Acetal linkage

The acetal polymers are probably never supplied for use without any additives being incorporated. As mentioned in Section 19.3.1 antioxidants of the phenol alkane type are present in both homopolymers and copolymers. Acid acceptors are also believed to be widely used to absorb traces of acidic materials which attack the acetal linkage. Epoxides, nitrogen compounds and basic salts have been successfully employed. 

Naturally occurring compounds called cyanogenic glycosides, such as lotau-stralin, release hydrogen cyanide, HCN, when treated with aqueous acid. The reaction occurs by hydrolysis of the acetal linkage to form a cyanohydrin, which then expels HCN and gives a carbonyl compound-fa) Show the mechanism of the acetal hydrolysis and the structure of the cyanohydrin that results. 

Mutarotation (Section 25.5) The change in optical rotation observed when a pure anomer of a sugar is dissolved in water. Mutarotation is caused by the reversible opening and closing of the acetal linkage, which yields an equilibrium mixture of anomcrs. 

The polymerized system gives similar results. The IR spectrum of the polymerized lipid shows two absorption peaks at 1805 cm-1 and 1735 cm"1 which correspond to the lactone carbonyl and ester groups, respectively. After the polymerized vesicle had been allowed to stand in an aqueous system for 2 weeks, the lactone carbonyl absorption peak at 1805 cm"1 disappeared as seen in Figure 3, which indicates the hydrolysis of the connecting acetal linkages has been completed. 

In the discussion above it has been shown that the lipid can been polymerized through UV irradiation of its aqueous suspension. The polymerization of the system improves the stability of the synthetic liposomes. Since there is an acetal linkage introduced between the polymer chain and the amphiphilic structure, this linkage can be slowly hydrolyzed in aqueous systems to separate the polymer chain from the lipid. 

In this paper two new polymerized vesicle systems have been presented. The first lipid can be polymerized in vesicle through UV irradiation. Because the second lipid contains a flexible spacer group it can be prepolymerized in benzene and then converted to vesicles by ultrasonication in water. The polymerization improves the stabilities of the synthetic liposomes. Since there is a acetal linkage between the... 

While starches are commonly used, they are relatively poor viscosifiers. Acids and bacterial enzymes readily attack the acetal linkages resulting in facile depolymerization. Both formaldehyde and isothiazolones have been used as starch biocides (17). Development of improved high temperature water viscosifiers for drilling and other oil field applications is underway. For the... 

There is some need for new pH indicators with improved characteristics which allow also covalent binding. P. Makedonski report about new kind of reactive azo dyes and their application as reversible pH sensors35. They prepare a new pH indicating sensors based on thin films prepared from azo dyes that are covalently bonded by an acetal linkage to a vinylalcohol ethylene copolymer (Figure 7). The absorption spectra of the polymer bond... 

Carbohydrates have been included in the wide range of molecules used in the parameterization of MM2 and of MM3. Alcohol and ether parameters have usually been determined from simple alcohols and ethers themselves. However, carbohydrates contain some unusual features in the acetal linkages, and in the many vicinal hydrogen-bonded hydroxyl groups. The "anomeric effect", first discovered by Edward (15) and popularized by Lemieux (16.), is best known in carbohydrates, although, of course, it occurs in other classes of compounds as well. One apparent result of this effect is that an axial alkoxy substituent is often more stable than the corresponding equatorial substituent when attached at the Cl position of a tetrahydropyranyl ring. This effect can be... 

Etoposide is an effective anticancer drug used in the treatment of small-cell lung cancer, testicular cancer and lymphomas. It is a semi-synthetic modification of the natural lignan podophyllotoxin, and contains three acetal linkages. Can you identify them ... 

The third acetal linkage is not so obvions, and it is in the five-membered ring fused onto the aromatic ring of the podophyllotoxin derivative. This is called a methylenedioxy group, and it is a common bidentate substitnent on many natural aromatic structures. However, it can be formally regarded as an acetal of formaldehyde. 

The main cyanogenic glycoside in laurel is prunasin, the P-o-glucoside of benzaldehyde cyanohydrin. The enzymic hydrolysis of prunasin may be visualized as an acid-catalysed process, first of all hydrolysing the acetal linkage to produce glucose and the cyanohydrin. Further hydrolysis results in reversal of cyanohydrin formation, giving HCN and benzaldehyde. 

Cellulose differs from amylose principally in the stereochemistry of the acetal linkages, which are a in amylose but P in cellulose. a-Amylase is specific for al 4 bonds and is not able to hydrolyse pi 4 bonds. An alternative enzyme, termed cellulase, is required. Animals do not possess cellulase enzymes, and thus cannot digest wood and vegetable fibres that are predominantly composed of cellulose. Ruminants, such as cattle, are equipped to carry out cellulose hydrolysis, though this is dependent upon cellulase-producing bacteria in their digestive tracts. 

The problem with the ether and ester linkage is that they are often hydrolytically unstable, and so there have been approaches to replace the CIO acetal linkage with more chemically robust linkages. Posner s group prepared a series of CIO olefinic non-acetal dimers and CIO saturated dimers that showed good antiproliferative activities . Dimers 108-110 were aU especially potent and selective at inhibiting leukaemia and colon cancer in the NCI assay (Scheme 38) . ... 

O Neill and coworkers have also sought to address the problem of the metabolically susceptible CIO acetal linkage . A series of CIO carba dimers were prepared and assayed for antitumour activity. The two most potent compounds that were prepared are two phosphate ester finked dimers 115 and 116 (Scheme 40). They are principally active against leukaemia, colon and certain melanoma and breast cancer cell lines in the NCI 60-cell line assay. 

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