Hemiacetals, hydrolysis chains

The lithium etiolate of acetaldehyde DMH has recently been utilized in the opening reaction of the ot-epoxide obtained by DM DO oxidation ofenol ether 142, to provide hemiacetal 143 after mild oxidative acid hydrolysis. The protected carbonyl functionality was subsequently used for the introduction of the trans enyne chain through a Wittig olefmation reaction to provide alcohol 144, which was then transformed into (+)-laurenyne (Scheme 8.37) [71]. 

The hydrolysis of glycosides is achieved by specific hydrolytic enzymes, e.g. p-glucosidasc for [)-glucosides and ft-galactosidase for ft-galactosides. These enzymes mimic the readily achieved acid-catalysed processes [Figure 2.27(b)], Under acidic conditions, the a- and (Vanomcric hemiacetal forms can also equilibrate via the open chain... 

The polyvinyl alcohol is soluble in hot water, and the solution is wet-spun into a coagulating bath consisting of a concentrated solution of sodium sulfate. The fibers are heat-treated to provide temporary stability so that they may be converted to the formal derivative by treatment with an aqueous solution of formaldehyde and sulfuric acid. This final product resists hydrolysis up to the boiling point of water. It seems reasonable to assume that it contains hemiacetal groups and some unreacted hydroxyls on the polymer chain as... 

Cellulose is a heterogeneous substrate that makes modeling cellulolytic enzyme hydrolysis difficult. Cellulose is composed of chains of glucose connected by P 1-4 glycosidic bonds. One chain end is termed the reducing end because the hemiacetal is able to open to expose the reducing aldehyde. The other chain end is called the non-reducing end because the 1 carbon in the hemiacetal is involv-... 

Two different procedures can be used to determine what size ring a monosaccharide forms. In the first procedure, treatment of the monosaccharide with excess methyl iodide and silver oxide converts all the OH groups to OCH3 groups (Section 22.12). Acid-catalyzed hydrolysis of the acetal then forms a hemiacetal, which is in equilibrium with its open-chain form. The size of the ring can be determined from the structure of the open-chain form because the sole OH group is the one that had formed the cyclic hemiacetal. 

In analogy with pyranoses, the furanoses are also in equilibrium with the open-chain forms. The new functionality which appears in the cyclic forms is hemiacetal, the structure already mentioned in our discussions on alcohol addition to the carbonyl group. The hemiacetals can easily undergo hydrolysis yielding the open chain monosaccharide molecule. 

An enzyme which catalyses the hydrolysis of high molecular weight carbohydrates such as starch and glycogen. Human amylases are referred to as a-amylases because they attack the a-l,4-hemiacetal linkages of starch in a random manner, as distinct from the jS-amylases of plant and bacterial origin which can act only at the terminal reducing end of a carbohydrate chain. Hydrolysis of straight chain carbohydrates results in the formation of the disaccharide, maltose. In the case of the... 

It already has been mentioned (Chapter 2) that the cyanohydrin reaction of adding hydrogen cyanide to carbohydrates played an important role in the elucidation of the configurational structures of the carbohydrates. The reaction increases the length of the carbohydrate chain by one carbon but, unfortunately, forms two epimeric products that must be separated. Sometimes one of the two products is produced in much larger amounts than the other, facilitating the separation, but sometimes also providing the unwanted product. The nitrile products can be converted into the aldonic acids by hydrolysis with sodium hydroxide. The aldonic acid can be converted into the lactone by treatment with sulfuric acid. The lactone can then be reduced by sodium borohydride to the hemiacetal (reaction 4.41). 

---