A combined experimental and computational strategy in the assignment of absolute configurations of 4-methyl-5-oxo-tetrahydrofuran-3-carboxylic acids and their esters

Enantiopure cis- and trans-4-methylparaconic acids and their alkyl esters were synthesized by a procedure involving the kinetic enzymatic resolution of diastereomeric lactonic esters. Thus, ethyl cis- and trans-4-methyl-5-oxo-tetrahydrofuran-3-carboxylates were isolated, at high conversion values, with 99% ee from the hydrolyses with HLAP and α-CT, respectively. The corresponding enantiopure cis- and trans-lactonic acids were also obtained. The absolute configurations of the products were assigned by chemical correlation, by analysis of their circular dichroism spectra and by electronic structure calculations. All three methodologies lead to the same assignment for the species considered, another example of successful interplay between experiment and theory.     

Synthesis of eupomatilone-6 and assignment of its absolute configuration

[Reaction: see text]. The Zn-mediated Barbier reaction of the biarylaldehyde 8 with crotyl bromide followed by hydroboration and oxidation provided the gamma-butyrolactones 4 and 5. The stereoselective installation of methyl group at C-3 by using LiHMDS and MeI completed the synthesis of racemic eupomatilone-6 (2) and its diastereomer 3. The spectroscopic data of 2 was in full agreement with reported spectra of natural product, thus confirming the revised relative configuration of eupomatilone-6. Similarly, an optically active (3R,4R,5S)-isomer of eupomatilone-6 (23) was prepared in which the aldol reaction with thiazolidinethione as a chiral auxiliary was employed as a key step. On the basis of the spectroscopic data and optical rotation values of 23, the absolute configuration of eupomatilone-6 was proposed.     

Synthesis and oxidation of 2,6-dimethyl-1,4-dihydropyridine-3, 5-dicarboxylic acid esters

Twenty-six esters of 2,6-dihydropyridine-3,5-dicarboxylic acid were obtained by means of the Hantzsch pyridine synthesis. The synthesized compounds were oxidized by chloranil to the corresponding pyridine derivative.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1230–1237, September, 1975.     

Direct determination of absolute configuration of methyl-substituted phenyloxiranes: combined experimental and theoretical approach

Three possible methyl-substituted phenyloxiranes have been synthesized in enantioenriched form (89-99% enantiomeric excess (ee)), and their vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra have been recorded. The experimental spectra are compared to theoretical spectra obtained from quantum mechanical calculations (density functional theory with the B3LYP hybrid exchange correlation functional with 6-31++G*, aug-cc-pVDZ, or aug-cc-pVTZ basis set) and related to the physical structure of the compounds. The absolute configuration could be established directly in each case by comparing experimental and theoretical spectra. In addition, we have been able to document the changes that occur both in structures and in the VA and VCD spectra due to substituent effects on the oxirane ring.     

Synthesis, resolution, and absolute configuration of chiral 4,4'-bipyridines

A chiral polyhalogenated 4,4'-bipyridine derivative is described allowing an easy access to a new family of chiral 4,4'-bipyridines by site-selective cross-coupling reactions. The absolute configurations of all the HPLC separated enantiomers were determined by X-ray diffraction and electronic circular dichroism coupled with time-dependent density functional theory calculations.     

Synthesis and characterization of Boc-protected 4-amino- and 5-amino-pyrrole-2-carboxylic acid methyl esters

Syntheses of Boc-protected 4-amino- and 5-amino-pyrrole-2-carboxylic acid methyl esters have been achieved and the structures of these compounds have been fully characterized by detailed NMR studies.     

