Replacement of the Aldehyde Group

Dakin reaction conditions have been used on 2-hydroxyisophthaldehyde in 85% aqueous potassium hydroxide added over 45 mins, to a cooled solution of 26% hydrogen peroxide at 5-10°C with continued stirring for 2.5 hours, to furnish pyrogaltol in 64% yield (ref.66). 

By contrast, acidic conditions for the transformation of 2-methoxybenzaldehyde in methanol containing 31% hydrogen peroxide and a little sulphuric acid were used at ambient temperature during 24 hours to give a 94% yield of 2- 

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Monocarboxylic acids formally derived from aldoses by replacement of the aldehydic group by a carboxy group are termed aldonic acids (see 2-Carb-20). 

The reactions of the phenolic aldehydes may be conveniently grouped under etherification of the phenolic hydroxyl group, replacement of the aldehyde group and condensation reaction of the latter. 

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. 

In 1982, a new reaction was reported by Tamura and Ono namely, allylic nitro compounds undergo replacement of the nitro group by various nucleophiles in the presence of a palladium (0) catalyst.17a b 18a b The details of these reactions are discussed in Ref. 2b here, only some typical examples are presented. Carbon, sulfur, nitrogen, and phosphorous centered nucleophiles replace the nitro groups at the allylic positions. The reaction of allylic nitro compounds with triphenylphosphine is applied to the highly stereoselective olefination of aldehydes (Eqs. 7.15-7.18).19... 

IR spectroscopy has been applied more than the UV-Vis, as this technique can establish unequivocally the presence of various functionalities. The strong IR and Raman features of DTT-A4, 78, at 1650 cm 1, due to the aldehyde y(C=0) stretching are no longer present after replacement of the -CHO group by the dicyanoethylene group in DTT-A5. New IR and Raman bands appeared at 2218 and 1570 cm-1 in case of DTT-A5, 79 due to y(C=N)... 

Replacement of the triphenyl group by trialkyl groups increases the trans-selectivity, but increase in the bulk of the customary phenyl groups can increase cis-selectivity. Tris(o,o -difluorophenyl)phosphine (m.p. 125-127°) is recommended as a replacement for triphenylphosphine in dr-selective Wittig reactions, particularly of aromatic and a,(l-unsaturated aldehydes. 

Suzuki and co-workers achieve aromatic substitution of fluoroarenes with a variety of aldehydes in good yields [91, 92], Imidazolilydene carbene formed from 143 catalyzes the reaction between 4-methoxybenzaldehyde 22a and 4-fluoroni-trobezene 141 to provide ketone 142 in 77% yield (Scheme 20). Replacement of the nitro group with cyano or benzoyl results in low yields of the corresponding ketones. The authors propose formation of the acyl anion equivalent and subsequent addition to the aromatic ring by a Stetter-like process forming XXVIII, followed by loss of fluoride anion to form XXIX. 

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... 

Tris(diethyl)amino]phosphoniuni 2,2,3,3,4,4-hexafluorocyclobutane ylide (2) is a very hydroscopic solid, which is stable for long periods of time when stored under anhydrous conditions.13 It is unreactive towards most functional groups such as C = C bonds, ketones, aldehydes, and esters but reacts smoothly under neutral conditions with alcohols or carboxylic acids with replacement of the hydroxy group by a fluorine atom.44,45 Other tris(dialkylammo-nium)phosphonium perfluorocyclobutanes have been synthesized however, they are less reactive fluorinating reagents.44,45... 

When a diazonium compound is boiled with an alcohol, oxidation of the latter to the corresponding aldehyde and simultaneous replacement of the diazonium group by hydrogen occurs. 

An important example of this effect is the decrease in equilibrium constants for ketones as compared to aldehydes. The replacement of the aldehyde hydrogen of acetaldehyde (K = 1.3) with a methyl group, to produce acetone (K = 2 X 10-3), results in a decrease in the equilibrium constant for hydration by a factor of approximately 1000. The inductive effect of the electron-donating alkyl group also helps shift the equilibrium for ketones toward the reactant. 

Carbonyl compounds are reduced to symmetrical ethers, probably by way of reduction of some of the starting material to a silyl ether (9), reaction to form the mixed ketal (10) and then reductive replacement of the silyloxy group. Some hydrocarbon may be obtained as a by-product by reduction of (9 Scheme 4). Among the acid partners that have been used are trifluoroacetic acid, trityl perchlorate (with aldehydes) and electrogenerated protons. With Nafion resin symmetrical ethers are obtained from aldehydes, but silyl ethers are obtained from ketones. ... 

Acids related to monosaccharides are also common for the formation of natural polysaccharides. There are three types of acids associated with monosaccharides aldonic acids generated by the oxidation of the aldehyde group of a monosaccharide, uronic acids generated by the replacement of the primary alcohol group of a monosaccharide by a carboxyl group, and saccharic acids generated by simultaneous oxidation and replacement. Aldonic acids have a marked tendency to eliminate water and form lactones. The structural formulas of two common hexuronic acids are... 

Dakin reaction. Replacement of the aldehyde or acetyl groups in phenolic aldehydes or ketones by a hydroxyl group by means of hydrogen peroxide. 

With furfural, the oxygen of the aldehyde group, as well as that of the furan ring, was replaced by sulfur. In addition, condensation and cleavage reactions led to a complex mixture of products. 

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