Urea, cyclized

The affinity for the S-HTj receptor of the cyclized urea derivatives is weak or absent (Table 18). 

Urea Derivatives. Sun et al. have developed a strategy to synthesize substituted ureas 102 from sodium benzenesulhnate resin 43 (Scheme 12.24). 2-Chloroethanol was hrst coupled with the resin 43 to afford resin 100, which can easily react with carboxylic acid to form ester linkages that can be cleaved under basic or acidic conditions. Treatment of resin 100 with Boc-protected glycine using DCC/DMAP afforded resin 101 whose sulfone linkage was cleaved using HCl in dioxane to yield the cyclized urea derivatives 102. 

Buthidazole and methazole possess heterocyclic rings containing a cyclized urea function, and, like the ureas, they act by inhibiting the Hill reaction in chloroplasts. It is believed that methazole is metabolized to a less lipophilic phenylurea that is the active compound, for methazole itself is a poor inhibitor of the Hill reaction and also is too lipophilic to be well transported to leaves from roots. ... 

Reactions. The reactions of dicyandiamide resemble those of cyanamide. However, cycUzations take place easily and the nitrile group is less reactive. Under pressure and ia the presence of ammonia, dicyandiamide cyclizes to melamine. Considerable toimages of melamine have been made ia this manner however, melamine is produced chiefly by the urea process (43). 

In contrast to the above additions A-allyl- and substituted A-allyl-amides, -urethanes, -ureas and -thioureas undergo intramolecular cyclization only in 6(3-96% sulfuric acid to give the corresponding oxazolinium and thiazolinium salts. Treatment of these cations with base yields 2-oxazolines and 2-thiazolines in moderate to good yields. The reaction is illustrated by the conversion of A-2-phenylallylacetamide (342) into 2,5-dimethyl-5-phenyl-2-oxazoline (343) in 70% yield 70JOC3768) (see also Chapter 4.19). 

Sodium borohydride reduction of 4-substituted isoquinolinium salts led to vinylogous cyanamides, ureas, and urethanes, as well as the corresponding tetrahydroquinolines (640). Hydrogenation of /8-acylpyridinium salts (641) to vinylogous ureas was exploited in syntheses of alkaloids (642), leading, for instance, to lupinine, epilupinine, and corynantheidine (643, 644). Similarly, syntheses of dasycarpidone and epidasycarpidone were achieved (645) through isomerization of an a,/0-unsaturated 2-acylindole and cyclization of the resultant enamine. 

The addition of phenylisocyanate to aldehyde-derived enamines resulted in the formation of aminobutyrolactams (438,439). As aminal derivatives these produets can be hydrolyzed to the linear aldehyde amides and thus furnish a route to derivatives of the synthetically valuable malonaldehyde-acid system. With this class of reactions, a second acylation on nitrogen becomes possible and the six-membered cyclization products have been reported (440). Closely related to the reactions of enamines with isocyanates is the condensation of cyclohexanone with urea in base (441). 

Forty years after the initial proposal, Sweet and Fissekis proposed a more detailed pathway involving a carbenium ion species. According to these authors the first step involved an aldol condensation between ethyl acetoacetate (6) and benzaldehyde (5) to deliver the aldol adduct 11. Subsequent dehydration of 11 furnished the key carbenium ion 12 which was in equilibrium with enone 13. Nucleophilic attack of 12 by urea then delivered ureide 14. Intramolecular cyclization produced a hemiaminal which underwent dehydration to afford dihydropyrimidinone 15. These authors demonstrated that the carbenium species was viable through synthesis. After enone 13 was synthesized, it was allowed to react with N-methyl urea to deliver the mono-N-methylated derivative of DHPM 15. 

Furthermore, Shutalev and coworkers reported a two-step modification. Urea 43a or thiourea 43b was condensed with 5 in the presence of p-toluenesulfonic acid to deliver a-tosylderivative 44. The enolate of 6 was then allowed to react with 44 to give a substitution product which then cyclized to give the hemiaminal 45. Dehydration of the hemiaminal with p-toluenesulfonic acid delivered 46. 

The 6-methyl derivative (98, R = Me) was an important intermediate in the synthesis of analogs (e.g., 183) of folic acid. Korte has shown that 2-aminopyrido[3,2-guanidine carbonate with 3-aminopicolinic acid and that treatment of the same acid with ammonium thiocyanate or potassium cyanate yields the thioureido and ureido derivatives (100, X = S and X = 0). In contrast to the pyrido[2,3-d]pyrimidine system bsoth of these compounds could be cyclized by heat and the latter (100, X = O) is a likely intermediate in the synthesis of the dione (98) by the fusion with urea. 

Melting of 2-aminomethyltetrahydroquinolines 499 with urea gave the imidazo derivative 500 that upon alkylation gave 501 (83JHC139). On the other hand, cyclization of 499 with ethyl acetamide hydrochloride gave 502... 

The ureas, e.g. 28 (R = NMe2), derived from the corresponding 2-(l-arylviny )benzylamines by reaction with (dimethylamino)carbamoyl chloride (Me2NCOCl) in the presence of triethyl-amine, undergo cyclization in refluxing phosphoryl chloride to the 5-aryl-3-(dimethylamino)-l//-2-benzazepin-3-amines. e.g. 29a.84 Prepared similarly are the 3-(4-methylpiperazin-l-yl) compound 29b and the 3-methyl derivative 29c from the corresponding urea and amide, respectively. 

