Synthesis and Dynamic NMR Study of Functionalized 1-(3-Furyl)-1H-indole-2,3-diones

Protonation of the highly reactive 1:1 intermediates produced in the reaction between alkyl(aryl) isocyanides and dibenzoylacetylene by isatin, leads to vinylnitrilium cations, which undergo carbon-centered Michael-type addition with the conjugate base of the NH-acid to produce highly functionalized 1-(3-furyl)-1H-indole-2,3-diones. A dynamic NMR effect is observed in the ¹H NMR spectra of these compounds as a result of restricted rotation around the single bond linking the indole moiety and the furan system. The free-energy of activation (ΔG ^#) for this process is 69–71 ± 2 kJ mol⁻¹.     

Synthesis of Stable Atropisomers of Dialkyl-4-Ethoxy-1-(8-((2-ethoxy-2-oxoacetyl)-amino)-1-naphthyl)-5-oxo-4,5-dihydro-1H-pyrrole-2,3-dicarboxylates

Summary.  Protonation of the reactive 1:1-intermediate produced in the reaction between triphenylphosphine and dialkyl acetylenedicarboxylates with diethyl N,N′-(naphthalene-1,8-diyl)-dioxamate leads to a vinylphosphonium salt which undergoes an intramolecular Wittig reaction to produce dialkyl-4-ethoxy-1-(8-((2-ethoxy-2-oxoacetyl)-amino)-1-naphthyl)-5-oxo-4,5-dihydro-1H-pyrrole-2,3-dicarboxylates in good yields. The title compounds exist as stable rotamers as a result of restricted rotation around the single bond linking the naphthalene moiety and the heterocyclic system. The calculated free energy of activation for interconversion of the atropisomers amounts to about 102±2 kJ · mol⁻¹. Corresponding author. E-mail: isayavar@yahoo.com Received February 5, 2002. Accepted March 19, 2002     

Regioselective hydrolysis of ketenimines derived from NH-acids and acetylenic esters in the presence of <Emphasis Type="Italic">tert</Emphasis>-butyl isocyanide under neutral conditions

The regioselective hydrolysis of ketenimines derived from NH-acids, such as 2,2,2-trichloro-N-phenylacetamide or ethyl-2-anilino-2-oxoacetates and acetylenic esters in the presence of tert-butyl isocyanide in a THF/H₂O system (1/1) without any catalysis leads to a diastereomeric mixture of dialkyl 2-[(tert-butylamino)carbonyl]-3-[(2,2,2-trichloroacetyl)anilino]succinates and dialkyl 2-[(tert-butylamino)carbonyl]-3-[2-ethoxy-2-oxoacetyl)anilino]-succinates in good yields. Dynamic NMR effects were observed in the ¹³C NMR spectra of diethyl 2-[(tert-butylamino)carbonyl]-3-[(2,2,2-trichloroacetyl)anilino]succinate as a result of restricted rotation around the N-aryl single bond. The free energy of activation (ΔG ^#) for this process is 37.9 kJ mol⁻¹ which leads to an observable atropisomerism. Correspondence: Dr. Farough Nasiri PhD, Department of Chemistry, Faculty of Sciences, University of Kurdistan, P.O. Box 66315-416, Sanandaj, Iran.     

Determination of the primary structure and carboxyl p<Emphasis Type="Italic">K</Emphasis><Subscript>A</Subscript>s of heparin-derived oligosaccharides by band-selective homonuclear-decoupled two-dimensional <Superscript>1</Superscript>H NMR

