Synthesis and Structure of Linear and Cyclic Oligomers of 3-Hydroxybutanoic Acid with Specific Sequences of (R)- and (S)-Configurations

To study the stereoselectivity of enzymatic cleavage of poly(3-hydroxybutyrates) (PHB) in a well-defined system (purified depolymerase and monodisperse substrate of specific relative configuration), linear and cyclic oligomers of HB (OHBs) containing (R)- and (S)-3-hydroxybutanoate residues were synthesized. The starting material (R)-HB was prepared from natural sPHB, and (S)-HB by enantioselective reduction of 3-oxobutanoate with yeast or with H₂/Noyori-Taber catalyst (Scheme 2). The HB building blocks were then protected (O-benzyl/tert-butyl ester; Scheme 3) and coupled to give dimers 3 , 4 , tetramers 5 – 9 , and octamers 10 – 18 ; for analytical comparison, a 3mer, 5mer, 6mer, and 7mer ( 19 – 22 ) were also prepared. Two of the tetramers were subjected to macrolactonization conditions (Yamaguchi) to give the cyclic tetramers 23 and 25 and octamers 24 and 26 . All new compounds were fully characterized (m.p., [α]_D, CD, IR, ¹H- and ¹³C-NMR, MS, elemental analysis). Single-crystal X-ray structure analyses were performed with oligolides 24 and 25 (Figs. 2 and 4), and the structures, as well as the crystal packing, were compared with those of analogs containing only (R)-HB units or consisting of 3-amino- instead of 3-hydroxybutanoic-acid moieties.     

Chemical structure of networks resulting from curing of N,N-diglycidylaniline-type resins with aromatic amines. IV. Characterization of TGDDM/DDS and TGDDM/DDM networks by high-resolution solid-state 13C-NMR

Cured N,N,N′,N′-tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) based epoxy resins were investigated by high-resolution solid-state ¹³C-NMR spectroscopy. Associated hardeners were the most commonly used low reactivity 4,4--diaminodiphenylsulphone (DDS), as well as, for comparisons reasons, the higher reactivity 4,4′-diaminodiphenylmethane (DDM) with, in each case, a1 to 1 or 1 to 0.6 epoxy/NH ratio. In order to interpret the spectra, the poorly resolved aliphatic region was decomposed into elementary lines, the structural assignments of which were made using solution ¹³C-NMR data resulting from a previous model compound study. The main structural feature of all investigated systems is the predominance of small cyclic units resulting from intramolecular reactions of N,N-diglycidylaniline groups. The resins are therefore far less crosslinked that it could be anticipated from the functionality of the reactants. Using the low reactivity DDS still increases this effect, due to a higher proportion of residual non reacted secondary amines. Reducing the initial ratio of hardener could on the contrary lead to a higher proportion of reacted amine function, and thus to a higher crosslinking degree. A qualitative picture of such networks is given at the end.     

Synthesis of Linear Oligomers of (R)-3-Hydroxybutyrate and Solid-State Structural Investigations by electron microscopy and X-ray scattering

Repetitive treatment of the biopolymer P(3-HB) (molecular weight > 10⁵ Dalton, storage or s-P(3-HB)), with lithium hexamethyl disilazanid (LHMDS) at ?70° in THF leads to a mixture of oligomers with increasingly sharp distribution around a 15-, 30-, and 45mer. Discrete fragments are also isolated when P(3-HB) is heated under reflux (89°) in neat Et₃N. Linear oligo(3-HB) derivatives ( 3-7 ) containing up to 96 3-HB units are synthesized using an exponential segment-coupling strategy. These oligomers are used to calibrate size-exclusion chromatography columns for the analysis of oligo(3-HB) samples from the different sources. The linear oligo-(3-HB) derivatives also served as a model with respect to the physical properties of high molecular weight P(3-HB) and were investigated as such by transmission electron microscopy (TEM) and by small- and wide-angle X-ray scattering (SAXS and WAXS). The thicknesses of the lamellar crystallites (long periods) formed by the 8mer, 16mer, and 32mer, are ca. 26, 52, and 53 Å, respectively, indicating that the 32mer molecules are folded once, very tightly, into a ‘hair-pin’-type conformation. High-molecular-weight P(3-HB), which was crystallized in a similar way, also has a lamellar crystallite thickness of ca. 50–65 Å. Thus, the treatment of P(3-HB) with LHMDS at low temperature causes etching of the amorphous regions, an effect well known in polymer science for studying the regularity of chain folding. The ca. 50-Å packing within the tight folds of P(3-HB) is discussed in view of its possible function in ion transport through cell membranes.     