Cyclization of 3,4,5,6-tetrachlorophthalic anhydride (8) in the presence of urea, Ihe appropriate metal salt and catalytic amounts of ammonium molybdate(VI) in nitrobenzene at high temperature leads to phthalocyanine 9. 

Barbituric acid can be considered as a cyclized malonic acid diamide (malonyl-urea). It is therefore a cyclic diketone that may be classified, in the sense of the compounds discussed in Section 12.6, as a coupling component with a methylene group activated by two carbonyl groups in the a- and a -positions. The reaction with arenediazonium salts was studied by Nesynov and Besprozvannaya (1971). These authors obtained coupling products (in good yield) that they considered to be arylhydrazones. Coupling with 4-(phenylazo)benzenediazonium chloride was studied by Chandra and Thosh (1991). The lH NMR spectra of these compounds are consistent with the arylhydrazone structure 12.68. 

Hydantoins have been prepared starting from aminoesters that were first transformed into the corresponding carbanilides (ureas) 144 under standard conditions and then cyclized under microwave irradiation in the presence of Ba(OH)2 in DMF in 2 min and in yields ranging from 91 to 80% (Scheme 51) [91,92], Simple and more complex hi- and tricyclic compounds, such as 145-147, have been prepared with this method that seems versatile and applicable to many substrates. 

The sulfonyl urea sulfometuron methyl is stable at neutral or alkaline pH values, but is hydrolyzed at pH 5 to methyl 2-aminosulfonylbenzoate that is cyclized to saccharin (Figure 1.21) (Harvey et al. 1985). The original compound is completely degraded to CO2 by photolysis. 

Following the enzymatic cleavage, azaquinone methide was rapidly eliminated and decarboxylation occurred, leading to internal cyclization that released a urea derivative and phenol 35. The latter was disassembled as previously described to generate two equivalents of phenol 36, which was further fragmented to release the four reporter groups. 

The phthalide 25, obtainable by condensation of 4,4 -bisdimethyl-aminobenzophenone-2-carboxylic acid with 3-dimethylaminoacetanilide and subsequent hydrolysis, was diazotized in sulfuric acid and the resultant diazonium salt treated with copper powder to yield 26. However, better yields are reportedly obtained by carrying out ring closure of the diazonium salt in phosphoric acid.103 A further synthetic route has also been described in which phthalides undergo intramolecular cyclization in the presence of aluminum chloride and urea.104,105 Thus, Crystal Violet lactone (2) has been directly converted into phthalide 26.106... 

A fused heterocyclic compound (146) distantly related to the antiinflammatory agent cintazone (Chapter 12), which itself can be viewed as a cyclized derivative of phenylbutazone, retains the activity of the prototype, in the synthesis of 146, reaction of the nitroaniline 139 with phosgene gives intermediate 140, which is then reacted with ammonia to afford the substituted urea (141). Cyclization of the ortho nitrourea function by means of sodium hydroxide leads to the N-oxide (142) this last reaction represents... 

In 1997, the controversial mechanism of the Biginelli reaction was reinveshgated by Kappe using NMR spectroscopy and trapping experiments [94], and the current generally accepted process was elucidated (see Scheme 9.23). The N-acyliminium ion 9-112 is proposed as key intermediate this is formed by an acid-catalyzed reaction of an aldehyde with urea or thiourea via the semiaminal 9-111. Intercephon of 9-112 by the enol form of the 1,3-dicarbonyl compound 9-113 produces the open-chain ureide 9-114, which cyclizes to the hexahydropyrimidine 9-115. There follows an elimination to give the final product 9-116. 

The yields of AEG were between 11 and 79%, at starting concentrations of 10 —10 4 M. AEG cyclizes to the lactam, monoketopiperazine, at neutral pH and around 373 K. The synthesis of the monomeric PNA units was carried out three years later (Nelson et al., 2000a, b). The synthesis of pyrimidine-A -acetic acid can be achieved if the reaction of cyanoacetaldehyde with high concentrations of urea mentioned above is modified slightly, with hydantoic acid replacing urea (Robertson and Miller, 1995). 

The described fluorous-tag strategy has also been applied to the synthesis of biaryl-substituted hydantoins (Scheme 7.81) [94]. 4-Hydroxybenzaldehyde was converted into the corresponding perfluorinated species, which was then subjected to a reductive amination. The resulting amine was treated with an isocyanate to produce the fluorous-tagged urea, which spontaneously cyclized to form the corresponding hydantoin. Finally, the fluorous tag was detached by a Suzuki-type carbon-carbon bond formation to furnish the desired target structure in good yield. 

Treatment of 2-benzoylpyridine 81 with p-toluene-sulfonamide gave 1-amino-2-benzoylpyridinium tosylate 82 (X = OTs), which was cyclized with formamide in the presence of triethylamine hydrobromide to give 83 (82FRP2486942). The reaction of the perchlorate 82 (X = C104) with urea in polyphosphoric acid afforded 3-hydroxy-l-phenylpyrido[2,l-/][l,2,4]triazinium perchlorate 84. Treatment of this salt with base led to the zwitterionic l-phenylpyrido[2,l-/][l,2,4]triazin-5-ium-3-olate 85 (86JHC375). Pharmaceutical compositions contain 83 (82FRP2486942). 

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