Acetal groups

The exocyclic 1,3-dioxolane ring is much more vulnerable to acid hydrolysis than the ring connected with the acetal group. Partial deprotection of the side-chain is easily achieved by treatment with sulfurie acid. 

Owing to the instability of a-halogenoaldehydes it is occasionally preferable to use more stable derivatives, such as enol acetate prepared according to Bedoukian s method (204) and a-bromoacetals (4, 8, 10, 16, 22, 67, 101, 426). An advantage is said to be in the yield however, this appears to be slight. The derivatives react in the same sense as the aldehydes themselves, that is, the acetal group as the more polarized reacts first and enters the C-4 position. It is likely that the condensation and cyclization occur by direct displacement of alkoxide ions. Ethyl-a,/3-dihalogeno ethers (159, 164, 177, 248) have also been used in place of the free aldehydes in condensation with thioamides. 

The resihency and dyeabihty of poly(vinyl alcohol) fibers is improved by a process incorporating -hydroxybenzaldehyde to provide a site for the formation of a stable Mannich base. Hydroxyl groups on the fiber are converted to acetal groups by -hydroxybenzaldehyde. Subsequent reaction with formaldehyde and ammonia or an alkylamine is rapid and forms a stable Mannich base that is attached to the polymer backbone (94). 

An economic synthesis of (3) has been patented (74,91). The process iavolves (/) synthesis of sucrose 6-acetate by way of sucrose 4,6-cychc orthoacetate (2), and (2) selective chlorination usiag thionyl chloride—pyridine—1,1,2-trichloroethane, followed by removal of the acetate group. 

The hydroxyl groups in poly(vinyl alcohol) contribute to strong hydrogen bonding both intra- and intermolecularly, which reduces solubiUty in water. The presence of the residual acetate groups in partially hydrolyzed poly(vinyl alcohol) weakens these hydrogen bonds and allow solubiUty at lower temperatures. 

The hydrophobic nature of the acetate groups results in a negative heat of solution (61—64), which increases as the number of acetate groups is increased. This means that the critical temperature or the 9 temperature is lower, ie, the solubiUty decreases as the temperature is increased. 

The Sb atom ia this tetraacetato compouad is hexacoordiaate with three moaodeatate acetate groups and one symmetrically bonded bidentate acetate group. 

C-Glycosyl compounds have a carbon atom ia place of the exocycHc oxygen atom of the acetal group and, therefore, are branched cycHc ethers. An example is the naturally occurring anthroquiaone dye, carminic acid [1260-17-9] (Cl Natural Red 4). 

Ester, ether, and cycHc acetal groups are used as blocking groups to allow regiospecific reactions to take place, ie, reaction at specific unblocked hydroxyl groups. 

The use of a catalytic quantity of alkah equivalent to only a small fraction of the acetate has the advantage that contamination of the poly(vinyl alcohol) with salts, which are difficult to remove, is minimized. A variant of the process is the use of a mixture of alcohol with the acetate ester produced by the alcoholysis as the alcoholyzing agent. This provides a means of controlling the completeness of removal of the acetate groups from the poly(vinyl acetate) (111). 

Me2BBr, CUjClj, -78°, then NaHC03/H20, 87-95% yield. This reagent also cleaves the MEM, MTM, and acetal groups. Esters are stable to this reagent. 

Vinyl acetate may be easily polymerised in bulk, solution, emulsion and suspension. At conversions above 30%, ehain transfer to polymer or monomer may occur. In the case of both polymer and monomer transfer two mechanisms are possible, one at the tertiary carbon, the other (illustrated in Figure 14.4) at the acetate group. 

The radical formed at either the tertiary carbon atom or at the acetate group will then initiate polymerisation and form branehed structures. 

Commercial polyfvinyl alcohol) (e.g. Gelvatol, Elvanol, Mowiol and Rhodo-viol) is available in a number of grades which differ in molecular weight and in the residual acetate content. Because alcoholysis will cause scission of branched polymers at the points where branching has proceeded via the acetate group, polyfvinyl alcohol) polymer will have a lower molecular weight than the poly (vinyl acetate) from which it is made. 

Residual acetate groups, due to incomplete hydrolysis of poly(vinyl acetate) to poly(vinyl alcohol). 

Ethylenevinylacetate (EVA) is the most used type (approximately 80%) it can be varied in viscosity (melt index) and content of acetate in broad ranges. It is semicrystalline. The vinyl acetate groups provide improved adherability towards many materials. The low heat stability, however, limits the areas of application. 

Acetic anhydride has also been used as the acylating agent. The formation of thiiranes from thiocyanatohydrins having a tertiary hydroxy group is best achieved by p-toluenesulfonic acid-catalyzed acetylation.The analogous thiocyanatohydrins with a secondary hydroxyl and a tertiary thiocyanate function give a predominance of epoxide from thiocyanatohydrin acetates since the hydrolysis rate of the secondary acetate grouping becomes competitive with that of the tertiary thiocyanate. 

The reaction of the iV-acetylenanime with perbenzoic acid at low temperature is much faster than attack at a A -double bond, so the latter can be preserved without prior protection. In addition, the A-acetyl compound is relatively stable to weak alkali and to Oppenauer conditions permitting, e.g., hydrolysis of a 3-acetate group and conversion to a 3-keto-A" function. These latter compounds also react selectively with perbenzoic acid at the A -double bond. 

Pentafluorobenzyl bromide [II] and a partially fluonnated alkyl iodide [12] react with potassium carboxylates An interesting replacement of iodine in a fluonnated alkyl iodide by an acetate group takes place with peroxyacetic acid [13] (equations 10-12)... 

The net reaction accomplished by the TCA cycle, as follows, shows two molecules of COg, one ATP, and four reduced coenzymes produced per acetate group oxidized. The cycle is exergonic, with a net AG° for one pass around the cycle of approximately —40 kj/mol. Table 20.1 compares the AG° values for the individual reactions with the overall AG° for the net reaction. 

AC2O, Pyr, 20°, 12 h, 100% yield. This is one of the most common methods for the introduction of acetate groups. By running the reaction at lower temperatures, good selectivity can be achieved for primary alcohols over secondary alcohols. Tertiary alcohols are generally not acylated under these conditions. 

HBF4, MeOH, 23°, 48 h, 83% yield. This system cleaves acetate groups in the presence of benzoate groups. HCl in methanol can also be used. ... 

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