Stevia Rebaudiana Bertoni, a Source of High-Potency Natural Sweetener-Biochemical and Genetic Characterization

Stevia rebaudiana is a natural sweetener herb that is increasingly used in herbal medicines in the food and cosmetics industries. Molecular methods can be combined with morphological techniques to identify stevia genotypes as a starting material to produce more reliable bioproducts. This study evaluated the level of the genetic and biochemical diversity in various stevia genotypes using HPLC (high performance liquid chromatography) analysis and random amplified polymorphic DNA (RAPD) markers. Stevia genotypes collected from different locations of the world showed clear variations at the biochemical and genetic level in Polish climate conditions. The influence of the genotypes on the content of steviol glycosides, antioxidants, phenols, flavonoids, and tannins was analyzed using phytochemical assays. Genotypes from Morocco, Poland, Egypt, and Nigeria can be defined as samples of higher quality compared to other genotypes analyzed in terms of the amount of steviol glycosides. Considering the rebaudioside A/stevioside ratio as a selection criterion, genotypes from Australia, China, India, and Pakistan should be considered to be valuable in terms of suitability for obtaining new varieties. The present results of RAPD marker analysis indicated differential banding pattern and considerable polymorphism among all ten stevia genotypes. Genotypes from Morocco, Egypt, Poland, Nigeria, China, and India, as genetically different, can be selected for further stevia breeding programs.

Genetic estimates of antioxidant properties in the conventionally and in vitro propagated strawberry (Fragaria × ananassa Duch.)

This study was undertaken in order to estimate the effect of in vitro propagation on antioxidant activity in strawberry. Results of this research exhibited differences between conventionally and in vitro propagated plants in respect of all traits analyzed. In spite of the decrease in range and mean content of vitamin C and polyphenols as well as antioxidant activity, the genetic gain expressed as percent of mean was higher in microplants regarding phenolics, flavonoids and antioxidant activity in contrast to conventional plants (22.39-20.83, 21.79-15.61, 9.52-3.39; resp.). Correlation and path coefficients showed changes of antioxidants inter-relations between micropropagated and conventional plants. Phenolics and vitamin C correlated positively with antioxidant activity in all genotypes. The highest positive direct effect on antioxidant activity was observed via vitamin C in microplants (0.705), while in conventional plants via phenolics (0.834). Flavonoids affected directly and positively antioxidant activity in microplants (0.103) and negatively in conventional plants (-0.143).

Solvent-assisted synthesis of a shape-persistent chiral polyaza gigantocycle characterized by a very large internal cavity and extraordinarily high amplitude of the ECD exciton couplet

A giant, chiral square-shaped macrocycle, characterized by a large hydrophobic cavity, has been obtained from readily available and inexpensive substrates.

Benzhydryl Ethers of Tartaric Acid Derivatives: Stereochemical Response of a Dynamically Chiral Propeller

The benzhydryl (diphenylmethyl) group is a molecular propeller that can act as a chirality reporter if it is introduced nearby a stereogenic center by making an ether bond. The hydrophobic character of the benzhydryl group allows transformation of insoluble natural tartaric acid derivatives into soluble entities in a nonpolar environment. Electronic circular dichroism spectra, recorded within the short-wavelength region of the phenyl ¹ B transitions (190-200 nm) shows strong bisignate Cotton effects. The signs and magnitudes of these Cotton effects are a function of absolute configuration and conformation of the molecule and do not primarily arise from exciton coupling of chiral benzhydryl chromophores. In crystals, the main-chain conformation is stabilized by intramolecular hydrogen bonds and CH-CO dipolar interactions. The number of the donor NH groups has a pronounced effect on the preferred conformations and inclusion properties of benzhydryl-(R,R)-tartaric acid diamides. Evidence is shown for the solvent dependency of the conformations of NH amides of tartaric acid diphenylmethyl ethers.

Chirality transfer through sulfur or selenium to chiral propellers

Steric and electronic effects to the structure of a trityl moiety due chirality transfer by ECD method and DFT calculation.

