Mesoionic l,3-oxazolium-5-olate

Under Dakin-West reaction conditions (trifluoroacetic anhydride-MeCN/80 °C/5 h), At-methoxycarbonylproline (128 R = Me) yielded Af-methoxycarbonyl-4-trifluoro-acetyl-2,3-dihydropyrrole (129 R = Me) and none of the expected Dakin-West product, the trifluoromethyl ketone (127). A possible mechanism proposed by the authors involves initial formation of a mesoionic l,3-oxazolium-5-olate (130 R = Me), but the pathway to the iV-methoxycarbonyl-2,3-dihydropyrrole (131 R = Me) and thence the final product (129 R = Me) was unexplained. ... 

One more example of 1,3-dipolar cycloaddition with nitroarenes is the reaction of nitrobenzazoles with mesoionic l,3-oxazolium-5-olates (miinchnones)... 

Examples of the conversion of indoline-2-carboxylic acid to indoles are known (Scheme 5, equations 1-3) [24-26]. The novel transformation in equation 1 is an anomalous Dakin-West reaction that proceeds through a mesoionic l,3-oxazolium-5-olate 3 (a miinchnone) [24],... 

Ring transformation of mesoionic l,3-oxazolium-5-olates into 2(lW)-pyrazinones occurs via an initial attack of TosMIC (p-toluenesulfonylmethylisocyanide) on the C(2) position of the ring. 0-labeling experiments reveal that C(2) carbonyl oxygen in the product is derived from O2 which is bubbled into the solvent DMF in the presence of base DBU (l,8-diazabicyclo[5.4.0]undec-7-ene)." ... 

Reaction of mesoionic l,3-oxazolium-5-olates with amidines afforded 5-(tri-fluoroacetyl)imidazoles (Scheme 38) [52], The mechanism for this novel ring transformation is presumably initiated by the nucleophilic attack of amidines onto C-2 of l,3-oxazolium-5-olates. Subsequent ring fragmentation followed by decarboxylation and recyclization of the enolate ion with concomitant loss of ammonia afforded 5-trifluoroacetylated imidazoles. Reaction of iV-benzoyl-Af-methylglycine with pentafluoropropionic anhydride or heptafluorobutyric anhydride followed by the treatment of the resulting l,3-oxazolium-5-olates with formamidine hydrochloride also afforded the corresponding 5-perfluoroacylimid-azoles in an one-pot fashion. 

Kawase M (1994) A novel ring transformation of mesoionic l,3-oxazolium-5-olates into... 

The proposed reaction mechanism involves formation of mesoionic l,3-oxazolium-5-olate 22 through the cyclodehydration of 18 by TFAA. The intermediate 22 undergoes trifluoroacetylation followed by decarboxylation to give enol trifluoroacetate (25). Cyclization of 25 leads to oxazolinium salt (26). If the group of 25 is easily removable, the reaction could proceed efficiently. This is the reason why A-benzyl derivatives are the best substrates for this transformation. 

Unsubstituted isoxazolium salts 96 react with hydrazines to yield 3-aminopyrazoles 97 (Scheme 53) <1988S203>. Likewise, 5-trifluoromethyl-3-hydroxypyrazoles 99 were obtained selectively through the regioselective attack of phenylhydrazine on mesoionic 4-trifluoroacetyl-l,3-oxazolium-5-olates 98 in refluxing benzene (Scheme 54) C1998TL663, CHEC-III(4.01.9.1.2)103>. 

Trifluoromethyl-3-hydroxypyrazoles 786 were obtained selectively through the regioselective attack of phenylhy-drazine on mesoionic 4-trifluoroacetyl-l,3-oxazolium-5-olates 785 in refluxing benzene the same reaction in DMF and 1,2-dichloroethane at room temperature, respectively, gave 6-trifluoromethyl-l,2,4-triazines and 3-trifluoro-methyl-5-pyrazolones (Equation 171) <1998TL663>. 

Oxazoles 1, benzoxazoles 2, oxazolium salts 3, and oxazole A -oxides 4 are fully conjugated compounds (Figure 1). In addition, the two mesoionic structures l,3-oxazolium-5-olates 5 and (l,3-oxazolium-4-olates) 6, commonly known as miinchnones and isomiinchnones, respectively, are also considered to be conjugated rings. There are five systems of hydroxyl-substituted oxazoles and they exist in their oxo forms the 2(3H)-, 2(5H)-, 4(5//)-, 5(2//)-, 5(4//)-oxazolones 7-11. Three forms of dihydrooxazoles are known 2,3-, 2,5-, and 4,5-dihydrooxazoles respectively 12-14. The fully saturated ring is called oxazoline 15. The monooxo derivatives are 2-oxazolidinone 16,4-oxazolidinone 17, and 5-oxazolidinone 18. The three variants of oxazolidinediones are 19-21 and the fully oxidized oxazolidi-netrione is 22. 

Kawase and Koiwai have developed an unusual ring-transformation methodology for the synthesis of trifluoromethyl-substituted 1,2,4-triazine derivatives (08CPB433). Mesoionic 4-trifluoroacetyl-l,3-oxazolium-5-olates... 

The use of sulfur yUdes in the synthesis of trifluoromethylpyrroles from mesoionic 4-triflnoroacetyl-l,3-oxazolium-5-olates 431 is prove to be also valuable. Reactions proceed to give alkyl(aryl)thio-4-trifluoromethylpyrroles 434 in good yields with an admixture of non-sulfur pyrroles 435 in small amonnts [142],... 

Saijo R, Hagimoto Y, Kawase M (2010) New synthesis of 3-trifluoromethylpyrroles by condensation of mesoionic 4-trifluoroacetyl-l,3-oxazolium-5-olates with phosphorus ylides. Org Lett 12 4776 779... 

Saijo R, Kawase M (2012) The use of sulfur ylides in the synthesis of 3-tilkyl(aryl)thio-4-trifluoromethylpyrroles from mesoionic 4-trilluoroacetyl-l,3-oxazolium-5-olates. Tetrahedron Lett 53 2782-2785... 

There are two common forms of mesoionic compound that belong to the oxazole ring system mlinchnones (1,3-oxazolium-5-olates) 86, named by Huisgen for the city Miinchen, and isomilnchnones (l,3-oxazolium-4-olates) 87 (Figure 4). These are highly versatile compounds which are often used as 1,3-dipoles in cycloaddition chemistry <2002HC(59)681>. 

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