Other Aldehyde Condensations

Melamine reacts with several other aldehydes as well as with alcohols to give etherified products. Most other aldehydes give resinous products that have not been fully characterized. Rudelburgher reacted melamine (2 moles) with glucose (1.0 mole) to obtain a derivative melting at 218°C [69]. The synthesis of a curable melamine-acetaldehyde resin was reported by Henkel Co. [70]. The reaction of melamine with aldehydes such as benzaldehyde, cinnamaldehyde, hexahydrobenzaldehyde, terephthalaldehyde, and acrolein have been reported [71]. 

TABLE XII Reaction of Melamine with Aldehydes Other than Formaldehyde 

Melamine (moles) Aldehyde (moles) Alcohol (or water) (gm) Temp. CC) Time (min) Yield (%) M.p. CC) Ref. 

Chemical Abstracts names this compound as follows l,r-[(6-amino-5-triazine-2,4-diyI)diimo]biS 2,2,2-trichloroellianol see Chcnu Abstr. 61, 831 If. (1964), and formula index under C7H8CJ6N6O2. 

To a flask equipped with a stirrer and condenser is added a solution of 132gm (0.8 mole) of chloral hydrate in 125 ml of water. The solution is stirred at 70 -80°C while 12.6 gm (0.1 mole) of pure melamine is added. Within about 2 min the melamine is all dissolved, and a granular solid begins to precipitate. After heating for another 5 min the suspension is cooled, and the product is filtered, washed with water, and dried to afford 35.0 gm (83 o). Heating the product on a spatula causes it to decompose to chloral and melamine. However, a determination of the melting point on a Fisher-John melting point apparatus indicates it melts at 327°-329 C (see Table XII). The literature [72] reports that it melts with decomposition. 

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Numerous other aldehyde condensation reactions of five-membered heterocycles have been utilized in the synthesis of the central pyridine core. Paulmier and co-workers employed the condensation of 3-aminothiophenes 114 with aldehydes to give bis-thiophenylpyridines 115 (Equation 25) <1996JHC9>. 

Just as most other aldehydes do, furfural condenses with compounds possessing active methylene groups such as aUphatic carboxyUc esters and anhydrides, ketones, aldehydes, nitriles, and nitroparaffins. 

A few other aldehydes have been used in the reaction, either under normal or pseudo-physiological conditions. Of these, glycolalde-hyde, 5-hydroxypentanal, phenylacetaldehyde, and benzalde-hyde condense readily, but hydroxy and methoxy derivatives of these aromatic aldehydes give the product in poor yield,presumably due to their instability, as evidenced by their tendency to undergo self-condensation in acid solution. Reaction with phthaldehydic acids, such as opianic acid, proceeded readily, whereas reaction with chloral did not occur,... 

Other aldehydes which have been used in the reaction are pro-panal, butanal, glycolaldehyde, 3-hydroxybutanal, and a number of phenylacetaldehydeand benzaldehyde derivatives. Whereas condensation of tryptophan with acetaldehyde takes place even at room temperature and pH 6.7, the reactions with chloral, chloroacetaldehyde, and crotonaldehyde fail entirely. 

The product of condensation of a hydrazine and an aldehyde or ketone is called a hydrazone. Hydrazine itself gives hydrazones only with aryl ketones. With other aldehydes and ketones, either no useful product can be isolated, or the remaining NH2 group condenses with a second mole of carbonyl compound to give an azine. This type of product is especially important for aromatic aldehydes ... 

Amino acid formation in the Urey-Miller experiment and almost certainly in the prebiotic environment is via the Stecker synthesis shown in Figure 8.3. This reaction mechanism shows that the amino acids were not formed in the discharge itself but by reactions in the condensed water reservoir. Both HCN and HCO are formed from the bond-breaking reactions of N2 and H2O in a plasma, which then react with NH3 in solution. The C=0 group in formaldehyde or other aldehydes is replaced by to form NH and this undergoes a reaction with HCN to form the cyano amino compound that hydrates to the acid. The Strecker synthesis does not provide stereo-control over the carbon centre and must result in racemic mixtures of amino acids. There is no room for homochirality in this pathway. 

Krausz el al. have synthesised two series of amino acid porphyrinylsugar derivatives (Fig. 5). One of them involves the coupling of adequate glycoporphyrin derivatives, prepared by pyrrole/aldehyde condensation methodology, with 9-fluorenylmethoxycarbonyl-L-alanine (Fmoc-L-alanine) to give the tri-, di-, and mono-alanine glycoporphyrin derivatives 54—57 after the deprotection step the other series (58) involves a glucosylamino acid moiety instead of the alanine in their preparation.21,44... 

