Asymmetric synthesis of new chiral long chain alcohols

Sixteen new chiral alcohols with alkyl (C₁₁–C₁₉) and aryl, substituted aryl, hetero aryl and biaryl groups 2a–2t were synthesized by three different asymmetric reduction methods from their corresponding ketones 1a–1t. Chiral NaBH₄ (method A), chiral BH₃ (method B) and chiral AIP (method C) were used as asymmetric reduction catalysts. Chiral NaBH₄ was modified by four different ligands 3a–3d, chiral BH₃ and chiral AIP by four different ligands 4a–4d. Ligand 4c was synthesized for the first time in this work. Chiral NaBH₄ generated chiral alcohols of (R)-configuration and chiral BH₃ and chiral AIP of (S)-configuration with high enantiomeric excesses, were analysed by chiral HPLC. In order to determine the ee values by chiral HPLC, sixteen corresponding racemic alcohols, synthesized by reducing their corresponding ketones via NaBH₄, were used for chiral resolution on a Daicel OD HPLC column. The sixteen starting ketones were synthesized in this study by Friedel–Craft acylation. The new chiral alcohols were characterized by IR, NMR, (¹H and ¹³C), MS, elemental analyses and specific rotation. The reduction methods A, B and C were applied to these ketones for the first time in this study and were compared with each other. The relationship between the structure of the ketone and the yield and the enantiomeric excess was discussed.     

Synthesis and transformations of metallacycles 43. One-pot synthesis of polycyclic 3-alkyl(phenyl)phospholane 3-oxides

A one-pot method for the synthesis of polycyclic phospholanes and phospholane 3-oxides from norbornenes was developed based on their Cp₂ZrCl₂-catalyzed cycloalumination. The in situ formed polycyclic aluminacyclopentanes were treated with organic dichlorophosphines (RPCl₂, where R = Me, Bu^t, Ph) to obtain polycyclic phospholanes. The phospholanes were oxidized with hydrogen peroxide to obtain their 3-oxides in 81–92% yields.     

Diastereoselective LiAlH4 reduction of chiral ketone hydrazones derived from (R)-(–)-2-aminobutan-1-ol

Various chiral N,N-dialkylhydrazines were prepared in four to five steps from (R)-(–)-2-aminobutan-1-ol 6. They reacted with various prochiral ketones, thus giving the corresponding hydrazones. Reduction of the latter by means of LiAlH₄ afforded N,N,N′-trisubstituted hydrazines whose d.e.s were in the range 43–100%. Interestingly, LiAlH₄ reduction of the four N-trifluoroethylhydrazones 34 and 38–40 yielded the hydrazines 46 and 48–50, respectively, and with d.e.s=100% by ¹H and ¹³C NMR.     

Optically active transition metal complexes. Part 117:Synthesis,crystal structure and properties of chiral (η6-arene)ruthenium complexes with N,O- and N,N-ligands

The complexes [(η⁶-p-ⁱPrC₆H₄Me)Ru(NO₂pesa)Cl] 2, [(η⁶-p-ⁱPrC₆H₄Me)Ru(oxazsa)Cl] 3 and [(η⁶-p-ⁱPrC₆H₄Me)Ru(pepy)Cl] 4, chiral in the chelate ligand and chiral at the ruthenium atom, have been prepared by reaction of [(η⁶-p-ⁱPrC₆H₄Me)RuCl₂]₂ with the anions of the (S)-configured bidentate N,O- and N,N-ligands. [(η⁶-p-ⁱPrC₆H₄Me)Ru(pesa)I] 5 was synthesized by halide exchange. The diastereomer ratios of compounds 2–4 with respect to the stereogenic ruthenium atom are in CDCl₃ 2a:2b=81:19, 3a:3b=77:23 and 4a:4b=61:39. Compound 5 is obtained diastereomerically pure. An X-ray structure analysis of 3 shows (R_Ru,S_C)-configuration     

