Mesoionic thiazoles cycloadditions with

Benzothiazole 3-oxide reacts with some dipolarophiles yielding 2-substituted ben-zothiazoles (Scheme 53) (70CPB1176). 1,4-Cycloadditions of dipolarophiles are observed with mesoionic thiazoles. The 4- and 5-hydroxythiazolylium hydroxide inner salts (95) and (98) react with dipolarophiles, such as electron-deficient alkynes, to give cycloadducts which react further by extrusion either of sulfur (96) or of a simple molecule (97, 99 Scheme 54). An example is given for the reaction of DMAD with 4-hydroxy-2,3-diphenylthiazolylium... 

Numerous structures containing the thiocarbonyl ylide dipole are conceivable. Incorporation of the thiocarbonyl ylide dipole into a bicyclic heterocyclic system is possible by the conversion of the cyclic thione (203) into the ring-fused mesoionic system (204). The thiocarbonyl ylide dipole (205) undergoes cycloaddition with both alkenic and alkynic electron-poor dipolarophiles in refluxing benzene or xylene so that, after extrusion of hydrogen sulfide or sulfur, respectively, from the initial 1 1 cycloadducts (206) and (207), a ring-fused pyridinone is formed. The method has been used for the annelation of pyridinones to the imidazole, 1,2,4-triazole, thiazole and 1,3,4-thiadiazole systems... 

Only 2,3-dihydropyrrolo[2,l-(>]thiazoles have been described. Reaction of 7V-acetylthiazolidine-2-carboxylic acid (303) in the presence of acetic anhydride provides mesoionic oxazolone intermediates (304) which undergo 1,3-dipolar cycloaddition with DMAD, to give unstable intermediate (305) which rapidly eliminates carbon dioxide to give 2,3-dihydro-5-methylpyrrolo[2,l-( ]thiazole-6,7-dicarboxylate (306) (Scheme 55) <88JMC1427>. 

Mesoionic thiazoles such as (69) can be considered to be potential precursors in basic media of cyclic azomethine ylids. These mesoionic thiazoles can be generated from 5-(tert-butylamino)thiazolium chlorides (68). They can undergo 1,3-dipolar cycloaddition reactions with carbon disulphide. The initial cycloadducts (70) and (71) rapidly eliminate tert-butyl isothiocyanate at room temperature to give the thiazolium-... 

An example of 1,3-dipolar cycloaddition involving a thiazole dioxide derivative was described (99T(55)201). A-Benzoyl-(R)-thiazolidin-4-carboxylic acid 5,5-dioxide 120 was cyclized to the bicylic mesoionic thiazolo-oxazolium 5,5-dioxide with Ac O and reacted with the imine 121 in DMF... 

Analogously, the mesoionic jV-methyl thiazol-5-ones and l,3-dithiol-4-ones afforded A-methyl-4-pyridones and thiapyran-4-ones when reacting with diphenyl cyclopropenone and its thione261. Benzonitrile oxide apparently gives a 1,3-dipolar cycloaddition to the C=0 group of diphenyl cyclopropenone rationalizing the formation of triphenyl-l,3-oxazin-6-one 41626i ... 

The cyclodehydration of 2-substituted-A/-acylthiazolidine-4-carboxylic acids yields bicyclic munchnones. This mesoionic ring system acts as a cyclic azomethine ylid and can undergo 1,3-dipolar cycloaddition reactions with dipolarophiles. A range of chiral pyrrolo[l,2-c]thiazoles have been prepared by this method both intermolecularly and intramolecularly. 

As early as 1963, interesting cycloadditions of 3,4-dihydroisoquinolinium methylides 76 (R = H, EWG = P-NO2C6H4 or PhCO) with carbon disulfide were reported (63TL1441). A spontaneous dehydrogenation of the initial cycloadducts gives rise to red-colored thiazole-type mesoionic compounds 224. 

Cycloaddition of a mesoionic selenazole with an alkyne forms a bicyclic intermediate that undergoes deselenization to form pyridones. This is in contrast to the analogous thiazole which can undergo a retro-Diels-Alder to liberate the thiophene <82T2129>. The reaction was also discussed in CHEC-I <75CC617, 77JOC1644). 

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