High-resolution infrared spectroscopy of the charge-transfer complex [Ar-N2]+: a combined experimental/theoretical study

A combined experimental and theoretical study of the charge-transfer complex [Ar-N(2)](+) is presented. Nearly 50 transitions split by spin-rotation interaction have been observed by means of infrared diode laser absorption spectroscopy in a supersonic planar plasma expansion. The band origin is at 2272.2563(18) cm(-1) and rotational constants in the ground and vibrationally (NN-stretch) excited state amount to 0.128701(8) cm(-1) and 0.128203(8) cm(-1), respectively. The interpretation of the data in terms of a charge switch upon complexation is supported by new ab initio calculations. The best estimate for a linear equilibrium structure yields R(e)(NN)=1.102 A and R(e)(Ar-N)=2.190 A. Predictions for molecular parameters not directly available from the experimental results are presented as well. Furthermore, the electronic structure and Ar-N bonding mechanism of [Ar-N(2)](+) have been analyzed in detail. The Ar-N bond is a textbook example of a classical 2-center-3-electron bond.     

Atropisomers of 1-(acyl or aroyl)-2-naphthylindolines. Isolation,X-ray crystal structure and conformational analysis

A series of pairs of stable diastereomeric atropisomers caused by restricted rotation around the Csp3-Csp2 bond of [2-(2-hydroxynaphthalen-1-yl)-3,3-dimethyl-2,3-dihydroindol-1-yl]-(3- or 4-substituted phenyl)-methanone or [2-(2-hydroxynaphthalen-1-yl)-3,3-dimethyl-2,3-dihydroindol-1-yl]-1-alkanone were isolated. The conformational analyses of the atropisomers were performed based on the X-ray crystallographic and (1)H-NMR spectral data. It became clear that rotation about the C2-naphthyl bond is restricted at room temperature, whereas the >NCO-Ar bond rotates freely.     

A method for determining the enantiomeric purity of profens

A simple method for determining the enantiomeric purity of profens (based on the carbon skeleton of 2-phenylpropionic acid) is discussed. The enantiomeric purity of a given profen can be determined by stereospecific DCC self-coupling to give a statistical diastereoisomeric mixture of racemic and meso- anhydrides. The relative ratio of diastereoisomers formed can be related to the enantiomeric excess of the original carboxylic acid.     

Application of a radial compression column to the high-performance liquid chromatographic separation of the enantiomers of some 2-arylpropionic acids as their diastereoisomeric s-(-)-1-(naphthen-1-yl)ethylamines

The enantiomers of 2-phenylpropionic acid and four congeneric anti-inflammatory drugs were separated as their diastereoisomeric amides with S-(-)-1-(naphthen-1-yl)ethylamine by high-performance liquid chromatography using a silica-packed radial compression cartridge. The order of elution of the diastereoisomeric amides was always R, S or -, S before S,S or +,S. The conditions for the derivatization, using 1-(3-dimethylaminopropyl)-3-ethyl-carbodiimide as coupling agent, were optimized, and it was found that the addition of 1-hydroxybenzotriazole rendered the reaction quantitative. Good calibration curves were obtained for the quantitation and determination of the enantiomeric composition of 2-phenylpropionic acid in urine, and the application of the method to the study of the metabolism of this acid in vivo is described.     

Microbial reduction of α-acetyl-γ-butyrolactone

Isomers of α-1′-hydroxyethyl-γ-butyrolactone can be considered as potential GHB receptor ligands designed by the molecular hybridization of GLB 2 and GHV 4. Using Aspergillus niger, Geotrichum candidum, and Kluyveromyces marxianus, it was possible to obtain (+)-(3R,1′S)-α-1′-hydroxyethyl-γ-butyrolactone in good to excellent conversions, diastereoisomeric and enantiomeric excesses. The corresponding enantiomer, (−)-(3S,1′S)-α-1′-hydroxyethyl-γ-butyrolactone was also produced in good conversion, and diastereoisomeric and enantiomeric excesses using Hansenula sp.     