N,N-1,2-Benzenedisulfonylimide,a new cyclic leaving group for the stereoselective nucleophilic substitution of amines

We hereby report the preparation and nucleophilic substitutions of the N,N-1,2-benzenedisulfonylimide derivatives 1a and 2a of the chiral amines 1 and 2. The nucleophilic attack of KNO₂ afforded the respective alcohols 3 and 4 with 84–90% inversion of configuration. Nucleophilic attack by the azide ion afforded the azide products 5 and 6 which were reduced to the corresponding inverted amines 1 and 2 (94–98.5% inversion). The improved leaving group ability of the N,N-1,2-benzenedisulfonylimides compared with previously reported N,N-disulfonylimides is discussed. Chiral GLC analysis of all products is summarized and the alternative chiral analysis of product 3 by ¹³C NMR using heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin as a chiral solvating agent (CSA) is discussed.     

Enantiodifferentiation of the antitumor alkaloid crispine A using the NMR chiral solvating agents (R)- and (S)-BINOL

The use of (R)- and (S)-BINOL as chiral solvating agents (CSA) to determine the enantiomeric purity of the antitumor alkaloid crispine A was investigated. The formation of diasteromeric host–guest complexes between the CSA and (±)-crispine A 1 produced significant ¹H NMR signal splitting which allowed the accurate and rapid determination of the enantiomeric composition in CDCl₃ or C₆D₆ solution.     

Synthesis of novel chiral Schiff base and amino alcohol derivatives of calix[4]arene and chiral recognition properties

In the present study, the synthesis and liquid phase extraction properties towards some amino acid methylesters and amino alcohols of Schiff base and amino alcohol substituted calix[4]arene are reported. The Schiff base substituted calix[4]arene 5 has been synthesized via condensation reaction involving 5,17-diformyl-11,23-di-tert-butyl-25,27-di[3-(4-formylphenoxy)propoxy]-26,28 dihydroxycalix[4]arene 4 and (R)-(?)-2-phenylglycine methyl ester in CHCl₃:MeOH. To give the amino alcohol substituted calix[4]arene 6, the synthesized chiral compound 5 was reduced by LiAlH₄. The new chiral Schiff base and amino alcohol derivatives of calix[4]arene have been characterized by a combination of FT-IR, ¹H NMR, ¹³C NMR, FAB-MS and elemental analysis. Also, the extraction behaviors of 5 and 6 towards some selected amino acid methylesters and amino alcohols have been studied by liquid–liquid extraction.     

Chiral molecular polygons based on the Pt-alkynyl linkage: Self-assembly, characterization, and functionalization

Treatment of 2,2′-diacetoxy-1,1′-binaphthyl-6,6′-bis(ethyne), L-H₂, with one equiv of trans-Pt(PEt₃)₂Cl₂ led to a mixture of different sizes of chiral metallocycles [trans-(PEt₃)₂Pt(L)]_n (n = 3-8, 1-6). Each of the chiral molecular polygons 1-6 was purified by silica-gel column chromatography and characterized by ¹H, ¹³C{¹H}, and ³¹P{¹H} NMR spectroscopy, MS, IR, UV-Vis, and circular dichroism (CD) spectroscopies, size exclusion chromatography, and microanalysis. Chiral molecular square 2 was also characterized by single-crystal X-ray diffraction. The acetyl groups of 2 were readily deprotected under mild conditions to generate 2a which possesses exposed chiral dihydroxy functional groups. The dihydroxy groups were functionalized with n-octadecyl chains or Fréchet-type dendrons to generate dendritic molecules built on a chiral molecular square core. This work shows the potential of generating interesting functional supramolecular systems based on Pt-alkynyl chiral molecular polygons.     

