1,3-Oxazolium 4-oxides

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. ... 

Cyclic amino acids 139, when heated in acetic anhydride, probably form initially mesoionic oxazolium 5-oxides (munchnones) subsequent 1,3-dip olar cycloaddition of 1,2-dicyanocyclobutene, loss of carbon dioxide, and opening of the cyclobutane ring lead to dinitriles 140 (80JHC1593). Pyridone 141 is the by-product (together with an indolizine) of the mono-cyclic pyridone dicarboxylate and acrylic ester (73JHC77). 

Mesoionic oxazolium-5-oxides 49 react with aminomalonic ester to give pyrrolidinones 50 as the major or exclusive products <99H(50)71> and the oxazolamine 51 is converted by sodium acetate in acetic acid into the hydantoin 52 <99JHC283>. The intramolecular Diels-Alder cycloaddition of the oxazole 53 and related compounds has been used as a route to substituted isoquinolines <99JOC3595>. ... 

The 1,3-dipolar cycloadducts (266) from cyclobutenes with oxazolium 5-oxides (265) (miinchnones) (80JHC1593) extrude carbon dioxide to yield 4,5-dihydro-lH-azepines as illustrated in Scheme 36. 

Other important pyrrole syntheses of this type are cycloadditions involving mesoionic oxazolium 5-oxides, azomethine ylides or isonitriles, e.g. (145 +146 — 147) or (148 +149 — 150). 

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]... 

The use of nitro aromatics as dipolarophiles in cycloaddition reactions has shown great utility in heterocyclic synthesis. The reaction of 3-substituted-4-nitroisoxazoles (299) with trisubstituted oxazolium 5-oxides (300) affords an intermediate adduct (301) which eliminates carbon dioxide and nitrous acid to afford 3,4,6-trisubstituted pyrrolo[3,4-r/]isoxazoles (302) (Scheme 56) <93G633>. 

In this chapter, oxazole and its derivatives are named and numbered as in Chemical Abstracts. Thus compound (6) is called 4,5-dihydrooxazole rather than 2-oxazoline or A2-oxazoline, (7) is 2,5-dihydrooxazole, the betaines (3) are named anhydro-5-hydroxy-oxazolium hydroxides and not oxazolium 5-oxides or oxazolium 5-olates, and the oxo derivatives (4) and (5) are 5(4//)-oxazolone and 5(2//)-oxazolone, respectively, the position of the extra hydrogen atom being indicated in parentheses. The fully saturated compound (8) is oxazolidine its oxo derivatives are named oxazolidinones and oxazolidinediones, e.g. compound (9) is 2-oxazolidinone and (10) is 4,5-oxazolidinedione. A formula such as (11) is not meant to imply that all the substituents are methyl groups it represents a general oxazolidine derivative and is used in place of the cumbersome expression (12 R-R = H, alkyl or aryl). 

Pyrrole-3-carboxylates were obtained by the regioselective 1,3-cycloaddition reaction between 1,3-oxazolium-5-oxides (Milnchnones) and a-acetoxyacrylates <05TL1061> under microwave irradiation. 

The palladium catalyzed coupling of imine, carbon monoxide and acid chloride is reported as a new route to prepare peptide-based imidazoline-carboxylates. Mechanistic studies suggest this process proceeds via the palladium catalyzed generation of l,3-oxazolium-5-oxide intermediates, which react with imine to generate the observed products. 

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. 

While wanning the catalysis mixture to 55 C (Step D, Scheme 1) leads to no other observable reaction intermediates, the generation of intermediate 8 would allow the series of steps shown in Scheme 1. Insertion of the coordinated CO into the palladium-carbon bond would lead to the overall coupling of acid chloride, imine and carbon monoxide in conq>lex 10. The subsequent loss of HCl from 10, either via direct deprotonation or P-H elimination, would form the a-amide substituted ketene 11. The latter is known to be in rapid equilibrium with its cyclic mesoionic l,3-oxazolium-5-oxide tautomeric 12 (14). These steps would lead to the liberation of the Pd(0) catalyst, which can return to the catalytic cycle. 

The meso-ionic oxazolium 5-oxide (536 R = Ph) reacts with methyl-p-benzoquinone to yield the isoindole-4,7-dione (537). The monophenyl-compound (536 R=H) is transformed photochemically into the valence-isomer (538), which has been intercepted as the ester PhC0NMeCH2C02Et by its reaction with ethanol.A-Benzoyl-A-benzylamino-a-phenylacetonitrile (539) may be regarded as an open-chain analogue of a Reissert compound it is transformed into the hydrofluoroborate (540) of the oxazolium 5-imide (541) ... 

Although several resonance structures can be imagined for munchnones (1,3-oxazolium 5-oxides or 1,3-oxazolium 5-olates) and isomunchnones (1,3-oxazolium 4-oxides or 1,3-oxazolium 4-olates), I will draw those forms depicted in 1 and 2, respectively, which capture the flavor of the 1,3-dipolar reactivity of these heterocycles. In addition, this chapter covers the relatively few new developments involving munchnone imines and isomunchnone rmines. The format follows that used by Gingrich and Baum. 

Oxazolium 5-Oxides (Munchnones) 4.2.2. Structure and Spectral Properties... 

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