Chiral homoenolate equivalents. I. Asymmetric synthesis of β-substituted aldehydes via metalated chiral allylamines

Metalated chiral allylamines of type 2 (M = Li, K) are used as chiral homoenolate equivalents and allow after alkylation and acidic hydrolysis asymmetric CC bond formations to β-substituted aldehydes in enantiomeric excesses up to 67%.     

PtCl

The ionic complex [PtCl(3)(C(2)H(4))](-)[(S,S)-(PhMeCH)(2)NH(2)](+) containing a chiral secondary amine is a new and versatile chiral derivatizing agent (CDA) for the determination of the enantiomeric composition of several unsaturated compounds including simple olefins: the diastereoisomeric mixtures arising from the exchange of ethylene by the unsaturated analytes are easily detected by (195)Pt NMR spectroscopy.     

Synthesis and Ruthenium-Catalyzed Enantioselective Hydrogenation of 3-O-Substituted 1,3-Dihydroxypropan-2-ones. Part 2

A number of 3-O-substituted 1,3-dihydroxypropan-2-ones have been synthesized in view of their potential use as prochiral precursors of optically active glycerols. Indeed, the oxo-ethers have been reduced to the corresponding 3-O-substituted glycerols via chiral Ru complexes derived from (S)-binap, ( =(?)-(S)-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene) with enantiomeric excesses up to 93%. The steric course of the catalytic reduction appears to be essentially dependent on the steric encumbrance of the substituents; indeed, a dramatic increase of the enantiomeric excess is observed when the bulky trityl group is substituted by the less encumbering benzyl or octadecyl groups.     

Diastereoisomeric cationic pi-allylpalladium-(P,C)-MAP and MOP complexes and their relationship to streochemical memory effects in allylic alkylation

The axially chiral ligands 2-(diphenylphosphanyl)-2'-methoxy-1,1'-binaphthalene (MOP; 6) and 2'-dimethylamino-2-(diphenylphosphanyl)-1,1'-binaphthalene (MAP; 7) coordinate to a cationic allylpalladium fragment in an unusual bidentate (P,C)-mode through the triarylphosphane and ipso-carbon atom (C1'). The readily prepared MAP and MOP complexes [Pd[(P,C)-(L)](n3-allyl)][OTf] (9 (L = 7) and 10 (L = 6)) have been characterised in solution (NMR), in which two diastereoisomeric rotamers are observed. The stereochemical identity of the rotamers is established by one- and two-dimensional NMR spectroscopy experiments. In both the solid state and in solution, the allyl unit is shown to coordinate in a slightly distorted n3-mode that results in a more alkene-like character at the allyl terminus trans to phosphane ligand. The opposite allyl terminus, which is trans to the ipsocarbon atom (C1'), is more strongly bound and the dominant allyl stereodynamic process involves C-C bond rotation in an n'-allyl intermediate bound through this carbon. Palladium complexes of MAP and MOP are very efficient catalysts for allylic alkylation of racemic cyclopentenyl pivalate with [NaCH(CO2Me)2] in THF. Isotopic desymmetrisation revealed that the reaction occurs with powerful stereochemical memory effects and consequently with low global ee values. The memory effect is suggested to arise through selective generation of diastereoisomeric [Pd[(P,C)-L](n3-cyclopentenyl)]+ ions (L = MAP or MOP) and subsequent capture by nucleophile before ion-pair collapse or equilibration occurs.     

Synthesis and crystal structures of ring A modified glycyrrhetinic acid derivatives derived from 2,3-oxirane and 2,3-thiirane intermediates

A general method for the introduction of thiol groups at positions 1, 2, and 3 of glycyrrhetinic acid has been developed starting from a protected 2α,3α-oxido-derivative. Conversion into the corresponding 2β,3β-epithio-derivative was followed by ring-opening leading to either 2- or 3-substituted thio derivatives. Conversely, 3α-configured allylic alcohol intermediates derived from the 2,3-epoxide provided efficient access to both diastereoisomeric 3-thio derivatives as well as 1α-thio derivatives. The stereochemistry of the newly formed stereogenic centers was rigorously proven using X-ray crystallography.     