Determination of the structure of heparin-derived oligosaccharides by ¹H NMR is challenging because resonances for all but the anomeric protons cover less than 2 ppm. By taking advantage of increased dispersion of resonances for the anomeric H¹ protons at low pD and the superior resolution of band-selective, homonuclear-decoupled (BASHD) two-dimensional ¹H NMR, the primary structure of the heparin-derived octasaccharide ∆UA(2S)-[(1 → 4)-GlcNS(6S)-(1 → 4)-IdoA(2S)-]₃-(1 → 4)-GlcNS(6S) has been determined, where ∆UA(2S) is 2-O-sulfated ∆^4,5-unsaturated uronic acid, GlcNS(6S) is 6-O-sulfated, N-sulfated β-d-glucosamine and IdoA(2S) is 2-O-sulfated α-l-iduronic acid. The spectrum was assigned, and the sites of N- and O-sulfation and the conformation of each uronic acid residue were established, with chemical shift data obtained from BASHD-TOCSY spectra, while the sequence of the monosaccharide residues in the octasaccharide was determined from inter-residue NOEs in BASHD-NOESY spectra. Acid dissociation constants were determined for each carboxylic acid group of the octasaccharide, as well as for related tetra- and hexasaccharides, from chemical shift–pD titration curves. Chemical shift–pD titration curves were obtained for each carboxylic acid group from sub-spectra taken from BASHD-TOCSY spectra that were measured as a function of pD. The pK _As of the carboxylic acid groups of the ∆UA(2S) residues are less than those of the IdoA(2S) residues, and the pK _As of the carboxylic acid groups of the IdoA(2S) residues for a given oligosaccharide are similar in magnitude. Relative acidities of the carboxylic acid groups of each oligosaccharide were calculated from chemical shift data by a pH-independent method.     

The Potential Energies of Cohesion of a Crystalline Organic Enantiomer and the Racemate; on the Energy for the Liquid Racemate [1]

Summary. The cohesion potential energy of the crystal of one enantiomer of ethyl 3-cyano-3-(3,4-dimethyloxyphenyl)-2,2,4-trimethylpentanoate, −47.7 ± 0.1 kJ mol⁻¹ (0–90°C), was found out from the heat of sublimation (123.2 ± 5.1 kJ mol⁻¹, 78.6°C) and the kinetic energies for the gas phase and the crystal. It was found that the entropy function of Debye’s theory of solids mathematically agreed with the vibrational entropy of the gas (variationally obtained), allowing to disclose the vibrational energy using the Debye energy function (E _vib 835.0 kJ mol⁻¹ (78.6°C), E ⁰ included). E _kin for the crystal (771.1 kJ mol⁻¹ (78.6°C)) was obtained by Debye’s theory with the experimental heat capacity. The cohesion energy represented a moderate part of the sublimation energy. The cohesion energy of the racemic crystal, −44.2 kJ mol⁻¹, was obtained by the heat of formation of the crystal in the solid state (3.0 kJ mol⁻¹, 83.3°C) and E _kin for the crystal (by Debye’s theory). The decrease in cohesion on formation of the crystal accounted for the energy of formation. The change in potential energy on liquefaction of the racemate from the gas state was disclosed obtaining added-up E _{vib + rot} for the liquid in the way as to E _vib for the gas, the Debye entropy function being increasedly suited for the liquid (E _{vib + rot} 763.4 kJ mol⁻¹ (115.4°C)). Positive ΔE _pot, 13.0 kJ mol⁻¹, arised from the increase in electronic energy (Δ _l ν_mean − 154.3 cm⁻¹, by the dielectric nature of the liquid), added to the cohesion energy.     

Synthesis and Dynamic NMR Spectroscopic Study of Dialkyl 4-Ethoxy-1-(1-naphthyl)-5-oxo-4,5-dihydro-1<Emphasis Type="Italic">H</Emphasis>-pyrrole-2,3-dicarboxylates

