Mesoionic derivatives

Treatment of 192 with dimethyl acetylenedicarboxylate yields a thiophene derivative (195) when R = Ph and a 2-p3Tidone (1S>6) derivative when R = H (Scheme 100). The proposed mechanism involves the formation of a mesoionic derivative (193) initially further dipolar addition yields adduct 194, the decomposition of which is dependent on the R substituent as described for related compounds (435). ... 

Reaction of pyridinium thiocyanatoacetamides (106) with a strong base (e.g potassium t-butoxide) in ethanol gave mesoionic Af-[2-(l,3,4-thiadiazolo[3,2-a]pyridino)]acetamidates (107) or (108) whose structures were confirmed by the X-ray analysis of (107 R = Me). Possible mechanisms for the formation of the mesoionic derivatives were discussed <96BCJ1769>. 

Mesoionic derivatives are generally synthesized from the parent 1,2,3-thiadiazoles. A new method based on the rearrangement of oxadiazoles under reductive conditions has been reported. For example, the oxadiazole 70 when... 

As for sydnonimines, the ability of mesoionic derivatives 82 and 83 to produce NO is strictly connected to their capacity to decompose to nitrosohydrazine intermediates. Mesoionic l,2,3,4-oxatriazolium-5-olates represent a thermally stable class of compounds, but they undergo a relatively facile nucleophilic ring-opening reac-... 

An alternative approach to 5-5 bicyclic fused ring systems utilizing a condensation reaction was described by Moderhack et al. <2000JPC591> (Scheme 10). The amino-substituted tetrazole derivatives 52 and 54 gave the neutral 53 as well as the mesoionic derivatives 55 in moderate to excellent yields (16-86% R = CH3 or Ph). 

Matsuda et al. described the cyclization to a mesoionic derivative of the title ring system <1995H(41)329>. The starting compound of the ring-closure reaction is the anion 416, which was easily obtainable from ethyl [l,2,4]triazol-1-ylacetate with potassium bromide. Upon heating this compound, ring closure to the zwitterionic 417 takes place in 40% yield. 

Structure of the mesoionic derivative of the imidazo[2,l-/][ l, 2,4]triazine ring system 55 has been determined <2005TL5325>. The X-ray analysis unequivocally revealed the zwitterionic character. 

The reaction of tetrahydro-l,3-thiazine-2-thione and diethyl 2-chloro-malonate in the presence of triethylamine in boiling methylene chloride for 1.5 hr gave tetrahydro-1,3-thiazin-2-ylidenemalonate (508) in 33% yield via 507 through Eschenmoser sulfur elimination, together with traces of the mesoionic derivative (509) [77JCS(P 1) 1107]. In a similar reaction, diethyl 2-bromomalonate afforded the mesoionic compound (509) in 80% yield. Tetrahydro-l,3-thiazin-2-ylidenemalonate (508) was also obtained in 42% yield from 509 by irradiation in the presence of tributylphosphine in ethanol for 15 hr under argon [77JCS(P1)1107]. 

If AAs are heated in acetic anhydride in the presence of 1,2-dicyanocyclo-butene as a dipolarophile, 4,5-dihydroazepines are formed in good yields. For example, the reaction with Ala is actually considered a cycloaddition of the intermediate mesoionic derivative (Scheme 43) (80JHC1593). 

The only reported derivative of this ring system (160) is prepared from 158 and 159 (83H1271). Only mesoionic derivatives of this ring system can be considered aromatic. 

Three systems (166, 167, and 168) are aromatic lOTrelectron systems. Mesoionic derivatives of 169 are aromatic as are salts of 170. 

Mesoionic derivatives of 214 are also aromatic, and so, synthetic approaches to 214 will be reported. None of the known derivatives of 215 have a lOTrelectron system, and syntheses will not be reported. 

Further examples of the ease of exchange of substituents on carbon by nucleophiles under mild conditions have been reported. The reactivity of the ring towards nucleophiles is increased in triazolium cations and mesoionic derivatives. 

An index of aromatic character based upon statistical evaluation of the deviations in peripheral bond orders has been devised and applied to five-membered ring heterocycles and their mesoionic derivatives including 1,2,3,4-thiatriazole <85TI409> and 5-phenyl-l,2,3,4-thiatriazol-3-oxide <93T8441>. See also <93QSAR146>. 