Highly diastereoselective alkylation,acylation and aldol condensation of cis- and trans-(N-acyloyl)hexahydrobenzoxazolidin-2-ones

The potential of hexahydrobenzoxazolidinones 1a–d as chiral auxiliaries was explored. N-Acylation of 1a–d, 2a–d and 3a–d was followed by methylation and benzylation via the corresponding sodium enolates generated by treatment with NaHMDS. Diastereoselectivities of 98% or higher were observed. The absolute configuration of the newly created stereogenic center was established by chemical correlation with 2-benzyl-1-propanol. The stereochemical results are congruent with addition to the electrophile from the less hindered face of a (Z)-configured enolate, the sodium cation being coordinated by both carbonyl oxygens of the substrate. cis- and trans-N-Propionyl derivatives 2a–d were treated with Bu₂BOTf/Et₃N to give dialkylboron enolates 6a–d, which were then reacted with acetaldehyde and benzaldehyde. ¹H and ¹³C NMR analyses showed the formation of a single diastereomeric aldol addition product, whose relative configuration was ascertained as syn from the measurement of the ³J_{H(2′)/H(3′)} coupling constants, and whose absolute configuration was determined by X-ray crystallographic analysis. The results are rationalized in terms of a Zimmerman–Traxler transition state, with a (Z)-configured enolate where boron is coordinated to the aldehyde carbonyl rather than the oxazolidinone carbonyl. Substrate 2a was also reacted with acyl chlorides via the sodium enolate (NaHMDS). The effect of reaction conditions on O- versus C-acylation, as well as the influence of solvent and additives on diastereoselectivity, are discussed.     

Übergangsmetall-substituierte phosphane,arsane und stibane: XXXVI. Diastereomere dreikernkomplexe mit verbrückenden dimethylarsino-, dimethylstibino- oder dimethylbismutino-einheiten

Interaction of the chiral organometallic Lewis bases Cp(CO)(Me₃P)Fe—EMe₂ (E = As, Sb, Bi) (1a–1c) with the norbornadiene metal complex (C₇H₈)Mo(CO)₄ yields the first examples of trinuclear complexes [Cp(CO)(Me₃P)Fe—EMe₂]₂Mo(CO)₄ (2a–2c), bearing two chiral metal atoms separated by a E—Mo—E-linkage. 2a–2c are generated as a mixture of two diastereomers (RS/SR, RR/SS), which gives rise to a resonance doubling in their ¹H and ³¹P NMR spectra. This phenomenon is not observed for the achiral, in part sterically more crowded derivatives [Cp(CO)₂Fe—SbMe₂]₂Mo(CO)₄ (4) and [Cp(CO)₂(Me₃P)Mo—EMe₂]₂Mo(CO)₄ (E = As, Sb (6a, 6b)), which excludes the existence of conformers resulting from restricted rotation about the FeE or MoE bond in the case of 2a–2c.     

Synthesis of enantiomerically pure spiro-cyclopropane derivatives containing multichiral centers

A novel chiral source, 5-(R)-[(1R,2S,5R)-(−)-menthyloxy]-3-bromo-2(5H)-furanone (5a), was obtained in 46% yield with d.e.≥98% from the epimeric mixture of 5-(l-menthyloxy)-3-bromo-2(5H)-furanone (5a+5b) obtained via the bromination of an epimeric mixture of 5-(l-menthyloxy)-2(5H)-furanone (3a+3b) followed by the elimination of hydrogen bromide. The asymmetric reaction of 5a with a nucleophilic alcohol afforded enantiomerically pure spiro-cyclopropane derivatives containing four stereogenic centers, 9a–9e, in 50–68% yield with d.e.≥98%. The enantiomerically pure compounds 9a–9e were identified on the basis of their analytical data and spectroscopic data, such as [α]_D²⁰, UV, IR, ¹H NMR, ¹³C NMR, MS and elementary analysis. The absolute configuration of the chiral spiro-cyclopropane compound 9a was established by X-ray crystallography.     