Cyclo-depolymerization of poly(propylene terephthalate): some ring-opening polymerizations of the cyclic oligomers produced

We report the cyclo-depolymerization of poly(propylene terephthalate) to give a mixture of cyclic oligomers in 94% yield, the characterization of the mixture by ¹H-NMR spectroscopy, matrix assisted laser desorption ionization time of flight mass spectrometry and gel permeation chromatography. The major cyclic oligomer in the mixture was shown to be the cyclic dimer. It was isolated and its X-ray crystal structure determined. Some entropically-driven ring-opening polymerizations of the cyclic oligomers were carried out. So too were some copolymerizations using mixtures of the cyclic oligomers and those derived similarly from poly(ethylene terephthalate) and poly(butylene terephthalate). ¹³C-NMR spectroscopic analysis showed that the copolymers were random. Copyright © 2003 John Wiley & Sons, Ltd.     

Synthese, 13C-NMR-Spektren und räumliche Struktur von ‘Push-Pull’-Diacetylenen

Synthesis, ¹³C-NMR Spectra, and X-Ray Investigation of ‘Push-Pull’ Diacetylenes Phenyl-substituted ‘push-pull’ diacetylenes 1f and 1g have been prepared by acetylation and benzoylation of the appropriate lithiodiynylamines 4 (Scheme 2). ¹³C-NMR spectra of diacetylenes 1a–g (Table 1) are discussed with respect to the expected polarisation of the diacetylene unit by ‘push’ and ‘pull’ substituents. X-Ray investigations of 1c , 1e , and 1f have been performed in view of the planned solid-state polymerisation of ‘push-pull’ diacetylenes. In the crystalline state, diacetylenes 1c and 1f are stacked, however, the stacking parameters do not allow a solid-state polymerisation.     

High-Yield Synthesis of 20-, 24-, and 28-Membered Macropentolide, -hexolide,and -heptolide,Respectively, from (R)- or (S)-3-hydroxybutanoic acid under Yamaguchi's macrolactonization conditions

The macrocyclic pentolide 1 , hexolide 2 , and heptolide 3 constitute ca. 80% of the oligomers formed in ca. 50% yield from enantiomerically pure 3-hydroxybutanoic acid under Yamaguchi's macrolactonization conditions. The FAB mass spectra of the MH⁺, M Na⁺, and MCs⁺ are reported (Figs. 2, 3, 5, and 6). No cyclic tetramer is detected. The ¹H-NMR spectra of the cyclic oligomers, of the monomer, and of the polymer (PHB) are very similar (Fig. 4). Directed synthesis of the open-chain dimer and tetramer of 3-hydroxybutanoic acid and attempted cyclization do not lead to the isolation of the cyclic tetramer.     

Synthese monodisperser linearer und cyclischer Oligomere der (R)-3-Hydroxybuttersäure mit bis zu 128 Einheiten

Monodisperse Linear and Cyclic Oligo[(R)-3-hydroxybutanoates] Containing up to 128 Monomeric Units Using benzyl ester/(tert-butyl)diphenylsilyl ether protection, (COCl)₂/pyridine esterification conditions, and a fragment-coupling strategy (with H₂/Pd-C debenzylation and HF · pyridine desilylation), linear oligomers of (R)-3-hydroxybutanoic acid (3-HB) containing up to 128 3-HB building blocks (mol. weight > 11 000 Da) are assembled (Schemes 1,2,5, and 6). In contrast to the previously employed protecting-group combination, and due to the low-temperature esterifying conditions, this procedure leads to monodisperse oligomers: all steps occur without loss of single 3-HB units. The product oligomers with two, one, and no terminal protecting groups (mostly prepared in multi-gram amounts) are characterized by all standard spectroscopic methods, especially by mass spectroscopy (Figs. 2 and 3), by their optical activity, and by elemental analyses. Cyclization of the oligo[(R)-3-hydroxybutanoic acids] with up to 32 3-HB units, using thiopyridine activation and CuBr₂ for the ring closure, produces oligolides consisting of up to 128 ring atoms (Scheme 7). Mixed oligolides containing 3-HB and (R)-3-hydroxypentanoic units are prepared from the corresponding linear trimers, using Yamaguchi's method for the ring closure (Scheme 8 and Fig.4 (X-ray crystal structures of two folded conformers)). Comparisons of melting points (Table 1), of [α] values (Tables 2 and 3), of ¹H-NMR coupling constants (Table 3), and of molecular volume/hydroxyalkanoate unit (Table 4) of linear and cyclic oligomer derivatives and of the high-molecular-weigh polymer show that the monodisperse oligomers appear to be surprisingly good models for the polymer. Besides this insight, our synthesis is supplying the samples to further test the role of P(3-HB) (ca. 140 units) as a component of complexes forming channels through cell-wall phospholipid bilayers.     

