Oxadiazol-2-enes

4-Oxadiazol-2-enes (Anhydro-2-alkyl-l,3,4-oxadiazolium Hydroxides) (164) 

Representatives of this novel class of meso-ionic compounds in which the exocyclic substituent f (see Table I) is a stabilized carbanionoid residue [-C(CN)C02Me or -C(CN)2] have been recently synthesized. Base-catalyzed (potassium carbonate or triethylamine) condensation of iV-acylhydrazines (147) with 3,3-dichloroacrylonitriles (165) yield the greenish-yellow meso-ionic l,3,4-oxadiazol-2-enes (164) directly.114 

An analogous meso-ionic l,3,4-oxadiazol-2-ene (168) in which the negative charge is associated with a fluorenyl residue has also been recently described.113 The N,N -diacylhydrazine (166) and acetic 

Comprehensive reviews of the chemistry of sydnones (1) have been published elsewhere,2-9 and a general discussion of this subject is not included here. Recent work on the photochemistry (Section IX), physical and theoretical studies (Section XI), and pharmacological activity (Section XII) of sydnones (1) is included in later sections. 

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The reaction of 5-[2-(iV,./V-dimethylamino)ethyl]-l,2,4-oxadiazole with methyl iodide forms the quaternary ammonium salt 170 (Scheme 22), which undergoes elimination in the presence of base (diisopropylethylamine (DIEA), TEA, l,8-diazabicyclo[4.3.0]undec-7-ene, etc.) to form an intermediate 5-vinyl-l,2,4-oxadiazole 171, which undergoes in situ Michael addition with nucleophiles to furnish the Michael adducts 172. As an example, also shown in Scheme 22, 3-hydroxy-pyrrolidine allows the synthesis of compound 172a in 97% yield. Mesylation followed by deprotonation of the 1,2,4-oxadiazole methylene at C-5 enables Sn2 displacement of the mesylate to give the 5-azabicycloheptyl derivative 173, which is a potent muscarinic agonist <1996JOC3228>. 

Azido derivatives of furazans have proved to be particularly useful for the synthesis of heterocyclic systems. Thus, by 1,3-dipolar addition of l-azido(4-amino-l,2,5-oxadiazol-3-yl)aldoxime 186 to propargyl alcohol and phenylacetyl-ene, bicyclic 4-amino-l,2,5-oxadiazol-3-yl(4-R-l,2,3-triazol-l-yl)ketoximes 187 were obtained (Equation 34) which in reaction with acetic anhydride afforded the corresponding 0-acyl derivatives <2003RJ0574>. 

Reaction of vinyl azides with nitrosyl tetrafluoroborate gives furazans in good yield, particularly when the alkene is 1,2-dialkyl-substituted. For example, 1-azidocyclooctene and 2-azidobut-2-enes yield (19 n = 6) and (1, R1 = R2 = Me), respectively 1,2,4-oxadiazoles may also be formed as by-products. 

Irradiation of acetyl azide (98) in benzonitrile produced a small yield of the 1,3-cycloaddition product, 2-methyl-5-phenyl-l,3,4-oxadiazole (99) Photolysis of 98 in the presence of phenylacetyl-ene resulted in a small yield of the oxazole 101. While the tetrazole... 

Oxadiazoles.—1,2,3-Oxadiazoles. 3-Phenylsydnone (594 R = H) is anodically chlorinated at position 4, while anodic oxidation yields mainly phenol and benzaldehyde. The foregoing sydnone reacts with tetracyanoethylene to give the ene-hydrazone (596), presumably via the cyclo-adduct (595), by loss of carbon dioxide and cleavage of the strained carbon-carbon single bond. Irradiation of diphenylsydnone (594 R = Ph) in the presence of methyl pro-piolate affords the pyrazole (597) by way of the nitrile imine PhC=N—NPh with benzonitrile, a mixture of the 1,2,3- and 1,2,4-triazoles (598) and (599), together with the 1,3,4-oxadiazolinone (600) and 7s/ -benzoyl-iV -phenylhydrazine, is formed. ATV -Dibenzoylphenylhydrazine is produced by photo-oxygenation of diphenylsydnone. 

The oxadiazole group is already present in 2,2 -(oxydi-4,l-phenyl-ene)bis[5-(4-fluorophenyl)-l,3,4-oxadiazole], c.f. Figure 10.1. The compound can he condensed with various naphthalenedioles, such as 1,5-na-phthalenediol to result in PODA types. AH polymers are amorphous materials and some of them are soluble in aprotic solvents. Alkylated 2,7-dibromo-9H-fluorene compounds, with pendent carhazole and oxadiazole units can be condensed with nickel catalysts. ... 

Poly(9,9 -dihexylfluorene-2,7-divinylene-m-phenylene vinylene-stat-p-phenyl-ene vinylene), 30, 31, 51 Poly(2,5-dimethoxy-l,4-phenylene vinylene), 95 Poly(2-(A, A -dimethylamino) phenylene vinylene), 100 Poly(2-dimethyloctylsilyl)-phenylene vinylene, 99 Poly(9,9-dioctylfluorene), 31, 110 Poly(9,9-dioctylfluorene-co-lluorenone), 30 Poly(4,4 -diphenyl ether-l,3,4-oxadiazole), 334 Poly(2,6-diphenyl-l,4-phenylene oxide), 152 Poly(2,6-diphenyl-l-4-phenylene oxide), 141 Poly(dithiathianthrene), 189 Poly(2-dodecyl-p-phenylene), 36 Poly(AT-epoxypropyl)carbazole, 13 Poly(ether ether ketone), 209 Poly(ether imide), 154, 214, 264, 376 Pol(yether ketone), 213 Poly(ether nitrile), 227 Poly(ethersulfone), 209, 264... 

J. Griiner, R. H. Friend, J. Huber, and U. Scherf, A blue-luminescent ladder-type poly(para-phenyl-ene) copolymer containing oxadiazole groups Chem. Phys. Lett. 257 204 (1996). 

Figure 28.3 Molecular structures of siloxane hybrid precursors. DCM= 4-dicyanomethy-lene-2-methyl-6-[p-(dimethylamino)styryl]-4/-/-pyran PBD= 2-(4-biphenylyl)-5-(4-ferf-butyl-phenyl)-1,3,4-oxadiazole NABUP= A/-(4-butyl-phenyl)-4-[(A/-2-hydroxyethyl)(methyl)amino] naphthalimide TPPy = 1,3,6,8-tetraphenylpyr-ene PHEMA= poly(2-hydroxyethyl...

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