Intramolecular interactions of trityl groups

Trityl group, Tr, is a molecular dynamic rotor of which the conformation and helicity depend on other groups in the close vicinity. Interactions with another covalently linked Tr group and with other substituents are analyzed in terms of transfer of chirality to the trityl group. Two trityl groups in a molecule can mutually interact at a distance of two, three, or five bonds. Despite its size, a Tr group attached to a cyclohexane or cyclopentane ring through an oxygen or nitrogen atom adopts either an axial or equatorial position, depending on additional stabilizing interactions, such as hydrogen bonding.

C-hexaphenyl-substituted trianglamine as a chiral solvating agent for carboxylic acids

Chiral hexaazamacrocycles with a trianglamine structure and C(3)-symmetry, containing six ring substituents and twelve stereocenters have been tested as chiral solvating agents (CSAs) for α-substituted carboxylic acids. Excellent results have been obtained with a hexaphenyl-substituted macrocycle. The optimal ratio between the macrocycle and racemic acid, allowing for baseline separation of the enantiomers' signals in the (1)H NMR spectrum, was dependent on the type of acid, in particular on its degree of acidity. The analyte and the CSA could be separated and recovered by a simple acid-base extraction and reused without purification. The conformations of the free and protonated hexaamino macrocycles were inferred by CD spectroscopic studies and DFT calculations.

Solid solution of two diastereomers of [3a(R,S),7a(R,S)]-3-[(1′R)-1-phenylethyl]perhydro-1,3-benzothiazol-2-iminium chloride

A mixture of two diastereomers with the configurations (3aS,7aS,1′R) and (3aR,7aR,1′R) forms co-crystals in which there is one unique molecule in the asymmetric unit, but the molecule displays disorder which is a result of the presence of the two diastereomers at the same crystallographic site. Theoretical calculations carried out by the DFT method with the 6-311++G(2df,p) basis set allowed for the estimation of the energy difference between the two diastereomers both in the isolated and the solid state, while the natural bond orbital (NBO), Mulliken, natural population (NPA) and CHelpG analyses helped to establish the electronic structure of the thazolidin-2-imine fragment.

Probing the shapes of chiral bis-(o-naphthalimidobenzoyl) systems using X-ray and circular dichroism methods

CD (circular dichroism) and X-ray investigations have been carried out in order to identify the prevalent conformations and define the forces that determine the molecular and supramolecular organization of the alkyl-bridged bichromophoric [NAB, ortho-(1,8-naphthalimido)benzoyl] units, each consisting of the benzoyl substituted in the ortho position with the 1,8-naphthalimide group. The results reveal that NAB bichromophores incorporated into the same molecule exist in a variety of conformation/helicity combinations. The molecular structures are largely stabilized by local 1,3-CH/CO dipole-dipole interactions, while the crystal packing besides dispersive H...H interactions is mostly governed by multiple C-H...O(=C) and C-H...pi interactions. The relatively small contribution of pi...pi interactions comes from a pairwise off-face stacking between naphthalimide rings or from pairwise carbonyl...pi interactions. All these types of intermolecular interactions have been summarized quantitatively by means of a Hirshfeld surface analysis.

Role of the gauche effect and local 1,3-dipole–dipole interactions in stabilizing an unusual conformation of tartarodinitriles

This paper reports the synthesis, X-ray and NMR investigations of chiral and meso dinitriles of tartaric acid (tartarodinitriles) and their O,O′-diacetyl and O,O′-dibenzoyl derivatives. While in chiral tartaric acid its esters and NH amides the four-atom carbon chain is overwhelmingly trans, it is gauche in chiral tartarodinitriles. Conversely, meso-tartaric acid, its esters and amides display a tendency for the gauche conformation, but meso-tartarodinitriles usually have the trans conformation. The NMR studies of tartarodinitriles reveal the presence of a conformational equilibrium in solution with a preference for those conformers found in crystals. The gauche conformation of meso-tartarodinitriles seems to be stabilized by local dipolar interactions, intramolecular C—H...O hydrogen bonds and by a tendency for maximization of the gauche effect, the latter effect also operating in chiral tartarodinitriles. Stabilization of the trans conformers of tartarodinitriles in the crystals seems to originate from specific intermolecular interactions.

