Acetal links

Link (Section 25.8) An acetal link between the Cl -OH group of one sugar and the C4 -OH group of another sugar. 

Glycosides are formed by condensation between the hydroxyl group of the anomeric carbon of a monosaccharide, or monosaccharide residue, and a second compound that may—or may not (in the case of an aglycone)—be another monosaccharide. If the second group is a hydroxyl, the O-glycosidic bond is an acetal link because it results from a reaction between a hemiacetal group (formed from an aldehyde and an —OH group) and an-... 

Rnorr V, Russ V, Allmendinger L, Ogris M, Wagner E (2008) Acetal linked oligoethy-lenimines for use as pH-sensitive gene carriers. Bioconjug Chem 19 1625-1634... 

Hydrolytic degradation is especially important in polymers with hydrolyzable links between the CRUs. Thus, polyesters can be saponified to yield the starting materials from which they were formed. Acetal links in synthetic polymers such as polyoxymethylene, or in natural polymers such as cellulose, can be hydrolyzed with acids. However, the resistance to hydrolysis depends very much on the structure of the polymer for example, polyesters of terephthalic acid are very difficult to hydrolyze while aliphatic polyesters are generally easily hydro-... 

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. 

Linen cellulose hydrolyzes at the acetal link. In acidic or neutral conditions, reducing oxycellulose is formed in alkaline conditions, a nonreducing oxycellulose is formed. The new end groups produced, then, are aldehydic in the first case and carboxylic in the second case. Hydrolysis and oxidation are nonuniform amorphous areas are attacked first (10). 

Nudelman ED, Levery SB, Igarashi Y, Hakomori S. Plas-malopsychosine, a novel plasmal (fatty aldehyde) conjugate of psychosine with cyclic acetal link. Isolation and characterization from human brain white matter. J. Biol. Chem. 1992 267 11007-11016. 

Solid-supported synthesis has rapidly emerged as an important strategy in synthetic organic chemistry. Solid-phase methodology is aimed at the direct synthesis of libraries of molecularly diverse compounds for biological evaluation in lead discovery. The asymmetric addition of polymer-supported chiral crotylsilanes to acetals and allylation of polymer-bound acetals linked through an ester with the chiral crotylsilanes has been investigated [44d] la can be employed in these crotylation reactions and results in the formation of polymer-supported homoallylic esters with diastereoselec-tivity similar to that of solution-phase reactions. 

Also in the field of glycolipids unusual types of glycosidic linkage have been discovered [14], such as an open-chain acetal-linked A-acetylgalactosamine in the core part of the lipopolysac-charides from Proteus microorganisms (O Fig. 1). 

Another versatile diene building block is cyclohexadiene diol [304], which can be obtained upon fermentation of halobenzenes. This building block is very versatile, and its potential as a synthon has not yet been much exploited in SPOS. However, its appHcability may be currently compromised by high price and lack of commercial accessibility in a stable pure form. The diene can be bound to a polymer bearing aldehyde or keto functions by transacetahzation. The acetal link to the polymer can readily be cleaved by dilute TFA, and this therefore serves as an excellent Hnkage system. The two double bonds of the cyclohexadiene have differ-... 

The sorption of formaldehyde from aqueous solution by cellulose depends on diffusion and, ultimately, on chemical substitution of accessible hydroxyl groups by hemiacetal (hydroxymethyl) groups. " Full acetal links are formed only when the pH of the aqueous formaldehyde is below 3, below which point, there is a linear relationship between the logarithm of the amount of chemically combined aldehyde and the pH. The reaction, which is slow at room temperature, is markedly affected by temperature, and the energies of activation (23 and 21 kcal./mole, for cotton and viscose rayon, respectively) are much higher than the value observed for the reversible sorption of formaldehyde (13 kcal./mole). 

Methylene acetal-linked dinucleosides have been prepared previously by condensation of a suitably protected 3 -Omethylthiomethylenethymidine (154) with the 5 -hydroxy group of... 

Glycogen A huge branched polysaccharide of glucose, a store of energy in humans and animals. Glycoside A sugar acetal, linking monosaccharides. 

V. Knorr, V. Russ, L. Allmendinger, M. Ogris, E. Wagner, Acetal linked oligoethylenimines for use as pH-sensitive gene carriers, Bioconjug. Chem. 19 (2008) 1625-1634. 

Floreancig completed the total synthesis of (+)-dactylohde via a sequential Peterson olefination and an intramolecular Hosomi-Sakurai-Prins cycli-zation of the acetal-linked substrate (Scheme 32). Macrocychzation was performed by Horner-Emmons olefination as Smith did (Sect. 3.2.1). The key element of 2,6-cfs-tetrahydropyran in 155 was constructed via the sequential cyclization starting from acetal 156, which involved aldehyde 157 and 1,3-diol 158, synthesized via Denmark s asymmetric aldol reaction and Stille coupling. 

The unusual oleanic add "seco lycoside 24 has been isolated fix)m si beet it was postulated that the acetal linked (ficaibm c add lactone fragment is derived from a pentoade or fixmi a glucuronide with concomitant decartxn lation. ... 

C287.9) and the nitrogen concentration N) does not follow the 1 1 relationship expected for the peptidic link. The deviation may be explained by the contribution at 287.9 eV of carbon making two single bonds with oxygen, as present in the acetal links of polysaccharides. The concentration of acetal carbon Cac may be evaluated by... 

A relationship close to 1 1 between the (O+iV) /C ratio and the proportion of carbon that is bound to a oxygen or nitrogen Cox/C (Fig. 34) has been discussed in the section Accuracy in connection with the sensitivity factors. It has been observed for many microorganisms. This is consistent with a dominance of alcohol, primary amide or amine, and ester functions. A high concentration of ether functions as in acetal links would lead to a ratio lower than 1 1. A high concentration of carboxyl, carboxylate, or ester phosphate would lead to a higher ratio. 

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