Six novel complexes based on 5-Acetoxy-1-(6-chloro-pyridin-2-yl)-1H-pyrazole-3-carboxylic acid methyl ester derivatives: Syntheses,crystal structures,and anti-cancer activity

A novel 5-Acetoxy-1-(6-chloropyridin-2-yl)-1H-pyrazole-3-carboxylic acid methyl ester derivatives Htcdodtta (1), and it’s five complexes, [Cu₂(L¹)₂]·(CH₃CN) (2), [Cu₂(L²)_1.63(L³)_0.37]·(CH₃OH)_0.5 (3), [Cu₂(L³)(L⁴)]·(C₂H₅OH)_0.5·(CH₃OH)_0.5 (4), [Cu₂(L⁴)(L⁵)]·(H₂O) (5) and [Cu₂(L¹)_1.18(L²)_0.82] (6) have been synthesized. The Htcdodtta, HL¹-HL⁵ were formed in-situ reaction. HL¹-HL⁵ are homologues which possess two chiral carbons. Compounds 1–6 were characterized using single-crystal X-ray diffraction, IR, and elemental analysis. Compounds 2–6 are dinuclear copper complexes. The in vitro cytotoxicities of compounds 1–4 against a variety of cell lines were evaluated by MTT assays. Hela cancer cell apoptosis assay of 1 and 2 were examined by flow cytometry. The cell apoptosis in NP69, A549, Capan-2, Hela, HepG2, and HUVECs cell lines induced by compound 2 was further affirmed by cellular morphology observations.     

Synthesis of chiral oligopeptides by means of catalytic asymmetric hydrogenation of dehydropeptides

Asymmetric hydrogenations of Ac-ΔTyr(Ac)-(S)-Ala-Gly-OMe (6), Ac-ΔTyr(Ac)-(R)-AlaGly-(S)-Phe-OMe (7), Ac-ΔPhe-NH-CH(R) CH₂-OCH₂Ph (10), HCO-ΔPhe-(S)-Leu-OMe (16), X-AA-ΔPhe-AA'-OMe (5: X ^tBOC, CBZ, CF₃CO; AA, AA' = α-amino acid), and ^tBOC-AA-ΔPhe-AA'-NH-Y (21: Y=H, NH-AA'-ΔPhe-AA-^tBOC, NHPh), catalyzed by cationic Rh complexes, [L*Rh(NBD)]⁺ClO₄ (L* = chiral diphosphine), were performed to give the corresponding chiral oligopeptides with high stereoselectivities. It was found that the nature of the N-protecting group of dehydrotripeptides (5) exerted a significant influence on the asymmetric induction as well as catalyst efficiency. The chiral centers in AA' and AA' amino acid residues in 5 were also found to have some influence on the catalytic asymmetric induction. Possible mechanism which can accommodate these effects are discussed. Asymmetric reduction of RCOCO-AA-OMe (13) via hydrosilylation was carried out to give chiral depsipeptide units. The asymmetric hydrogenation of dehydropeptides was successfully applied to the synthesis of enkephalin analogs, Ac-(R)Tyr-(R) -Ala-Gly-(S)-Phe-(S)-Leu-OMe (23) and Ac-(S)Tyr-(R) -Ala-Gly-(S)-Phe-(S)- Leu-OMe (29)     

A modular design of ruthenium(II) catalysts with chiral C2-symmetric phosphinite ligands for effective asymmetric transfer hydrogenation of aromatic ketones

Hydrogen-transfer reduction processes are attracting increasing interest from synthetic chemists in view of their operational simplicity. The new chiral C₂-symmetric ligands N,N′-bis-[(1S)-1-sec-butyl-2-O-(diphenylphosphinite)ethyl]ethanediamide, 1 and N,N′-bis-[(1S)-1-phenyl-2-O-(diphenylphosphinite)ethyl]ethanediamide, 2 and the corresponding ruthenium complexes 3 and 4 have been prepared and their structures have been elucidated by a combination of multi-nuclear NMR spectroscopy, IR spectroscopy, and elemental analysis. ¹H–³¹P NMR, DEPT, ¹H–¹³C HETCOR, or ¹H–¹H COSY correlation experiments were used to confirm the spectral assignments. The catalytic activity of complexes 3 and 4 in transfer hydrogenation of acetophenone derivatives by iso-PrOH has also been studied. Under optimized conditions, these chiral ruthenium complexes serve as catalyst precursors for the asymmetric transfer hydrogenation of acetophenone derivatives in iso-PrOH and act as excellent catalysts, giving the corresponding chiral alcohols in 99% yield and up to 75% ee. This transfer hydrogenation is characterized by low reversibility under these conditions.     