Design of radical-resistant amino acid residues: a combined theoretical and experimental investigation

Ab initio calculations have been used to design radical-resistant amino acid residues. Optimized structures of free and protected amino acids and their corresponding alpha-carbon-centered radicals were determined with B3-LYP/6-31G(d). Single-point RMP2/6-31G(d) calculations on these structures were then used to obtain radical stabilization energies, to examine the effect of steric repulsion between the side chains and amide carbonyl groups on the stability of alpha-carbon-centered peptide radicals. Relative to glycine, the destabilization for alanine and valine residues was found to be approximately 9 and 18 kJ mol(-1), respectively, which correlates with the reactivity of analogous amino acid residues in peptides toward hydrogen atom abstraction in conventional free radical reactions. To design amino acid residues that would resist radical reactions, strategies by which the steric effects could be magnified were considered. This resulted in the identification of tert-leucine and 3,3,3-trifluoroalanine as suitable molecules. With these amino acid residues, the destabilization of the alpha-carbon-centered radicals relative to that of glycine is increased substantially to approximately 36 and 41 kJ mol(-1), respectively. The theoretical predictions have been supported by experimental observations: a tert-leucine derivative was shown to be very slow to react with N-bromosuccinimide, while the corresponding trifluoroalanine derivative was found to be inert.     

Synthesis of optically active α-benzyl paraconic acids and their esters and assignment of their absolute configuration

The cis- and trans-4-benzylparaconic acids and their ethyl esters were synthesized with high enantiomeric excess by hydrolysis of the corresponding diastereomeric lactonic esters using α-chymotrypsin. Thus, at low conversion values, cis- and trans-4-benzyl-5-oxo-3-tetrahydrofurancarboxylic acids were separately isolated with 99% ee and 92% ee, respectively. Both ethyl ester diastereomers were also obtained in enantiopure form. The absolute configuration of the trans-lactonic acid was assigned by ¹H NMR analysis of its ester derivatives with both enantiomers of 1-(9-anthryl)-2,2,2-trifluoroethanol, while that of the cis-lactonic acid was assigned by means of X-ray analysis of a crystalline derivative. The circular dichroism curves of the products obtained are also reported.     

Ethyl esters of coumarin-3-phosphonic acid and 1,2-benzoxaphosphorine-3-carboxylic acid: crystal structure,spectroscopic and theoretical elucidation

IR-spectroscopic characterization of the coumarin-3-phosphonic acid and 1,2-benzoxaphosphorine-3-carboxylic acid ethyl esters has been carried out by means of linear-polarized IR (IR-LD) spectroscopy of oriented colloid suspensions in a nematic host. Quantum chemical DFT calculations at the B3LYP level of theory and 6-311++G** basis set were performed. The electronic structure and vibrational properties of both compounds are discussed. The spectroscopic data for 2-benzoxaphosphorine-3-carboxylic acid ethyl ester are in accordance with the crystal structure determined by single crystal X-ray diffraction. The compound C₁₃H₁₅O₅P crystallizes in the noncentrosymmetric space group P2₁2₁2₁, and its structure consists of a 3D network formed by short contacts of the type P=O···HC_(Ar) with distances of 3.420 and 2.467 Å. The geometry of the PO₃C fragment exhibits a pseudo T _d symmetry.     

Thermodynamic study of sesamol, piperonyl alcohol, piperonylic acid and homopiperonylic acid: a combined experimental and theoretical investigation

The standard (p(o)= 0.1 MPa) molar energies of combustion in oxygen, at T= 298.15 K, of four 1,3-benzodioxole derivatives (sesamol, piperonyl alcohol, piperonylic acid and homopiperonylic acid) were measured by static bomb calorimetry. The values of the standard molar enthalpies of sublimation, at T= 298.15 K, were derived from vapour pressure-temperature measurements using the Knudsen effusion technique. Combining these results the standard molar enthalpies of formation of the compounds, in the gas phase, at T= 298.15 K, have been calculated: sesamol (-325.7 +/- 1.9) kJ mol(-1); piperonyl alcohol (-329.0 +/- 2.0) kJ mol(-1); piperonylic acid (-528.9 +/- 2.6) kJ mol(-1) and homopiperonylic acid (-544.5 +/- 2.9) kJ mol(-1). The most stable geometries of all the compounds were obtained using the density functional theory with the B3LYP functional and two basis sets: 6-31G** and 6-311G**. The nonplanarity of the molecules was analyzed in terms of the anomeric effect, which is believed to arise from the interaction between a nonbonded oxygen p orbital and the empty orbital sigma*(CO) involving the other oxygen atom. Calculations were performed to obtain estimates of the enthalpies of formation of all the benzodioxoles using appropriate isodesmic reactions. There is a perfect agreement between theoretical and experimental results.     