Summary. Protonation of the reactive 1:1 intermediate produced in the reaction between triphenylphosphine and dialkyl acetylenedicarboxylates with ethyl 2-(1-naphthylamino)-2-oxoacetate, leads to a vinylphosphonium salt, which undergoes intramolecular Wittig reaction to produce dialkyl 4-ethoxy-1-(1-naphthyl)-5-oxo-4,5-dihydro-1H-pyrrole-2,3-dicarboxylates in excellent yields. A dynamic NMR effect is observed in the ¹H NMR spectra of the title compounds as a result of restricted rotation around the single bond linking the naphthalene moiety and the heterocyclic system, which is attributed to the peri interaction between the pyrrole residue and the peri CH group. The free energy of activation (G ) for this process is 58±2kJmol^–1.Received December 6, 2002; accepted December 11, 2002 Published online June 12, 2003     

Study of the reactions of cisplatin with ranitidine and nizatidine by means of 1H NMR spectroscopy in D2O

The reactions of cisplatin with nizatidine and ranitidine were studied in D₂O at pD 7.4 and 298 K by means of ¹H NMR spectroscopy. The second order rate constants, k ₂, for the reaction of cisplatin with nizatidine is (2.71 ± 0.11) × 10⁻⁴ M ⁻¹ s⁻¹, and for the reaction with ranitidine (6.72 ± 0.17) × 10⁻⁴ M ⁻¹ s⁻¹. The reactions of nizatidine and ranitidine were also studied with other Pd(II) and Pt(II) complexes. The set of the complexes was selected because of their difference in reactivity, steric hindrance, and binding properties. Correspondence: Prof. Dr. Živadin D. Bugarčić, Faculty of Science, University of Kragujevac, Radoja Domanovića 12, 34000 Kragujevac, Serbia.     

Solvothermal Syntheses,Crystal Structures,and Properties of New [Mn(<Emphasis Type="BoldItalic">dien</Emphasis>)<Subscript>2</Subscript>]<Superscript>2+</Superscript> Complexes

Summary.  The two new compounds Mn(dien)₂[MoS₄] (1) and Mn(dien)₂[Mo₂O₂S₆] (2) (dien = diethylenetriamine) were prepared under solvothermal conditions. Both compounds were obtained as phase-pure products. The structures consist of new [Mn(dien)₂]²⁺ cations and isolated tetrahedral [MoS₄]²⁻ (1) or [Mo₂O₂S₆]²⁻ (2) anions. Between the anions and the cations, hydrogen bonding is observed. Compound 1 crystallizes in the tetragonal space group I (a = 10.219(2), c = 9.259(2) ?, Z = 2), whereas 2 crystallizes in the monoclinic space group P2₁/c (a = 8.703(2), b = 18.390(4), c = 14.603(3) ?, β = 103.18(3)°, Z = 4). The thermal behaviour of the thiomolybdates was investigated using difference thermoanalysis (DTA) and thermogravimetry (TG). Both compounds decompose under argon with a single endothermic signal in the DTA curve (peak maximum: 252 (1) and 242°C (2)). Received November 5, 2001. Accepted December 27, 2001     

Conformational Properties of (Z,Z)-, (E,Z)-, and (E,E)-Cycloocta-1,4-dienes

Summary.  Ab initio calculations at the HF/6-31G^* level of theory for geometry optimization and the MP2/6-31G^*//HF/6-31G^* level for a single point total energy calculation are reported for (Z,Z)-, (E,Z)-, and (E,E)-cycloocta-1,4-dienes. The C ₂-symmetric twist-boat conformation of (Z,Z)-cycloocta-1,4-diene was calculated to be by 3.6 kJ·mol⁻¹ more stable than the C _S-symmetric boat-chair form; the calculated energy barrier for ring inversion of the twist-boat conformation via the C _S-symmetric boat-boat geometry is 19.1 kJ·mol⁻¹. Interconversion between twist-boat and boat-chair conformations takes place via a half-chair (C ₁) transition state which is 43.5 kJ·mol⁻¹ above the twist-boat form. The unsymmetrical twist-boat-chair conformation of (E,Z)-cycloocta-1,4-diene was calculated to be by 18.7 kJ·mol⁻¹ more stable than the unsymmetrical boat-chair form. The calculated energy barrier for the interconversion of twist-boat-chair and boat-chair is 69.5 kJ·mol⁻¹, whereas the barrier for swiveling of the trans-double bond through the bridge is 172.6 kJ·mol⁻¹. The C _S symmetric crown conformation of the parallel family of (E,E)-cycloocta-1,4-diene was calculated to be by 16.5 kJ·mol⁻¹ more stable than the C _S-symmetric boat-chair form. Interconversion of crown and boat-chair takes place via a chair (C _S) transition state which is 37.2 kJ·mol⁻¹ above the crown conformation. The axial- symmetrical twist geometry of the crossed family of (E,E)-cycloocta-1,4-diene is 5.9 kJ·mol⁻¹ less stable than the crown conformation. Corresponding author. E-mail: isayavar@yahoo.com Received March 25, 2002; accepted April 3, 2002     