Reaction of 2-chlorophenylacetylene with 2,2,2-trichloro-1,3,2λ5-benzodioxaphosphole

According to the ¹³C and ¹H NMR data (one-and two-dimensional spectra), o-chlorophenylacetylene reacts with 2,2,2-trichloro-1,3,2λ⁵-benzodioxaphosphole under mild conditions to give a mixture of diastereoisomeric 2,6-dichloro-4-(2-chlorophenyl)-1,2λ⁵-benzoxaphosphinin-2-ones differing in the configuration of the phosphorus atom and rotation of the o-chlorophenyl group about the C⁴-C_arom bond. Hydrolysis of that mixture leads to 6-chloro-4-(2-chlorophenyl)-2-hydroxy-1,2λ⁵-benzoxaphosphinin-2-one as a single stereoisomer.     

Asymmetric synthesis with chiral hydroxylamines.: Synthesis of optically pure 4-substituted azetidinones

The reaction between β-substituted acrylate esters and α-methylbenzyl hydroxyl amine affords diastereoisomeric 5-isoxazolidinones, convenient precursors of simple optically pure 2-azetidinones.     

Arylthiazylamides: syntheses, structures, and bonding properties

Air-sensitive, thermally unstable tris(dimethylamino)sulfonium (TAS) salts (3) of the title anions [ArNSN]- have been prepared from corresponding sulfurdiimides Ar-N=S=N-SiMe3 (2) by Si-N bond cleavage with [(Me2N)3S]-[Me3SiF2]- (TASF). They are characterized by low-temperature X-ray crystallography as Z isomers. Because of the very short terminal S-N distance (144.2 (3h)-147.9 (3i)pm) and the relatively long internal S-N distance (158.3 (3i)-160.3 (3c) pm) the [ArNSN]- ions should be regarded as thiazylamides 1b, rare species containing a S triple bond N triple bond. A bonding model is developed and the experimental results are compared with those of restricted Hartree-Fock (RHF), density functional theory (DFT), and M?ller-Plesset second-order (MP2) calculations.     

endo-1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hept-5-ene-2-carboxylic acid, a superior resolving agent for the high-performance liquid chromatographic separation of enantiomers of hydroxylated derivatives of two azaaromatic hydrocarbons

The high-performance liquid chromatographic (HPLC) separation of enantiomers of oxide and hydroxy derivatives of dibenz[a,j]acridine and 7-methylbenz[c]acridine was investigated on a chiral stationary phase chromatography column using commercially available columns. In most cases either poor or no separation of enantiomers was achieved. Normal-phase separation of diastereoisomeric ester derivatives of the hydroxy compounds, prepared from commercially available (-)-menthoxyacetic acid or (+)-alpha-methoxy-alpha-(trifluoromethyl)phenylacetic acid, was investigated. No separation of the diastereoisomeric esters of trans-3,4-dihydroxy-3,4-dihydrodibenz[a,j]acridine was observed. However, diastereoisomeric esters prepared from (+)-endo-1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hept-5-ene-2-carboxyl ic acid [(+)-HCA] were easily separated. Using the three chiral acids, diastereoisomers were prepared from sixteen hydroxy derivatives of dibenz[a,j]acridine and 7-methylbenz[c]acridine. (+)-HCA esters gave good to excellent HPLC separations which were superior to those achieved using other chiral acids in most cases. The enantiomeric composition of trans-3,4-dihydroxy-3,4-dihydrodibenz[a,j]acridine formed as a major rodent liver microsomal metabolite of dibenz[a,j]acridine was determined using (+)-HCA.     

