4-PHENYL-l,2,4-TRIAZOLINE

The product 18 does not give a Diels Alder adduct with 4-phenyl-l,2,4-triazoline-3,5-dione, indicating the absence of its bicyclic tautomer. 

The values of X = NH, OH, F, Cl, and CH3 are smaller than that of X = H, in accordance with the observed selectivity. Excellent correlation was found for all other cyclopentadienes described above. Syn rr-facial selectivity in the reactions between 4-phenyl-l,2,4-triazoline-3,5-dione and cyclopentadiene having simple alkyl group at 5 positions are reported by Burnell and coworkers [46] (Scheme 37). 

Dipolar cycloaddition of pyrido[2,l-A][l,3]thiazinium betaine 507 (R = Me) with 1-diethylamino-l-propyne afforded cycloadduct 508, from which quinolizin-4-one 509 formed by a rapid cheletropic extrusion of COS (Scheme 53) <1995T6651>. 1,4-Dipolar cycloaddition of 507 and 4-phenyl-l,2,4-triazoline-3,5-dione yielded 511 (via 510) <1995H(41)1631> and 512 <1995T6651>. 

Electrocyclic reactions have been performed with three of the bridged [ 11 Jan-nulenones. Both 11 and 13, which both contain a cycloheptatriene unit, undergo Diels-Alder additions with dienophiles via their norcaradiene valence tautomers 41 and 43 and yield adducts of the type 42 and 44. Annulenone 13 was found to react only with 4-phenyl-l,2,4-triazoline-3,5-dione whilst 11 underwent reaction with a variety of dienophiles. 3,8-Methano[ 11 Jannulenone 12 contains a tetraene system and undergoes addition reactions, apparently of the (8 + 2)-type, at the termini of the tetraene system. Thus with maleic anhydride the adduct 46, the valence tautomer of the initial adduct 45, was isolated. 

The most powerful azo dienophile is the cisoid 4-phenyl-l,2,4-triazoline-3,5-dione 264, which is surpassed in reactivity only by singlet oxygen. The dione adds rapidly to all types of dienes and the process can be followed visually since the bright-red color of the reagent is discharged when the reaction is complete136. 

The (diphenylmethylene)aminocyclobutenecarboxylates 109 obtained by rearrangement of the DMPA-H adducts of 1-Me, 2-Me, contain a 2-azadiene unit and a cyclobutene moiety. Indeed, the parent compound 109 a reacted with 4-phenyl-l,2,4-triazoline-3,5-dione (PTAD, [80]) at room temperature in a [4-1-2] cycloaddition mode to yield the tricyclic tetraazaundecene 132 in almost quantitative yield (Scheme 44) [8]. As substituted cyclobutenes, compounds 109 should be capable of opening up to the corresponding butadienes [1, 2b, 811. When compounds 109 were subjected to flash vacuum pyrolysis, the dihydro-isoquinolines 135 were obtained, presumably via the expected ring-opened intermediates 133, which subsequently underwent bn electrocyclization followed by a 1,5-shift, as is common for other 3-aza-l,3,5-hexatrienes [82]. 

C. Heteroatomic dienophiles 111 Esters of azodicarboxylic acid 12k 4-Phenyl-l,2,4-triazoline-3,5-dione 131 hninocarbamates... 

Phenyl-l,2,4-triazoline-3,5-dione has been prepared by oxidizing 4-phenylurazole with lead dioxide,7 and with ammoni-acal silver nitrate followed by an ethereal solution of iodine.8 The yields are low for both methods. 4-Substituted triazoline-diones can also be made by oxidation of the corresponding urazole with fuming nitric acid9 or dinitrogen tetroxide.10 Oxidation by <-butyl hypochlorite in acetone solution has also been described 1112 it, however, yields an unstable product, even after sublimation. Either dioxane12 or ethyl acetate are preferred as solvents for the reaction, since the product is obtained in a stable form. The latter solvent is superior since... 

In common with other azodicarboxylic acid derivatives, the uses of 4-phenyl-l,2,4-triazoline-3,5-dione are many. It undergoes a Diels-Alder reaction with most dienes11-14 and is, in fact, the most reactive dienophile so far reported.15 16 As with the formation of all Diels-Alder adducts the reaction is reversible, and in the case of the adduct with 3-j3-acetoxy-17-cyano-5,14,16-androstatriene, the reverse reaction can be made to proceed under especially mild conditions.14 An instance has also been reported of the dione photochemically catalyzing other retro Diels-Alder reactions.17 Along with the proven use of azodicarboxylic ester,18-18 the dione should be potentially important in the preparation of strained ring compounds. 

