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1.
《Tetrahedron: Asymmetry》2007,18(16):1975-1980
l-Proline catalyzed direct diastereoselective aldol reactions of α-amino aldehydes with cyclic ketones have been described and utilized further for the stereoselective synthesis of the 2-amino-1,3,4-triol unit as the phytosphingosines base backbone. This leads to the formal synthesis of (2S,3S,4S)-lyxo-phytosphingosine.  相似文献   

2.
(S)-Valine amides containing (S)- or (R)-α-phenylethyl substituents at N1 atom efficiently catalyze asymmetric aldol reactions between cyclic (heterocyclic) ketones and aromatic aldehydes in water, predominantly giving rise to the aldol anti-diastereomers in high yields (up to 98%) and enantiomeric excess (up to 94%).  相似文献   

3.
An N4-p-methoxybenzyloxadiazinone has been prepared from (1R,2S)-norephedrine through a process of reductive amination, N-nitrosation, reduction, and cyclization. The oxadiazinone was acylated and employed in the asymmetric aldol addition reaction with aromatic and aliphatic aldehydes to yield aldol adducts in isolated yields ranging from 54% to 90%. Selected aldol adducts were treated with ceric ammonium nitrate in aqueous acetonitrile to afford the desired β-hydroxycarboxylic acids through a tandem process of oxidative cleavage of the N4-p-methoxybenzyl group and acidic hydrolysis of the N3-acyl side chain. The β-hydroxycarboxylic acids were recovered in high diastereomeric purity as determined by 500 MHz 1H NMR spectroscopy and the absolute configuration was confirmed by polarimetry. The chiral auxiliary unit, the 3,4,5,6-tetrahydro-2H-1,3,4-oxadiazin-2-one (oxadiazinone), was converted into its corresponding 3,6-dihydro-2H-1,3,4-oxadiazin-2-one (oxadiazinone) through an oxidative pathway promoted by the ceric ammonium nitrate.  相似文献   

4.
An efficient, solvent-free ball-milling protocol for the asymmetric aldol reaction between cyclohexanone and cyclopentanone with various aromatic aldehydes using a novel series of (S)-proline-containing dipeptides and thiodipeptides 1af as organocatalysts is reported. In general, (S)-proline-containing thiodipeptides proved to be better organocatalysts relative to their analogous amides. In particular, thiodipeptide (S,S)-1d catalyzed the stereoselective formation of the expected aldol products with excellent diastereo- and enantioselectivity (up to 98:2 anti/syn dr and up to 96% ee). This observation may be ascribed to the increased N–H acidity of the thioamide segment that leads to stronger H-bonding interaction with the aldehyde carbonyl at the transition state and thus higher stereoinduction. Furthermore, thiodipeptide 1f proved to be an efficient organocatalyst for the aldol reaction of acetone with isatin and isatin derivatives (ee 56–86%).  相似文献   

5.
The first enantioselective synthesis of (8S,13S,14R)-7-oxa-estra-4,9-diene-3,17-dione with the trans-C/D ring junction is described. Key features of the synthesis include Ag2O-mediated C-O bond formation, thermodynamically controlled axial-equatorial inversion, one-pot of halogen exchange and Co/Cr-mediated C-C bond formation, and intramolecular aldol condensations.  相似文献   

6.
Room temperature ionic liquid (bmim)PF6 was evaluated for recycling an organocatalyst (4S)-phenoxy-(S)-proline for direct asymmetric aldol reactions. The desired aldol products were obtained with good yields up to 93.2% and enantioselectivities up to 88.5%, and isolation of the products by simple extraction allowed recycling the ionic liquid containing the immobilized catalyst in subsequent reactions without significant decrease of yields and enantioselectivities.  相似文献   

7.
《Tetrahedron: Asymmetry》2001,12(10):1403-1406
An efficient and stereocontrolled process is described for the preparation of (6S,7S,9S,10S)-6,9-epoxynonadec-18-ene-7,10-diol, a marine epoxy lipid isolated from the brown alga, Notheia anomala. The key 2,3,5-trisubstituted tetrahydrofuran ring was constructed by stereoselective hydrogenation of the hemiketal derivative elaborated through nucleophilic addition of Grignard reagent in the presence of CeCl3 to the highly functionalized lactone derived from L-galactono-1,4-lactone.  相似文献   

