Aldehyde and Keto Groups

The most commonly used protected derivatives of aldehydes and ketones are 1,3-dioxolanes and 1,3-oxathiolanes. They are obtained from the carbonyl compounds and 1,2-ethanediol or 2-mercaptoethanol, respectively, in aprotic solvents and in the presence of catalysts, e.g. BF (L.F. Fieser, 1954 G.E. Wilson, Jr., 1968), and water scavengers, e.g. orthoesters (P. Doyle, 1965). Acid-catalyzed exchange dioxolanation with dioxolanes of low boiling ketones, e.g. acetone, which are distilled during the reaction, can also be applied (H. J. Dauben, Jr., 1954). Selective monoketalization of diketones is often used with good success (C. Mercier, 1973). Even from diketones with two keto groups of very similar reactivity monoketals may be obtained by repeated acid-catalyzed equilibration (W.S. Johnson, 1962 A.G. Hortmann, 1969). Most aldehydes are easily converted into acetals. The ketalization of ketones is more difficult for sterical reasons and often requires long reaction times at elevated temperatures. a,/9-Unsaturated ketones react more slowly than saturated ketones. 2-Mercaptoethanol is more reactive than 1,2-ethanediol (J. Romo, 1951 C. Djerassi, 1952 G.E. Wilson, Jr., 1968). 

The only acid-resistant protective group for carbonyl functions is the dicyanomethy-lene group formed by Knoevenagel condensation with malononitrile. Friedel-Crafts acylation conditions, treatment with hot mineral acids, and chlorination with sulfuryl chloride do not affect this group. They have, however, to be cleaved by rather drastic treatment with concentrated alkaline solutions (J.B. Bastus, 1963 H. Fischer, 1932 R.B. Woodward, 1960, 1961). 

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Free aldehyde and keto groups [1 — 3] e.g. in aldoses and ketoses... 

Hydrazines may be nucleophiles such as when they interact with aldehyde and keto groups to form hydrazones. This is the basis for the inhibition of enzymes such as transaminases, which rely on pyridoxal phosphate as a coenzyme. Mono-substituted hydrazines can be formed as metabolites when azo groups are reduced, dialkylated hydrazines are dealkylated or hydrazides are hydrolysed. 

An important issue is the differentiation between aldehyde and keto groups in cellulosic materials. Different approaches are theoretically conceivable a selective derivatization of either aldehyde and keto groups, a mathematical... 

There is no differentiation between aldehyde and keto groups. 

Figure 5.1. Derivatization of glucose for GC-MS analysis. Aldehyde and keto groups are converted into oximes using methoxyamine, followed by conversion of hydroxyl groups into trimethylsilyl (TMS) groups. This example depicts the derivatization of two tautomeric forms, syn and anti, formed during rotation along the C=N bond.

The same authors found that with Cu-Al catalysts similar results were obtained where aldehyde and keto groups are concurrently reduced, with yields of 75-85% and 90-95%, respectively209 ... 

The function groups of organic material (humus) can react with cyanide as described in the following equations. Under natural conditions (pH near neutral, 0°C-40°C), cyanide ion can react with aldehyde and keto groups, and carboxylic salt is formed in the presence of metal ions ... 

In conclusion, the salient features of the light-induced oxidation of polymers are the formation of hydroperoxide, peroxide, and carbonyl groups, the latter in the form of both aldehyde and keto groups. Moreover, certain reactions, such as reaction (d) in Scheme 7.20 and reaction (b) in Scheme 7.21, result in main-chain cleavage as far as the oxidation of linear macromolecules is concerned. Main-chain cleavage leads to a deterioration in certain important mechanical properties. Therefore, the photo-oxidation of polymers is deleterious and should be avoided in commercial polymers. Appropriate stabilization measures are discussed in Section 9.3. 

DlnitroplieiiyUiydiaziiie for free aldehyde and keto groups and for ketoses. 

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