The Henry condensation of nitroform and terminal dinitromethyl compounds with formaldehyde and other aldehydes, followed by nitration of the resulting alcohol functionality, has been used to synthesize numerous explosives. The nitrate esters (104), (105), (106) " and (107)155 ij yg ijggjj synthesized from the action of absolute nitric acid on the parent alcohols. In a similar manner, NMHP (109) is synthesized from the condensation of TNHP (108) with formaldehyde, followed by O-nitration with absolute nitric acid. ... 

In far too many instances trade-name polymer nomenclature conveys very little meaning regarding the structure of a polymer. Many condensation polymers, in fact, seem not to have names. Thus the polymer obtained by the step polymerization of formaldehyde and phenol is variously referred to a phenol-formaldehyde polymer, phenol-formaldehyde resin, phenolic, phenolic resin, and phenoplast. Polymers of formaldehyde or other aldehydes with urea or melamine are generally referred to as amino resins or aminoplasts without any more specific names. It is often extremely difficult to determine which aldehyde and which amino monomers have been used to synthesize a particular polymer being referred to as an amino resin. More specific nomenclature, if it can be called that, is afforded by indicating the two reactants as in names such as urea-formaldehyde resin or melamine-formaldehyde resin. 

Aldehydes condense with DISN to give 2-alkylimidazoles, but in low yield (73USP37099(X)). A related reaction with ketones produces 2,2-dialkyl-4,5-dicyano-2//-imidazoles. When one alkyl group is methyl and the other is a... 

In a survey of chemical plants (without prior hypothesis) in the German Democratic Republic, nine cancer cases were found in a factory where the main process was dimerization of acetaldehyde and where the main exposures were to acetaldol (3-hydroxybu-tanal), acetaldehyde, butyraldehyde, crotonaldehyde (IARC, 1995) and other higher, condensed aldehydes, as well as to traces of acrolein (lARC, 1985). Of the cancer cases, five were bronchial tumours and two were carcinomas of the oral cavity. All nine patients were smokers. The relative frequencies of these tumours were reported to be higher than those expected in the German Democratic Republic. [The Working Group noted the mixed exposure, the small number of cases and the poorly defined exposed population.]... 

The preparation of the first unsaturated azlactone was reported in 1883 by Plochl/40 who condensed benzaldehyde with hippuric acid in presence of acetic anhydride. This approach was later used by Erlenmeyer/41 who extended the procedure to include other aldehydes and also established the usefulness of azlactones as intermediates in the synthesis of DHAs. The method involves the condensation of an A-acylglydne 4 with aldehydes and ketones in the presence of acetic anhydride and anhydrous sodium acetate (Scheme 2)J41 t5l Other catalysts such as copper(II) acetate/46 lead acetate/47,48 potassium carbonate/49 or potassium hydrogen carbonate 50 have also been used. The reaction proceeds via formation of an azlactone 5, which then condenses with the appropriate aldehyde or ketone to give unsaturated azlactone 6. Reaction of 6 with a nucleophile such as OH, OR, or NHR leads to the corresponding A-acyl-DHA derivatives 7. Reaction with the sodium salt of an amino acid gives a DHA containing dipeptide acid. 51 ... 

Xanthene itself results from the dehydration of 2,2 -dihydroxydiphenylmethane (43JOC316), and this route provides access to the important derivative, 3,6-bis(dimethyl-amino)xanthene (192). The reactive m -dimethylaminophenol condenses readily with formaldehyde and subsequent cyclization occurs in sulfuric acid (Scheme 37) (1896JPR(54)217). Other aldehydes behave in a similar manner to yield 9-substituted xanthenes (12LA(391)308). 

Most of the reactions of aromatic aldehydes, ArCHO, are those expected of aldehydes with no a hydrogens and most of these will not be reviewed here. One reaction that usually is regarded as being characteristic of aromatic aldehydes (although, in fact, it does occur with other aldehydes having no a hydrogens), is known as the benzoin condensation. This reaction essentially is a dimerization of two aldehyde molecules through the catalytic action of sodium or potassium cyanide ... 

Melt poly condensation is also the most popular method for other thermotropic condensation polymers, including the polyazomethines where the reaction between aromatic aldehydes or ketones and primary amines with elimination of water leads to azomethine (Schiffs base) formation 48). 

The data in Table III illustrate that condensation of VII with isobutyraldehyde is a satisfactory synthesis of VIII which can be hydrolyzed to the desired phosphonomethylphosphinic acid. This method has been tested with several other aldehydes and found to give results similar to those in Table III. 

These aldehyde-condensation products are solvents for the catalyst. This allows the unit to operate without need for other solvents and forestalls the need for a catalyst-recycle step. 

Figure 13.50 outlines how esters in general (not shown) and especially lactones (shown) can be prepared for a one-step aldol condensation with an aldehyde they are exposed to a mixed ( crossed ) Claisen condensation with formic acid methyl ester (cf. Figure 13.59, first line). Like all Claisen condensations (Section 13.5.1), this also first leads to the formation of the enolate of the acylated ester. Unlike other Claisen condensations, this enolate is isolated. 

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