Enantiomer最新文献

alpha-Alkylation of 2-phenylacetate ester of 2-alkylamino-2'-hydroxy-1,1'-binaphthyls with various alkyl iodides proceeded with good stereoselectivities. An intramolecular hydrogen bonding between N-H group and the carbonyl oxygen seemed to play an important role for asymmetric induction. The auxiliary was also applicable to Diels-Alder reaction of an acrylate.

A series of copper(Schiff-base) complexes with two chiral centers derived from 1,2-diphenyl-2-amino-ethanol were synthesized and applied to catalyze the asymmetric cyclopropanation of ethenes with diazoacetates. A mechanism that can explain the observed results was proposed. Some of these complexes were also efficient catalysts for asymmetric cyclopropanation of 1,1-diphenylethene with ethyl diazoacetate, affording high e.e. of up to 98.6%. An e.e. of 80.7% was achieved when no solvent was used.

Two crystalline modifications of cinchonine cobalt complex, C19H23Cl3CoN2O, were obtained from mixture of saturated alcohol solutions of CoCl3 x 6H2O and cinchonine. The X-ray structure analysis revealed that the asymmetric unit of one modification, CoCn1, contains only zwitterionic molecules of the complex. In the asymmetric unit of the other, CoCn2, there are two molecules of the title compound and two molecules of ethanol. The influence of the absolute configuration, the CoCl3 coordination with quinoline, and the presence of alcohol molecules on the studied structures was established by comparison of the crystal and molecular structures of both cobalt complexes with the analogous quinine complex and zinc complex of cinchonine. The interactions that dominate in the packing of the molecules in both structures are intermolecular hydrogen bonds. They form characteristic ring systems, depending on the presence of the alcohol molecules. The ring features are also related to the absolute configuration of the alkaloid.

This paper reports CD spectroscopic studies on acridino-18-crown-6 ligands (RR)-2 and 2a (see Figure 1), and their complexes with the enantiomers of alpha-naphthyl)ethylamine hydrogenperchlorate (1-NEA), 1-phenylethylamine hydrogenperchlorate (PEA) and alpha-2-naphthyl)ethylamine hydrogenperchlorate (2-NEA), and also with the achiral guests (1-naphthyl)methylamine hydrogenperchlorate (1-NMA), benzylamine hydrogenperchlorate (BA), methylamine hydrogenperchlorate (MA) and 1-methylnaphthalene (1-MN). The general feature of the CD spectra of complexes of (RR)-2 with MA, BA, (R)- and (S)-PEA is the replacement of the oppositely signed 1Bb doublet of the host by one positive band near 265 nm. The CD spectra of the heterochiral and homochiral complexes of phenazino and acridino hosts (R,R)-1, 1a, (R,R)-2 and 2a with (R)- and (S)-1-NEA and 1-NMA are governed by exciton interaction. Surprisingly, the heterochial [(R,R)/(S)] complexes of the structural isomeric 2-NEA gave rise to a positive couplet in contrast to the negative couplet measured in the spectrum of the heterochiral [(R,R)/(S)] complexes of 1-NEA.

A new complex of diastereoisomeric pair, quinine and quinidine (QQd), was obtained from a mixture of saturated ethanol solutions of quinine and quinidine (0.5:1). The complex crystallises in the triclinic system, space group P1, and contains two molecules of quinine, two molecules of quinidine and four water molecules in the asymmetric unit. The X-ray structure analysis of a single crystal revealed that quinine and quinidine molecules occur in the so-called open conformation, characteristic for Cinchona alkaloids, whenever they are engaged in intermolecular hydrogen bonds. Quinine and quinidine molecules are organized in two very similar kinds of chains. In each chain the links that contain 14-membered rings can be distinguished. Within these rings quinine and quinidine molecules interact via intermolecular hydrogen bonds between the quinuclidine nitrogens and hydroxyl groups, mediated by water molecules. The links are connected with each other by hydrogen bonds between water molecules and nitrogens of the quinoline moieties, which interact via pi-pi stacking. The architecture of the hydrogen bond system in QQd, compared to those observed in the crystal structures of nonhydrated quinidine, cinchonine and cinchonidine, reveals the effect of the co-crystallizing water on the molecular packing. In nonhydrated alkaloid structures the hydrogen-bonded molecules form helical chains, different from those observed in the hydrated QQd complex and hydrated quinine toluene solvate (QTol). Comparison of QQd structure with that of QTol suggests that while the intermolecular hydrogen bonds in the system quinine-water-quinidine-water are very similar to those in quinine-water-quinine-water system, the mode of pi-pi interaction between their quinoline moieties depends on the absolute configuration of the interacting alkaloid molecules.

