Acid-catalyzed condensation with urea

Crotonylidene Diurea. (CDU). Crotonjlidene [1129 2-6] is produced by the acid catalyzed reaction of urea with either crotonaldehyde or acetaldehyde. The condensation reaction produces a ring-stmctured compound. Table 4 hsts select properties. 

Plywood and particle board are often glued with cheap, waterproof urea-formaldehyde resins. Two to three moles of formaldehyde are mixed with one mole of urea and a little ammonia as a basic catalyst. The reaction is allowed to proceed until the mixture becomes sympy, then it is applied to the wood surface. The wood surfaces are held together under heat and pressure, while polymerization continues and cross-linking takes place. Propose a mechanism for the base-catalyzed condensation of urea with formaldehyde to give a linear polymer, then show how further condensation leads to cross-linking. (Hint The carbonyl group lends acidity to the N—H protons of urea. A first condensation with formaldehyde leads to an inline, which is weakly electrophilic and reacts with another deprotonated urea.)... 

The first step in the mechanism of the Biginelli reaction is the acid-catalyzed condensation of the urea with the aldehyde affording an aminal, which dehydrates to an A/-acyliminium ion intermediate. Subsequently, the end form of the 3-keto ester attacks the A/-acyliminium ion to generate an open chain ureide, which readily cyclizes to a hexahydropyrimidine derivative. 

Heterocycles. Base-catalyzed condensation of urea with diethyl oxalate gives parabanic acid (Murray ) condensation with diethyl malonate gives barbituric acid (Dickey and Gray ). The first procedure uses a solution prepared from sodium... 

Acid-catalyzed Condensation of Sugars With Urea. 217... 

The glycosyl ureide and thioureide obtained through the isocyanate and isothiocyanate routes were prepared from tetra-O-acetyl-a-D-glucopyranosyl bromide. The glycosyl ureides derived from this source are identical with those prepared by the acid-catalyzed condensation of D-glu-cose with urea and with thiourea. 

Some 1-aryl-substituted hydantoins have been synthesized by acid-catalyzed condensation of monosubstituted ureas and glyoxal with moderate yields.34... 

Another belt-shaped molecule is cucurbituril (53) which was presumably synthesized as early as 1905 by Behrend et al. [42]. The structural formula of 53 was resolved in 1981 by Freeman. Mock, and Shih [43] only when modern spectroscopic methods were available. Cucurbituril (53) is easily obtained from urea, glyoxal, and formaldehyde in an acid-catalyzed condensation reaction with glycoluril (52) as an intermediate. The initially formed polycondensation product, insoluble in all common solvents, is treated with hot concentrated sulfuric acid. After dilution with water and subsequent boiling a solid is obtained in a yield of 40-70%. The H-NMR spectrum shows only three signals of equal intensity and the infrared spectrum suggests retention of the glycoluril moieties. The compound is not suitable for usual mass spectrometry due to its low volatility. However, an X-ray crystal structure of the calcium complex obtained from sulfuric acid solution was undertaken. 

Oxalacetic acid 264 in the acid-catalyzed Biginelli condensation with urea/thiourea and an aldehyde gave C-5 unsubstituted 265 bearing a carboxylic acid at C-6 (Scheme 103) (OOJOC6777,11T1294). 

The acid-catalyzed condensation of ethyl levulinate with urea was investigated. After the mixture was heated, ethanol evolved, so it is presumed that the condensation involved both carbonyl groups of the levulinate ester. The reaction produced a sticky resin that is soluble in alcohol. 

Closely related to the Pinner protocol are (a) the base-catalyzed condensations of ureas with malonates to afford derivatives of barbituric acid (e.g., 18, cf p. 473), (b) the condensations of ureas with cyanoacetates promoted by Ac2 O (-> cyanoacetyl ureides 19) followed by base-induced cyclization to give 4-aminouracil derivatives (19 — 20) [258] ... 

In 1893, the Italian chemist Pietro Biginelli (University of Florence) for the first time reported on the acid-catalyzed cyclocondensation reaction of ethyl acetoacetate 1, benzaldehyde 2, and urea 3 [1], The reaction was carried out by simply heating a mixture of the three components dissolved in ethanol with a catalytic amount of HC1 at reflux temperature. The product of this novel one-pot, three-component synthesis that precipitated on cooling the reaction mixture was identified as 3,4-dihydropyrimidin-2(lH)-one 4 (Scheme 4.1) [2]. This reaction is nowadays referred to as the Biginelli reaction , Biginelli condensation or as the Biginelli dihydropyrimidine synthesis . 

