The diels-alder reaction of 5-aminopyrazole-4-carbonitrile with dmad: Synthesis and structural determination of dimethyl 2-(p-toluenesulfonylamino)-3-cyano-4-imino-1,4-dihydropyridine-5,6-dicarboxylate

The Diels-Alder reaction of 5-amino-1-(p-toluenesulfonyl)pyrazole-4-carbonitrile with dimethyl acetylenedi-carboxylate was carried out in the presence of potassium carbonate in dimethyl sulfoxide. The reaction gave dimethyl 2-(p-toluenesulfonylamino)-3-cyano-4-imino-1,4-dihydropyridine-5,6-dicarboxylate. The product was formed by transformation of the original Diels-Alder adduct followed by rearrangement of the p-toluenesul-fonylamino group into the 2-position of the pyridine ring. The structure of the product was irrefutably established by X-ray crystallography. This reaction is the first example of a pyrazole ring serving as the diene in a [4 + 2] cycloaddition reaction.     

Reactions of urocanic acid (UCA) methyl esters with singlet oxygen and 4-methyl-1,2,4-triazoline-3,5-dione (MTAD)

Singlet oxygen adds to the imidazole ring of cis- and trans-methyl urocanate (MUC) to yield the corresponding 2,5-endoperoxides, which are modestly stable at low temperature but decompose upon warming to form complex reaction mixtures. MTAD, a singlet oxygen mimic, reacts with cis- and trans-MUC to yield stereospecific [4+2] reaction products involving the olefinic side chain and the C₄-C₅ double bond of the imidazole ring. trans-MUC forms a 1:2 MTAD adduct while the cis isomer yields only the 1:1 adduct at 25 °C. The stereospecificity and absence of MeOH trapping adducts indicate that these reactions may not involve open or trappable dipolar intermediates.     

Synthesis of 1,2,4-oxadiazines and their rearrangement to pyrimidines

Several amide oximes underwent condensation reactions with dimethyl acetylene dicarboxylate to afford 1:1 adducts. Under basic conditions, these adducts underwent ring closure to afford several methyl [3-(substituted)-4,5-dihydro-5-oxo-6H-1,2,4-oxadiazin-6-ylidene]acetates. The reactions of these compounds with a variety of amines resulted in addition-rearrangement reactions with the formation of the corresponding methyl 2-substituted-5-substituted amino-1,6-dihydro-6-oxo-4-pyrimidine carboxylates.     

Additionen von 2,2-Dimethyl-3-dimethylamino-2H-azirin an 2-Formyl-cycloalkanone und Sulfinsäuren

2,2-Dimethyl-3-dimethylamino-2H-azirine ( 1 ) reacts with the formyl-cycloalkanones 4 – 8 in boiling benzene to give the 1:1 adducts 13 – 17 in 60–99% yield (Table). These adducts are N′-[(2-oxo-cycloalkylidene)-methyl] derivatives of 2-amino-N, N-dimethylisobutyramide. The reaction mechanism (Scheme 6) is analogous to the mechanism of the reaction of 1 with carboxylic acids and cyclic enolisable 1,3-diketones [1]. Sulfinic acids and 1 undergo a similar reaction at ?15° to yield 2-sulfinamido-N, N-dimethylisobutyramides (Schemes 4 and 7), while sulfonic acids and the azirine 1 lead to a dimeric salt of type 20 , which with sodium hydroxide gives the dihydropyrazine 21 (Scheme 5).     

Metal‐induced B–H Activation. Addition of Methyl Acetylene Carboxylates to Cp*Rh‐ and Cp*Ir 16 e Halfsandwich Complexes containing a Chelating 1,2‐Dicarba‐closo‐dodecaborane‐1,2‐diselenolato Ligand

