohydrates possess alcohol characters in that they undergo distinctly alcoholic reactions. Like all alcohols they react with acetyl chloride or acetic anhydride. In.practice the latter reagent is used. They form acetyl derivatives, or esters, just as ethyl alcohol does. 

The substance to be identified must be in a pure condition in order that its physical properties may be accurately determined and the conclusions drawn from its reactions may be trustworthy. If the substance does not show the characteristics of a pure compound, it must be purified by the methods already outlined (7, 8). A qualitative analysis is next made to determine what elements are present. The action of the compound is then studied with water, concentrated sulphuric acid, a hot and a cold solution of sodium hydroxide, dilute hydrochloric acid, acetyl chloride, bromine, sodium carbonate, phenylhydrazine, and Schiff s reagent. The carbohydrate test (386) should be applied, and if the compound contains sulphur the reactions given above under sulphur compounds should be studied. It is sometimes advisable and is often necessary to use other reactions than those just mentioned. A few examples will now be given. 

Several methods have been described for rapid and high-yielding microwave-assisted protection of carbohydrate hydroxyl functionalities with acetic anhydride, acetyl, chloroacetyl, pivaloyl, dodecanoyl, and benzoyl chlorides, and with a conventional base or a resin-linked amine. The use of solid supported reagent in these reactions resulted in reactions which are slightly slower but still comparable with conventional methods. 

By means of an ETHOS MR oven, Nuchter et al. [33] accomplished scaling-up of a microwave-assisted Fischer glycosylation to the kilogram scale with improved economic efficiency. In batch reactions, carbohydrates (o-glucose, o-mannose, d-galactose, butyl o-galactose, starch) were converted on the 50-g scale (95-100% yield, from 95 5 to 100%) with 3-30-fold molar excesses of an alcohol (methanol, ethanol, butanol, octanol) in the presence of a catalytic amount of acetyl chloride under pressure (microwave flow reactor, 120-140 °C, 12-16 bar, 5-12 min) or without applying pressure (120-140 °C or reflux temperature, 20-60 min). Furano-sides are not stable under these reaction conditions. 

One of the best-known carbohydrates of the anhydro class is 1,2-anhydro-3,4,6-tri-O-acetyl-a-D-glucopyranoside, commonly called Brigl s anhydride. It is prepared from p-D-glucopyranoside pentaacetate by reaction with phosphorous pentachloride in carbon tetrachloride to give 3,4,6-tri-0-acetyl-2-0-trichloro-acetyl-P-D-glucopyranosyl chloride from which 1,2-anhydro-tri-O-acetyl-a-D-glucopyranoside is obtained by further treatment with dry ammonia in benzene [63,64] (reaction 4.57). 

Complete acetylation of totally O-unpiotected mono- and disaccharides was carried out under microwave heating in 90 sec at 720 W under closed vessel conditions using indium (111) chloride catalyst. Das et al. (2005) studied reactions in acetonitrile with stoichiometric amounts of acetic anhydride, which were quantitative and afforded predominantly the a-peracetylated form. Normally partial acetylation of carbohydrates is achieved with acetyl chloride and pyridine. 

The Staudinger [2+2] cycloaddition of chiral carbohydrate Schiff base with phthalimidoacetyl chloride has yielded the sugar-based monocyclic (3-lactam as a single isomer [88]. This latter could be transformed in several (3-lactams variously functionalized through ozonolysis, reduction, hydrolysis, and acetylation reactions, (Scheme 26). 

The relation which the oxygen atoms in dextrose bear to the rest of the molecule, is indicated by the fact that when the carbohydrate is treated with acetic anhydride, in the presence of a small amount of zinc chloride, five acetyl groups are introduced into the molecule. This reaction shows that dextrose contains five alcoholic hydroxyl groups. Taken in connection with the reactions just mentioned, it leads to a structure for dextrose which is given in the following formula —... 

Esters of cellulose with interesting properties such as bioactivity and thermal and dissolution behavior can be obtained by esterification of cellulose with nitric acid in the presence of sulfuric acid, phosphoric acid, or acetic acid. Commercially important cellulose esters are cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate. Cellulose esters of aliphatic, aromatic, bulky, and functionalized carboxylic acids can be synthesized through the activation of free acids in situ with tosyl chloride, iV,iV -carbonyldiimidazole, and iminium chloride under homogeneous acylation with DMA/LiCl or DMSO/TBAF. A wide range of cellulose esters that vary in their DS, various substituent distributions, and several desirable properties can be obtained through these reactions. Recently, a number of enzymes that degrade cellulose esters have been reported. Some of them are acetyl esterases, carbohydrate esterase (CE) family 1, and esterases of the CE 5 [169-172] family. 

The tosylation of cellobiose by tosyl chloride in pyridine and thermal analysis gf the products using DTA and isothermal TGA has been carried out. Selective tosylation of 6,1, 6 -tri-O-trityl sucrose gave the 2-tosylate and not the 3-toaylate as previously claimed by I.Jeze ( Chem. Zvesti. 1971, 25, 369). Base treatment of this gave 40% 2,3-manno-epoxide (54), which was opened with a variety of nucleophiles in all cases the 3-nucleophilo-altro product resulted (Scheme 9). With ammonium thiocyanate, the acetyl-ated epoxide (55) gave the 2.3-allo-epithio disaccharide (56). Imidazolylsulphonyl derivatives of carbohydrates and their reactions are mentioned in Chapter 10. 

The 1,2-cw-glycosides are prepared by a modification of the Koenigs-Knorr reaction. Reaction of the peracetylated carbohydrate with phosphorous pen-tachloride in carbon tetrachloride gives 3,4,6-tri-( -acetyl-2-0-trichloroacetyl-3-D-glucopyranosyl chloride [77] (reaction 4.57), which reacts with alkyl and aryl nucleophiles to form the a-glycopyranoside (reaction 4.62). 

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