1,3-Oxazolium 4-oxides 1,3-dipolar cycloadditions

The stereochemical characterization of the adduct 53 follows from its NMR spectrum and a comparison with that of the l-(2-thienyl) compound (54). The aSY-exo configuration for the adducts 51 and 52 is consistent with the NMR spectra (hydrogen atoms at C-2, C-3, C-5, and C-6 all equivalent), with the proposed mechanism of formation, and with the failure of the related tetramethyl ester to xmdergo N-acetylation even in very vigorous conditions. N-substituted derivatives of compounds such as 51-53 may be obtainable directly from similar dipolar cycloaddition reactions of mesoionic N-substituted oxazolium 5-oxides, although the formation of only the N-methyl derivative of (52) has so far been reported. ... 

Huisgen and coworkers have also described the cycloaddition behavior of the munchnones , unstable mesoionic A2-oxazolium 5-oxides with azomethine ylide character.166 Their reactions closely parallel those of the related sydnones. These mesoionic dipoles are readily prepared by cyclodehydration of N-acyl amino acids (216) with reagents such as acetic anhydride. The reaction of munchnones with alkynic dipolarophiles constitutes a pyrrole synthesis of broad scope.158-160 1,3-Dipolar cycloaddition of alkynes to the A2-oxazolium 5-oxide (217), followed by cycloreversion of carbon dioxide from the initially formed adduct (218), gives pyrrole derivative (219 Scheme 51) in good yield. Cycloaddition studies of munchnones with other dipolarophiles have resulted in practical, unique syntheses of numerous functionalized monocyclic and ring-annulated heterocycles.167-169... 

The synthesis of various heterocyclic systems via 1,3-dipolar cycloaddition reactions of 1,3-oxazolium-5-oxides (32) with different dipolarophiles was reported. The cycloaddition reactions of mesoionic 5H,7H-thiazolo[3,4-c]oxazolium-l-oxides (32), which were prepared from in situ N-acyl-(/J)-thiazolidine-4-carboxyIic acids and N,N -dicyclohexylcarbodiimide, with imines, such as N-(phenylmethylene)aniline and N-(phenylmethylene)benzenesulfonamide, gave 7-thia-2,5-diazaspiro[3,4]octan-l-one derivatives (33) and lH,3H-imidazo[ 1,5-cJthiazole derivative (35). The nature of substituents on imines and on mesoionic compounds influenced the reaction. A spirocyclic p-lactam (33) may be derived from a two-step addition reaction. Alternatively, an imidazothiazole (35) may be obtained from a typical 1,3-dipolar cycloaddition via a tricyclic adduct (34) which loses carbon dioxide and benzenesulfinic acid. [95T9385]... 

This imidazoline-carboxylate synthesis involves the coupling of four separate cont5)onents (two imines, an acid chloride and carbon monoxide), and the generation of at least five separate bonds, all via a one-pot, palladium catalyzed process. From an analysis of the structure of the imidazoline carboxylate, the individual constituents can be seen (Figure 3). This stmcture might be considered to arise from the dipolar cycloaddition of an imine with a mesoionic l,3-oxazolium-5-oxide (5) intermediate, which itself could be generated from imine, acid chloride and carbon monoxide. Consistent with this potential formulation, performing the catalytic reaction with CO leads to the incorporation of the carbon-13 label into the carboxylate position of 4. 

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