Reactions of the 16e halfsandwich complexes Cp*M[Se₂C₂(B₁₀H₁₀)] ( 5 M = Rh, 6 M = Ir) with both methyl acetylene monocarboxylate and dimethyl acetylene dicarboxylate were studied in order to obtain information on the influence of the chalcogen (selenium versus sulfur), as well as further evidence for B–H activation, ortho‐metalation and substitution of the carborane. In the case of the rhodium‐selenium complex 5 , the reaction with methyl acetylene monocarboxylate gave products which were all structurally different compared to those of the sulfur analogue of 5 : a polycyclic derivative 12 with a B(6)‐substituted carborane cage was obtained as one of the final products; in addition, both geometrical isomers containing a Rh–B bond ( 10 , 11 ) and isomers without a Rh–B bond ( 8 , 9 ) were isolated, the latter being the result of twofold insertion into one of the Rh–Se bonds. In the case of the iridium‐selenium complex 6 , the reaction with methyl acetylene monocarboxylate led to the geometrical isomers 13 and 14 (similar to 10 and 11 ) with structures possessing an Ir–B bond. Both 5 and 6 reacted with dimethyl acetylene dicarboxylate at room temperature to give the complexes 15 and 16 which are formed by addition of the C≡C unit to the metal center and insertion into one of the metal‐selenium bonds. The proposed structures in solution were deduced from NMR data (¹H, ¹¹B, ¹³C, ⁷⁷Se, ¹⁰³Rh NMR), and an X‐ray structural analysis was carried out for the rhodium complex 12 .     

Additionsreaktionen von 3-Dimethylamino-2,2-dimethyl-2H-azirin an Phenylisocyanat und Diphenylketen

Addition Reaction of 3-Dimethylamino-2,2-dimethyl-2H-azirine with Phenylisocyanate and Diphenylketene 3-Dimethylamino-2,2-dimethyl-2H-azirine ( 1a ) reacts with carbon disulfide and isothiocyanates with splitting of the azirine N(1), C(3)-double bond to give dipolar, fivemembered heterocyclic 1:1 adducts. In some cases, these products can undergo secondary reactions to yield 1:2 and 1:3 adducts. In this paper it is shown that the reaction of 1a with phenylisocyanate also takes place by cleavage of the N(1), C(3)-bond, whereas with diphenylketene N(1), C(2)-splitting is observed. The reaction of 1a and phenylisocyanate in hexane at room temperature yields the 1:3 adduct 2 in addition to the trimeric isocyanate 3 (Scheme 1). A mechanism for the formation of 2 is given in Scheme 5. Hydrolysis experiments with the 1:3 adduct 2 , yielding the hydantoins 4–6 and the ureas 7 and 8 (Schemes 3 and 5), show that the formation of this adduct via the intermediates d , e and f is a reversible reaction. The aminoazirines 1a and 1b undergo an addition reaction with diphenylketene to give the 3-oxazolines 14 (Scheme 8), the structure of which has been established by spectral data and oxidative degradation of 14a to the 3-oxazolin-2-one 15 (R¹ ? R² ? CH₃, Scheme 9).     

Ultrasound-assisted synthesis of novel spiro[indoline-3,5′-pyrido[2,3-d]pyrimidine] derivatives using Fe3O4@Propylsilane@Histidine[HSO4−] as an effective magnetic nanocatalyst

A simple, economical, three-component, and one-pot synthesis of novels spiro[indoline-3,5′-pyrido[2,3-d]pyrimidine] derivatives by condensation of dimethyl acetylene dicarboxylate, isatin derivatives, and 6-amino-1,3-dimethyluracil in presence of Fe₃O₄@Propylsilane@Histidine[HSO₄⁻] as an impressive catalyst via reflux and ultrasound procedure is described. The results show that ultrasound method has several advantages such as milder condition, shorter reaction time, and higher reaction yield.     

