Rings, mesoionic

Organo trihalogenomethyl-mercury compounds PhenyKtrihalogeno-methyDmercury as reagent 21, 523 —(II) fluoride 21, 617 —(II) iodide 21, 685 -(II) oxide 21, 321 —(II) sulfate 21,173 Merrifield syntheses 19, 33 Mesitoyl chloride as reagent 17, 169 suppl. 21 Mesityleiie as medium 13, 844 suppl. 21 Mesoionic rings s. Rings, mesoionic... 

Organomercuric sulfonamides 19, 364 Organotrihalogenomethyl-mercury compounds Phenyl(trihalogeno-methyl)mercury as reagent 19, 712 —(II) oxide 19, 281, 354, 552 —(II) salt 19, 122 —(I) sulfate 19, 713 Mesoionic rings s. Rings, mesoionic... 

The direct connection of rings A and D at C l cannot be achieved by enamine or sul> fide couplings. This reaction has been carried out in almost quantitative yield by electrocyclic reactions of A/D Secocorrinoid metal complexes and constitutes a magnificent application of the Woodward-Hoffmann rules. First an antarafacial hydrogen shift from C-19 to C-1 is induced by light (sigmatropic 18-electron rearrangement), and second, a conrotatory thermally allowed cyclization of the mesoionic 16 rc-electron intermediate occurs. Only the A -trans-isomer is formed (A. Eschenmoser, 1974 A. Pfaltz, 1977). 

Formamidinoyl isothiocyanates (157) combine with 2-aminothiazoles the ring nitrogen attacks the spC part of the electrophilic reagent (312) further reaction then yields aza-condensed thiazolo-s-triazines (158) (Scheme 99) (313). Mesoionic S-alkvlthiazolo[3.2-fl]-i-tria2ine-5,7-diones (159) are obtained when 2-alkylaminothiazoles react with phenoxycar-bonyl isocyanate (304). 

The thiazolyl-2-thioglycollic acid (119) undergoes intramolecular ring closure to give mesoionic compound 120 under treatment with acetic anhydride and triethylamine (Scheme 60) (192). The parent acid (119) can be recovered from 120 by hydration with hot 50% aqueous sulfuric add. Compound 120 affords monohydrate of bis(-cyclopentenothiazolyi-2-thio)acetone (121) (192). 

The reasonable stable products are characterized by an ir-absorption near 1615 cm". The 4-protons resonate near 6.2 ppm in the H NMR spectrum (23). NMR spectra exhibit a carbonyl atom signal near 173 ppm, whereas C-4 resonates near 8 108 these positions are characteristic of other mesoionic ring carbon atoms (24). In the mass spectra, decomposition with loss of CO, rupture of the 1,5 and 2.3 bonds with elimination of R NC2R 0 and cleavage of the 1,2 and 3,4 bonds with elimination of C2R 0S is observed (11)... 

Substitution of the nitrogen atom in (289) and subsequent ring closure of (293) under acid cyclodehydration conditions gave the mesoionic system anhydro-5-hydroxythiazoIium hydroxide (294). These reactions are analogous to the cyclodehydration of the A-nitrosogly-cines (295) with acetic anhydride to give the sydnones (296) (see Chapter 4.21). 

Use of mesoionic ring systems for the synthesis of five-membered heterocycles with two or more heteroatoms is relatively restricted because of the few readily accessible systems containing two heteroatoms in the 1,3-dipole. They are particularly suited for the unambiguous synthesis of pyrazoles as the azomethine imine is contained as a masked 1,3-dipole in the sydnone system. An attractive feature of their use is that the precursor to the mesoionic system may be used in the presence of the cyclodehydration agent and the dipolarophile, avoiding the necessity for isolating the mesoionic system. 

Pyrazolino[2,3-c][l,2,3]triazoles, 5, 702 Pyrazolium hydroxide, l,2-dimethyl-3,5-diphenylanhydro-4-hydroxy-IR spectra, 5, 201 Pyrazolium salts dequatemization, 5, 269 H NMR, 5, 185 hydrogen exchange at ring carbon, 5, 245 mesoionic compounds, 5, 171 nitrodebromination, 5, 237 reactivity, 5, 217 reduction, 5, 68, 243 synthesis, 5, 156 UV spectra, 5, 199 Pyrazolium salts, amino-reactions, 5, 262 Pyrazolium salts, bromo-nucleophilic displacements, 5, 266 Pyrazolium salts, 1,2-dimethyl-deuteration, 5, 175, 245 hydrogen exchange, 5, 71 acid-catalyzed, 5, 239 reactions... 

Ollis and Ramsden state that A compound may be appropriately called mesoionic if it is a five-membered heterocycle which cannot be represented satisfactorily by any one covalent or polar structure and possesses a sextet of electrons in association with the five atoms comprising the ring . From the point of view of systematic nomenclature, compounds of this type are difficult to deal with, since most available nomenclature systems are designed so as to name one particular bond- and charge-localized canonical form. 

Heating the mesoionic l-amino-2-thioxo-l,2,4-triazolo[l,5-c]quinazo-lines 59 with aromatic aldehydes and ethanolic hydrochloric acid resulted in the formation of Schiff bases and simultaneous pyrimidine ring cleavage... 

Flash vacuum thermolysis (FVT) of 2-substituted 4//-pyrido[l,2-n]pyrimidin-4-ones 126 above 800 °C afforded (2-pyridyl)iminopropadie-none (130) (99JCS(P2)1087). These reactions were interpreted in terms of reversible ring opening of 4//-pyrido[l,2-n]pyrimidin-4-ones to imidoyl-ketenes 127. A 1,5-H shift in 127 generated the N(l)H-tautomeric methylene ketene 128, in which facile elimination of HX took place via a six-membered cyclic transition state 129 to yield 130. In the case of 2-methoxy derivative 126 (X = OMe) another competing pathway was also identified at lower temperature, which resulted in the formation C3O2 and 2-methylaminopyr-idine via mesoionic isomer 131 (Scheme 9). The products were identified by IR spectroscopy. 

Only noncondensed thiazoles in which mesoionic charge delocalization involves atoms directly bonded to the thiazole ring are considered here. Two such systems. 1 and 2. exist (Scheme 1). Structure 1 (X = Ol is... 

The mesoionic tetrazole dehydrodithizone is transformed by iron penta-carbonyl into 4-phenyl-2-phenylazo-A2-l,3,4-thiadiazolin-5-one, presumably by a mechanism of ring opening, complexation, carbonyl insertion and subsequent ring closure (Scheme 128).193 Unfortunately, analogous processes do not occur on other mesoionic compounds in the 1,2,3-oxadiazole, s-triazole or tetrazole series, and the scope of this unusual carbonylation is probably limited. 

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