Dynamic Covalent Optical Chirality Sensing with a Sterically Encumbered Aminoborane

A sterically encumbered aminoborane sensor is introduced and used for quantitative stereochemical analysis of monoalcohols, diols and amino alcohols. The small-molecule probe exhibits a rigid ortho-substituted arene scaffold with a proximate boron binding site and a triarylamine circular dichroism (CD) reporter unit which proved to be crucial for the observed chiroptical signal induction. Coordination of the chiral target molecule produces strong Cotton effects and UV changes that are readily correlated to its absolute configuration, enantiomeric composition and concentration to achieve comprehensive stereochemical analysis within a 5 % absolute error margin. The sensing method was successfully applied in the chromatography-free analysis of less than one milligram of a crude asymmetric reaction mixture and the advantages of this chiroptical sensing approach, which is amenable to high-throughput experimentation equipment and automation, over traditional methods is discussed.     

Remote induction of stereoselective 1,2-addition of aryl Grignard reagents to β-sulfinyl enones

Optically active tert-allylic alcohols constitute important and often challenging targets in organic synthesis. In this work, we employed a β-sulfinyl moiety as a remote chiral auxiliary to effect asymmetric 1,2-addition of aryl Grignard reagents to enones to form a variety of optically active tert-allylic alcohols. The absolute configuration of a representative alcohol product was determined by X-ray crystallography.     

Chiral secondary alcohol-induced asymmetric autocatalysis: correlation between the absolute configuration of the chiral initiators and the product

In the presence of various chiral secondary alcohols as chiral initiators, an enantioselective alkylation of a pyrimidine-5-carbaldehyde using diisopropylzinc was examined: a pyrimidyl alkanol was obtained in high yield and enantiomeric excess. The correlation between the absolute configuration of the chiral secondary alcohols and the pyrimidyl alkanol is discussed.     

手性溶解剂在NMR法测定对映体比率和绝对构型的研究进展

综述了手性溶解剂在NMR法测定对映体比率及绝对构型的研究进展.按照手性溶解剂的结构类型,包括胺、酰胺、羧酸、醇、氨基醇、大环化合物,对手性溶解剂的结构特点与手性识别性能进行了详细的介绍.     

Cytochrome P450 Catalyzed Oxidative Hydroxylation of Achiral Organic Compounds with Simultaneous Creation of Two Chirality Centers in a Single CH Activation Step

Regio‐ and stereoselective oxidative hydroxylation of achiral or chiral organic compounds mediated by synthetic reagents, catalysts, or enzymes generally leads to the formation of one new chiral center that appears in the respective enantiomeric or diastereomeric alcohols. By contrast, when subjecting appropriate achiral compounds to this type of C? H activation, the simultaneous creation of two chiral centers with a defined relative and absolute configuration may result, provided that control of the regio‐, diastereo‐, and enantioselectivity is ensured. The present study demonstrates that such control is possible by using wild type or mutant forms of the monooxygenase cytochrome P450 BM3 as catalysts in the oxidative hydroxylation of methylcyclohexane and seven other monosubstituted cyclohexane derivatives.     

Cytochrome P450 Catalyzed Oxidative Hydroxylation of Achiral Organic Compounds with Simultaneous Creation of Two Chirality Centers in a Single C?H Activation Step

Regio‐ and stereoselective oxidative hydroxylation of achiral or chiral organic compounds mediated by synthetic reagents, catalysts, or enzymes generally leads to the formation of one new chiral center that appears in the respective enantiomeric or diastereomeric alcohols. By contrast, when subjecting appropriate achiral compounds to this type of C H activation, the simultaneous creation of two chiral centers with a defined relative and absolute configuration may result, provided that control of the regio‐, diastereo‐, and enantioselectivity is ensured. The present study demonstrates that such control is possible by using wild type or mutant forms of the monooxygenase cytochrome P450 BM3 as catalysts in the oxidative hydroxylation of methylcyclohexane and seven other monosubstituted cyclohexane derivatives.     

手性螺-环丙烷双内酯化合物的合成与结构

本文进一步研究了5-(l-孟氧基)-3-溴-2(5H)-呋喃酮(1)与氧的亲核试剂,如二苯甲醇、苯甲醇、α-甲基苯甲醇、薄荷醇、冰片醇发生新颖的串联不对称双Michael加成/分子内亲核取代反应,合成了一般方法难以合成的含有多个手性中心的螺[1-溴-4-l-孟氧基-5-氧杂-6-氧代双环[3.1.0]己烷-2,3'-(4'-亲核基-5'-孟氧基丁内酯)](4a-4e)。通过元素分析,IR,UV,^1HNMR,^1^3CNMR,MS,[α]~D^2^0波谱分析数据以及X四圆衍射确定了4a-4e的化学结构和绝对构型。     