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.     

ab initio molecular orbital and density functional analysis of acetylene + O2 reactions with CHEMKIN evaluation

A number of researchers have indicated that a direct reaction of acetylene with oxygen needs to be included in detailed reaction mechanisms in order to model observed flame speeds and induction times. Four pathways for the initiation of acetylene oxidation to chain propagation are considered and the rate constants are compared with values used in the mechanisms:

• 1 ³O₂ + HCCH to triplet adduct and reaction on the triplet surface
• 2 ³O₂ + HCCH to triplet adduct, conversion of triplet adduct to singlet adduct via collision in the reaction environment, with further reaction of the singlet adduct
• 3 ¹O₂ + HCCH to singlet adduct
• 4 Isomerization of HCCH to vinylidene and then vinylidene insertion reaction with ³O₂

Elementary reaction pathways for oxidation of acetylene by addition reaction of O₂(³Σ) on the triplet surface are analyzed. ab initio molecular orbital and density functional calculations are employed to estimate the thermodynamic properties of the reactants, transition states, and products in this system. Acetylene oxidation reaction over the triplet surface is initiated by addition of molecular oxygen, O₂(³Σ), to a carbon atom, forming a triplet peroxy‐ethylene biradical. The reaction path to major products, either two formyl radicals or glyoxal radical plus hydrogen atom, involves reaction through three transition states: O₂(³Σ) addition to acetylene (TS1), peroxy radical addition at the ipso‐carbon to form a dioxirane (TS2), and cleavage of O O bond in a three‐member ring (TS3). Single‐point QCISD(T) and B3LYP calculations with large basis sets were performed to try to verify barrier heights on important transition states. A second pathway to product formation is through spin conversion of the triplet peroxy‐ethylene biradical to the singlet by collision with bath gas. Rapid ring closure of the singlet peroxy‐ethylene biradical to form a four‐member ring is followed by breaking of the peroxy bond to form glyoxal, which further dissociates to either two formyl radicals or a glyoxal radical plus hydrogen atom. The overall forward rate constant through this pathway is estimated to be k_f = 2.21 × 10⁷ T^1.46e^{−33.1(kcal/mol)/RT}. Two additional pathways from the literature, HCCH + O₂(¹Δ) and pressure‐dependent isomerization of acetylene to vinylidene and then vinylidene reaction with O₂(³Σ), are also evaluated for completeness. CHEMKIN modeling on each of the four proposed pathways is performed and concentration profiles from these reactions are evaluated at 0.013 atm and 1 atm over 35 milliseconds. Through reaction on the triplet surface is evaluated to be not important. Formation of the triplet adduct with conversion (via collision) to a singlet and the vinylidene paths show similar and lower rates than those used in mechanisms, respectively. Our implementation of the HCCH + O₂(¹Δ) pathway of Benson suggests the need to include: (i) reverse reaction, (ii) barriers to further reaction of the initial adduct plus (iii) further evaluation of the O₂(¹Δ) addition barrier. The pathways from triplet adduct with conversion to singlet and from vinylidene are both recommended for initiation of acetylene oxidation. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 623–641, 2000     

Container Systems II: An Experimental Study of the High‐Pressure Intramolecular Cycloaddition of Tethered Furans and Anthracenes onto Norbornane Cyclobutene‐1,2‐diesters

A new class of container molecules is described and the first steps in producing protypes are reported. Central to the approach is the formation of polynorbornanes with cyclobutene‐1,2‐difurfuryl esters at the terminus or similar functionality at the bridgehead of a central norbornane subunit. The synthesis of the furfuryl starting materials is described as well as their anthracenyl counterparts. Conversion to the container systems involved the intermolecular linking of the furfuryl or anthracene by treatment with dimethyl acetylene dicarboxylate (DMAD) in a Diels–Alder (DA) protocol under thermal or high‐pressure (HP) conditions. In practice, no intermolecular linking occurred between the norbornane substrates and only products from DA 1:1‐addition with DMAD were produced. Intramolecular addition of one of the furfuryl units onto the cyclobutene π‐bond was detected under HP conditions, and this intermolecular product was capable of isolation and characterization by working at room temperature or below, but reverted to starting material above room temperature. When conducted in the presence of DMAD, a single 1:1‐adduct was obtained in which one furfuryl moiety was intramolecularly cyclized and the other present as the DMAD adduct; again this product underwent retro‐DA reaction at 40°C. Similar intermolecular cyclization was observed with the bis‐anthracenyl esters. The stereoselectivity of the intermolecular attack of the furfuryl diene with the dienophilic cyclobutene gave a single adduct by endo‐face attack in which the oxa‐bridge is endo‐positioned. Quantum chemical DFT calculations (B3LYP) predict that the formation of the endo‐isomer is kinetically favored and that relief of ring strain enhances the rate of retro‐Diels–Alder in the tethered system.     