Synthesis of esters of metallocene alcohols and 4,5-dichloroisothiazol-3-carboxylic and 5-arylisoxazole-3-carboxylic acids

Acylation of alkyl- and 1,1′-dialkylferrocene alcohols and diols as well as (3,4,4-trichlorobut-3-ene-1-ol-1-yl)-4,5-cymantrene with dichloroisothiazole- and 5-arylisoxazole-3-carbonyl chlorides has afforded esters containing 1,2-azoles fragments. Some of the obtained compounds have exhibited potentiating action in the binary mixtures with insecticides.     

One-pot synthesis of helical aromatics: stereoselectivity, stability against racemization, and assignment of absolute configuration assisted by experimental and theoretical circular dichroism

Helical aromatics (1) were synthesized via one step in good quantity by solvent-free condensation of N,N'-p-phenylenediamine (2) and various carboxylic acids in the presence of a Lewis acid. Microwave irradiation greatly facilitated the condensation reaction to furnish 1 with a 100% diastereo- and a 50% enantioselectivity, when a chiral carboxylic acid was utilized. 1f, derived from 2-methylglutaric acid, was quite stable, no racemization taking place even at 200 degrees C. The assignment of the absolute configurations to the helical aromatics has been achieved by experimental and theoretical CD spectra calculated by time-dependent density functional theory.     

3-Menthoxybiphenyl-4-carboxylic acid: a versatile resolving agent and reagent for determination of the absolute configuration of benzylic alcohols

A versatile reagent for the chiral resolution and determination of the absolute configuration of benzylic alcohols, (?)-3-menthoxybiphenyl-4-carboxylic acid, is reported. (?)-3-menthoxybiphenyl-4-carboxylic acid was condensed with racemic benzylic alcohols to yield diastereomeric 3-menthoxybiphenyl-4-carboxylic esters, which were separated by reversed-phase HPLC. The absolute configurations of the 3-menthoxybiphenyl-4-carboxylic esters were unambiguously determined by exciton-coupled circular dichroism. Hydrolysis of the 3-menthoxybiphenyl-4-carboxylic esters yielded enantiopure alcohols, and their absolute configurations were simultaneously determined.     

Determination of absolute configuration and enantiomeric purity of 2- and 3-hydroxycarboxylic acid esters

Relative magnitude of lanthanide induced shift of COOMe(LIS_COOMe) and methine proton (LIS_CH) of (R)-MTPA derivatives of 2- and 3-hydroxycarboxylic acid esters by Eu(fod)₃ has been successfully correlated to absolute configuration of the original hydroxy acid. This method also allows simultaneous determination of enantiomeric purity of the original hydroxy acid.     

Circular dichroism studies on absolute configuration assignment of peptidomimetics with a terminal chiral 3-arylpropionic acid unit

The relationship between the molecular structure of peptidomimetics 1 and their chiroptical properties has been studied using circular dichroism spectroscopy. It was demonstrated that the sign of the Cotton effects occurring around 230 and 270 nm enables the direct assignment of stereochemistry at C-1 and C-3 carbon atoms. The TD-DFT calculations of the circular dichroism (CD) spectrum of one of the model compounds confirm the absolute stereochemistry assignment made experimentally. Thus, CD spectroscopy proved to be a simple, reliable and general tool for the determination of the absolute configuration of the stereogenic centers of peptidomimetics with a terminal 3-arylpropionic group.     

