Tieftemperaturbestrahlungen von 3-Phenyl-2H-azirinen

2, 2, 3-Triphenyl-2H-azirine ( 4a ) in a matrix of 2, 2-dimethylbutane/pentane 8:3 (DMBP) at ?185° gave rise on irradiation with light of 250–350 nm to a new UV.-maximum at 350 nm (Fig. 1). We assign the dipole benzonitrildiphenylmethylide ( 1a ) to this new maximum. Irradiation with monochromatic light of 366 nm destroyed this maximum and the initial absorption curve reappeared (Fig. 2). When the azirine 4a was photolysed in DMBP at ?185° in the presence of methyl trifluoracetate (TFEM), the maximum at 350 nm was obtained again. This maximum vanished upon increasing the temperature to ?160°. Through gas chromatography we were able to show that 5-methoxy-5-trifluormethyl-2, 2, 4-triphenyl-3-oxazoline ( 6a ) was produced. 6a was also obtained upon irradiation of 4a at room temperature in the presence of TFEM (scheme 1 and table 1). Modification of the previously described experiment, in which the maximum at 350 nm was extinguished in the matrix due to irradiation at 366 nm gave, after warm up, almost no dipole adduct 6a (table 1). From these experiments, an extinction coefficient of 17, 000 for the 350 nm maximum of 1a , was calculated. These experiments have shown that irradiation of triphenylazirine 4a leads to the dipole 1a , which can be reversed photochemically – but not tharmally – into azirine 4a. 1a reacts at less than ?160° with TFEM to give adduct 6a . The results which were obtained with triphenylazirine 4a could be correspondingly obtained with 2, 3-diphenyl-2H-azirine 4b (Fig. 3, scheme 2 and table 2). The dipole 1b showed two UV.-maxima at 330 nm (ε = 17, 500) and 343 nm (ε = 21, 000). Later experiments established, that the two maxima belonged to a single dipole species. The dipole 1c obtained upon irradiation of 2, 2-dimethyl-3-phenyl-2H-azirine ( 4c ) in DMBP at ?190°, appears to absorb in the same region as the azirine 4c . The presence of the dipole 1c was univocally established by low temperature trapping experiments with TFEM. The dipole 1a showed no ESR.-spectrum characteristic for a triplet state. We assume therefore, that 1a is in a singlet state. Photolysis of oxazolinone 7 at ?190° in DMBP led to the dipole 1a with loss of CO₂. 1a recombines apparently in considerable amount with the CO₂ trapped in the matrix to give starting oxazolinone 7 because the 350 nm-maximum of 1a appeared with low extinction. Irradiation with light of 366 nm into this matrix produccd triphenylazirine 4a . Low temperature trapping experiments with TFEM led to small amounts of 5-methoxy-5-trifluormethyl-triphenyl-3-oxazolinc ( 6a ).     

Zur Photodimerisierung von 3-Phenyl-2H-azirinen. Vorläufige Mitteilung

Irradiation of 3-phenyl-2H-azirine ( 2 ) in benzene solution with a high-pressure mercury lamp yields 4,5-diphenyl-1,3-diazabicyclo[3,1,0]hex-3-ene ( 4 ) and not 3-phenylimino-4-phenyl-1-azabicyclo[2,1,0]pentane ( 1 ), as had been reported previously by others [2]. 2-Methyl-3-phenyl-2H-azirine ( 3 ) yields on irradiation a 2:1 mixture of 2-exo, 6-exo- and 2-exdo, 6-exo-dimethyl-4,5-diphenyl-1,3-diazabicyclo[3,1,0]hex-3-ene (2-exo,6-exo- and 2-endo, 6-exo- 5 ). Irradiation of 2,3-diphenyl-2H-azirine ( 8 ) leads to the formation of 2,4,5-triphenyl-imidazole ( 9 ) and tetra-phenylpyrazine ( 10 ). The suggested reaction path for the generation of 9 and 10 is shown in Scheme 2.     

