1.3- Dipolar cycloaddition reactions mesoionic systems

The 1,3-dipolar cycloaddition reactions of several other mesoionic heterocycles have been investigated since Potts review (1). Kato et al. (113,114,123) found that the l,3-thiazohum-5-olate (358) ring system affords low yields or complex reaction mixtures with benzocyclopropene (113), benzocyclobutadiene (114), and tropone (123). Likewise, Vedejs and Wilde (209) isolated in low yields a cycloadduct of 358d with thiopivaldehyde, along with a ring-opened thiamide. Also, 1,3-thiazo-lium-5-olate 359 reacts with thiopivaldehyde to give 360 (209). 

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 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. 

Prompted by our earlier work dealing with the internal dipolar cycloaddition reaction of mesoionic oxazolium ylides of type 74, we subsequently studied the rhodium(II) catalyzed reactions of the related a-diazo ketoamide system 154 <97JOC2001 04OL3241 05JOC2206>. Attack of the amido oxygen at the rhodium carbenoid produces a push-pull carbonyl ylide dipole (i.e., 155) that is isomeric with the isomiinchnone class of mesoionic betaines. 

Mesoionic ring systems in 1,3-dipolar cycloaddition reactions 02HC(59)681. 

Anhydro-5-hydroxyoxazolium hydroxides lacking substituents at C(4) dimerize spontaneously by a process in which one molecule acts as an electrophile and the other as a nucleophile (Scheme 21). This accounts for the fact that dimeric products of this type are obtained by the action of dicyclohexylcarbodiimide on acylamino acids of the general formula R1C0NR2CH2C02H. Substituents at position 4 stabilize the mesoionic system the first compounds to be prepared were the acetyl derivatives (220) (B-49MI41800) and (221) (58Cl(L)46l) and much of the more recent work has been carried out with the relatively stable methyldiphenyl compound (222). This miinchnone decomposes above 115 °C to yield the allene (225) with loss of carbon dioxide. The mechanism proposed for this remarkable reaction (Scheme 22) involves valence isomerization to the ketene (223), which undergoes a 1,3-dipolar cycloaddition with the miinchnone. The product loses carbon dioxide to form a new betaine (224), which collapses to the allene as shown. 

Recent developments in the chemistry of mesoionic compounds85 include cycloaddition-elimination reactions, which afford novel synthetic routes to a variety of heterocyclic systems. These reactions may be seen as involving 1,3-dipolar cycloadditions, following Huisgen,86 or alternatively as 1,4-cycloadditions to heterodiene systems,87 depending on the choice of canonical structure to represent the mesoionic compound. Benzyne has been employed in such reactions less frequently than more stable acetylenic or ethylenic dipolarophiles. 

In a similar fashion, the Pummerer rearrangement has been used to generate mesoionic dipoles which can then undergo cycloaddition reactions to afford azapolycyclic ring systems. This is illustrated in the reaction of imidosulfoxide 58 which, when treated under Pummerer reaction conditions, underwent a tandem Pummerer-induced cyelization-isomunehnone dipolar... 

The mesoionic compounds 3 give six-membered ring systems on reaction with 2tt-dipolar-ophiles. The addition reaction involves a sequential cycloaddition and sigmatropic rearrangement. None of the desired intermediates were isolated or detected. 

---