Ultra-trace level determination of nitrite in human saliva by spectrofluorimetry using 1,3,5,7-tetramethyl-8-(3,4-diaminophenyl)-difluoroboradiaza-s-indacene

A rapid, highly sensitive and selective fluorogenic method for the determination of traces of nitrite is described. It is based on the reaction of weakly fluorescent 1,3,5,7-tetramethyl-8-(3,4-diaminophenyl)-difluoroboradiaza-s-indacence (DAMBO) and nitrite in acidic aqueous solution to give 1,3,5,7-tetramethyl-8-(5-benzotriazolyl)-difluoroboradiaza-s-indacene (DAMBO-T), which is highly fluorescent. The optimum reaction conditions and other analytical parameters are investigated to enhance the sensitivity of the method. The fluorescence enhancement at 507 nm is linearly related to the concentration of nitrite in the range of 6.0 × 10⁻⁹–5.0 × 10⁻⁷ mol L⁻¹ with a correlation coefficient of R = 0.9995 (n = 10) and a detection limit of 1.0 × 10⁻¹⁰ mol L⁻¹. The R.S.D. is 1.12% (n = 10). The method is applied to the determination of nitrite in human saliva samples with the recoveries of 96. 24–105.30%. Correspondence: Ke-Jing Huang, Department of Chemistry, Wuhan University, Wuhan 430072, P.R. China     

A new triterpene glycoside from fruit of <Emphasis Type="Italic">Phytolacca americana</Emphasis>

Glycosides H and I, the structures of which were established by modern physicochemical analytical methods (PMR, ¹³C NMR, COSY, TOCSY, HMBC, MS) and acid-base hydrolysis, were isolated from the purified total saponins from fruit of Phytolacca americana containing at least 10 triterpene glycosides by rechromatography of enriched fractions over a column of silica gel. Glycoside H was a bidesmoside of phytolaccageninic acid, which was isolated earlier from cell culture of Phytolacca acinosa. Glycoside I was 3-O-(β-D-xylopyranosyl-(1 → 3)-β-D-galactopyranosyl-(1 → 3)- β -D-xylopyranosyl)-28-O- β -D-glucopyranosyl phytolaccagenin, which was isolated by us for the first time.     

Stable Monopyrrole Atropisomers

Summary.  Malonic ester derivatives of ethyl and methyl 3,5-dimethyl-4-(1′-iodoneopentyl)-1H-pyrrole-2-carboxylate exhibit restricted rotation about the pyrrole C(4)–C(1′) bond due to the bulky 1′-tert-butyl and malonic ester groups and the ortho effect at C(4) of the sterically crowded 3,5-dimethylpyrrole. The malonates belong to a rare class of atropisomers with restricted rotation about an sp³–sp² C–C bond, and they undergo diastereomeric separation by TLC and crystallization: the diastereomers are stable in solution at room temperature. A crystal of one of the diastereomers, suitable for X-ray crystallography, gave the relative configuration of the chiral axis and stereogenic center at C(1′). Dynamic NMR studies of the purified diastereomers provide kinetic and thermodynamic parameters associated with the atropisomerism: ΔG ^‡ = 132–134 kJ/mol (∼32 kcal/mol) at 383 K in C₂D₂Cl₄ solvent. Corresponding author. E-mail: lightner@scs.unr.edu Received July 1, 2002; accepted July 8, 2002     