Zur Reaktionsweise von Enaminen mit Cyclopropenonen IV. Einsatz von Ketenacetalen

A new interpretation – based on a reevaluation of the spectroscopic properties of products 16 to 27 – is proposed for the reaction of diphenyl-cyclopropen-one 14 and -thione 15 with ketene-A, N-diacetals 8 to 13 (A ? R₂N, RO and RS) originally reported by Sauer & Krapf. It is concluded that the previous structural assignments (see the a-structures), made on the assumption of a prevailing “C,C-insertion” reaction, must be rcplaced as follows: (1) All the “secondary adducts” are, in fact, derivatives (amides and lactams) of 2,3-diphenyl-penta-2, 4-dienoic acid and thioacid (structures 16b to 24b ); (2) the “isomerization products”, differ from the latter only in the configuration of the α,β-double bond (structures 25b and 26b ); (3) the common “hydrolysisproduct” is α,β-diphenyl-γ-methyl-γ-hydroxy-Δ^α-butenolide ( 27b ), The above cyclopropenone-ketcneacetal reactions represent, therefore, cases of “C, N-insertion”. This is rationalized with a reaction scheme, in which the “acylide” structure of the “primary adducts” plays a role.     

An efficient ultrasonic-assisted synthesis of ethyl-5-(aryl)-2-(2-alkokxy-2-oxoethylidene)-7-methyl-3-oxo-3, 5-dihydro-2H-thiazolo [3, 2-a] pyrimidine-6-carboxylate derivatives

Dihydropyrimidinone derivatives were prepared by tri-component reaction of ethyl aceto acetate, aldehydes and thiourea in the presence of modified montmorillonite nanostructure as a catalyst and used as key intermediates for the synthesis of ethyl-5-(aryl)-2-(2-alkokxy-2-oxoethylidene)-7-methyl-3-oxo-3,5-dihydro-2H-thiazolo[3,2-a]pyri midine-6-carboxylate derivatives with use of diethyl and dimethyl acetylene dicarboxylate by two methods: (a) in methanol as a solvent under ultrasonic irradiation at ambient temperature (b) in methanol as a solvent at ambient temperature (conventional magnetic stirring). Ultrasound-assisted synthesis provides excellent yields in short reaction times (15–25 min) at room temperature.     

Hydroxyalkylierungen von Cystein über das Enolat von (2R,5R)-2(tert-Butyl)-1-aza-3-oxa-7-thiabicyclo[3.3.0]octan-4-on und unter Selbstreproduktion des Ciralitätszentrums

Hydroxyalkylations of Cysteine through the Enolate of (2R,5R)-2(tert-Butyl)-1-aza-3-oxa-7-thiabicyclo[3.3.0]octan-4-one with Self-Reproduction of the Center of Chirality The heterobicyclic compound 1 specified in the title is readily prepared as a single stereoisomer from (R)-cysteine, formaldehyde, and pivalaldhyde. While it is not possible to generate the enolate 10 from 1 qunatitatively – due to β-elimination of thiolate (→6) – an in-situ addition to aromatic aldehydes such as benzaldehydes (→13–16) , pyrrol-, furan-, and thiophen-2-carbaldehydes (→17–19) , pyridine-3-carbaldehyde (→21) , as well as to other non-enolizable aldehydes like cinnamaldehyde (→22) , can be achieved in yields of ca. 50%. The adducts ( 8 and 9 ) of lithium diisopropylamide or t-butoxide to these aldehydes are acting, probably as bases for deprotonation and as in-situ sources of the electrophilic aldehyde species (cf. 11, 12 ). - Of the four possible diastereoisomeric products, one is usually formed with >90% selectivity (Table). It is assumed that the preferred stereochemical course of the reaction corresponds to that observed previously with the analogous proline-derived enolate (See 23,24 ). A chemical correlation with l-α-methyl-β-phenylserine (25) proves the relative configuration of the benzaldehyde adduct 13 . All hydroxyalkylated products (13–19, 21, 22) are obtained as crystalline, diastereoisomerically pure compounds and are fully characterized. – The benzaldehyde derivative 13 was used to exemplify the various possible transformations of these products to monocyclic or acyclic amino-acid derivatives such as the oxazolidionenes 26 and 29 (cleavage of the ring containing the S -atom), the thiazolidines 28 , 31 , and 32 (cleavage of the cyclic N,O-acetal) and the α-branched cysteine 27 and the phenylserines 25 and 30 (cleavage of both rings to give open-chain aminoacids).     

