1.3- dipolar cycloaddition reactions sydnones

The Weintraub reaction was revisited to form additional members of the series of diazatetracycloundecanes by tandem 1,3-dipolar cycloaddition of sydnones and 1,5-cyclooctadiene (Equation 11) <1996JHC719>. 

Nan ya et al. (93,94) also reported the reaction of sydnones with Al-methyl- and N-phenylmaleimides. Gribble and Hirth (206) extended the work of Weintraub (207) on the tandem dipolar cycloaddition of sydnones with 1,5-cyclooctadiene to afford diazatetracycloundecanes 305. 

Thermal reactions and 1,3-dipolar cycloaddition reactions Thermally mesoionic oxatriazoles are relatively stable. Heating of (85) in tolane (97) at 200 °C for 20 d in the presence of LiCl induces C02 fragmentation and formation of cycloadduct (99) in 37% yield. There is no bimolecular reaction as found in the case of the sydnones and isosydnones (equation 56) (68CB536). 

SCHEME 5.24 1,3-Dipolar cycloaddition reaction between sydnones and 1,5-cyclooctadiene. 

Padwa A, Burgess EM, Gingrich HL, Roush DM (1982) On the problem of regioselectivity in the 1,3-dipolar cycloaddition reaction of munchnones and sydnones with acetylenic dipo-larophiles. J Org Chem 47 786-791... 

Photochemical cycloaddition reactions between sydnones (1) and 1,3-dipolarophiles take place to give products which are different from, but isomeric with, the thermal 1,3-dipolar cycloaddition products. These results are directly interpreted in terms of reactions between the 1,3-dipolarophiles and Ae nit mine (316). The photochemical reactions between sydnones and the following 1,3-dipolarophiles have been reported dicyclopentadiene, dimethyl acetylene dicarboxylate, dimethyl maleate, dimethyl fumarate, indene, carbon dioxide, and carbon disulfide. ... 

Dumitrascu and co-workers (52) transformed 4-halosydnones into 5-halopyr-azoles by cycloaddition with DMAD and methyl propiolate followed by retro-Diels-Alder loss of CO2. Turnbull and co-workers (194) reported that the cycloadditions of 3-phenylsydnone with DMAD and diethyl acetylenedicarboxylate to form pyrazoles can be achieved in supercritical carbon dioxide. Nan ya et al. (195) studied this sydnone in its reaction with 2-methylbenzoquinone to afford the expected isomeric indazole-4,7-diones. Interestingly, Sasaki et al. (196) found that 3-phenylsydnone effects the conversion of l,4-dihydronaphthalene-l,4-imines to isoindoles, presumably by consecutive loss of carbon dioxide and A-phenylpyrazole from the primary cycloadduct. Ranganathan et al. (197-199) studied dipolar cycloadditions with the sydnone 298 derived from A-nitrosoproline (Scheme 10.43). Both acetylenic and olefinic dipolarophiles react with 298. In... 

Huisgen and coworkers have also described the cycloaddition behavior of the munchnones , unstable mesoionic A2-oxazolium 5-oxides with azomethine ylide character.166 Their reactions closely parallel those of the related sydnones. These mesoionic dipoles are readily prepared by cyclodehydration of N-acyl amino acids (216) with reagents such as acetic anhydride. The reaction of munchnones with alkynic dipolarophiles constitutes a pyrrole synthesis of broad scope.158-160 1,3-Dipolar cycloaddition of alkynes to the A2-oxazolium 5-oxide (217), followed by cycloreversion of carbon dioxide from the initially formed adduct (218), gives pyrrole derivative (219 Scheme 51) in good yield. Cycloaddition studies of munchnones with other dipolarophiles have resulted in practical, unique syntheses of numerous functionalized monocyclic and ring-annulated heterocycles.167-169... 

This is exactly what has been observed for the cycloaddition reactions of sydnones 6.336 with all three kinds of dipolarophile. An intermediate is produced in the first instance with the general structure 6.337 this loses carbon dioxide in a retro 1,3-dipolar cycloaddition, followed by tautomerism giving the 3-substituted pyrazolines 6.338 as the major products.860... 

Sydnones undergo smooth cycloaddition with acetylenes to give pyrazoles in high yield. The reaction involves a 1,3-dipolar cycloaddition of the sydnones. 

Also in this field, Larock et al. have described that sydnones 29 undergo [3+2] dipolar cycloadditions with arynes affording 2//-indazoles 30 nndo" mUd reaction conditions. These heterocyclic derivatives are formed by a spontaneons extmsion of CO from the initial cycloadduct (Scheme 12.19) [30]. The same authors have developed a simple route to pyrido[l,2-f>]indazoles 31 via an aryne [3+2] cycloaddition with A(-tosylpyridininm imides and subsequent flnoride-induced elimination of the tosylate anion (Scheme 12.19) [31]. 

The chemistry of the 1,3-dipole cycloaddition reaction, so elegantly elucidated by Professor Huisgen by the early 1960 s, appeared to be especially suited for the synthesis of aromatic polymers since the monomers could be recdily synthesized, and many of the dipolar additions gave high yields of 5-membered heterocyclic aromatic compounds. An inspection of the literature revealed that nitrili-mines, sydnones and nitrile oxide dipoles were especially suited. 

The addition to alkenes normally leads to unstable adducts that lose carbon dioxide under the reaction conditions. The intramolecular cycloaddition of the sydnone (30) takes place at room temperature, however (Equation (5)) and the cycloadduct (31) has been characterized <86HCA927>. The unstable species formed by the loss of carbon dioxide are also azomethine ylides. It is therefore possible for a second 1,3-dipolar addition to take place, as illustrated in Scheme 6 for the reaction of 3-phenylsydnone with Al-phenylmaleimide <86TL317,92JA8414>. This 2 1 addition has been used as the basis of a synthesis of polyimides. Imides of the type (32) were used as the dipolarophiles and their reaction with 3-phenylsydnone gave linear polymers <87MM726>. 

Six articles cover general aspects of heterocyclic chemistry 1,3-dipolar cycloreversions, syntheses with arylnitrenes and a-metallated isocyanides, and photo-oxygenation of nitrogen heterocycles,while others deal with more specialized subjects, i.e. preparation and use of halogeno-lactones, aspects of the chemistry of furan, 1-hydroxy-indoles, ring-opening of azoles by the action of amines, " the use of 2-chlorobenzoxazolium (1) and other heterocyclic onium salts for dehydration and condensation reactions," the synthesis of monosub-stituted tetrathiafulvalenes (2), cycloadditions of azoles containing three heteroatoms,sydnone imines (3), the conversion of acyl-benzofuroxans into nitro-indazoles (cf. p. 199), and advances in the chemistry of pyrrolizidine " and indolizine." ... 

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