8.
The reaction of trialkylphoshanes and amino-phosphanes with isothiocyanates yields adducts containing the zwitterionic thioamidyl-phosphonium P+C(S)N? functional group. Ligands containing this group were not previously studied, probably due to their instability towards dissociation, in the presence of metal species able to coordinate the P atom. The ligand EtNHC(S)Ph2PNPPh2C(S)NEt (HEtSNS) was obtained by reaction of Ph2PNHPPh2 with ethylisothiocyanate and proved to be very versatile: it can be protonated giving cation H2EtSNS+ and deprotonated forming the dianion–cation EtSNS?. HEtSNS and its derivatives behave as ligands showing five possible coordination fashions, S,N,S tridentate and S,S-bidentate (with a bite-angle varying from 180° to 90°), S-monodentate, S,S bridging and N,N,N interaction. Here we describe the coordination chemistry of HEtSNS, in particular towards Rh, Cu, Ag and Au, and some properties of its complexes which are still the only examples containing the P+C(S)N? zwitterionic group.  相似文献   

9.
The stereoselective synthesis of (2S,3S,7S)-3,7-dimethylpentadecan-2-ol (diprionol) and its propionate, the sex pheromones of pine sawflies (Neodiprion sp. and other), in high enantiomeric purity was achieved from (1R,3S)-2,2-dichloro-3-methylcyclopropanecarboxylic acid. The carbon skeleton of diprionol was formed via copper-catalyzed cross-coupling reactions and diastereoselective methylation of the intermediate chiral α-methylbranched aldehyde with MeTi(Oi-Pr)3 in the presence of [(R,R)-TADDOL]Ti(Oi-Pr)2. The latter transformation leads to a syn-adduct with high stereoselectivity, which depends on the presence and configuration of the α-stereogenic center in the aldehyde. The diastereomeric purity of (2S,3S,7S)-diprionol can be substantially increased via crystallization of its 3,5-dinitrobenzoate.  相似文献   

10.
《Tetrahedron: Asymmetry》2003,14(13):1837-1845
The highly stereoselective synthesis of (1S,2S)-2-amino-1-hydroxyalkylphosphonic acids was achieved by addition of dimethyl phosphite to N-protected aminoaldehydes. Relative configuration and solution conformations of (1S,2S)-2-amino-1-hydroxy-alkylphosphonic acids (in D2O) and their dimethyl esters (in CDCl3 and CD3OD) were established by means of NMR basing on the dependence between observed values of coupling constants (3JHH, 3JPC, 3JHP) and corresponding dihedral angles. Potentiometric and spectroscopic methods were used for the evaluation of the structure of the complexes of (1S,2S)-2-amino-1-hydroxy-alkylphosphonic acids with Zn(II) and Cu(II) ions in aqueous solutions.  相似文献   

11.
A T-x diagram is designed for the Yb2S3-In2S3 system using physicochemical methods. A complex chemical reaction occurs in the system to yield ternary compounds Yb3InS6 (S1), YbInS3 (S2), Yb3In5S12 (S3), and YbIn3S6 (S4). In2S3-based limited solid solutions are found. Phases S1, S3, and S4 are formed by peritectic reactions at 1260, 1200, and 1100 K, respectively. Compound S2 melts congruently at 1390 K. Compound S3 crystallizes in monoclinic system (a = 10.90 Å, b = 21.01 Å, c = 3.846 Å, β = 96.2°). Compounds S1 and S4 crystallize in orthorhombic system (for S1, a = 16.76 Å, b = 13.70 Å, c = 3.88 Å; for S4, a = 3.86 Å, b = 11.64 Å, c = 20.98 Å, d exp = 4.62 g/cm3). Compound S2 crystallizes in cubic system (a = 10.68 Å).  相似文献   

12.
Phase diagrams for the Sc2S3-Ln2S3 (Ln = Dy, Er, or Tm) systems were designed in the range from 1000 K to melting temperatures. The Ln3ScS6 compounds that are formed in these systems crystallize in monoclinic space group P21/m, and melt congruently: for Dy3ScS6, a = 1.118 nm, b = 1.262 nm, c = 0.354 nm, β = 94.7°, 1800 K, H = 2600 MPa; for Er3ScS6, a = 1.113 nm, b = 1.258 nm, c = 0.353 nm, β = 94.5°, 1830 K, H = 2800 MPa; for Tm3ScS6, a = 1.112 nm, b = 1.229 nm, c = 0.352 nm, β = 94.3°, 1835 K, H = 2940 MPa. The LnScS3 (Ln = Dy or Er) complex sulfides, with orthorhombic structures, space group Pnma, melt incongruently: for DyScS3, a = 0.700 nm, b = 0.637 nm, c = 0.943 nm, 1810 K, H = 3800 MPa; and for ErScS3, a = 0.697 nm, b = 0.633 nm, c = 0.942 nm, 1800 K, H = 3800 MPa. As the ionic radii rLn3+ and rSc3+ approach Ln Sc to each other in the row Dy-Er-Tm, the solubility in Sc2S3 increases, at 1670 K being equal to 13 mol % Dy2S3, 30 mol % Er2S3, and 40 mol % Tm2S3. LnScS3 (Ln = Dy or Er) forms a two-sided homogeneity region, at 1670 K lying in ranges of 43–56 mol % Ln2S3. The eutectic temperatures and compositions are as follows: 1700 K and 66 mol % Dy2S3, 1730 K and 81 mol % Dy2S3, 1740 K and 65 mol % Er2S3, 1700 K and 83 mol % Er2S3, 1730 K and 70 mol % Tm2S3, and 1755 K and 84 mol % Tm2S3.  相似文献   