Stereochemical Course of the Reaction between Thiocarbonyl Compounds and Oxiranes: Reaction with cis‐ and trans‐2,3‐Dimethyloxirane

The reactions of thiocarbonyl compounds with cis‐2,3‐dimethyloxirane ( 1a ) in CH₂Cl₂ in the presence of BF₃⋅Et₂O or SnCl₄ led to trans‐4,5‐dimethyl‐1,3‐oxathiolanes, whereas with trans‐2,3‐dimethyloxirane ( 1b ) cis‐4,5‐dimethyl‐1,3‐oxathiolanes were formed. With the stronger Lewis acid SnCl₄, the formation of side‐products was also observed. In the case of 1,3‐thiazole‐5(4H)‐thione 2 , these side‐products are the corresponding 1,3‐thiazol‐5(4H)‐one 5 and the 1 : 2 adduct 8 (Schemes 2 – 4). Their formation can be rationalized by the decomposition of the initially formed spirocyclic 1,3‐oxathiolane and by a second addition onto the C=N bond of the 1 : 1 adduct, respectively. The secondary epimerization by inversion of the configuration of the spiro‐C‐atom (Schemes 5 – 7) can be explained by a Lewis‐acid‐catalyzed ring opening of the 1,3‐oxathiolane ring and subsequent ring closure to the thermodynamically more stable isomer (Scheme 12). In the case of 2,2,4,4‐tetramethyl‐3‐thioxocyclobutanone ( 20 ), apart from the expected spirocyclic 1,3‐oxathiolanes 21 and 23 , dispirocyclic 1 : 2 adducts were formed by a secondary addition onto the C=O group of the four‐membered ring (Schemes 9 and 10).     

Inverse‐electron demand [π2 + σ2 + σ2] cycloaddition reaction of dimethyl oxaquadricyclane‐2,3‐dicarboxylate with cyclooctyne

A first example of an inverse‐electron demand [_π2 + _σ2 + _σ2] cycloaddition reaction of dimethyl oxaquadricyclane‐2,3‐dicarboxylate was reported: cyclooctyne underwent cycloaddition with dimethyl oxaquadricyclane‐2,3‐dicarboxylate to afford the corresponding adducts one of whose structure was confirmed by a single crystal X‐ray analysis.     

Pyrrole studies—XXIV: The reactions of 1-methyl-2-vinylpyrrole and 1-phenyl-2-vinylpyrrole with dimethyl acetylenedicarboxylate

The reaction of dimethyl acetylenedicarboxylate with 1-methyl-2-vinylpyrrole is temperature dependent. At 80° the predominant reaction is the [4π + 2π] cycloaddition to give dimethyl 1-methyl-6,7-dihydroindole-4, 5-dicarboxylate, whereas at room temperature Michael addition of the acetylenic ester at the 5-position of the pyrrole ring to give fumaric and maleic ester derivatives also occurs. Unequivocal assignment of the configurations of the Michael adducts have been made on the basis of their ³J_CO,h coupling constants. 1-Phenyl-2-vinylpyrrole reacts with dimethyl acetylenedicarboxylate to give only the [4π + 2π] cycloadduct at room temperature and at elevated temperatures.     

