Hemi-acetals and -ketals

Hemi-acetals and -ketals, cyclic s. Lactols Hemiketals... 

Acetals and ketals are also called glycosides. Acetals and ketals (glycosides) are not in equilibrium with any open chain form. Only hemi-acetals and hemiketal s can exist in equilibrium with an open chain form. Acetals and ketals do not undergo mutarotation or show any of the reactions specific to the aldehyde or ketone groups. For example, they cannot be oxidized easily to form sugar acids. As an acetal, the carbonyl group is effectively protected. 

The aldehyde products can be easily distinguished from the ketone products by the lone hydrogen. The hemi products can be distinguished from the acetals and ketals because the hemi products both have alcohols while the full acetals and ketals don t. Hemi formation is catalyzed by acid or base. In formation of acetal and kctal from the hemi forms the hydroxyl group must be protonated to make a good leaving group, thus this part of the reaction is catalyzed by acid only. 

Hemi-acetal and hemi-ketal functional groups (Fig. 26) are very susceptible to base-catalyzed hydrolysis and are also susceptible to acid catalyzed hydrolysis. In contrast, acetals and ketals are extremely resistant to hydrolysis by base. For acetals and ketals, only acid-catalyzed hydrolysis occurs... 

We saw in the previous chapter that aldehydes react with alcohols to form hemi-acetals, and hemiacetals react with another alcohol to afford acetals. Well, the intramolecular reaction of the aldehyde carbon of an aldose monosaccharide with one of the hydroxyl groups in the same molecule affords a cyclic hemiac-etal. An mferaiolecular reaction of this hemiacetal with the hydroxyl group of another sugar molecule provides an acetal functional group. This kind of acetal linkage connects monosaccharides into polysaccharides. Biochemists refer to these acetal (and ketal) bonds as glycoside bonds. 

Recall from organic chemistry studies that in addition to forming hemi-acetals and hemiketals, aldehydes and ketones can go a step further and form acetals and ketals ... 

I LEARNING CHECK 14.5 Draw the structural formula for the hemiacetal and hemi-ketal intermediates and for the acetal and ketal products of the following reactions. Label each structure as a hemiacetal, hemiketal, ketal, or acetal. 

For our present purposes, we are interested in the ways in which hemi-acetals, acetals, hemiketals, and ketals are formed. Hemiacetals and hemi-ketals can be regarded as products of the addition of alcohols to the carbonyl... 

Figure 26 Basic structures of ketals, hemi-ketals, acetals, and hemi-acetals. ...

Derivatives of sugars generated by transformation of the carbonyl group with alcohols are conventionally named as (hemi)acetals, (hemi)ketals and so on. 

First the isopropylidene ketal at C-3b and C-4b is generated using acetone and an ion-exchange resin at first. There is no further ketal formation at other positions of the lactose. It is also possible to use 2,2-dimethoxypropane (Me)2C(OMe)2 instead of acetone. The problem in this step is the partial formation of hemi-ketals with the other hydroxy groups which lower the yield. They are destroyed by heating to 100 °C in glacial acetic acid.20... 

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