13.
《Tetrahedron: Asymmetry》2005,16(7):1305-1307
The natural mosquito attractant pheromone, (−)-(5R,6S)-6-acetoxyhexadecan-5-olide 1, was synthesized from readily available aldehyde 3 and cyclopentanone 4 using l-proline catalyzed asymmetric aldol reactions as the key step.  相似文献   

14.
The energy spectra of electrons released in thermal energy (≈ 50 meV) ionizing collisions of He*(21 S, 23 S) with H2 have been measured with high resolution and low background. Based on a detailed data analysis, we report accurate H 2 + (v′) vibrational populationsP(v′) for both He*(21 S)+H2(v′=0–10) and He*(23 S)+H2(v′=0–15) and the spectral shapeS(ε) for the individual vibrational peaks. The vibrational populationsP(v′) are quite similar to the Franck-Condon factorsf v ′0 for unperturbed H2(v″=0)→H 2 + (v′) transitions, but, more in detail, the ratiosP(v′)/f v ′0 show a characteristically differentv′-dependence for He*(23 S), He*(21 S), and HeIα(58.4 nm) ionization. The vibrational level separations in the He*(21 S, 23 S)+H2 spectra agree with those in the HeI photoelectron spectrum to within 1–2 meV. The spectral shapesS(ε) are characteristically different for He*(21 S)+H2 and He*(23 S)+H2, reflecting the respective differences in the entrance channel potentials, as determined previously in ab initio calculations and from scattering experiments.  相似文献   

15.
《Tetrahedron letters》2004,45(28):5379-5382
Stereoselectivity of the titanium-mediated aldol process based on (S)-2-benzyloxy-3-pentanone, 1, is dramatically modified by the presence of a Lewis acid. Among the Lewis acids surveyed, TiCl4 has given access to the corresponding 2,4-anti-4,5-syn aldol adducts with the highest diastereomeric ratios. The excellent stereocontrol exerted by the aforementioned ketone has been demonstrated in double asymmetric reactions involving chiral α-methyl-β-OTBDPS aldehydes.  相似文献   

16.
《Tetrahedron》2006,62(2-3):311-316
A convenient method for proline-catalyzed asymmetric aldol reactions using synthons of straight-chain aliphatic aldehydes, and aldehydes bearing a 1,3-dithiane moiety at the β-position, has been developed. This method was successfully applied to the synthesis of (−)-(5R,6S)-6-acetoxyhexadecanolide, an oviposition attractant pheromone of the female Culex mosquito.  相似文献   

17.
Single-crystal X-ray diffraction analysis was used to determine the structure of a racemic diastereomer of the agricultural fungicide propiconazole [1-(2-(2,4-dichlorophenyl)-4-n-propyl-1,3-dioxolane-2-yl-methyl)-1-H-1,2,4-triazole] and of two by-products (a symmetrical 1,3,4-triazole racemic-constitutional isomer and a propiconazole ditriazole analogue). All three crystalline racemic-diastereomers had (2R,4S)/(2S,4R)-stereochemistry in which then-propyl group was observed in atrans-to-phenyl disposition. Propiconazole (2R,4S)/(2S,4R)-diastereomer gives crystals belonging to the monoclinic space group P21,/a, and, at 293 K,a=8.1192(3),b=18.9769(6),c=10.7137(4) å,Β=99.765(3)?,V=1626.8(1) å3, Z=4,R(F)=0.060, andR w(F)=0.058. The constitutional isomer by-product (2R,4S)/(2S,4R)-1-(2-(2,4-dichlorophenyl)-4-n-pro-pyl-1,3-dioxolane-2-yl-methyl)-1-H-1,3,4-triazole gives crystals belonging to the monoclinic space group P21/n, and, at 293 K,a=11.1763(6),b=10.7716(4),c=14.5804(8) å,Β=107.445(4)?,V=1674.6(1) å3, Z=4,R(F)=0.043, andR w(F)=0.043. The ditriazole byproduct (2R,4S)/(2S,4R)-1-(2-(2-chloro-4-(1,2,4-triazole-1-yl)phenyl)-4-n-propyl-1,3-dioxolane-2-yl-methyl)-1-H-1,2,4-triazole gives crystals belonging to the triclinic space group 1, and, at 193 K,a=5.3329(8),b=8.3738(7),c=20.240(2) å, α=84.213(6)?,Β=87.20(1)?,γ=86.23(1)?,V=896.5(2) å3, Z=2,R(F)=0.046, andR w(F)=0.051. The presence of both propiconazole (2R.4S)- and (2S,4R)-enantiomers enables the formation of a crystalline racemic modification, while the diastereomeric propiconazole (2R,4R)- and (2S,4S)-enantiomers are viscous oils. In the absence of its enantiomorphic partner, the propiconazole (2R,4S)- or (2S,4R)-enantiomers remain as viscous oils rather than form chiral crystals.  相似文献   