Novel Synthesis of Oxothiazolidine Derivatives

(Z)‐Methyl 2‐[3‐(arylideneamino)‐2‐(arylidenehydrazono)‐4‐oxothiazolidin‐5‐ylidene]acetate prepared during the reaction between thiocarbonohydrazides and dimethyl acetylene dicarboxylate. Rational for there conversations involving the nucleophilic addition on C/C triple bond of dimethyl acetylenedicarboxylate are presented.     

Synthese von Pyrimido[1,2-a]benzimidazolen durch Umsetzung von 2-Aminobenzimidazol mit Acetylendicarbonsäure-dimethylester und ihre Überführung in Imidazo[1,2-a]benzimidazole

The main product of the reaction between 2-aminobenzimidazole and dimethyl acetylenedicarboxylate is shown to be methyl 1, 2-dihydro-2-oxo-pyrimido[1, 2-a]benzimidazole-4-carboxylate ( 6 ), which can be methylated at position 1 to give 7 . Catalytic hydrogenation of 7 leads to the 1,2,3,4-tetrahydro derivative 8 , whereas NaBH₄ reduces the ester and, to some extent, the double bond to yield a mixture of 9 and 10 . When a 1-substituted 1, 2-dihydro-4-hydroxymethyl-pyrimido[1, 2-a]benzimidazol-2-one (for example 11 ) is catalytically hydrogenated, the double bond is unaffected, but hydrogenolysis of the alcohol group occurs instead to give 13 . The lactam group is less stable in the tetrahydro series than in the dihydro compounds. For example, the lactam is cleaved when 8 is treated with amines containing a small amount of water, and the monoamides 16 and 17 are formed. Similarly, sodium hydroxyde cleaves the lactam under mild conditions to the dicarboxylic acid 19 , which can be converted to 2, 3-dihydro-1-methyl-2-oxo [1H]imidazo[1, 2-a]benzimidazole-3-acetic acid (4-methyl)piperazide ( 20 ) with thionyl chloride and N-methylpiperazine. However, when 7 is treated with methylamine at low temperature, the amide 22 is formed, whilst at room temperature the amine attacks both the ester and the double bond to give 23 . The structure of 8 was confirmed by X-ray analysis.     

Reaction of 5-(2-furyl)-1-methyl-1<Emphasis Type="Italic">H-</Emphasis> and 1-methyl-5-(2-thienyl)-1<Emphasis Type="Italic">H</Emphasis>-imidazoles with electrophilic reagents

5-(2-Furyl)-1-methyl-1H- and 1-methyl-5-(2-thienyl)-1H-imidazoles were synthesized. The electronwithdrawing effect of the 5- and 2-imidazole substituents on the furan ring was studied by ¹H NMR spectroscopy and quantum-chemical calculations. Some electrophilic substitution reactions were investigated (nitration, bromination, sulfonation, hydroxymethylation, formylation, and acylation). In some cases, depending on the reaction conditions, both the furan and thiophene ring and the imidazole fragment undergo electrophilic attack.     

Die Acylierung von 5-Amino-1 H-1, 2, 4-triazolen. Eine 13C-NMR.-Studie

The Acylation of 5-Amino-1 H-1,2,4-triazoles. A ¹³C-NMR. Study The acylation of 3-substituted-5-amino-1 H-1,2,4-triazoles (1) with methyl chloroformate or dimethylcarbamoyl chloride yielded mainly 1-acyl-5-amino-1,2,4-triazoles ( 2 and 3 ). Acylation of 3-methyl-, 3-methoxy- and 3-methylthio-5-amino-1 H-1,2,4-triazole ( 1b , 1c and 1d ) with methyl chloroformate gave up to 10% of the 1-acyl-3-amino-1,2,4-triazoles. For the unsubstituted 5-amino-1,2,4-triazole (1a) , a (1:1)-mixture of the 3- and 5-isomers 2a and 4 was obtained in dioxane in the presence of triethylamine. No 4-acylated product was detected in contrast to earlier reports. The structures of the reaction products were determined with the aid of proton coupled ¹³C-NMR. spectra using the corresponding N-methyl-1,2,4-triazoles as reference compounds.