The reaction of -halo carbonyl compounds with primary amides is appropriate for oxazoles containing one or more aryl groups . Ureas form 2-aminooxazoles. Formamide can be used resulting in a free 2-position in the oxazole. A convenient synthesis of 5-substituted-4-cyanooxazoles 223 is based on the condensation of -hydroxy—cyanoenamines 222 with trimethyl orthoformate (Scheme 109). The cyanoenamine intermediates 222 are derived from Lewis acid-catalyzed Passerini reactions between /-butyl isonitrile and aldehydes <2002S1969>. 

Lipides, 238, 239, 241-243, 261 Lipidosis, 239, 242, 243 Lobry de Bruyn-Alberda van Ekenstein transformation, 63, 291 acid catalysis of, 79 aldolization in, 77 base catalysis of, 79-81 catalysis of, by metal ions, 81 dealdolization in, 77 dehydration reactions in, 73 enzyme-catalyzed, 66, 70 formation of reductones in, 79 of or-hydroxy aldehydes, 71 mechanism of, 84 of noncarbohydrate a-ketols, 71 non-enzymic, 66, 67, 83 in paper chromatography, 81 rearrangement of carbon chain, 79 scope of, 65 of steroids, 72 use of, for synthesis, 82 Lyxonic acid, 3-deoxy-D-, 300 Lyxose, D-, condensation of, with urea, 218... 

A cationic starch can be prepared by blending a condensate of formaldehyde plus an inorganic ammonium salt with starch and heating at 80 °C for up to 1 h.1316 A great interest was developed with urea-formaldehyde resins that were made to react with such polyalcohols as starch.1317-1319 It was reported that the reaction of starch with urea-formaldehyde resin required an acidic catalyst592,1320-1325 and the use of such dispersants as polyphosphates.1326 Ammonium metaphosphate catalyzed the reaction without the addition of any other acid catalyst.1327 A product that formed a paste was produced by controlling the proportion of starch to aldehyde (either up to 20% of formaldehyde or up to 30% of acetaldehyde) and urea (up to 15%) at pH 2 1.1328... 

Glacial acetic acid-catalyzed Biginelli condensation of 2-(2-dimethyla-minovinyl)-5,6-dicyanobenzofurans 227 (R = Me, OMe) with aldehydes and urea/thiourea furnished 5-(benzofuran-2-yl) substituted 228 (X = O, S R = Me, OMe) (Scheme 87) (11MC46). 

Condensation of the vinylogous carbamate (40) with acetaldehyde and isocyanic acid afforded the pyrimidine derivative (41) which was converted to the urea derivative (42) by Curtius rearrangement in 65% overall yield. Treatment of (42) with acetic acid at 50° afforded exclusively the desired product (39), whereas treatment with trifluoroacetic acid gave a mixture of (39) and stereoisomer (43). The stereochemistry was established by comparing the coupling constants involving H-5 and H-6 with those of saxitoxin (1). Different cyclization mechanisms were suggested for the acetic acid and trifluoroacetic acid catalyzed cyclization to account for the different stereochemistry of the products. 

Furan no-bake resins are two-component, r.t.-cur-ing, acid-catalyzed systems of furfiiryl alcohol prepolymer with additional monomer. Hot and warm box resins are mainly based on urea-modified fur-ftiryl alcohol-formaldehyde condensates that are cured at 100-170 °C. Also phenol-formaldehyde resins, modified with furfiiryl alcohol, are used. In cold box binder systems, the sand is mixed with a low-viscosity fiiran resin and a peroxide. The core is formed, and SO2 is blown into or generated in situ in the sand to cure the resin rapidly. 

Recently, enantioselective organo-catalytic procedures for the aza-Henry reaction have been disclosed. The presence of either an acidic or a basic function appears to be a requisite of the catalyst. In fact, the condensation of ni-tromethane with M-phosphinoyl arylimines 72 is catalyzed by the chiral urea 85 derived from (R,R)-l,2-diaminocyclohexane and gives the product (R)-74 with good yield and moderate enantioselectivity (Scheme 15) [50]. The N-phosphinoyl substituent is determinant, as the addition of nitromethane to the N-phenyl benzaldimine failed and the reaction of the N-tosyl ben-zaldimine gave the expected adduct with quantitative yield but almost no... 

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