The tris(didentate) chelates [E(OCR1R2CR3R4O)3], with E = Se and Te, display both configurational (delta or lambda; R or S) and conformational (delta or lambda) chirality. In order to assess the contributions of these three chiral arrays to the Cotton effects of the chelates and to elucidate their stereochemistry in the gas phase and in solution, calculations of the UV and CD spectra (down to 180 nm), and also of the relative stability of the chelates, have been performed at the TDDFT/TZVP/B-P86 level. An extensive conformational analysis has supplied additional information on the relevant conformers in the conformational manifold. It was found that the dominant CD effect reflects mostly the delta/lambda twists of the three five-membered ligand rings, and less so the influence of the A/A core configuration, while the contributions of any R/S chiral carbon atoms of the ligand rings are negligible. The sign, the intensity, and the energy of this dominant CD band are found to depend on the stereochemistry of the chelates in a predictable way. Among the conformers, those with equatorially disposed methyl substituents are much preferred. These results make it possible to determine the absolute configuration (A/A) and conformation (delta/lambda) of the chelates from the CD data.

Rotamer population of S-tyrosinato and S-phenylalaninato ligands side groups in diastereomers of (1,2-diaminoethane)bis-(S-aminocarboxylato)cobalt(III) complexes is calculated by vicinal alpha and beta proton coupling constant analysis. The effect of noncovalent intra- and interligand interactions on the population of rotamers in D20 solution is discussed. It has been established that in all the complexes investigated the most abundant is rotamer t, in which aromatic voluminous moiety and carboxylic group are in an anti position. In almost all complexes the lowest content is of rotamer g, in which these two groups are in the nearest position. Relatively high population of rotamer h in complex 5 tyr, in spite of high steric hindrances, is due to intra- and interligand NH...pi interactions.

X-ray crystallographic and semiempirical PM3 and AM1 studies of 1,1'-dichlorobi-9H-fluoren-9-ylidene (5) are reported. The X-ray molecular structure of (Z)-5 indicated an approximately C2 symmetric conformation with pure twist around C9 = C9' of 40.4 degrees. The fjord regions are somewhat overcrowded: r(C8...C8') = 315.3 pm, r(Cl(1)...Cl(1') = 341.7 pm, r(C(8)...H(8')) = 259.0 pm. The four chlorine atoms of two neighboring molecules of (Z)-5 form a chain. The PM3 calculations showed that the global minimum of 5 is the C2 symmetric twisted conformation t(E)-5, which is 2.4 kJ/mol more stable than its diastereomer C2-t(Z)-5. The corresponding AM1 relative stability is reversed: C2-t(Z)-5 is 1.1 kJ/mol more stable than C2-t(E)-5. The pure twists of t(Z)-5 and t(E)-5 are 37.0 degrees and 37.2 degrees (PM3) and 40.5 degrees and 39.1 degrees (AM1). The corresponding (E) --> (Z) (PM3) and (Z) --> (E) (AM1) energy barriers of diastereomerization are 80.6 kJ/mol (PM3) and 75.8 kJ/mol (AM1). Two anti-folded local minima conformations C2-a(Z)-5 and C(i)-a(E)-5 were found to be 21.2 and 29.5 kJ/mol (PM3) and 25.8 and 35.2 kJ/mol (AM1) less stable than t(E)-5. The syn-folded conformations C(S)-s(Z)-5 and C2-s(E)-5 are transition states for the enantiomerization processes of C2-tz-5 and C2-tE-5, respectively, and lay 79.8 and 94.1 kJ/mol (PM3) and 108.3 and 107.4 kJ/mol (AM1) higher in energy than their corresponding twisted conformations. An alternative pathway for enantiomerization of C2-t(E)-5 via the anti-folded achiral intermediate C(i)-a(E) has a barrier of 56.0 kJ/mol (PM3) and 68.5 (AM1). An alternative pathway for enantiomerization of C2-t(Z)-5 via C2-t(E) and C(i)-a(E) has a barrier of 80.6 (PM3) and 75.8 (AM1) kJ/mol.

Circular dichroism spectra of a series of 4-(tetra-hydroxy-tetryl-1-yl)2-phenyl-2H-1,2,3-triazoles were measured and the sign of their Cotton effects was correlated to the absolute configuration of the hydroxyl group alpha to the triazole base moiety. Those having the D-glycero-configuration in their Fischer projection formula show positive Cotton effect and those with the L-glycero-configuration show negative Cotton effect. This correlation was extended for the assignment of the anomeric configuration of the corresponding glycofuranosyl-C-nucleosides.