Übergangsmetall-katalysierte Additionsreaktionen von 3-Phenyl-2H-azirinen und Acetylencarbonsäureestern

Transition Metal Catalyzed Addition Reactions of 3-Phenyl-2H-azirines and Alkyl Acetylene Carboxylates In the presence of molybdenum hexacarbonyl, the 3-phenyl-2H-azirines 1 and 7 react with alkyl acetylene carboxylates 2 via the cleavage of the C, N-double bond to give 2H-pyrroles 5 or pyrrole 9 (Table), whose structures were deduced from the spectra data, in particular ¹³C-NMR. data. The 2H-pyrrole 5a was also obtained by treatment of a mixture of 1 and 2a with tungsten hexachloride. A tentative mechanism for the formation of the 2H-pyrroles is formulated.     

Thermische Reaktionen mit 3-Phenyl-2H-azirinen; 1, 3-dipolare Cycloadditionen und En-Reaktionen

Benzonitrile p-nitrobenzylide ( 5 ) undergoes 1,3-dipolar cyclo-additions in the presence of 3-phenyl-2H-azirines ( 1 ), yielding in benzene at 0° 2-(p-nitrophenyl)-4,5-diphenyl-1,3-diazabicyclo[3.1.0]hex-3-enes ( 7 , scheme 2). Under the basic conditions of the reaction mixture, 7 a and 7 b are partially converted to 2-(p-nitrophenyl)-4,5-diphenyl-1,6-dihydropyrimidines ( 8a, b ) which are dehydrogenated by oxygen to the corresponding pyrimidines 9a and 9b , respectively. 3-Phenyl-2H-azirines ( 1 ) form, on heating at 145° in xylene in the presence of the azalactone 32 (2,4-diphenyl-Δ²-oxazolin-5-one), 4-(aziridin-2′-yl)-2,4-diphenyl-Δ²-oxazolin-5-ones ( 33 , scheme 11). 33 arises from an ene reaction of the enol form of 32 with 1 . Similar ene reactions are observed with the azirines 1 and dimedone ( 37 , scheme 12). Under the ene reaction conditions (xylene, 145°), the non-isolated intermediate primary adducts ( 38a and 38b ) undergo rearrangements of the vinylcyclopane-cyclopentene type to give 6,6-dimethyl-4-oxo-1,3-diphenyl-4, 5, 6, 7-tetrahydroisoindole ( 40 ) and 6, 6-dimethyl-4-oxo-3-phenyl-4, 5, 6, 7-tetrahedroindole ( 42 ), respectively.     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen mit aktivierten Doppelbindungen. Vorläufige Mitteilung

Irradiation of 2-methyl- ( 1a ), 2,2-dimethyl- ( 1b ) and 2,3-diphenyl-2H-azirine ( 1c ) in the presence of diethyl mesoxalate yields the corresponding 4-phenyl-5,5-diethoxycarbonyl-3- oxazolines 3a–c . Similar cycloadducts are observed (cf. 6 ) by irradiation of 1b and 1c in the presence of trifluoroacetophenone. When ethyl cyanoformate is used as trapping agent photolysis of 1b or 1c leads to cycloadducts with the carbonyl and nitrile group, respectively which are present in the cyanoformate.     

4,4-Disubstituierte Imidazol-Derivate aus der Umsetzung von 3-Amino-2H-azirinen mit Salicylamid

4,4-Disubstituted Imidazole Derivatives from the Reaction of 3-Amino-2H-azirines with Salicylamide Reaction of 3-amino-2H-azirines 1a–c with salicylamide ( 7 ) in MeCN leads to imidazoles 10 and 11 in different rates, depending on the conditions. In the case of 1a and 1b, 11a and 11b , respectively, have been obtained as the main product at 50°; in reactions at 80°, 10a and 10b are the favored products (Tables 1 and 2). 2,2-Dimethyl-3-(N-methyl-N-phenylamino)-2H-azirine ( 1c ) reacts with 7 in MeCN mainly to 2-(2-hydroxyphenyl)-5,5-dimethyl-3,5-dihydroimidazol-4-one ( 10a ); in boiling toluene, 11c is formed with low preference (Table 3). The structure of the products has been established by spectroscopic means, and in the case of 10b and 11c , by X-ray crystallography. Two different reaction mechanisms for the formation of the products are discussed (Scheme 2).     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen mit kumulierten Doppelbindungen. Vorläufige Mitteilung