Nylon 6 structure in solid state as studied by high-resolution 13C-NMR spectroscopy

High resolution ¹³C-NMR spectra of nylon 6 samples crystallized under various conditions and of a drawn sample were measured at room temperature by the cross polarization/magic angle spinning (CP/MAS) and pulse saturation transfer/magic angle spinning techniques. Additionally, ¹³C-NMR spectra of the drawn sample were measured at temperatures from 20 to 100°C by the CP/MAS technique and at 20 and 100°C by the low-power decoupling/magic angle spinning technique. The nylon 6 structure in the solid-state is discussed on the basis of these results. The solid-state ¹³C chemical shift data are used for reference in a study of conformation in solution.     

Channel-Forming Activity of 3-Hydroxybutanoic-Acid Oligomers in Planar Lipid Bilayers

Monodisperse and polydisperse oligomers and polymers of 3-hydroxybutanoic acid (3-HB) containing 8, 16, ca. 28, 32, ca. 60, 64, 96, and ca. 3000 monomer units were incorporated into palmitoyl-oleoyl-phosphatidyl choline (POPC) planar bilayers. At concentrations of 0.1–5% of oligo(3-HB), the resulting phospholipid bilayers showed typical single-channel behavior for Rb⁺ and Ba²⁺ ions, using the patch clamp technique. Thus, channel-forming activity of a pure polyester has been demonstrated for the first time (Figs. 1, 3, and 6). Single-channel activity depends upon the following structural parameters of the 3-HB derivatives: unprotected OH and COOH groups on the chain ends; chain length ⩾ 16 monomer units; no high-molecular-weight as in P(3-HB). The results are discussed in view of the Ca²⁺-specific channel formed with the P(3-HP)/Ca · PPi complex from genetically competent Escherichia coli and in view of the ubiquitous occurrence of low-molecular-weight P(3-HB) in prokaryotic and eukaryotic organisms. A simple model for the channel-causing structure is proposed, based on the proven tendency of oligo- and poly(3-HB) to form ca. 50-Å thick lamellar crystallites.     

Peak area measurements of cross-polarization magic-angle-spinning 13C-NMR spectra of crosslinked polystyrenes

Cross-polarization magic-angle-spinning ¹³C-NMR spectra of polystyrenes crosslinked with 1–20% of methine vinyl carbon ¹³C-labeled p-divinylbenzene and of Friedel–Crafts crosslinked poly(chloromethylstyrene)s have been obtained with both glossy solid and CDCl₃-swollen gel samples. The spectra of natural abundance, uncrosslinked, glassy polystyrene, and the spectra of the solid labeled networks give aliphatic and aromatic peak areas only 0.7 times as large per ¹³C atom as that of poly(oxymethylene). Similarly the crosslinked poly(chloromethylstyrene) gave peak areas about 0.6 times that of internal poly(oxymethylene). The labeled gels give peak areas 0.2–0.6 times as large per ¹³C atom as glassy polystyrene, and the peak areas in spectra of gels increase with the divinylbenzene content     

Cyclische Oligomere von (R)-3-Hydroxybuttersäure: Herstellung und strukturelle Aspekte