Structure elucidation and absolute stereochemistry of isomeric monoterpene chromane esters

Six novel monoterpene chromane esters were isolated from the aerial parts of Peperomia obtusifolia (Piperaceae) using chiral chromatography. This is the first time that chiral chromane esters of this kind, ones with a tethered chiral terpene, have been isolated in nature. Due to their structural features, it is not currently possible to assess directly their absolute stereochemistry using any of the standard classical approaches, such as X-ray crystallography, NMR, optical rotation, or electronic circular dichroism (ECD). Herein we report the absolute configuration of these molecules, involving four chiral centers, using vibrational circular dichroism (VCD) and density functional theory (DFT) (B3LYP/6-31G*) calculations. This work further reinforces the capability of VCD to determine unambiguously the absolute configuration of structurally complex molecules in solution, without crystallization or derivatization, and demonstrates the sensitivity of VCD to specify the absolute configuration for just one among a number of chiral centers. We also demonstrate the sufficiency of using the so-called inexpensive basis set 6-31G* compared to the triple-ζ basis set TZVP for absolute configuration analysis of larger molecules using VCD. Overall, this work extends our knowledge of secondary metabolites in plants and provides a straightforward way to determine the absolute configuration of complex natural products involving a chiral parent moiety combined with a chiral terpene adduct.     

Small amounts of achiral beta-amino alcohols reverse the enantioselectivity of chiral catalysts in cooperative asymmetric autocatalysis

An asymmetric autocatalytic reaction has been catalyzed by a mixture of chiral and achiral beta-amino alcohols. The absolute configuration of the highly enantioenriched obtained product (>98% ee) was shown to depend not only on the absolute configuration of the chiral catalyst but also on the structure and the amount of achiral catalyst. Even in default versus the chiral catalyst, achiral catalysts were shown to be able to reverse the enantioselectivity of the reaction.     

Diastereomeric non-equivalence of (−)-ω-camphanic esters of chiral secondary alcohols in 13C NMR spectroscopy

The diastereomeric differences (Δδ) were measured by ¹³C NMR spectroscopy for a series of 11 (?)-ω-camphanic esters of secondary alcohols of known absolute configuration. This method cannot be used for the determination of the absolute configuration, but it can be applied to the estimation of the enantiomeric purity of chiral alcohols.     

Intramolecular vs. intermolecular induction in the diastereoselective catalytic reduction of enantiomerically pure ketones with borane in the presence of cyclic β-amino alcohols

Asymmetric reduction of various enantiomerically pure ketones was carried out by using oxazaborolidine catalysts with a variety of achiral and chiral ligands. The efficiency of chiral 1,2-amino alcohols as well as the effect of the stereogenic centers in the substrate on the catalytic asymmetric reduction were studied. It was found that the corresponding secondary alcohols were obtained with extremely high stereoselectivities with the proper choice of chiral ligands although a considerably large double asymmetric induction was observed in some cases.     

Diastereoselective zinco-cyclopropanation of chiral allylic alcohols with gem-dizinc carbenoids

The highly diastereoselective zinco-cyclopropanation of chiral allylic alcohols using gem-dizinc carbenoids is described. The reaction produces three contiguous stereogenic centers, and the resulting chiral cyclopropylzinc derivatives can be trapped with electrophiles with retention of configuration. Simple functional group manipulations lead to the efficient synthesis of orthogonally protected 1,2,3-substituted cyclopropane derivatives.     

Determination of absolute configuration of secondary alcohols using new chiral auxiliary and chemical shift calculation

A new chiral carboxylic acid 1 was synthesized for determination of absolute configuration of secondary alcohols. Several chiral alcohols were converted to esters with 1. Their experimental esterification shift changes were successfully reproduced by the chemical shift simulation method combined with molecular dynamic simulation.     

Chiral Cooperativity: The Effect of Distant Chiral Centers in Ferrocenylamine Ligands upon Enantioselectivity in the Gold(I)-Catalyzed Aldol Reaction