Chiral Macrocyclic Aliphatic Oligoimines Derived fromtrans-1,2-Diaminocyclohexane

Aliphatic dialdehydes of rigid structures having a cyclohexane, a bicyclo[2.2.2]octane or a [7]triangulane skeleton, have been condensed with enantiomerically pure trans-1,2-diaminocyclohexane to give [3+3] or [2+2] macrocyclization products. Unlike acyclic aliphatic imines, these macrocyclic oligoimines show enhanced stabilities and their structures in the crystals could be determined by X-ray diffraction analyses. The enantiomerically pure [7]triangulane dialdehyde showed remarkable diastereoselectivity in the condensation with the two enantiomers of trans-1,2-diaminocyclohexane: only one of the enantiomers gave a [2+2] macrocyclization product. Circular dichroism measurements combined with computational analysis show that the lowest energy electronic transition in these cyclic oligoimines is of n-pi* type.

Unprecedented selectivity in the formation of large-ring oligoimines from conformationally bistable chiral diamines

[reaction: see text] Stereoselective formation of large macrocycles in "click-type" reactions is a current challenge. Chiral macrocycles of differing size and shape (e.g., rectanglimine or loopimine) were selectively obtained by cyclocondensation of terephthalaldehyde or isophthalaldehyde with conformationally bistable chiral diamines derived from trans-1,2-diaminocyclohexane and aromatic dianhydrides. This opens new opportunities for the programmed synthesis of large-ring molecular assemblies.

Asymmetric syntheses and transformations--tools for chirality multiplication in drug synthesis

A review of currently used methods for the synthesis and resolution of enantiomers of drugs and their precursors is presented. For the synthesis part the methods of diastereoselective as well as enantioselective synthesis are discussed, with particular consideration given to enantioselective catalysis with either metal complexes or biocatalysts. Desymmetrization processes are also included as methods to access enantiomerically pure compounds. Racemate resolution still remains an important method to obtain pure enantiomers and methods involving kinetic resolution in enzymatic or chemical systems, and particularly in connection with racemization (dynamic kinetic resolution) are on the rise in fine chemical industry, when applicable.

New Macrocycles with Planar Chirality—Synthesis and Determination of Absolute Configurations

A versatile system for preparing macrocyclic compounds with planar chirality is presented. In this system chiral diazacoronands are synthesized readily from lariat-type diesters 13 and 14 and unsymmetrical diamines 7, 8, 12, 15, and 16 under non-high-dilution conditions. The title compounds were subjected to structural analysis by X-ray crystallography and circular dichroism spectroscopy, and absolute configurations for two representative examples (compounds 2 and 3) were assigned by molecular modeling. The correctness of the theoretical approach was verified by the crystallographic results obtained experimentally.

RhombiminesCyclic Tetraimines of trans-1,2-Diaminocyclohexane Shaped by the Diaryl Ether Structural Motif

[reaction: see text] Rhombimines, chiral macrocyclic tetraimines, are preferentially formed because of the structural bias in the reaction of aromatic ether-linked dialdehydes with enantiomerically pure trans-1,2-diaminocyclohexane.

