Anhydro- hydroxides

AK(30)26l). 2-Acetoxyfuran-3(2i/)-ones react with hydrazine to give 3,6-disubstituted-4-ethoxycarbonylpyridazin-4(li/)-ones (184) as the main product, but with mono-substituted hydrazines in addition to these pyridazines anhydro-5-hydroxypyridazinium hydroxide (185) derivatives and some pyrazole derivatives are also formed (Scheme 102) (79JOC3053). The... 

Bielectrophiles have found appreciable applications in the synthesis of ring-fused systems, especially those involving [5,6] fused systems. The following serve to illustrate these applications. Reaction of pyrazole with (chlorocarbonyl)phenyl ketene (214) (Type 1, Scheme 6) readily formed the zwitterionic pyrazolo[l,2-a]pyrazole derivative (215) (80JA3971). With l-methylimidazole-2-thione (216), anhydro-2-hydroxy-8-methyl-4-oxo-3-phenyl-4//-imidazo[2,l-6][l,3]thiazinium hydroxide (217) was obtained (80JOC2474). 

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). 

Reaction of the A-nitrosoglycine (394) with acetic anhydride gave the anhydro-5-hydroxy-l,2,3-oxadiazolium hydroxide (395). Reaction with DMAD resulted in formation of the intermediate 1 1 cycloadduct (396) which was not isolated and which lost CO2 under the thermal reaction conditions to give dimethyl l-phenylpyrazole-3,4-dicarboxylate (397) (83MI40300). This reaction is capable of considerable variation in terms of the substituents... 

A similar product is obtained from the reaction of anhydro-4(5)-hydroxy-l,2,3-triazolium hydroxide (398). In this case reaction with DMAD occurred in 1 hour in boiling benzene. Extrusion of methyl isocyanate from the initial 1 1 cycloadduct (399) occurred during the reaction giving (400). 

Anhydro-3-hydroxy-2-phenylthiazolo[2,3-6]thiazolylium hydroxide (407) underwent ready thermal reaction with alkynic and alkenic dipolarophiles in refluxing toluene. With the former dipolarophile sulfur was lost from the intermediate 1 1 cycloadduct (408) to give the substituted 5H-thiazolo[3,2- i]pyridin-5-ones (409). With the latter, the intermediate (410) lost H2S, also forming (409). 

Decomposition of the diazoimide (551) by heating in the presence of copper acetylaceton-ate also generated a ketocarbene (552). This undergoes an intramolecular condensation to give the anhydro-4-hydroxy-3-methyl-4-p-nitrophenyl-2-phenyloxazolium hydroxide (553), which cannot be prepared by more classical means (75CL499). 

The mesoionic compounds are derived from pyrazolium salts (22) when R is replaced by a negatively charged heteroatom, like the anhydro-4-hydroxypyrazolium hydroxide (28). According to Ollis and Ramsden (76AHC(l9)l) they belong to the mesoionic class B type. 

Imidazo[2,l-6]thiazolylium hydroxide, anhydro-3-hydroxy-7-methyl-2-phenyl-synthesis, 5, 633... 

Oxazolium hydroxide, anhydroamino-reactions, 6, 206-211 Oxazolium hydroxide, anhydro-4-amino-mesoionic IR spectra, 6, 186... 

Oxazolium hydroxide, anhydro-2-m-bromophenyl-5-hydroxy-3-methyl-4-trifluoroacetyl-X-ray structure, 6, 180 Oxazolium hydroxide, anhydro-4-hydroxy-cycloaddition reactions, 6, 208 IR spectra, 6, 186 mesoionic structures, 6, 179 tautomerism, 6, 185 reactions, 6, 206-211 synthesis, 6, 225... 

Oxazolium hydroxide, anhydro-5-hydroxy-aromaticity, 6, 184 cycloaddition reactions, 6, 209 dimerization, 6, 207 1,3-dipolar cycloaddition reactions with alkynes, 6, 210 electrophilic reactions, 6, 207 mesoionic reactions, 6, 188 reactions, 6, 206-211 synthesis, 6, 225-227... 

Pyridinium hydroxide, anhydro-4-mercapto-2,8-methoxyphenyl-6-oxo-1,3-diphenyl-synthesis, 6, 343... 

Selenazolium hydroxide, anhydro-4-hydroxy-2,3,5-triphenyl-synthesis, 6, 343 Selenazolo[3,2-b][ 1,2,4]triazine synthesis, 6, 347... 

Thiadiazolo[4,5-d]pyridinium hydroxide, anhydro-3,6-dimethyl-5-hydroxy-7(6)-oxo-... 

Thiazolo[2,3-a]isoquinolinium hydroxide, anhydro-3-(4-bromophenyl)-2-mercapto-X-ray diffraction, 6, 669 Thiazololactams fused, 6, 683-684... 

Thiazolo[2,3-/][l,6]naphthyridin-4-ium hydroxide, anhydro-3,8-dicarboxy-6-hydroxy- — see Beminamycinic acid Thiazolonaphthyridinium salts desulfurization, 6, 687 Thiazolones mesoionic... 

Thiazolo[3,2-a]pyridinium hydroxide, anhydro-7-bromo-3a-carboxy-2,3-dihydro-8-hydroxy-2ft5-dimethyl-l 0-0X0-X-ray diffraction, 6, 670... 

Thiazolo[3,2,-a]pyridinium hydroxide, anhydro-8-hydroxy-photolysis, 6, 682... 

Thiazolo[3,2-6][l,2,4]thiadiazolium bromide, 2-ami nosynthesis, 5, 136 6, 1016 Thiazolo[2,3-6][l,3,4]thiadiazolium hydroxide, anhydro-5-hydroxy-2-methyl-6-phenyl-synthesis, 6, 743... 

Various methods have been employed to overcome this difficulty. For example, a method used by Chemical Abstracts involves naming the dihydro derivative of the heterocyclic ketone (or imine or exocyclic alkene) form, and adding the words mesoionic didehydro derivative (example 131). A similar approach, favoured by Ollis (76AHQ19)1), involves naming the corresponding cation hydroxide, with the prefix anhydro (indicating removal of the elements of water) (example 132). 

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