Acetal bonds

Disaccharide (Section 25.8) A carbohydrate formed by linking two simple sugars through an acetal bond. 

Polysaccharide (Section 25.1) A carbohydrate that is made of many simple sugars linked together by acetal bonds. 

An acidic salt must be present so that the acetal bonds will be hydrolyzed. Other salts, such as ammonium formate, may be substituted for ammonium chloride. 

A different situation obtains in compounds such as the basic beryllium monocarboxylates (140-142). In particular the acetate derivative, Be40(02CCH3)6, has been known in the solid state for nearly 100 years (143). A partial structure of this compound is in Fig. 13 (144). The central oxide ion is surrounded by a tetrahedron of beryllium atoms. The acetate ions form bridges across the six edges of the Be4 tetrahedron, with each acetate bonding with two Be atoms. Each berylium atom in surrounded by an approximate tetrahedron of... 

Pikho and co-workers50 found that nonanomeric [6,5]-spiroketals (having a pyranoside moiety with an equatorial CO acetal bond) can be formed under conditions of kinetic control. For instance, 62 undergoes acid catalyzed spiroacetalization giving the anomeric (most stable) acetal 63... 

Polymers of formaldehyde with semiacetal end groups are thermally unstable they decompose at temperatures as low as 150 °C, splitting off monomeric formaldehyde. Upon acetylation of the hydroxy end groups, thermal stability up to 220 °C is achieved alkylation also provides stability against alkali, but not against acids since these are capable of splitting the acetal bonds in the polymer chains (see Example 5-13). 

The reversal of the well-known transformation of sugars into pyrans has been detailed as a method for assembling simple monosaccharides from simple furans (71T1973). A compound of the 2-furylcarbinol type was converted by the Br2/MeOH procedure into a mixture of the cis and trans isomers of the corresponding 2,5-dimethoxy-2,5-dihydrofuran derivative (129). Mild acid hydrolysis of (129) resulted in cleavage of the acetal bonds with formation of the dicarbonyl compound (130) which underwent immediate cyclization to 2,3-dideoxy-DL-alk-2-enopyranos-4-ulose (131 Scheme 29). 

Polystyrene-polybutadiene-polystyrene block-copolymers were also heparinized via formation of acetal bonds. The procedure involved oxidation of the copolymer to glycol 90) ... 

The simplest interpretation might be that the lignin and carbohydrate are joined through a straightforward acetal link (IV) (/7), but no direct evidence has ever been found for the existence of acetal groups in lignin. Nevertheless, in our work we have continued to consider the acetal bond as an alternative explanation of the observations. 

With regard to the type of thermally induced cross-links, the following conclusions may be drawn from our results Formation of C—C bonds can be definitely excluded due to the cleavage observed by aqueous acid. Intramolecular ether bonds within the glucose unit might explain the change in cuprammonium viscosity, but they should be cleaved also by aqueous alkali. The most probable conclusion is, according to our opinion, a formation of intercatenary acetal bonds (12). 

An increasing number of surfactants contains ester bonds, such as the CnS04, the CnEOmS04, the polyol ester surfactants, and the replacements of the dialkyldimethylam-monium surfactants, the esterquats. The ester bond is susceptible to base hydrolysis (Holmberg, 1996) in contrast to the acetal bond in, for instance, the alkylpolyglucoside, which is acid-susceptible (Steber et al. 1995). 

A way round these difficulties is to use an ether or an amide that has a built-in weakness so that the over-vigorous conditions are not needed. This Achilles heel for an ether is commonly the THP group that makes the ether into an acetal. Dihydropyran, DHP 26, is protonated on carbon 27 to give the cation 28 that captures the alcohol to give the mixed acetal 29, usually referred to as the THP derivative . After the reaction the hydrolysis needs only the weak aqueous acid used for acetals. The secret is that the weak acetal bond (b in 30) is cleaved4 rather than the strong ether bond (a in 30). 

In a series of six conformationally restricted axial tetrahydropyranyl ethers [55], Briggs et al. (1984) observed a good linear relationship between the length of both the exocyclic and the endocylic acetal bond with the pAa-value of ROH, according to (5) and (6). 

Aldehyde starches are prepared by treatment with periodic acid/periodate ions, which selectively oxidize the adjacent hydroxyl groups on carbon atoms 2 and 3 to aldehyde groups. Dialdehyde starch can react with cellulose by forming covalent hemiacetal and acetal bonds.40 It is primarily used as a wet strength agent in the production of tissue and other sanitary grades. 

Dialdehyde starch,176-178 block reactive starch179 and cationic aldehyde starch180 181 comprise a special class of starches that can react with cellulose through the formation of covalent hemiacetal or acetal bonds. The zwitterion group is another functional group that has been utilized in conjunction with cationic and acetal groups. 

A general term for an acetal bond from an anomeric carbon joining two monosaccharide units, (pp. 1120, 1132)... 

A glycosidic linkage using an acetal bond from the anomeric carbon of glucose. 

Of several anhydrides studied, acetic anhydride reacted the most readily. Reactions were carried out by refluxing the wood in a xylene/acetic anhydride solution or with acetic anhydride vapors alone at 120°C. With this system, for each mole of acetate bonded onto the wood a mole of acetic acid is generated as a byproduct. Although this byproduct generation is a disadvantage of the process, the chemical system does penetrate and react quickly with wood, without a catalyst. It is not... 

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