Synthesis, structure, and contrasting chiroptical properties of large trianglimine macrocycles

We have synthesized a number of large trianglimine macrocycles (3a-3f) using our methodology of (3+3) cyclocondensation of (R,R)-trans-1,2-diaminocyclohexane (1) with rigid aromatic dialdehydes (2a-2f), having benzene, biphenyl, terphenyl, styrene, or divinylbenzene spacers. The structures of these chiral macrocycles have been analyzed using computational methods as well as CD spectroscopy. Whereas trianglimines 3a-3d gave uniformly negative exciton Cotton effects, as expected, styrene- and divinylbenzene-based trianglimines 3e and 3f gave opposite-sign exciton Cotton effects. Their contrasting CD behavior was traced by computational methods to subtle differences in the relative orientation of the electric dipole transition moments in the chromophores of the trianglimines. The presence of low-intensity long-wavelength electronic transitions having large rotational strengths in the electronic spectra of 3e and 3f can be accounted for by the symmetry properties of the trichromophoric macrocycles.

Structure and unexpected chiroptical properties of chiral 4-pyrrolidinyl substituted 2(5H)-furanones

Planar 2(5H)-furanones substituted at C4 with a chiral pyrrolidinyl group show CD spectra which are apparently due to the distortion of the C4-N1 bond of sp2 character from the plane defined by the 2(5H)-furanone ring atoms and/or due to the presence of substituents in the pyrrolidine ring. This is a new, previously not encountered structural factor determining the chiroptical properties of 2(5H)-furanones and emerging from the analysis of X-ray diffraction data and quantum mechanical DFT computations. In the presence of a C5 pseudoaxial substituent in the furanone ring, the sign of the furanone n-pi* and pi-pi* transition Cotton effects is determined primarily by the previously postulated allylic helicity rule.

The First Nonempirical Circular Dichroism Determination of the Absolute Configuration of N-Phthalimidosulfoximines Based on Exciton Coupling Mechanism and a Correlation with the Absolute Configuration of Chiral Sulfoxides

A simple nonempirical method for the assignment of absolute configuration of N-phthalimidosulfoximines and related S-chiral sulfoxides, based on exciton-coupled circular dichroism, is presented.

The significance of induced circular dichroism

[reaction: see text] Achiral molecules or ions can display induced circular dichroism (ICD) within their absorption bands on association with chiral inducing molecules. The reverse process is equally important, although less appreciated: chiral inducing molecules, such as tartrates, can show significantly differing CD bands in different achiral environments. Therefore true ICD due to achiral molecules can only be demonstrated in the spectral region outside the absorption bands of the chiral inducing molecules. The case of tartrate-trisimidazoline helical assembly in aqueous solvent is discussed.

Helicity induction in a bichromophore: a sensitive and practical chiroptical method for absolute configuration determination of aliphatic alcohols

[reaction: see text] A practical method for the determination of the absolute configuration of aliphatic secondary alcohols, based on the circular dichroism of the readily available N-(1,8-naphthaloyl)-2-aminobenzoyl (NAB) derivative, is presented. The origin of the induced Cotton effects is traced by ab initio calculations to the dominant helicity of the NAB pi-electron system.

Architecture and function of atropisomeric molecular triads

Molecular triads, B-A-B, in which the central unit A is aromatic or heteroaromatic, can exist as syn or anti conformers or atropisomers of correspondingly C or S shape. These stereoisomers are either discrete conformer species, often distinguished by NMR spectroscopy, or can be separated at ambient temperature as stable atropisomers. In the latter case syn and anti stereoisomers exhibit different (e.g., complex-forming) properties. Practical applications of atropisomeric triads as ligands in asymmetric dihydroxylation are also discussed.

Circular Dichroism of 9,10-Dihydrophenanthrene Derivatives Reveals both the Absolute Configuration and Conformation: A Novel Approach to Mislow's Helicity Rule

The absolute configuration and the conformation of 9,10-trans-disubstituted 9,10-dihydrophenanthrenes, known chiral metabolites of phenanthrene-9,10-oxide, have been determined by circular dichroism. The absolute configuration assignment is based on the sign of the long-wavelength Cotton effect (A-band), which is conformation invariant and originates from benzylic chirality. This provides a new interpretation of the Mislow biphenyl-helicity rule for the case of the 9,10-dihydrophenanthrene chromophore. The sign of the B-band Cotton effect reflects the conformation of the biphenyl chromophore in 9,10-dihydrophenanthrenes. It is shown that the origin of chiroptical properties of 9,10-dihydrophenanthrenes is closely related to those of 5,6-trans-disubstituted 1,3-cyclohexadienes.