Nucleophilic substitution reactions of acetylides on substituted tricarbonyl(η6-fluoroarene)chromium and reactions of tricarbonyl[η6-(2-trimethylsilylethynyl)toluene]chromium and tricarbonyl[η6-(p-ethynyl-phenylethynyl)benzene]chromium with dicobalt octacarbonyl

Nucleophilic substitution reactions of various acetylides on substituted tricarbonyl(η⁶-fluoroarene)chromiums were pursued. The reaction presumably underwent a more complicated mechanism rather than the direct substitution on the fluorine-bearing carbon. The organometallic compounds (η⁶-C₆H₃R¹R²R³)Cr(CO)₃ (R¹: CC–C₆H₄CH₃, R²: o-Me, R³: H (5a), R¹: CC–C₆H₄CH₃, R²: o-OMe, R³: H (6a), R¹: CC–C₆H₄CH₃, R²: m-OMe, R³: H (6b), R¹: CCPh, R²: o-Me, R³: o-OMe (8b), R¹: CCPh, R²: m-Me, R³: m-OMe (8c), R¹: CCSiMe₃, R²: o-Me, R³: H (9a), R¹: CC–C₆H₄CCH, R²: H, R³: H (12), R¹: CC–C₆H₄CCH, R²: o-Me, R³: H (13)) as well as the organometallic dimmer [{(η⁶-o-Me-C₆H₄)Cr(CO)₃(di-ethynyl)] (di-ethynyl: CC–C₆H₄CC (14)) have been synthesized from nucleophilic substitution reactions of tricarbonyl(η⁶-fluoroarene)(chromium) compounds with suitable acetylides. The products have been characterized by spectroscopic means. In addition, (8b) and (8c) were characterized by X-ray diffraction studies. Further reactions of (9a) and (12) with appropriate amount of Co₂(CO)₈ yielded μ-alkyne bridged bimetallic complexes, Co₂(CO)₆{μ-Me₃SiCC–(o-tolueneCr(CO)₃} (10) and (Co₂(CO)₆)₂{μ-HCC–C₆H₄–CC–(benzene)Cr(CO)₃)}(15), respectively. Both (10) and (15) were characterized by spectroscopic means as well as single crystal X-ray crystallography. The core of these molecules is quasi-tetrahedron containing a Co₂C₂ unit. A two-dicobalt-fragments coordinated di-enyls complex, (Co₂(CO)₆)₂{μ-HCC–C₆H₄–CC–H} (17), was synthesized from the reaction of 1,3-diethynylbenzene with Co₂(CO)₈. Crystallographic studies of (17) also show that it exhibits a distorted Co₂C₂ quasi-tetrahedral geometry.     

Preparation and temperature-dependent enantioselectivities of homochiral phenolic crown ethers having aryl chiral barriers: thermodynamic parameters for enantioselective complexation with chiral amines

Homochiral crown ether (S,S)-1 containing 1-naphthyl groups as chiral barriers together with the phenol moiety was prepared by using (S)-3 as a chiral subunit which was resolved in enantiomerically pure form by lipase-catalyzed enantioselective acylation of (±)-3. Homochiral phenolic crown ether (S,S)-2, containing phenyl groups as chiral barriers, was also prepared from (S)-5 which was derived from (S)-mandelic acid. The association constants for their complexes with chiral amines in CHCl₃ were determined at various temperatures by the UV–visible spectroscopic method demonstrating that the crown ethers (S,S)-1 and (S,S)-2 displayed the large Δ_R−SΔG values of 6.2 and 6.4 kJ mol⁻¹, respectively, towards the amine 21 at 15°C. Thermodynamic parameters for complex formation were also determined and a linear correlation between TΔ_R−SΔS and Δ_R−SΔH values was observed.     