A Dehydrogenated Cycloadduct of N-Phenylmaleimide and the Azomethine Ylide Derived from Ninhydrine and Phenylalanine: Structure and Conformation

Summary.  The structure of the dehydrogenation product 1′,3a′-dihydro-3′-((1,3-dioxoindan-2-ylidene)-phenyl-methyl)-5′-phenyl-spiro-(indan-2,1′-pyrrolo[3,4-c]pyrrole)-1,3,4′,6′-(5′H, 6a′H)-tetrone derived from the cycloadducts (±)-(3a′S,6a′R)-1′,3a′-dihydro-3′-((R)-α-(1,3-dioxoindanyl)-benzyl)-5′-phenyl-spiro-(indan-2,1′-pyrrolo[3,4-c]pyrrole)-1,3,4′,6′(5H,6a′H)-tetrone and/or (±)-(3a′S,6a′R)-1′,3a′-dihydro-3′-((S)-α-(1,3-dioxoindanyl)-benzyl)-5′-phenyl-spiro-(indan-2,1′-pyrrolo[3,4-c]pyrrole)-1,3,4′,6′(5H,6a′H)-tetrone, which were synthesized by 1,3-dipolar cycloaddition of N-phenylmaleimide to 2-((2-(1,3-dioxoindan-2-yl)-2-phenyl-ethenyl)-imino)-indan-1,3-dione, was determined by X-ray analysis. Crystal data (CCD, 180 K): rhombohedral, R&3macr;;, a = 34.0871(7), c = 13.9358(5) ?, Z = 18; the structure was solved by direct methods and refined by full-matrix least-squares procedures to R(F, I ≥ 3σ(I)) = 0.053. The molecule contains a central folded ring system of two cis-fused 5-membered heterocyclic rings; each ring is nearly planar, and the angle between the rings amounts to 59.0°. Dynamic ¹H NMR spectroscopy of the product revealed an exchange process caused by restricted rotation of the double bonded 1,3-indandione moiety and the phenyl group about the C_sp2-C_sp2 single-bonds. Molecular modeling and complete lineshape analysis indicated a four site exchange process for which free energies of activation and free energies could be established. ΔG ^‡ values for the barriers of rotation are in the range of 57–59 kJ · mol^− 1 at 273 K, which is unusually high for an unsubstituted phenyl group. Received May 3, 2001. Accepted (revised) June 8, 2001     

Synthesis,Crystal Structure and Raman Spectrum of Hydrazinium(+2) Fluoroarsenate(III) Fluoride,N<Subscript>2</Subscript>H<Subscript>6</Subscript>AsF<Subscript>4</Subscript>F

Summary.  Hydrazinium(+2) fluoroarsenate(III) fluoride was prepared by the reaction of hydrazinium(+2) fluoride and liquid arsenic trifluoride. N₂H₆AsF₄F is stable at 273 K, but decomposes slowly at room temperature. N₂H₆AsF₄F crystallizes in the orthorhombic space group Pnn2 with a = 774.0(2) pm, b = 1629.2(4) pm and c = 436.6(1) pm; V = 0.5506(3) nm³, Z = 4 and d _c  = 2.461 g cm⁻³. The structure consists of N₂H₆ ²⁺ cations, AsF₄ ⁻ anions, and F⁻ anions and is interconnected by a hydrogen bonding network. Distorted trigonal-bipyramidal AsF₄ ⁻ units are very weakly interconnected and form chains along the b axis. Bands in the Raman spectrum are assigned to the vibrations of N₂H₆ ⁺² cations and AsF₄ ⁻ anions. Corresponding author. E-mail: adolf.jesih@ijs.si Received April 18, 2002; accepted July 15, 2002     

