Cycloaddition reactions thiazoles, mesoionic

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. 

Mesoionic thiazoles represent masked ylides which can undergo cycloaddition reactions. 1,3-Thiazolium-5-olate (73) and thioisomunchnone (74) can be considered as an azomethine ylide dipole and a thiocarbonyl ylide dipole, respectively. The cycloaddition chemistry of these compounds have been studied and their utility for the synthesis of heterocycles demonstrated <94S993>. 

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). 

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-... 

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... 

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>. 

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