Irradiation of 2-methyl- ( 1c ) and 2,2-dimethyl-3-phenyl-2H-azirine ( 1d ) in benzene solution in the presence of carbon dioxide yields 2-methyl-4-phenyl- ( 3c ) and 2,2-dimethyl-4-phenyl-3-oxazolin-5-one ( 3d ), respectively. Similar cycloadducts are observed (see table) when 2,3-diphenyl-2H-azirine ( 1b ) and 1d are irradiated in the presence of phenylisocyanate, o-tolylisocyanate, phenylisothiocyanate or di-o-tolyl-carbodiimide.     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen mit Triphenyl-vinylphosphoniumbromid. 36. Mitteilung über Photoreaktionen

On irradiation in acetonitrile 3-phenyl-2H-azirines of type 1 react with triphenyl vinyl phosphonium bromide to form in approximative 50% yield 2H-indoles of type 4 (Scheme 1). In analogy to other photochemical reactions with 2H-azirines [2] [3] it is assumed that the photochemically generated dipoles 2 react with the triphenyl vinyl phosphonium salt (Scheme 1). The conversion of 1 to 4 represents a new synthesis for 2H-pyrroles.     

Photoinduzierte Reaktionen von 3-Phenyl-2H-azirinen mit Carbonsäureestern 40. Mitteilung über Photoreaktionen

Irradiation (280–350 nm light) of a benzene solution of 3-phenyl-2H-azirines 1a – e in the presence of carboxylate esters, whose carbonyl groups are activated by electron withdrawing groups situated in the acyl or alkyl moiety, produces 5-alkoxy-3-oxazolines (Tab. 1 and 4, Scheme 2) isolated in 18–82% yield. These heterocycles undoubtedly originate by regiospecific addition of the ester carbonyl group to the azirine-derived benzonitrile-methylide ‘dipole’ (Scheme 1). The 5-(2,′ 2′, 2′-trifluoroethoxy)-3-oxazolines, derived from 2′, 2′, 2′-trifluoroethyl carboxylic esters, on treatment with methanolic hydrogen chloride at low concentration, are smoothly transformed into the corresponding 5-methoxy-3-oxazolines (e.g. 16 → 17 , Tab. 5). Utilizing this process, various hitherto relatively unknown 9. 5-alkoxy-3-oxazolines become accessible. The constitution of the adducts is based essentially on spectral data. The structure of trans-5-methoxy-2,4-diphenyl-5-trifluoromethyl-3-oxazoline (trans- 14 ), the addition product of methyl trifluoroacetate and the benzonitrile-benzylide from 2,3-diphenyl-2H-azirine ( 1d ), was determined by X-ray crystallography (Section 5). Benzonitrile-isopropylide ( 22 ), resulting from the photochemical transformation of 2,2-dimethyl-3-phenyl-2H-azirine ( 1a ), also reacts with S-methyl thiobenzoate to give 2,2-dimethyl-5-methylthio-4,5-diphenyl-3-oxazoline ( 26 ). Ethyl cyanoacetate protonates predominantly the dipolar species derived from 1a at the nitrile C-atom and yields after work-up ethyl α-cyano-cinnamate ( 29 ) and ethyl isopropylidene-cyanoacetate ( 30 ) (Scheme 4). The relative rate of addition (k_rel) of benzonitrile-isopropylide ( 22 ) to methyl α-haloacetates and dimethyl oxalate was determined by competition experiments (Section 6). Log k_rel correlated satisfactorily (r = 0.97) with the pK_a of the acide derived from the ester reactant: log k_rel = ? 1.72 pK_a + 2.58 or with Taft's substituent constants σ*: log k_rel = 2.06 σ* ? 4.11 [k_rel(methyl dichloroacetate) = 1; Section 7.1]. On the basis of the results obtained, the mode of reaction of the so-called benzonitrile-methylide ‘dipole’ is discussed and a model for the transition state of addition of ester-carbonyl groups is proposed that accounts for the observed regiospecifity and steroselectivity.     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen an Carbonsäurechloride. 35. Mitteilung über Photoreaktionen

