Aldehydic Group

Figure B2.4.2. Eyring plot of log(rate/7) versus (1/7), where Jis absolute temperature, for the cis-trans isomerism of the aldehyde group in fiirfiiral. Rates were obtained from tln-ee different experiments measurements (squares), bandshapes (triangles) and selective inversions (circles). The line is a linear regression to the data. The slope of the line is A H IR, and the intercept at 1/J = 0 is A S IR, where R is the gas constant. A and A are the enthalpy and entropy of activation, according to equation (B2.4.1)...

Lactose on hydrolysis gives glucose and an isomeric monosaccharide galactose, which may be given the symbol Ga-r. The lactose molecule may be represented as Ga-r-G-r, and it has therefore also a free potential aldehyde group and is a reducing sugar like maltose. 

This reaction is given by most aromatic aldehydes having the aldehyde group directly joined to the benzene ring it is also given by formaldehyde, with the formation of methanol and formic acid. Other aliphatic aldehydes do not give Cannizzaro s reaction under these conditions. 

After the umpolung of an aldehyde group by conversion to a l,3 dithian-2-ide anion (p. 17) it can be combined with a carbonyl group (D. Seebach, 1969, 1979 B.-T. GrO-bel, 1977 B). Analogous reagents are tosylmethyl isocyanide (TosMIC), which can be applied in the nucleophilic formylation of ketones (O.H. Oldenziel, 1974), and dichloromethyl lithium (G. KObrich, 1969 P. Blumbergs, 1972 H. Taguchi, 1973),... 

An example of an intermolecular aldol type condensation, which works only under acidic catalysis is the Knoevenagel condensation of a sterically hindered aldehyde group in a formyl-porphyrin with a malonic ester (J.-H. Fuhrhop, 1976). Self-condensations of the components do not occur, because the ester groups of malonic esters are not electrophilic enough, and because the porphyrin-carboxaldehyde cannot form enolates. 

The decarboxylation of allyl /3-keto carboxylates generates 7r-allylpalladium enolates. Aldol condensation and Michael addition are typical reactions for metal enolates. Actually Pd enolates undergo intramolecular aldol condensation and Michael addition. When an aldehyde group is present in the allyl fi-keto ester 738, intramolecular aldol condensation takes place yielding the cyclic aldol 739 as a main product[463]. At the same time, the diketone 740 is formed as a minor product by /3-eIimination. This is Pd-catalyzed aldol condensation under neutral conditions. The reaction proceeds even in the presence of water, showing that the Pd enolate is not decomposed with water. The spiro-aldol 742 is obtained from 741. Allyl acetates with other EWGs such as allyl malonate, cyanoacetate 743, and sulfonylacetate undergo similar aldol-type cycliza-tions[464]. 

In a more elaborate and specific synthesis, the terpenoid indole skeleton found in haplaindole G, which is isolated from a blue-green alga, was constructed by addition of a nucleophilic formyl equivalent to enone 6.5A. Cyelization and aromatization to the indole 6.6B followed Hg -catalysed unmasking of the aldehyde group[6]. 

The aldotetroses are the four stereoisomers of 2 3 4 trihydroxybutanal Fischer pro jections are constructed by orienting the molecule m an eclipsed conformation with the aldehyde group at the top The four carbon atoms define the mam chain of the Fischer projection and are arranged vertically Horizontal bonds are directed outward vertical bonds back... 

Generating Haworth formulas to show stereochemistry m furanose forms of higher aldoses is slightly more complicated and requires an additional operation Furanose forms of D ribose are frequently encountered building blocks m biologically important organic molecules They result from hemiacetal formation between the aldehyde group and the C 4 hydroxyl... 

When the aldehyde group is directly attached to a carbon atom of a ring system, the suffix -carbaldehyde is added to the name of the ring system, e.g., 2-naphthalenecarbaldehyde. When the aldehyde group is separated from the ring by a chain of carbon atoms, the compound is named (1) as a derivative of the acyclic system or (2) by conjunctive nomenclature, for example, (1) (2-naphthyl)propionaldehyde or (2) 2-naphthalenepropionaldehyde. 

An aldehyde group is denoted by the prefix formyl- when it is attached to a nitrogen atom in a ring system or when a group having priority for citation as principal group is present and part of a cyclic system. 

Ring stmcture and location of the aldehyde group were estabUshed by the efforts of Baeyer, Markwald, and Harries (11—14). For some time, furfural was a laboratory curiosity and a compounding ingredient of perfumes. 

The common method of naming aldehydes corresponds very closely to that of the related acids (see Carboxylic acids), in that the term aldehyde is added to the base name of the acid. For example, formaldehyde (qv) comes from formic acid, acetaldehyde (qv) from acetic acid, and butyraldehyde (qv) from butyric acid. If the compound contains more than two aldehyde groups, or is cycHc, the name is formed using carbaldehyde to indicate the functionaUty. The lUPAC system of aldehyde nomenclature drops the final e from the name of the parent acycHc hydrocarbon and adds al If two aldehyde functional groups are present, the suffix -dialis used. The prefix formjlis used with polyfunctional compounds. Examples of nomenclature types are shown in Table 1. 

The location of the hydroxyl and aldehyde groups ortho to one another in saUcylaldehyde permits intramolecular hydrogen bonding, and this results in the lower melting point and boiling point and the higher acid dissociation constant observed relative to -hydroxybenzaldehyde. 

The effect of the aldehyde group on the phenoHc hydroxyl group is primarily an increase in its acidity both 2-hydroxy- and A- diX< > Lyben ldehydes are stronger acids than phenol (pK 20°C) = 9.89 see Table 2 for comparison). The aldehyde group, however, has Httie effect on the reaction of the... 

Another synthesis of Lyral (51) consists of the reaction of myrcene with acrolein to give the myrac aldehyde [37677-14-8] (52). The aldehyde group, which is sensitive to acid hydration conditions with strong acids, has to be protected by formation of the morpholine enamine. The enamine is then hydrolyzed on workup after the acid-catalyzed hydration to produce Lyral (93—95). 

Aldol reaction of campholenic aldehyde with propionic aldehyde yields the intermediate conjugated aldehyde, which can be selectively reduced to the saturated alcohol with a sandalwood odor. If the double bond in the cyclopentene ring is also reduced, the resulting product does not have a sandalwood odor (161). Reaction of campholenic aldehyde with -butyraldehyde followed by reduction of the aldehyde group gives the aHyUc alcohol known commercially by one manufacturer as Bacdanol [28219-61 -6] (82). 

Other methods of protecting the aldehyde group include formation of an enol acetate, an enamine, or an imine (174,175). In the enamine route, regeneration of the aldehyde is accompHshed simply by the addition of water. 

Vanillin is a compound that possesses both a phenoHc and an aldehydic group. It is capable of undergoing a number of different types of chemical reactions. Addition reactions are possible owing to the reactivity of the aromatic nucleus. 

Because vanillin is a phenol aldehyde, it is stable to autooxidation and does not undergo the Cannizzarro reaction. Numerous derivatives can be prepared by etherification or esterification of the hydroxy group and by aldol condensation at the aldehyde group. AH three functional groups in vanillin are... 

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