Determination of the enantiomeric purity of oxfenicine by high-performance liquid chromatography

Summary Reaction of the methyl esters of the -amino acids, R-(–)-, and S-(+)-4-hydroxyphenylglycine (oxfenicine) with the enantiomerically pure acid anhydride of R-(+)-1-methoxy-1-trifluoromethyl-phenylacetic acid (i.e. R-(+)-MTPA) yields diastereoisomeric amides which are separable by high-performance liquid chromatography (HPLC). The ratios of the diastereoisomeric amides may be measured chromatographically and are shown to correspond to the enantiomeric purities of the amino acids. This method is particularly well suited for detecting low levels of an enantiomeric impurity in -amino acids which possess a base labile -methine hydrogen atom.     

Carbon-13 NMR spectra of 3-aryl-2-thioxo-4-imidazolidinones. Conformational isomerism and substituent effects

A ¹³C NMR study of a series of 3-aryl-2-thioxo-4-imidazolidinones, which are capable of existing as enantiomeric or diastereomeric rotational isomers about the aryl C? N bond, shows that rates of internal rotation are slow at the probe temperature. The effects of hetero- and aryl-ring substituents on hetero-ring carbon atoms have been established.     

Synthesis and optical resolution of a new flavin with axial chirality and redox-dependent racemization

A new flavin (3-methyl-10-(2′-phenyl)phenylisoalloxazine) with restricted rotation about the C(1′)?N(10) single bond was synthesized and optically resoluted for the first time. (+)- Isomer (100% enantiomeric purity) did not racemize at ambient conditions, whereas it racemized invariably when it was reduced (by four different methods). The kinetic studies proved that the novel racemization mode is attributed to conversion of the “planar” oxidized form to the “bent” reduced form.     

Asymmetric synthesis of 4,5,6- and 3,4,5,6-substituted azepanes by a highly diastereoselective and enantioselective lithiation-conjugate addition sequence

Asymmetric syntheses of 4,5,6- and 3,4,5,6-substituted azepanes have been achieved by highly diastereoselective and enantioselective (-)-sparteine-mediated asymmetric lithiation-conjugate additions of N-Boc-N-(p-methoxyphenyl)-2,3-substituted allylamines to a beta-aryl alpha,beta-unsaturated ester followed by hydrolysis, cyclization, and reduction. Access to the enantiomeric adduct is provided by an invertive lithiation-stannylation-lithiation sequence.     

Divergent synthesis of 2-C-branched pyranosides and oxepines from 1,2-gem-dibromocyclopropyl carbohydrates

The ring opening of 1,2-(gem-dibromo)cyclopropyl carbohydrates by two different modes leads to either 2-C-(bromomethylene)pyranosides (using base) or 2-bromooxepines (using silver salts), as shown previously by us for a d-glucal-derived cyclopropane. The base-promoted ring opening is extended to encompass additional alcohol, thiol and amine nucleophiles, and diastereoisomeric cyclopropane precursors. Cross-coupling of the 2-C-(bromomethylene)pyranosides leads to extended 2-C-branched pyranosides. Silver-promoted ring expansion of the cyclopropyl carbohydrates in the presence of various alcohols is described. Cross-coupling of the resulting benzyl 2-bromooxepines affords 2-C-substituted oxepines.     

Synthesis of enantiomeric (+)- and (-)-6-(1-methylethylidene)-3,3a,6,6a-tetrahydro-2H-cyclopenta[b]furan-1-ones

Baeyer-Villiger oxidation of racemic [2 +2 ]-cycloadduct derived from dichloroketene and dimethylfulvene gave 3,3-dichloro-6-(1-methylidene)-3,3a,6,6a-tetrahydro-2H-cyclopenta[b]furan-2-one, and opening of the lactone ring in the latter with (+)-α-methylbenzylamine produced diastereoisomeric amides which can be readily separated by chromatography on silica gel. The subsequent lactonization and reductive dechlorination afforded enantiomeric (?)- and (+)-6-(propan-2-ylidene)-3,3a,6,6a-tetrahydro-2H-cyclopenta[b]-furan-1-ones.