Irradiation of 2, 3-diphenyl-2H-azirine ( 1a ) and 1-azido-1-phenyl-propene, the precursor of 2-methyl-3-phenyl-2H-azirine ( 1b ), in benzene, with a high pressure mercury lamp (pyrex filter) in the presence of acid chlorides yields the oxazoles 5a–d (Scheme 2). Photolysis of 2, 2-dimethyl-3-phenyl-2H-azirine ( 1c ) under the same conditions gives after methanolysis the 5-methoxy-2, 2-dimethyl-4-phenyl-3-oxazolines 7a, b, d , while hydrolysis of the reaction mixture leads to the formation of the 1, 2-diketones 8a, c, d (Scheme 4). The suggested reaction path for all these reactions is a 1, 3-dipolar cycloaddition of the photochemically generated benzonitrilemethylides 2 to the carbonyl double bond of the acid chlorides to give the intermediates 4 , followed by either elimination of hydrogen chloride or solvolysis (Schemes 2 and 4). Irradiation of 1c in the presence of acetic acid anhydride leads via the intermediate 9 to the 5-hydroxy-3-oxazoline 10 and the 5-methylidene-3-oxazoline 11 (Scheme 5).     

4-Amino-1,5-dihydro-2H-pyrrol-2-one aus Bortrifluorid-katalysierten Umsetzungen von 3-Amino-2H-azirinen mit Carbonsäure-Derivaten

4-Amino-1,5-dihydro-2H-pyrrol-2-ones from Boron Trifluoride Catalyzed Reactions of 3-Amino-2H-azirines with Carboxylic Acid Derivatives Reaction of 3-amino-2H-azirines 1 with ethyl 2-nitroacetate ( 6a ) in refluxing MeCN affords 4-amino-1,5-dihydro-2H-pyrrol-2-ones 7 and 3,6-diamino-2,5-dihydropyrazines 8 , the dimerization product of 1 (Scheme 2). Thus, 6a reacts with 1 as a CH-acidic compound by C? C bond formation via C-nucleophilic attack of deprotonated 6a onto the amidinium-C-atom of protonated 1 (Scheme 5). The scope of this reaction seems to be rather limited as 1 and 2-substituted 2-nitroacetates do not give any products besides the azirine dimer 8 (see Table 1). Sodium enolates of carboxylic esters and carboxamides 11 react with 1 under BF₃ catalysis to give 4-amino-1,5-dihydro-2H-pyrrol-2-ones 12 in 50–80% yield (Scheme 3, Table 2). In an analogous reaction, 3-amino-2H-pyrrole 13 is formed from 1c and the Li-enolate of acetophenone (Scheme 4). A reaction mechanism for the ring enlargement of 1 involving BF₃ catalysis is proposed in Scheme 6.     

Umsetzung von 3-Amino-2H-azirinen mit Salicylohydrazid

Reaction of 3-Amino-2H-azirines with Salicylohydrazide 3-Amino-2H-azirines 1a–g react with salicylohydrazide ( 7 ) in MeCN at 80° to give 2H, 5H-1,2,4-triazines 10 , 1,3,4-oxadiazoles 12 and, in the case of 1d , 1,2,4-triazin-6-one 11a (Scheme 3). The precursor of these heterocycles, the amidrazone of type 9 , except for 9c and 9g , which could not be isolated, has been found as the main product after reaction of 1 and 7 in MeCN at room temperature. 3-(N-Methyl-N-phenylamino)-2-phenyl-2H-azirin ( 1g ) reacts with 7 to give mainly the aromatic triazines 15b₁ and 15b₂ . In this case, two unexpected by-products, 16 and salicylamide ( 17 ), occurred, probably by disproportionation of a 1:1 adduct from 1g and 7 (Scheme 8). Oxidation of 10f with DDQ leads to the triazine 15a . The structure of 10c, 11a, 12c, 13 (by-product in the reaction of 1b and 7 ), the N′-phenylureido derivative 14 of 9d (Scheme 4) as well as 15b₂ has been established by X-ray crystallography. The ratio of 10/12 as a function of substitution pattern in 1 and solvent has been investigated (Tables 1, 3, 4, and 7). A mechanism for the formation of 10 and 12 is proposed in Scheme 7.     

