1,3-Oxazolium 5-imines 1,3-dipolar cycloadditions

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... 

This French group (145) has also been able to divert the usual Diels-Alder cycloaddition pathway of Reissert salts with oleftnic esters to a 1,3-dipolar cycloaddition pathway by the addition of triethylamine. In addition, mtinchnone imine 243 can be trapped with dipolarophiles to furnish 244 (146). No Diels-Alder cycloadducts derived from the oxazolium salt were detected. In contrast, fumarate... 

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]... 

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. 

This group " also diverted the usual Diels-Alder cycloaddition pathway of Reissert salts with olefinic esters to a 1,3-dipolar cycloaddition pathway by the addition of triethylamine. Thus treatment of munchnone imine 364 with ethyl acrylate and triethylamine affords the 1,3-dipolar cycloaddition product 366 (30%) as the major product, formed by fragmentation of cycloadduct 365 (Fig. 4.121). The Diels-Alder product (not shown) is formed in 15% yield. Similar products to 366 are formed with dimethyl and diethyl maleate and fumarate. Laude and coworkers " also were able to trap munchnone imine 367 with dipolarophiles to furnish 368 (Fig. 4.122). No Diels-Alder cycloadducts derived from the oxazolium salt were detected. In contrast, fumarate and acrylate esters give only Diels-Alder cycloadducts from the tautomeric oxazolium salt (not shown). However, benzo-quinones and 1,4-naphthoquinone react in a 1,3-dipolar fashion with munchnone imine 372 derived from Reissert compound 369 to give 373 (Scheme 4.11). " Diels-Alder cycloadducts derived from oxazolium salt 371 were not observed. In a... 

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