Cyclic Oligomers of (R)-3-Hydroxybutanoic Acid: Preparation and Structural Aspects The oligolides containing three to ten (R)-3-hydroxybutanoate (3-HB) units (12-through 40-membered rings 1–8 ) are prepared from the hydroxy acid itself, its methyl ester, its lactone (‘monolide’), or its polymer (poly(3-HB), mol. wt. ca. 10⁶ Dalton) under three sets of conditions: (i) treatment of 3-HB ( 10 ) with 2,6-dichlorobenzoyl chloride/pyridine and macrolactonization under high dilution in toluene with 4-(dimethylamino)pyridine (Fig. 3); (ii) heating a solution (benzene, xylene) of the β-lactone 12 or of the methyl ester 13 from 3-HB with the tetraoxadistanna compound 11 as trans-esterification catalyst (Fig. 4); (iii) heating a mixture of poly(3-HB) and toluene-sulfonic acid in toluene/1,2-dichloroethane for prolonged periods of time at ca. 100° (Fig. 6). In all three cases, mixtures of oligolides are formed with the triolide 1 being the prevailing component (up to 50% yield) at higher temperatures and with longer reaction times (thermodynamic control, Figs. 3–6). Starting from rac-β-lactone rac- 12 , a separable 3:1 to 3:2 mixture of the l,u- and the l,l-triolide diasteroisomers rac- 14 and rac- 1 , respectively, is obtained. An alternative method for the synthesis of the octolide 6 is also described: starting from the appropriate esters 15 and 17 and the benzyl ether 16 of 3-HB, linear dimer, tetramer, and octamer derivatives 18–23 are prepared, and the octamer 23 with free OH and CO₂H group is cyclized (→ 6 ) under typical macrolactonization conditions (see Scheme). This ‘exponential fragment coupling protocol’ can be used to make higher linear oligomers as well. The oligolides 1–8 are isolated in pure form by vacuum distillation, chromatography, and crystallization, an important analytical tool for determining the composition of mixtures being ¹³C-NMR spectroscopy (each oligolide has a unique and characteristic chemical shift of the carbonyl C-atom, with the triolide 1 at lowest, the decolide 8 at highest field). The previously published X-ray crystal structures of triolide 1 , pentolide 3 , and hexolide 4 (two forms), as well as those of the l,u-triolide rac- 14 , of tetrolide ent- 2 , of heptolide 5 , and of two modifications of octolide 6 described herein for the first time are compared with each other (Figs. 7–10 and 12–15, Tables 2 and 5–7) and with recently modelled structures (Tables 3 and 4, Fig. 11). The preferred dihedral angles τ₁ to τ₄ found along the backbone of the nine oligolide structures (the hexamer and the larger ones all have folded rings!) are mapped and statistically evaluated (Fig. 16, Tables 5–7). Due to the occurrence of two conformational minima of the dihedral angle O? CO? CH₂? CH (τ₃ = + 151 or ?43°), it is possible to locate two types of building blocks for helices in the structures at hand: a right-handed 3₁ and a left-handed 2₁ helix; both have a ca. 6 Å pitch, but very different shapes and dispositions of the carbonyl groups (Fig. 17). The 2₁ helix thus constructed from the oligolide single-crystal data is essentially superimposable with the helix derived for the crystalline domains of poly(3-HB) from stretched-fiber X-ray diffraction studies. The absence of the unfavorable (E)-type arrangements around the OC? OR bond (‘cis-ester’) from all the structures of (3-HB) oligomers known so far suggests that the model proposed for a poly(3-HB)-containing ion channel (Fig. 2) must be modified.     

Synthesis of novel organic oligomers containing Si?H bonds

Several organic oligomers (M_w = 10³–10⁴ order) containing Si?H bonds of the general formula 1 have been successfully synthesized by platinum-catalyzed partial hydrosilylation reaction of an allyloxy (or an allyl carbonate) end-blocked linear organic oligomer 2 with 2,4,6,8-tetramethylcyclotetrasiloxane, [CH₃(H)SiO]₄ ( 3 ) (hereafter called hydrocyclotetrasiloxane). ¹H-NMR spectroscopy confirmed the introduction of hydrocyclotetrasiloxane moiety into the oligomers through Si? C linkage by hydrosilylation reaction. ¹³C-NMR analysis revealed that the cyclic structure of the starting hydrocyclotetrasiloxane 3 was retained intact in product 1 . As the precursor for 1 , allyloxy (or allyl carbonate) end-blocked oligomers 2 could be prepared from hydroxyl-terminated oligomers 4 . The storage stability of product 1 was significantly influenced by the platinum catalyst still remaining in it. The poor stability was improved by decreasing the amount of the platinum catalyst and/or by adding coordinating compounds. As a result, an excellent stability of product 1 was obtained. © 1993 John Wiley & Sons, Inc.     