3-Phenyl-2-benzofuranon

Ohne Zusammenfassung     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen mit Ketonen,Acylcyaniden und Ketoestern. 33. Mitteilung über Photoreaktionen

Similarly to aldehydes [6] ketones form 3-oxazolines via cyclo-addition to the benzonitrile-methylides 2 that arise photochemically from the 3-phenyl-2H-azirines 1 . With various ketones benzonitrile-isopropylide ( 2a ) gives cyclo-addition products in very good preparative yields (scheme 1). Benzonitrile-ethylide ( 2c ) and benzonitrile-benzylide ( 2b ) [8] react, however, sluggishly with ketones, smooth cyclo-addition being observed in their case only with «activated» ketones (2,2,2-trifluoro-acetophenone, 1,1,1-trifluoro-2-propanone). With 1a acetonyl-acetone forms the bis-adduct 12 While the azirine 1a reacts with cyclohexanone to yield essentially only the spiro-(3-oxazoline) 13 , it gives with cyclopentanone, depending on the reaction conditions, either the spiro-(3-oxazoline) 14 or the butenyl-3-oxazoline 15 (scheme 3). The formation of 15 has to be preceded by the photochemical formation of 4-pentenal from the ketone. Norcamphor and camphor react in a similar way (schemes 4 and 5). The azirines 1a–c react smoothly with the keto groups in acylcyanides and α-keto-esters, giving with the former 5-cyano-3-oxazolines and with the latter 5-ethoxycarbonyl-3-oxazolines (schemes 6 and 7). β-Keto-esters (acetoacetic ester) form with the dipole arising from 1a the expected addition product 31 and, via the protonated dipole d (scheme 8), finally the benzylidene-acetoacetic ester. Analogous results are obtained with malonodinitrile, trifluoro-acetamide and other weak acids such as alcohols [29] [30] (scheme 9). The light-induced rearrangement of the bicyclic isoxazoline 37 into the oxazoline 38 is visualized as an intramolecular cyclo-addition reaction (scheme 10). The cyclo-addition in this case proceeds with the aldehyde group inversed as compared to the related intermolecular benzonitrile–methylide addition to aldehydes.     

Photochemisch induzierte Reaktionen von 3-Amino-2H-azirinen

Photochemically Induced Reactions of 3-Amino-2H-azirines Irradiation of 3-(N-methylanilino)-2H-azirines with a mercury low pressure lamp induces the cleavage of the C(2), C(3)-ring bond thus affording nitrilio-methanide dipols, substituted by an amino group at C(1). Depending on the substitution pattern at C(3), these intermediates can be trapped by dipolarophiles to yield five-membered heterocycles with high regioselectivity, or they undergo a 1,4-H-shift forming 2-azabutadiene derivatives. Further, the dipol is protonated at C(1) even by weak CH–acids.     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen mit Benzoyl-, Äthoxycarbonyl- und Vinylphosphonaten 34. Mitteilung über Photoreaktionen

The irradiation of the 3-phenyl-2H-azirines 1a–c in the presence of diethyl benzoylphosphonate ( 8 ) in cyclonexane solution, using a mercury high pressure lamp (pyrex filter), yields the diethyl (4, 5-diphenyl-3-oxazolin-5-yl)-phosphonates 9a–c (Scheme 3). In the case of 1b a mixture of two diastereomeric 3-oxazolines, resulting from a regiospecific but non-stereospecific cycloaddition of the benzonitrile-benzylide dipole 2b to the carbonyl group of the phosphonate 8 , was isolated. Benzonitrile-isopropylide ( 2a ), generated from 2,2-dimethyl-3-phenyl-2H-azirine ( 1a ), undergoes a cycloaddition reaction to the ester-carbonyl group of diethyl ethoxycarbonylphosphonate ( 15 ) with the same regiospecificity to give the 3-oxazoline derivative 16 (Scheme 5). The azirines 1a–c , on irradiation in benzene in the presence of diethyl vinylphosphonate ( 17 ) give non-regiospecifically the Δ¹-pyrrolines 13a–c and 14a–c (Scheme 6).     

Photoinduzierte 1, 3-dipolare Cycloaddition von 3-Phenyl-2H-azirinen an Azodicarbonsäure-diäthylester. 32. Mitteilung über Photoreaktionen