On the Chemistry of the Resveratrol Diastereomers

Summary.  The (E)- and (Z)-diastereomers of resveratrol were investigated with respect to their photochemical and thermal diastereomerization reactions. The free enthalpy difference between the two diastereomers was estimated to be in the order of common stilbenes, with the (E)-diastereomer more stable by about 11–14 kJ mol⁻¹. The Arrhenius activation barrier of about 280 kJ mol⁻¹ was found to be quite high and implies that thermal equilibration cannot account for the (Z)-diastereomer found in nature. A preparative access to the (Z)-diastereomer by photodiastereomerization is described. The ¹H and ¹³C NMR spectra of the two diastereomers were assigned and their absorption spectra and fluorescence quantum yields of the neutral and monodeprotonated species were determined. Corresponding author. E-mail: heinz.falk@jku.at Received October 15, 2002; accepted November 7, 2002 Published online February 20, 2003     

Flavonoid oligosides from georgian <Emphasis Type="Italic">Astragalus falcatus</Emphasis>

New flavonoid oligosides were isolated from leaves and flowers of Astragalus falcatus Lam. It was found on the basis of chemical transformations, UV, IR, PMR, ¹³ C NMR, HMBC, HSQC, 1D-TOCSY, and mass spectral properties that falcoside C had the structure quercetin 3-O-[β-D-glucopyranosyl(1 → 3)-α-Lrhamnopyranosyl(1 → 6)]-β-D-galactopyranoside 7-O-β-D-glucopyranoside; falcoside D, isorhamnetin 3-O-[β-D-xylopyranosyl(1 → 3)-α-L-rhamnopyranosyl(1 → 6)]-β-D-galactopyranoside 7-O-α-Lrhamnopyranoside.     

Concerning the Chiral Discrimination and Helix Inversion Barrier in Hypericinates and Hypericin Derivatives

Summary.  It was found that the hypericinate salts of (R)-1-phenylethylamine and (S)-1-(1-naphthyl)-ethylamine display a small chiroptical signal of the same sign only at high concentrations in an apolar solvent. No further indications of a chiral discrimination between the helical conformers of hypericinate could be found in these cases. However, upon esterification of the 3-hydroxyl group of hypericin with (1S)-camphanic chloride, the two diastereomers were found in an 1:1 ratio equilibrating rather fast at temperatures above 30°C with one diastereomer in excess. From the temperature dependence of the equilibrium positions (measured by means of CD and ¹H NMR), a ΔG ⁰ value of 5.8±0.5 kJ·mol⁻¹ was derived. Accordingly, the chiral discrimination of the (M)-configured enantiomer of the helix by the (S)-configured auxiliary occurred at an intermediate level. From the temperature dependence of the equilibration kinetics an activation energy of E _a = 70±0.5 kJ·mol⁻¹ was derived, which thus defines the upper limit of the helix inversion of hypericin and hypericinate. This value is by about 10 kJ·mol⁻¹ lower than the recently estimated limit. Corresponding author. E-mail: heinz.falk@jku.at Received March 22, 2002; accepted April 3, 2002     

Derivative Spectrophotometric Determination of Nd, Ho and Er in Rare Earth Mixtures with 1-Ethyl-6,8-difluoro-7-(3-methyl-1-piperazinyl)-4-oxo-1,4-dihydro-3-quinoline carboxylic acid

 The second derivative spectrophotometric method has been developed as a procedure for the determination of neodymium, holmium and erbium in mixed rare earths. It was found that the 1-ethyl-6, 8-difluoro-7-(3-methyl-1-piperazinyl)-4-oxo-1,4- dihydro-3-quinoline carboxylic acid forms stable complexes with neodymium, holmium and erbium ions in the pH 9.2–10.5 range. In the second derivative spectra the optimum analytical signals for neodymium, holmium and erbium are at 576.2 (+)−574.5 (−)nm, 444.2 (+) −447.8 (−)nm and 516.0 (+) −517.2(−)nm, respectively. Beer’s law is obeyed from 5.0×10⁻⁵ M to 2.5×10⁻⁴ M of neodymium, holmium and erbium. The quantification limits (10 Sb) were 1.2×10⁻⁵ M for Nd, 9.7×10⁻⁵ M for Ho and 3.0×10⁻⁶ M for Er. Received April 22, 1998. Revision March 8, 1999.     