Synthesis,chiroptical properties and absolute configuration of α-phenylglycidic acid

The (+)- and (-) enantiomers of potassium α-phenylglycidate, an irreversible inhibitor of the enzyme mandelate racemase, were synthesized by resolution of the diastereomeric esters with R-(-)-2-octanol. Base-catalyzed ring-opening of the resolved α-phenylglycidate esters gave the enantiomers of 2,3-dihydroxy-2-phenylpropanoic acid, also obtained by resolution of the racemic dihydroxy acid using ephedrine. A comparison of the chiroptical properties of the esters of α-phenylglycidic and 2,3-dihydroxy-2-phenylpropanoic acids with those of the structurally similar atrolactic and mandelic acids and their 2-methoxy-derivatives showed that the (-)-methyl 2,3-dihydroxy-2-phenylpropanoate corresponding to the (+)-enantiomer of potassium α-phenylglycidate, as well as the esters of α-phenylglycidic acid derived from the same (+)-potassium salt, were all configurationally related to S-(+)-atrolactic and mandelic acids. The configurational assignments made on the basis of the chiroptical data were confirmed by lithium aluminum hydride reduction of the (-)-2-octyl S- and R-α-phenylglycidates, which led exclusively to the R-(-)- and S-(+)-2-phenyl-1, 2-propanediols, respectively, previously related configurationally to R-(-)- and S-(+)-atrolactic acids.     

Antituberculosis agent diaportheone B: synthesis, absolute configuration assignment, and anti-TB activity of its analogues

First synthesis of diaportheone B, an antituberculosis agent isolated from endophytic fungus Diaporthe sp. P133 is reported using two complementary routes, a one step and a three-step sequence. The absolute configuration of diaportheone B was determined by using X-ray crystal structure analysis of its dibromo derivative. In addition, we have prepared several close analogues of diaportheone B and determined their anti-TB potential using Alamar-blue assay (H(37)Rv).     

3-Bromo-4,4-dimethyl-2-oxazolidinone: Preparation and investigation of a new brominating agent

A new brominating agent, 3-bromo-4,4-dimethyl-2-oxazolidinone (NBDMO) was prepared and its reactions compared to that of N-bromosuccinimide (NBS). NBDMO was found to have a lower “bromine potential” relative to NBS and, consequently possesses a less polar N-bromine bond. NBDMO was found to be equivalent or better than NBS as a brominating and oxidizing agent in the reactions investigated.     

(R)-2-硫代四氢噻唑-4-羧酸及其酯的合成

L-半胱氨酸盐酸盐与二硫化碳, 氢氧化钠在硝酸铅存在下进行成环反应得到(R)-四氢噻唑-2-硫酮-4-羧酸产率76% , 后者与 醇在硫酸铁水合物催化下反应得到(R)-四氢噻唑-2-硫酮-4-羧酸酯, 产率40～82% .     

UV photodissociation of ethylamine cation: a combined experimental and theoretical investigation

Direct current (DC) slice imaging of state-selected ions is combined with high-level ab initio calculations to give insight into reaction pathways, dynamics, and energetics for ethylamine cation photodissociation at 233 nm. These reaction pathways are of interest for understanding the rich chemistry of Titan's ionosphere recently revealed by the Cassini mission. The result for the H-loss product has a bimodal translational energy distribution, indicating two distinct H-loss pathways: these are assigned to triplet CH(3)CH(2)NH(+) product ions and the singlet CH(3)CHNH(2)(+) species. The distribution shows a modest fraction of energy available in translation and is consistent with barrierless dissociation from the ground state. HCNH(+) formation is observed as the dominant channel and exhibits a bimodal translational energy distribution with the faster component depicting a significant angular anisotropy. This suggests a direct excited-state decay pathway for this portion of the distribution. We have also observed the H + H(2) loss product as a minor secondary dissociation channel, which correlates well with the formation of CH(2)CNH(2)(+) ion with an exit barrier.