Irradiation of 2, 2-dimethyl-3-phenyl- ( 1a ), 2, 3-diphenyl-2H-azirine ( 1b ) or the azirine-precursors 1-azido-1-phenyl-propene ( 2a ) and 1-azido-1-phenyl-ethylene ( 2b ), respectively, in benzene in the presence of azodicarboxylic acid diethylester, yields the corresponding 1, 2-carbethoxy-3-phenyl-Δ³-1, 2, 4-triazolines 4a–d (Scheme 1). Refluxing 4 ( a, c or d ) in 0, 2–0, 4M aqueous ethanolic potassium hydroxide leads to the formation of the 1-carbethoxy-3-phenyl-Δ²-1, 2, 4-triazolines 6 ( a, c or d ). Under the same conditions 4b is converted to 3, 5-diphenyl-1, 2, 4-triazole ( 7b , Scheme 2). In 10M aqueous potassium hydroxide solution heating of either 4 ( c or d ) or 6 ( c or d ) yields the 3-phenyl-1, 2, 4-triazoles 7 ( c or d ). Photolysis of 1-carbethoxy-5, 5-dimethyl-3-phenyl-Δ²-1, 2, 4-triazoline ( 6a ) in benzene in the presence of oxygen and trifluoroacetic acid methylester gives the 5-methoxy-2, 2-dimethyl-4-phenyl-5-trifluoromethyl-3-oxazoline ( 13 , Scheme 5). 5, 5-Dimethyl-3-phenyl-1, 2, 4-triazole seems to be the intermediate, which on losing nitrogen gives the benzonitrile-isopropylide ( 3a ).     

Reaktionen der valenzpolaromeren Ketenform mesoionischer Heterocyclen mit 3-Dimethylamino-2H-azirinen

Reactions of valencepolaromeric ketenes of mesoionic heterocyles with 3-dimethylamino-2H-azirines Reactions of the 3-dimethylamino-2H-azirines 1a and 1b with the mesoionic oxazole 5 and the mesoionic dithiole 6 in acetonitrile at room temperature yield the 1:1 adducts 11 , 12 , 19 and 20 , respectively (Schemes 5 and 8). These products can be formulated as adducts of the aminoazirines and the ketenes 5a and 6a , which are valence polaromeric forms of the mesoionic heterocycles 5 and 6 (Scheme 2). The structure of the adducts has been elucidated by spectral data and their comparison with the data of (Z)- 11 , the structure of which has been established by X-ray [19]. Oxidation of the 1:1 adducts with KMnO₄ in a two-phase system yields 4-dimethylamino-3-oxazolin-2-ones (cf. Scheme 6) by clevage of the exocyclic C,C-double bond. A mechanism for the formation of the adducts is given in Scheme 9: Nucleophilic attack of 1 on the ketene leads to a primary adduct of type a , which undergoes clevage of the former N(1), C(2)-azirine bond to give adducts of type 11 or 19 . The N(1), C(2)-ring opening of 1a in the reaction with ketenes contrasts with the N(1), C(3)-opening of 1a in the addition with, for instance, isothiocyanates. These different ring openings are explained by the difference in nucleophilicity of the heteroatoms X and Y in a ′ (Scheme 10).     

Dipolare (1:1)-Addukte aus der Reaktion von 3-Amino-2H-azirinen mit 1,3,4-Oxadiazol- und 1,3,4-Thiadiazol-2(3H)-onen

Dipolar 1:1 Adducts from the Reaction of 3-Amino-2H-azirines with 1,3,4-Oxadiazol- and 1,3,4-Thiadiazol-2(3H)-ones 3-Amino-2H-azirines 1 react with 5-(trifluoromethyl)-1,3,4-oxadiazol-2(3H)-one ( 2 ) as well as with different 5-substituted 1,3,4-thiadiazol-2(3H)-ones ( 5a–e ) in 2-propanol at room temperature to give dipolar 1:1 adducts of type 3 and 6 , respectively, in reasonable-to-good yields (Schemes 3 and 6, Tab. 1 and 2). The structure of two of these dipolar adducts, 6a and 6e , which are formally donor-acceptor-stabilized azomethin imines, have been elucidated by X-ray crystallography (Figs. 1-4). In the reaction of 2 and sterically crowded 3-amino-2H-azirines 1c–e with a 2-propyl and 2-propenyl substituent, respectively, at C(2), a 4,5-dihydro-1,2,4-triazin-3(2H)-one of type 4 is formed as minor product (Scheme 3 and Table 1). Independent syntheses of these products proved the structure of 4 . Several reaction mechanisms for the formation of compounds 3 and 4 are discussed, the most likely ones are described in Scheme 4: reaction of 2 as an NH-acidic compound leads, via a bicyclic zwitterion of type A , to 3 as well as to 4 . In the latter reaction, a ring-expanded intermediate B is most probable.