5 -Oxazolones dimerization

Dehydration of 4-hydroxy-4,5,5-trimethyl-2-oxazolidinone 301 with a catalytic amount of p-toluenesulfonic acid affords the isomeric dimers 303 and 304 in a ratio of 1 2. The former type of dimer, that is, 303 likely results from coupling of the isomeric 2(5//)-oxazolone 302A with 4-methylene-2-oxazolidinone 302B, which are the initially formed dehydration products. On prolonged heating in CH2CI2, the 2(5//)-oxazolone dimer 303, completely isomerizes to the 4-methylene-2-oxazoh-dinone dimer 304 (Fig. 5.77). " ... 

Oxidative dimerization of various 2-benzyloxy-2-thiazoline-5-ones (222) catalyzed by iodine and triethylamine is another example of the nucleophilic reactivity of the C-4 atom (469) (Scheme 112). Treatment of 212 with pyrrolidinocyclohexene yields the amide (223) (Scheme 113). The mechanism given for the formation of 223 is proposed by analogy with the reactivitx of oxazolones with enamines (4701. 4-Substituted 2-phenylthiazol-5(4Hi-ones react with A -morphoiino-l-cyclohexene in a similar manner (562j. Recently. Barret and Walker have studied the Michael addition products... 

Methyl-2-phenyl-5-oxazolone (22) reacts with excess phenyl-magnesiuin bromide to give 2-benzainido-1,1-diphenyl-1-propanoP (23). With excess ethylmagnesium bromide, 22 forms a dimer, N,N -... 

Cycloadditions have been carried out to 37/-indoles (222, 223) (125,126), N-arylmaleimides (224) (127,128), l,2), -azaphospholes (225) (129), 5(47/)-oxazo-lones (226) (130), and 4,5-dihydrooxazoles (230) (131). The primary cycloadducts from the reaction of oxazolones (e.g., 226 with diaryl nitrile imines), derived from tetrazoles in refluxing anisole, do not survive. They appear to lose carbon dioxide and undergo a dimerization-fragmentation sequence to give the triazole 228 and the diarylethene 229 as the isolated products (130). In cases where the two aryl substituents on the oxazole are not the same, then, due to tautomerism, isomeric mixtures of products are obtained. 

Some unusual reactions have been described for 2-(4-chlorophenyl)-2-(3,3-dimethylallyl)-4-phenyl-5(277)-oxazolone 84. This compound undergoes a Lewis acid-catalyzed rearrangement to give a tetrahydrofuropyrrole 85. On the other hand, depending on the reaction conditions, thermolysis of 84 produces the azabicyclohexene 86 or a substituted 2,3-dihydropyridine 87 together with the caged compound 88 formed by dimerization of the 2,3-dihydropyridine and the azabicyclohexene (Scheme 7.21). " ... 

Dimerization of 5(4//)-oxazolones affords two different products depending on the reaction conditions. In one case, 4-benzyl-2-phenyl-5(4F/)-oxazolone 291 was converted to the pyrrolidinedione 292 with potassium carbonate followed by acidic hydrolysis (Scheme 7.96). ... 

A kinetic study of the base-catalyzed dimerization of 5(477)-oxazolones has shown that 5(477)-oxazolones with sterically demanding or electron-donating substituents at C-2 are less prone to dimerization. ... 

Dimerization can also be achieved using nickel peroxide " or through a photooxidation reaction. In these cases, 4-monosubstituted-5(4//)-oxazolones 293 are converted to the corresponding 4,4 -bis(oxazolones) 294 (Scheme 7.97 ... 

TABLE 7.26. 4,4 -BIS(OXAZOLONES) FROM DIMERIZATION OF SATURATED 5(477)-OXAZOEONES... 

The 4-acyl-l,2,3-triazole (96) gave an interesting pinacol-type dimer (97) by photoreduction in isopropanol118 [Eq. (6)]. When a heterocycle has an exocyclic C=C bond, a photodimer of type B2 can be formed for example, the dimer (99) was obtained1183 from the pseudo-oxazolone (98) [Eq, (7)]. 

Oxidation of 2,4-diaryl-5(4//)-oxazolones (94) with molecular oxygen gives dimeric products (Equation (7)) <9iT568i>. The reaction rate is dependent on the solvent and oxygen concentration. The oxidation is faster in highly polar solvents, such as DMSO and DMF, that favor the mesoionic form. The dimers are also formed by nickel peroxide oxidation <85T234l>. 

Methylene-3-phenyl-2-oxazolidone in dry ether treated with a catalytic amount of H2SO4 oxazolone-oxa-zolidone dimer. Y 95%. 

Martinez, Urriolabeitia, and coworkers have studied the regioselective C-H activation of (Z)-2-aryl-4-arylidene-5(4//)-oxazolones, 22, at Pd(OAc)2 in acidic media [27]. As seen in related studies on Af-methylbenzylimine [19], reactivity is enhanced by increased acidity and favors the arylidene position (i.e., C-H ) to give, unusually, a six-membered palladacycle, isolated as a dimer. B3LYP(MeC02H) calculations (X = Y=H) showed reduced barriers to C-H activation upon sequential protonation of the Pd active species and... 

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