Novel C2-symmetric chiral O,N,N,O-tetradentate 2,2-bipyridyldiolpropane ligands: synthesis and application in asymmetric diethylzinc addition to aldehydes

First synthesis of C₂-symmetric chiral O,N,N,O-tetradentate 2,2-bipyridyldiolpropane ligands is described. The Mukaiyama–Michael reaction was applied as an important reaction for the synthesis of 2,2-bipyridylpropane 9. Among the ligands synthesized, ligand 11 exhibits excellent chiral induction (up to 97% ee) in diethylzinc addition to various aldehydes. The use of additional Lewis acid such as Ti(OⁱPr)₄ in diethylzinc addition reaction is not required for the present catalytic system.     

Differentiation of planar chiral enantiomers of 〚Cp*M(2-alkyl-Phenoxo)〛 〚BF4〛 {M = Rh,Ir} by the trisphat anion

Precursor oxo-dienyl rhodium and iridium complexes 〚(η⁵-Cp*)M(η⁵-2-alkyl-oxodienyl)〛 〚BF₄〛 (2a–c) were prepared according to literature procedure. Addition of 〚n-Bu₄N〛 〚Δ-trisphat〛 (6) to a CD₂Cl₂ solution of these chiral derivatives has led to the NMR differentiation of the enantiomers. These results pave the way towards the preparation of enantiomerically pure o-quinone methide complexes.     

Synthesis of novel chiral lipophilic pyridyl-containing β-amino alcohol ligands and enantioselective hydrolysis of α-amino acid esters by chiral metallomicelles

Seven novel chiral lipophilic pyridyl-containing β-amino alcohol ligands have been synthesized by coupling of 6-alkoxymethyl-2-chloromethylpyridine 3 with the corresponding chiral β-amino alcohols or l-cysteine. Their metal ion complexes have been investigated as catalysts for the enantioselective hydrolysis of N-protected α-amino acid esters in aqueous comicellar solution. The results indicate that the hydrophobic interactions between substrate and metallocatalyst, the rigidity of the ligand, the hydroxyl group of the ligand acting as a nucleophile for the transacylation process, and the micellar microenvironment are important factors for the activity and enantioselectivity. Large rate accelerations (up to three orders of magnitude) and moderate enantioselectivities (up to 7.81 (k_R/k_S)) employing 4a–Cu²⁺ have been observed.     

Determination of enantiomeric excess for organic primary amine compounds by chiral recognition fast-atom bombardment mass spectrometry

Enantiomeric excess (ee) of organic primary amine compounds such as phenylglycine methyl ester hydrochloride (2) has been determined by fast-atom bombardment (FAB) mass spectrometry (NBA matrix). Chiral recognition in host–guest complexation systems between crown ethers [H] and amino acid ester ammonium ions [G] has been extended to the ee determination. The method characteristically uses a 1/1 mixture of a pair of enantiomeric hosts whose enantiomer is isotopically labeled [(RRRR)-1 and (SSSS)-1-d₆]. Chiral recognition of a given guest is simply measured with the given host–pair reagent from the relative peak intensities of the two corresponding diastereomeric host–guest complex ions in I[(H_RRRR · G)⁺]/I[(H_SSSS-d6 · G)⁺ = I_R/I_S-d6, so called IRIS value. The IRIS value varies in a linear fashion with the ee quantitiy of 2 and produces a symmetric linear V-shaped plot, indicating that in the case of a primary amine guest (such as 2) with unknown ee, one can determine the ee by this type of chiral recognition FAB mass spectometry. Further, based on the observed concentration effects on the IRIS values, it is suggested that the present IRIS value reflects the concentration ratio of the diastereomeric complex ions formed in the matrix.