Mesoionic thiazolones

Potts and Roy555 showed that cycloaddition of DM AD occurs with ease to the mesoionic thiazolones 42 and 43. After 15 hours reflux in... 

JOC3803). Good yields of these condensed mesoionic thiazolones (204) are obtained by simply reacting stoichiometric amounts of 2,2-dicyanooxiranes with the cyclic thioamides (203) (81S981). 

Mesoionic thiazolones obtained by the reaction of cyclic thioamide and a-halogenoacid possess an acidic hydrogen atom. This hydrogen migrates to the carbon a with respect to the carbonyl functionality. Thus 3//-7-chloro-5-phenyl-l,4-benzodiazepine-2-thione (203) reacts with bro-moacetic acid, and subsequent cyclization of intermediate 204 with acetic anhydride and triethylamine affords 8-chloro-6-pyenylthiazolo[3,2-a]-[l,4]benzodiazepin-l(2//)-one (205) (76USP3897446) (Scheme 46). 

Oxazolones are simply cyclic anhydrides of A-acyl-a-amino acids, and are constructed in the way that this implies. If the nitrogen also carries an aUcyl group, cyclisation " can only lead to an overall neutral product by adopting a zwitterionic stracture, for which no neutral canonical form can be written - a mesoionic structure. Mesoionic oxazolones (named miinchnones by Huisgen after their discovery at the University of Munchen, Germany) undergo ready dipolar cycloadditions,with loss of carbon dioxide from the initial adduct the examples" show the conversion of a miinchnone into a mesoionic thiazolone and into an imidazole. 

Thiazolo[2,3-/][l,6]naphthyridin-4-ium hydroxide, anhydro-3,8-dicarboxy-6-hydroxy- — see Beminamycinic acid Thiazolonaphthyridinium salts desulfurization, 6, 687 Thiazolones mesoionic... 

Beminamycinic acid Thiazolonaphthyridinium salts desulfurization, 6, 687 Thiazolones mesoionic... 

There are two mesoionic sulfur heterocycles which have been shown to undergo intramolecular cycloaddition as thiocarbonyl ylides 1,3-dithiolones130 and 1,3-thiazolones.131 Thus, the alkenyl 1,3-di-thiolone (235) gave a 90% yield of cyclization product (Scheme 70). The analogous alkynyl... 

Certain mesoionic derivatives of thiazole (21) rearrange into thiazolone (22) under irradiation. When the photolysis is conducted in methanol, the corresponding -lactam (23) is obtained. 

Cycloaddition-extrusion-ring expansion reactions of five-membered mesoionic oxazolones and thiazolones with 2,3-diphenylcyclopropenethione produced pyridine-4(l//)-thiones... 

Irradiation of mesoionic 2-alkyl-4-thiazolones gives highly strained thiirans <77JCS(Pi)ii07>. Similarly, triphenylthiazolium-4-olate in the presence of BujP leads to the quinolinone via desulfuration to an azetinone, isomerization of the latter to a ketene and intramolecular cyclization <88JCS(Pi)189>. 

Mesoionic selenazolones (cf. 180) react with dimethylacetylene dicarboxylate to give the corresponding pyridones (cf. 181) while the analogous thiazolones instead form thiophene derivatives. 

In what the authors suggest might be the first example of a [1 + 3]dipolar cycloaddition, mesoionic-4>thiazolones (130) give imidazolinones (132) upon reaction with ethyl azidoformate, in a multistep process which may proceed via the strained bicycle (131). A similar loss of sulphur occurred during an attempted synthesis of the thienodiazepinone (133), the isolated product being the pyrrolo[l,2-a]imidaz-2(3//)-one derivative (134)/ ... 

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