Polymerization of cyclic acetals. I. Polymerization of 1,3,6-trioxacyclooctane and copolymerization with p-methoxystyrene

The polymerization of 1,3,6-trioxacyclooctane initiated by trityl salts with various counterions in CH₂Cl₂ was investigated. The reaction mixtures and the isolated polymers were analyzed by GPC (double detection—IR and UV at 254 nm),¹H-, and¹³C-NMR spectroscopy. In the early stage of polymerization only oligomers (mainly cyclic) were formed. With longer reaction times, linear polymers (yield 86–94%, M = 70,000–80,000) were obtained. The concentration of each individual oligomer passed through a maximum and decreased, reaching its equilibrium concentration. The time interval necessary to reach the maximum concentration increased with n. The total concentration of the oligomers was 0.2 mol L^?1 regardless of the initiator used. Conditions for polymerization with virtually no termination were found. Addition of p-methoxystyrene to the “living” polyacetals resulted in block copolymers. GPC,¹H- and ¹³C-NMR and acidolytic degradation were used to prove the formation of AB block copolymers. The reactive alkoxycarbenium growing species are responsible for the formation of block polyacetal-polymethoxystyrene copolymer.     

Strukturaufklärung von N6-, 9- und 7-Acyladeninen durch 1H-und 13C-NMR-Spektroskopie von Festkörpern und in Lösung

Structure Determination of N⁶-, 9-and 7-Acyladenines by ¹H- and ¹³C-NMR Spectroscopy of Solids and in Solution . Adenine (1) reacts with carboxylic acid anhydrides or chlorides 2 to yield the acyladenine isomers 3–5 . The isomeric structures were determined by ³³C- and ¹H-NMR spectroscopy in solution and by solid-state ¹³C-NMR spectroscopy.     

Preparation and Structure of Oligolides from (R)-3-Hydroxypentanoic Acid and comparison with the hydroxybutanoic-acid derivatives: A small change with large consequences

Cyclic oligomers of (R)-3-hydroxyvaleric acid (3-HV) are prepared from the monomer by three different methods, giving various ratios of the oligomers. The macrocycles containing three to twelve 3-HV units (12- to 48-membered rings) are isolated in pure form by chromatography. The triolide 3 can be separated by distillation and isolated on large scale. Biopol, the copolymer of (R)-3-hydroxybutanoic acid (3-HB) and (R)-3-hydroxyvaleric acid (3-HV), is degraded to mixtures of Me- and Et-substituted triolides (‘mixolides’) with high crystallization tendency. The X-ray crystal structures of the tetrolide 4 , pentolide 5 , hexolide 6 , heptolide 7 , and of two ‘mixolides’ (with inclusions of solvent) have been determined (Figs. 3–7, 10, and 11) and are compared with those of the corresponding 3-HB derivatives reported previously. From the structural data, a 3₁ and a 2₁ helix of 3-HV can be modelled, and the latter one compared with helix structures of P9(3-HB) and P(3-HV) derived from stretch-fibre X-ray scattering. Crystals of a water-containing NaSCN complex of the triethyl triolide 3 were obtained in good quality for X-ray analysis. The structure (Figs. 12, 13, and Table 6) contains an interesting array of C?O and H₂O O-atoms around the Na⁺ ions along a channel-type tube (a-axis of the crystal) which may be relevant to the role of P(3-HB) and P(3-HV) as components of cellular ion channels.     

Application of gel-phase 13c-nmr to monitor solid phase peptide synthesis

The results presented in this article show the general applicability of gel-phase ¹³C-NMR to monitor solid phase peptide synthesis on the most commonly used polystyrene-based resins. The optimal conditions to acquire gel-phase ¹³C-NMR spectra of copoly(styrene-l%-divinylbenzene) has been determined. The technique proved to be applicable to characterize polystyrene-based starting supports as well as to the determination of their degree of functionality and purity. The stage-by-stage ¹³C-NMR characterization of the growing peptide chain during the synthesis of tripeptide H-Asn-(N-Me)Ala-Thr-NH₂ on a benzhydrylamine resin is described. The structural information, derived from this technique is relevant for the synthetic process as the spectra can be acquired under similar conditions. The characteristics of the spectra facilitate monitoring of coupling and deprotection reactions.     