18.
《Tetrahedron: Asymmetry》2007,18(11):1272-1277
Recoverable (Sa)-BINAM-l-prolinamide in combination with benzoic acid catalyzed the direct aldol reaction between α-chloroacetone and several aldehydes in different solvents, including water. It is possible to obtain mainly one of the isomers with good regio-, diastero-, and enantioselectivity by choosing the appropriate solvent and reaction conditions. Thus, α-chloroacetone mainly gives the anti-aldol isomer in DMF/H2O with up to 97% ee. The crude α-chloro-β-hydroxy ketones obtained are transformed stereospecifically into the corresponding enantioenriched trans-α,β-epoxy ketones derivatives with up to 97% ee through an SN2 displacement reaction by treatment with Et3N.  相似文献   

19.
Brominations of mono addition products of 1,4-cyclohexadiene with 2,4-dinitrobenzenesulfenyl chloride (RSCl) selectively gave 1R(S),2R(S),4R(S),5R(S)-4,5-dibromo-2-(2,4-dinitrophenylthio)cyclohexyl acetate and 1R(S),2R(S),4R(S),5R(S)-4,5-dibromo-2-chlorocyclohexyl)(2,4-dinitrophenylthio)sulfane. From series reactions 7-oxabicyclo[4.1.0]hept-3-ene with reagents, such as RSCl, NaN3, and H2SO4/Ac2O, six 1,2,4,5-tetrasubstituted cyclohexane derivatives including S, O, N, Cl, and Br were selectively obtained. Formations and structures of the products were discussed.  相似文献   

20.
《Tetrahedron: Asymmetry》2006,17(4):674-686
New chiral NCN-pincer palladium complexes containing proline ester moieties as chiral auxiliaries have been synthesized. The parent ligands 2,6-bis{[(S)-2-(methoxycarbonyl)-1-pyrrolidinyl]methyl}-1-bromobenzene LMe and 2,6-bis{[(S)-2-(benzoxycarbonyl)-1-pyrrolidinyl]methyl}-1-bromobenzene LBn were prepared in a single synthetic step and were obtained enantiomerically pure. Neutral arylpalladium bromide complexes 1a and 1b, formed upon treatment of the respective ligands LMe and LBn with [Pd2(dba)3]·CHCl3, were isolated as mixtures of three stereoisomers (SNSNSCSC, RNSNSCSC and RNRNSCSC). The ratio of stereoisomers is approximately 1:1:0.6 in the case of methyl ester derivative 1a, whereas the bulkier benzyl ester derivative 1b predominantly forms the (SNSNSCSC)-stereoisomer. Upon abstraction of the bromide ion from unresolved mixtures of 1a and 1b, cationic complexes 2 and 3, respectively, form as single diastereoisomers in which one of the ester prolinate carbonyl groups is coordinated to palladium according to X-ray crystal structure determination. This coordination of a carbonyl group to the metal has a substantial influence on the stereochemistry and results in the formation of a single diastereoisomer, having the (RNRNSCSC)-configuration, regardless of the stereochemistry or ratio of stereoisomers of the starting bromide compound. The structures of compounds 2 and 3 were somewhat unexpected since formation of the corresponding cationic [Pd(NCN)(OH2)]+ complexes was anticipated. In preliminary tests of these cationic complexes as catalysts in the enantioselective aldol condensation of benzaldehyde with methyl isocyanoacetate, modest selectivities were observed.  相似文献   

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