5 2// -Oxazolones decarboxylation

On heating, 4-(isopropoxy)-2-phenyl-2-(trifluoromethyl)-5(2/i/)-oxazolone 65 underwent decarboxylation to the alkoxy-substituted nitrile ylide 66 that was trapped in a 1,3-dipolar cycloaddition by trifluoroacetophenone to generate 68." Other dipolarophiles reacted similarly. In the absence of a dipolarophile, cyclization of 66 yielded the isoindole 67 (Scheme 7.16 Table 7.11, Fig. 7.12). 

The oxidation by molecular oxygen or the base-catalyzed oxidative decarboxylation of saturated 5(4//)-oxazolones 278 yields diacylamines and provides an efficient procedure to prepare imides 279 from Al-acylamino acids (Scheme 7.90). 

In a similar manner, an intramolecular Friedel-Crafts reaction of 2-aryl-4-(2-thienylidine)-5(477)-oxazolones 600 has been reported and, in this case, cycliza-tion and decarboxylation generates thienopyridines 601 (Scheme 7.192). ... 

The Diels-Alder reaction of (Z)-4-arylidene-2-phenyl-5(4//)-oxazolone 707 and Danishefsky s diene is best conducted in toluene at reflux to produce both the endo and the exo stereoisomers of 708. Base treatment of the cycloadduct mixture promotes aromatization through spontaneous oxidative decarboxylation to give 3-aryl-4-benzamidophenols that are converted to 3-aryl-4-aminophenols 709 by acid... 

A number of recent papers have appeared in the literature related to the synthesis of saturated 5(477)-oxazolones that were not yet covered in our contribution. 4-Acyl-2,4-dialkyl-5(477)-oxazolones 776 have been obtained from A-acylglycines. Thus, cyclization of an A-acylglycine in the presence of thionyl chloride affords a monosubstituted 5(477)-oxazolone 775. Acylation of 775 with an aroyl chloride in the presence of magnesium chloride occurred at C-4 to produce 776. Hydrolysis and decarboxylation of 776 gave the A-acylamino ketones 777 that are valuable intermediates to prepare oxazoles 778 (Scheme 7.236). 

Bis(trifluoromethyl)-5(2//)-oxazolone undergoes decarboxylation on treatment with trimethyl phosphite. The resulting 1,3-dipole may be trapped by methyl propio-late [171]. The initial P-C bond formation is favored because the alternative mode of reaction would form a weak P-N bond. This counter-Michael addition also generates an a-d-ard system which becomes fragmentable. 

A synthesis of the marine antibiotic from Pseudomonas bromoutilis has been achieved by 1,3-dipolar cycloaddition of the oxazolone (388) with diethyl acetylenedicarboxylate (73CI(L)275). Hydrolysis and decarboxylation gave the pyrrole (390), identical with a sample prepared previously by other workers, and converted by them to pentabromopseudilin (391 Scheme 85) (66JA4509). 

Olefinic dienophiles such as maleic anhydride and maleimide add to 4,6-dimethyl-2-pyrone to afford mono Diels-Alder adducts. These on irradiation afford, by a photo-decarboxylation, a diene which can be trapped by a dienophile to afford the products (311). - - Acetone-sensitized photoaddition of the oxazolones (312) to maleic and dimethyl maleic anhydride yields the adducts (313). - ... 

Equilibration of the Z-E isomers also occurs in acetonitrile.25 With 2-acetoxyphenylmethylene oxazolones, isomerization to the E isomer was observed.63 Isomerizations to benzamidocoumarin derivatives under photochemical conditions have also been reported 64,65 While the 2-oxazolin-5-ones are thermally stable, the introduction of a 4-acyl or aroyl group, a potential site for further reaction, markedly decreases the stability. Thus, when 2,4-dialkyl-4-aroyl-2-oxazolin-5-ones are heated to 180°, decarboxylation occurs readily with the formation of trisubstituted oxazoles in yields of 71-95%66 [Eq. (13)). 

These structural assignments appear to be incorrect based on reports (Bachi and Vaya, 1977 Brandt et al., 1977) that treatment of 36 with base produced the thiazoline (135) via cleavage of the p-lactam ring followed by decarboxylation and cyclization. A similar cleavage was also obtained with methanol when 136 and 137 were the products. In contrast, the thione (36) was remarkably stable under acidic conditions (Bachi and Vaya, 1977). The original authors have recently corrected the structural assignment of 134 to the oxazolone (138) (Bassignani et al., 1979). 

The reaction of oxazolones 134 with DMAD led to adducts 140, which were transformed into pyrroles 141 after decarboxylation [58]. 

As it was mentioned previously, mesoionic oxazolones were used as dipolaro-philes in the synthesis of 2-CF3-pyrroles. A tandem addition of 1,2-binucleophiles to oxazolones also led to pyrroles, but bearing the CFs-group in 3-position. For instance, the reaction of the oxazolones 426 with aminomalonate 427 gave the pyrrolidine derivative 428, which formed 429 by reaction with acetic anhydride. Treatment of 429 with lithium hydroxide resulted in a decarboxylation giving the pyrrole 430 [140]. 

An iV-acylamino acid (1) is reacted with thionyl chloride yielding the oxazolone (2), which by treatment with an aroyl chloride gave (3). The latter compound was hydrolysed and decarboxylated to yield (4), which reacted with chlorosulfonic acid-thionyl chloride to give the sulfonyl chloride, and subsequent reaction with ammonia afforded the sulfonamide (5) (Scheme 1). 

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