Phenyl-l,2,4-triazoline-3,5-dione also undergoes addition-abstraction reactions (e.g., with acetone17). As would be expected for such a species, it will oxidize alcohols to the corresponding aldehydes or ketones.20 This oxidation is especially mild (room temperature in benzene, chlorobenzene or ethyl acetate) and so is a valuable method of oxidizing, or preparing, compounds sensitive to acid, base, or heat. 

PHENYL-l,2,4-TRIAZOLINE-3,5-DIONE (A1-l,2,4-Triazoline-3,5-dione, 4-phenyl-)... 

New examples involve the reaction of 5-vinyl-17/-imidazole 293 with 4-phenyl-l,2,4-triazoline-3,5-dione 294. In this way, the imidazo[4,5-i ]pyridazine skeleton was smoothly constructed (Equation 72) <1998TL4561>. Another example using in situ formed 294 can be found in Section 8.01.6.5. 

Substituted l,2,4-triazoline-3,5-diones are excellent dienophiles which react rapidly at room temperature with oxepins, but particularly with the arene oxide valence tautomer. A similar [4+2] cycloaddition reaction between the episulfide tautomer of thiepin (44) and 4-phenyl-l,2,4-triazoline-3,5-dione has been reported (74AG(E)736>. Benzene episulfide (the valence tautomer of thiepin 44) was generated in situ by thermal decomposition of the diepisulfide (151) at 20 °C and trapped as a cycloadduct at the same temperature (equation 34). A 1,3-dipolar cycloaddition reaction between thiepin (152) and diazomethane has been reported (56CB2608). Two possible cycloadduct products are shown since the final structure has not been unequivocally established (equation 35). 

Chlorine and bromine add normally to the double bond in (256) and the dibromo compound can be mono-dehydrobrominated using sodium methoxide in methanol to give a mixture of 5-bromo-4,7-dihydro-l,3-dioxepin and 5-bromo-4,5-dihydro-l,3-dioxepin (which is converted in situ to the 5-methoxy derivative). More interestingly it can be fully dehydrobrominated using HMPA at 140 °C to give the fully unsaturated 2H- 1,3-dioxepin (261) (76TL2113). This is the only known route to this compound. It gives a Diels-Alder adduct with 4-phenyl-l,2,4-triazoline-2,5-dione in 95% yield and on UV irradiation it isomerizes to (262). 

Dipolar cycloaddition of anhydro pyrido[2,l-b][l,3]thiazinium hydroxides (128) with aryl isocyanates and dimethyl acetylenedicarboxylate gave pyrido[l,2]pyrimidines (129) and quinolizine-l,2-dicarboxylates (130), respectively (76CB3668). 1,4-Dipolar cycloaddition of pyrido[2,l-h][l,3]thi-azinium betaine (131, R = Me) with 1-diethylamino-l-propyne afforded cycloadduct 132, from which quinolizin-4-one 133 formed by a rapid cheletropic extrusion of carbonyl sulfide (93TL5405 95T6651). 1,4-Dipolar cycloaddition of anhydro 4-hydroxyl-2-oxo-6,7,8,9-tetrahydro-2//-pyrido-[2,l-b][l,3]thiazinium hydroxides (131) and 4-phenyl-l,2,4-triazoline-3,5-dione yielded 135 via 134 [94H(39)219 95H(41)1631] and 136 (95T6651). 

Where the carbon-carbon double bond is a part of an aromatic system, in general, cyclopropanation of diazoketones results in the formation of unstable cyclopropane adducts. For example, Saba140 has shown that in the intramolecular cyclopropanation of diazoketone 57 the norcaradiene ketone 58 can be detected by low-temperature NMR and can be trapped in a Diels Alder reaction with 4-phenyl-l,2,4-triazoline-3,5-dione (equation 69). In addition, Wenkert and Liu have isolated the stable norcaradiene 60 from the rhodium catalysed decomposition of diazoketone 59 (equation 70)105. Cyclopropyl ketones derived from intramolecular cyclopropanation of hetereoaromatic diazoketones are also known and two representative examples are shown in equations 71 and 72106. Rhodium(II) compounds are the most suitable catalysts for the cyclopropanation of aromatic diazoketones. 

Phenyl-l,2,4-triazoline-3,5-dione10 undergoes 1,3-cycloaddition at room temperature across the N-N double bond with vinyl azides to form bicyclic A3-l,2,3-triazolines (Scheme 2) (Table I)6 the relative rates of addition of... 

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