Catalytic reduction of the derivatives of triazolopyridines invariably attacks the pyridine ring, leaving the triazole. There is no report of reduction of compound 1, but 3-nitrotriazolopyridine (154) gives as major product the tetrahydrotriazolopyridinamine 155.229 Reduction with palladium-charcoal takes a different course (see Section IV,F). The mesoionic derivatives of [l,2,4]triazolo[l,5-a]pyridine 156-15854 and 159230 are reduced to py-tetrahydro derivatives, as is compound 3,231 although it had previously been reported that 3-substituted derivatives of compound 3 were not reduced.65 Chloro[l, 2,3]triazolo[4,5-6]pyridine (160) is reductively dehalogenated with reduction of the pyridine ring by platinum or palladium catalysts.146 By addition of base the reduction can be stopped at compound 4 (see Section... 

As shown in Section II,C,l,b, photolysis or thermolysis of l-azido-8-arylazonaphthalenes gives rise to heterocyclization of these compounds, resulting in a mixture of mesoionic derivatives of naphtho[rf ]triazine 233... 

In the last series of azapentalenes (the less experimentally known) we have studied bimanes (248-251) [189, 190] and mesoionic derivatives 222-246. Only the last compound 256, being a polymorph of dinitrogen, has been studied much [191-201], We have calculated (Fig. 7) the molecular electrostatic potentials (MEPs) of compounds 222 and 256 [202],... 

This chapter covers the literature from mid-1995 to mid-2007. Recent advances include the preparation of the first A2-l,2,3,4-oxatriazolines 7 by intramolecular [3+2] cycloaddition (Section 6.08.9), synthesis of mesoionic derivatives 4 using bromonitroformaldehyde iV-arylhydrazones as the starting materials (Section 6.08.9), and detailed studies of the biologically active mesoionic oxatriazoles in the context of their nitric oxide donating capabilities (Section... 

Of the possible structures 1-16 shown, the known structures are represented by 1,2,3,4-oxatriazolium salts 2, mesoionic derivatives 3-6, and 1,2,3,5-oxatriazolidines 15 and 16. Derivatives of A -1,2,3,4-oxadiazoline 7 have now been synthesized and are the first representatives of this particular ring system. 

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. 

Although thiadiazoles have been known for a long time, of the four possible isomers, 1,2,3-thiadiazoles account for the fewest literature citations. A substantial amount of this literature has dealt with thermal and photochemical reactions of 1,2,3-thiadiazoles, and most recently attention has been focused on mesoionic derivatives. Still, the many gaps that exist in our knowledge leave this field a fertile area for further research. 

Protonation of triazoles as bases or deprotonation to afford triazolates or mesoionic derivatives are aspects of reactivity but usually, also here, mentioned under acidity (Section 4.12.2.4.2). 

From a qualitative viewpoint there is no doubt that the compounds classified as aromatic in Section 4.04.1.1 (pyrazoles, indazoles, isoindazoles, pyrazolones, indazolones, mesoionic derivatives) indeed have aromatic properties to a greater or lesser extent. Dihy-drobenz[cd]indazole (115) is also formally an aromatic compound since it has an odd number of electron pairs (n =7). However, due to the presence of two adjacent nitrogen atoms the aromatic tautomer (115a) is not stable, and the compound exists as the 1,5- or 1,3-dihydro tautomers (115b) or (115c) <72JCS(P2)68). The antiaromatic benz[cd]indazole (116), although unstable, has been fully characterized by N, H and NMR spectroscopy... 

Polymethyne dyes of benzazoles, synthesis and properties 84MI7. Quaternary salts of azoles and their mesoionic derivatives, photochemistry... 

Evidence in support of zwitterionic intermediates is provided by the reaction of the C-acyl 1,3-dipoles 34 (R=Me, Ph) with o-chloranil (Scheme 10) (80LA1836). The initial intermediate 35 undergoes acyl transfer to give a new mesoionic derivative 36, which reacts with a second molecule of o-chloranil, via a new zwitteiion 37, to give the final adduct 38. 

Molteni G (2004) Synthesis of the new pyrazolo[4,3-c]pyrrolizine skeleton via intramolecular nitrilimine cycloaddition. Heterocycles 63 1423-1428 52. Padwa A, Hertzog DL et al (1994) Studies on the intramolecular cycloaddition reaction of mesoionics derived from the rhodium(II)-catalyzed cyclization of diazoimides. J Org Chem 59 1418-1427... 

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