Aldehydes, replacement hydroxylation

Oxidation. Oxidation of hydroxybenzaldehydes can result in the formation of a variety of compounds, depending on the reagents and conditions used. Replacement of the aldehyde function by a hydroxyl group results when 2- or 4-hydroxybenzaldehydes are treated with hydrogen peroxide in acidic (42) or basic (43) media pyrocatechol or hydroquinone are obtained, respectively. 

The amide nitrogen readily adds across the carbonyl group of an aldehyde yielding N-hydroxyalkyl-substituted pyrrohdinones (68), eg, A/-methylol-2-pyrrohdinone [15438-71-8] (34). In the presence of secondary amines or alcohols, the hydroxyl groups are replaced (69), eg, if diethylamine is present the product is A/-diethylaminomethyl-2-pyrrohdinone [66297-50-5] (35). 

In contrast to phenolic hydroxyl, benzylic hydroxyl is replaced by hydrogen very easily. In catalytic hydrogenation of aromatic aldehydes, ketones, acids and esters it is sometimes difficult to prevent the easy hydrogenolysis of the benzylic alcohols which result from the reduction of the above functions. A catalyst suitable for preventing hydrogenolysis of benzylic hydroxyl is platinized charcoal [28], Other catalysts, especially palladium on charcoal [619], palladium hydride [619], nickel [43], Raney nickel [619] and copper chromite [620], promote hydrogenolysis. In the case of chiral alcohols such as 2-phenyl-2-butanol hydrogenolysis took place with inversion over platinum and palladium, and with retention over Raney nickel (optical purities 59-66%) [619]. 

The monooxygenase group of enzymes includes the non-P450 hydroxylases which catalyze the insertion of a hydroxyl group to replace a hydrogen atom at a saturated carbon [6-8] and the non-heme-dependent oxygenases such as the flavin-molybdenum-cobalt-dependent xanthine oxidase and aldehyde oxidase... 

Several condensation routes to 2,3 -bipyridinones have been reported. Thus 3-acetylpyridine and nicotinaldehyde were condensed to the a,/ -unsat-urated ketone 45, which reacted in a Michael condensation with l-(car-bamoylmethyl)pyridinium chloride (46) to give 2,4-di(3-pyridyl)-6-pyridone (47). Compound 47 was converted to the alkaloid nicotelline 10 by replacement of the hydroxyl group of 47 by chlorine, followed by reductive dehalogenation. Related condensations have been described, including the synthesis of 4,6-diphenyl-2,3 -bipyridine. Similarly, aldehyde 48 was condensed with cyanoacetamide (49) to afford 2-(3-pyridyl)-5-cyano-6-pyridone (50), the cyano group of which was hydrolyzed and decarbox-ylated to 2,3 -bipyridin-6-one. Several modifications and extensions of... 

An ester is an organic molecule similar to a carboxylic acid except that in the ester the hydroxyl hydrogen is replaced by a carbon. Unlike carboxylic acids, esters are not acidic because they lack the hydrogen of the hydroxyl group. Like aldehydes, many simple esters have notable fragrances and are used as flavorings. Some familiar ones are listed in Table 12.4. 

We should also expect stereoelectronic control when the hydroxyl group is replaced by another nucleophile in the reaction with cyclic oxonium ions. A recent report (110) shows that hydride transfer to cyclic oxonium ion is subject to stereoelectronic control. Tricyclic spiroketal 140 (Fig. 19) undergoes an acid-catalyzed oxidation-reduction reaction to give the equatorial bicyclic aldehyde 147 stereospecifically. Similarly, spiroketals 148 and M9 gave the corresponding equatorial bicyclic ketone 150. 

In the Vilsmeier formylation of iV-( 3-hydroxyethyl)anilines the hydroxyl group is simultaneously replaced by chlorine [60], The resulting aldehydes condense with l,3,3-trimethyl-2-methyleneindoline to form very brilliant dyes [61], With 4-(W-methyl-iV- 3-chloroethylamino)benzaldehyde a commercial dye is obtained. A methyl group in the 2-position to the methine chain produces a bluish-red color (27). 

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