A 13C-NMR. Study of cis-trans isomeric vitamins A, carotenoids and related compounds.

The ¹H-decoupled ¹³C-NMR. spectra of 35 all-trans, 17 mono-cis vitamin A compounds (acetates, alcohols, aldehydes, acids and esters) and of one 11, 13-di-cis compound (11, 13-di-cis retinol) are reported. Included in this investigation are desmethyl-, desmethylethyl, and aryl-vitamin A analogues and others as well as 30 reference compounds of smaller molecular weight. Furthermore, the ¹³C-NMR. spectra of 23 β-apo- and other carotenoids were studied. A complete assignment of the signals of all 106 compounds to the specific carbon atoms was achieved by extensive application of lanthanide shift reagents, mainly Yb(dpm)₃, by CW-offset and selective ¹H-decoupling experiments, by comparison of the shifts of related compounds, and in three cases by utilization of specifically deuteriated compounds (11, 12-D₂-retinol and retinyl acetate, 15, 15′-D₂-β-carotene). The chemical shift differences between the cis- and trans-vitamin A compounds and the applicability of the shift reagents for the assignment of the ¹³C-NMR. spectra are discussed.     

Synthesis and characterization of a Schiff base derived from 2-aminobenzylamine and its Cu(II) complex: electropolymerization of the complex on a platinum electrode

A precursor (H₃A) was synthesized by the mono condensation of 2-aminobenzylamine with salicylaldehyde and then a tetradentade Schiff-base ligand (H₂L) prepared by using H₃A and 3-methoxysalicylaldehyde. The copper(II) complex of this new ligand was prepared and characterized by elemental analysis, electronic absorption, Fourier transform infrared (FT-IR), and magnetic susceptibility. For the ligand, ¹H- and ¹³C-NMR and liquid chromatography mass spectrometry (LC–MS) spectra were obtained. The tetradentate ligand is coordinated to Cu(II) through the phenolic oxygen and azomethine nitrogen. The use of this metal complex in the preparation of a modified electrode is also described. CuL was electropolymerized on a platinum electrode surface in a 0.1 mol dm^?3 solution of lithium perchlorate in acetonitrile by cyclic voltammetry between 0 and 1.6 V versus Ag/Ag⁺. Electrochemical properties of the electroactive polymeric film have been investigated and a surface confined polymerization mechanism was proposed.     

High-Resolution Solid-State 13C-NMR Spectroscopy of Polymers [New Analytical Methods (37)]

Until a few years ago, solid-state nuclear resonance yielded spectra containing broad lines only. Meanwhile, CP/MAS-NMR spectroscopy has provided a method which gives narrow nuclear resonance lines from a solid-state specimen as well. Using this technique, it is now possible to produce spectra of “rare” nuclei (¹³C, ²⁹Si, ¹⁵N etc.) which are resolved in terms of chemical structure. The analytical capabilities of NMR spectroscopy can be applied to the solid state: it may be that it is necessary to identify compounds in the solid state because, for example, a solvent would alter the coordination sphere, or that it is desired to monitor chemical reactions in the solid state, for example the baking of an enamel. Where a substance in the solid state is concerned, high-resolution ¹³C-NMR spectroscopy provides not only information about the chemical structure, but also about the solid state itself. To mention just a few examples, information on the conformation, crystal structure and molecular dynamics, as well as molecular miscibility is given. This opens up a broad spectrum of applications, from a statement concerning the crystal modification of an active substance in ready-to-use pharmaceutical preparations, e.g. tablets, to the question of whether two polymers are miscible with one another at a molecular level.     

Notiz über die Reaktion von Methylhydrazin mit N-Cyano-azomethinen. Abhängigkeit des Reaktionsverlaufs von der Natur der Abgangsgruppe

The reaction of methylhydrazine with N-cyanoazomethines 1 containing a thioalkyl leaving group yields the 3-amino-1,2,4-triazole derivatives 2 , whereas the N-cyanoazomethines 1 containing an alkoxy leaving group give the isomeric 5-amino-1,2,4-triazoles 3. The yields are excellent and the position selectivity is high. The structures of the 1,2,4-triazole derivatives were determined with the aid of proton-coupled ¹³C-NMR. spectra.