Long-range chiral cooperativity in enantiomerically pure ferrocenylamine ligands containing both planar and multiple centers of chirality (multiple stereogenic C-atoms) was demonstrated in the Au^I-catalyzed reaction of aldehydes and isocyanoesters. Synthetic methodology was developed for the synthesis of ferrocenylamine ligands with two and three chiral centers of known absolute configuration in the C-side chain in addition to the planar chirality of the molecule. The diastereo- and enantioselectivity of the Au^I-catalyzed formation of the trans- and cis-dihydrooxazoles 5 and 6 , respectively, from benzaldehyde ( 1 ) and methyl isocyanoacetate ( 2 ) depend upon the sequence of chirality (absolute configuration of the chiral centers) in the side chain of the ferrocenylamine ligands. Particularly significant effects were observed upon the enantioselectivity for the minor cis-dihydrooxazole 6 , for which, in certain cases, resulted in a change in the enantiomeric dihydrooxazole 6 produced in excess with a change in the absolute configuration of a distant chiral center. Significant effects upon diastereo- and enantioselectivity were observed when chiral ferrocenylamine ligands containing free OH groups were utilized. Using ligands containing a free OH group gave 6 with an absolute configuration opposite to that produced by the corresponding ester and carbamate derivatives. The possible mechanisms for the transmission of chiral information in the proposed stereoselective transition state (TS) was discussed, including both the formation of a stereogenic N-atom and steric effects based upon Newman's rule of six.     

Trianglamine as a new chiral shift reagent for secondary alcohols

Chiral trianglamines 1 and 2 were found to be useful as NMR chiral shift reagents for the determination of enantiomeric purity and absolute configuration of several kinds of secondary alcohols, cyanohydrins, and propargyl alcohols.     

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.     

Predetermined chirality at metal centers of various coordination geometries: a chiral cleft ligand for tetrahedral (T-4), square-planar (SP-4), trigonal-bipyramidal (TB-5), square-pyramidal (SPY-5), and octahedral (OC-6) complexes

Two tetradentate bispinene-bipyridine type ligands, each with six stereogenic carbon centers, were synthesized from (-)-alpha-pinene. Their ability to predetermine chiral configurations at metal centers was studied. The two diastereoisomers, L1 and L2, differ in their absolute configuration at the bridgehead position. These ligands form metal complexes with Ag(I), Pd(II), Zn(II), Cu(II), and Cd(II), with coordination numbers four, five, and six and with complete control of chirality at the metal centers. Using L1 rather than L2 leads to complexes of inverted absolute configuration at the metal centers. These diastereomeric coordination species can be obtained either as separate compounds or, in some cases, as solids containing them in a 1:1 ratio. Ligands L1 and L2 thus show that the pinene-bipyridines are versatile molecules for the formation of metal complexes with predetermined chirality. In all cases, absolute configurations were determined in the solid state by X-ray diffraction methods and in solution by CD spectroscopy. The sign of exciton couplets from the pi-pi* transitions always agrees with the expectations for a given local configuration at the metal center. The five-coordinate, inherently chiral species of Zn(II) and Cu(II) described in this article are the first examples of trigonal-bipyramidal metal complexes with predetermined absolute configuration containing topologically linear ligands.     

A characteristic CH band in VCD of methyl glycosidic carbohydrates

Novel vibrational circular dichroism (VCD) studies in the CH region of a series of methyl glycosidic carbohydrates were examined. The specific CH stretching VCD band predicts absolute stereochemistries of their anomeric positions. The C-1 chiral information was extracted to the methoxy substituent as a probe. The concept of the vibrational chirality probe from a single chiral center in the presence of numerous such centers might be useful in determining the absolute configuration, when a multiplicity of chiral centers is present in a molecule such as a carbohydrate.     

Stereoselective addition of alpha-sulfinyl carbanions to N-p-tolylsulfinylketimines: synthesis of optically pure 1,2,2'-trialkyl-2-aminoethanols

The reactions of lithium carbanions derived from both enantiomers of methyl (1) and ethyl p-tolyl sulfoxide (2) with (S)-N-arylsulfinylketimines 3 and 4 took place in a highly stereoselective manner and good isolated yields. The configuration of the carbon bonded to nitrogen relies exclusively on the N-sulfinylimine configuration. When ethyl p-tolyl sulfoxide (2) is use as nucleophile, two chiral centers are created simultaneously, where the configuration of the carbon bonded to the sulfur is mainly controlled by 2. The asymmetric induction increases with the temperature, being total at room temperature in the case of the matched pair of reactants. A non-oxidative Pummerer reaction on the obtained aminosulfoxides allows a straightforward synthesis of optically pure 1,2-ethanolamines with one or two chiral centers, including amino alcohols with a bulky quaternary carbon bonded to the amine group.     

Synthesis of novel norephedrine-based chiral ligands with multiple stereogenic centers and their application in enantioselective addition of diethylzinc to aldehyde and chalcone

Novel norephedrine-based chiral ligands with multiple stereogenic centers were conveniently prepared from norephedrine and N-substituted pyrrole. These novel chiral ligands were used to catalyze the enantioselective addition of diethylzinc to aldehydes and to chalcone in high yields and with good to high enantioselectivities. The absolute configuration of products was found to be affected by the stereogenic centers on the norephedrine part of the novel chiral ligands.