Novel chiral pyromellitdiimide (1,2,4,5-benzenetetracarboxydiimide) dimers and trimers: exploring their structure, electronic transitions, and exciton coupling

The chiral but highly symmetrical acyclic and cyclic pyromellitic diimide dimers and trimers 2-5 have been obtained and characterized for the first time. The pyromellitdiimide chromophores in these molecules are linked by a rigid diequatorially 1,2-disubstituted cyclohexane skeleton. The structures of the compounds have been determined in detail by molecular modeling and, in the case of cyclic dimer 4 and trimer 5, by means of X-ray diffraction analysis. The electronically excited states of the pyromellitdiimide chromophore (1a) have been studied in these and other model compounds by means of linear dichroism (LD), magnetic circular dichroism (MCD), and circular dichroism (CD) spectroscopy. CD spectra of the rigid cyclic trimer 5 have provided the most detailed information on the excited states of the pyromellitdiimide chromophore. The low-energy tail (340-360 nm) of the absorption envelope can be assigned to out-of-plane polarized n-pi* transitions (I, II). The higher energy bands are due to contributions from up to six pi-pi* transitions, these being polarized either along the long (IV-VI, VIII) or short axis (III, VII). The results of ab initio CIS/cc-pVDZ and semiempirical INDO/S-CI calculations have been compared with the experimental data. CD Cotton effects in the region 200-260 nm, which result from exciton interactions between electric dipole allowed transitions of two pyromellitdiimide chromophores in compounds 2-5, provide reliable and useful information concerning the conformation and absolute configuration of these molecules, which may be extrapolated to other oligoimide systems.

The use of benzamide derivatives of secondary amines for stereochemical studies by circular dichroism

The benzamide chromophore is widely used as a Cottonogenic derivative of primary amines for stereochemical studies by circular dichroism. The assignments based on the exciton chirality method are reliable since the benzamide group has well-defined geometry and conformation. A recent report (J.D. Chisholm, J. Golik, B. Krishnan, J.A. Matson, D.L. Van Vranken, J. Am. Chem. Soc. 1999, 121: 3801-3802) claimed a caveat in the application of the exciton chirality method to benzamides derived from secondary amines. By the use of benzoyl derivatives of amino alcohols (1-4) and diamines (5, 6) of known absolute configuration we demonstrate that the 250-210 nm range exciton Cotton effects due to secondary and tertiary benzamides are generally of opposite sign. The origin of such disparity is traced to different conformational equilibria of the amide C-N bond in secondary and tertiary benzamides, as shown by semiempirical molecular modelling and NMR data. This feature can be useful in the determination of absolute configuration by analysis of the CD spectra due to exciton coupling of tertiary benzamides.

Lactones. 6. Microbial lactonization of gamma,delta-epoxy esters

The ability of 19 microorganisms to perform the enantioselective lactonization of racemic gamma,delta-epoxy ester 3a and its 7-methyl homolog 3b was checked. It was found that Rhodotorula rubra preferentialy transformed both substrates to (-)-enantiomers of trans delta-hydroxy-gamma-lactones with ee 76% for 3a and 24% for 3b. The best efficiency (20-30%) and enantioselectivity (ee 60-100%) of formation of (-)-gamma-hydroxy-delta-lactones 6a and 6b was observed for lactonization by Botrytis cinerea and Fusarium semitectum, respectively.

ChiralC andS conformers of aromatic diimide triads

Chiral triads composed of a central aromatic diimide moiety and two outer aromatic molecules connected by one- or two-carbon linkers exist in an equilibrium of C and S conformers, as inferred from analysis of circular dichroism and nuclear magnetic resonance spectra, as well as from molecular modeling.