Synthesis of poly[2-(perfluorooctyl)ethyl acrylate-<Emphasis Type="Italic">co</Emphasis>-poly(ethylene glycol) methacrylate] and its control of enzyme activity in supercritical carbon dioxide

Poly[2-(perfluorooctyl)ethyl acrylate-co-poly(ethylene glycol) methacrylate], P(POA-co-PEGm) was prepared as a new surfactant for scCO₂. The random copolymer was obtained by the radical polymerization of 2-(perfluorooctyl)ethyl acrylate (POA) and poly(ethylene glycol) methacrylate (PEGm) in DMF. The molar ratio of the POA and PEGm units in the copolymer was POA/PEGm = 0.972/0.028 by ¹H NMR. The molecular weight and molecular weight distribution were estimated by size exclusion chromatography to be Mn = 133,000 and Mw/Mn = 8.25, respectively. It was suggested that the copolymer formed micellar aggregates with the cores of the PEGm chains in scCO₂, based on the analyses of the copolymer in hexafluorobenzene by ¹H NMR and dynamic light scattering. The copolymer was soluble in scCO₂ and had a cloud point at a much higher pressure than the critical pressure. It was found that the copolymer solubilized CO₂-insoluble proteins such as bovine serum albumin and subtilisin Carlsberg in scCO₂. The solubility of the copolymer was not influenced by the presence of the proteins; however, the solubility decreased in the presence of a small amount of water along with the protein. The activity of the subtilisin slightly decreased when only placed in scCO₂, whereas a marked decrease in the activity was observed for the subtilisin in the presence of the copolymer in scCO₂. The subtilisin activity decreased as the CO₂ pressure increased.     

<Emphasis Type="Italic">Ab initio</Emphasis> Study of Structural and Conformational Properties of Five- to Nine-Membered Cyclic 1,2,3-Trienes

Summary.  The structures and relative energies of fundamental conformations of cyclopenta-1,2,3-triene, cyclohexa-1,2,3-triene, cylohepta-1,2,3-triene, cycloocta-1,2,3-triene, and cyclonona-1,2,3-triene were calculated by the HF/6-31G^* and MP2/6-31G^*//HF/6-31G^* methods. Only a C _2v symmetric planar conformation is available to cyclopenta-1,2,3-triene and cyclohexa-1,2,3-triene. The calculated energy barrier for ring inversion of the C _S symmetric puckerd conformation of cyclohepta-1,2,3-triene via the planar geometry is 62.2 kJ·mol⁻¹. The C ₂ symmetric twist conformation of cycloocta-1,2,3-triene was calculated to be the most stable one. Conformational racemization of the twist form takes place via the C _S symmetric half-chair geometry, which is by 60.8 kJ·mol⁻¹ less stable than the twist conformer. The C _S symmetric chair and unsymmetrical twist-boat conformations of cyclonona-1,2,3-triene were calculated to have similar energies; their interconversion takes place via an unsymmetrical low-energy (18.4 kJ·mol⁻¹) transition state. The twist (C ₂) and boat (C _S) geometries of cyclonona-1,2,3-triene are higher in energy by 13.2 and 33.9 kJ·mol⁻¹, respectively. Ring inversion in chair and twist-boat conformations takes place via a twist form as intermediate and requires 33.6 kJ·mol⁻¹. Corresponding author. E-mail: isayavar@yahoo.com Received March 25, 2002; accepted April 4, 2002