Chromatographic Resolution, Circular Dichroism Spectra, Absolute Configurations, and Crystal Structures of Bridged Biphenyls, Containing Sulfide, Sulfoxide, and Sulfone Groups in the Bridge

Absolute Configuration Assignment to Chiral Natural Products by Biphenyl Chiroptical Probes: The Case of the Phytotoxins Colletochlorin A and Agropyrenol

The application of flexible biphenyls as chiroptical probes for the absolute configuration assignment to chiral natural products is described. The method is straightforward and reliable and can be applied to conformationally mobile and ECD silent compounds, not treatable by computational analysis of chiroptical data. By this approach, the (6'R) absolute configuration of the phytotoxin colletochlorin A (1) was confirmed, while the absolute configuration of the phytotoxin agropyrenol (2), previously assigned by the NMR Mosher method, was revised and assigned as (3'S,4'S). Moreover, with the biphenyl method the configurational assignment can be obtained simply by the sign of a diagnostic Cotton effect at 250 nm in the ECD spectrum, thus allowing application without the need of advanced knowledge of chiroptical spectroscopy and computational protocols.

Absolute Configuration Assignment from Optical Rotation Data by Means of Biphenyl Chiroptical Probes

A non-empirical approach for the assignment of the absolute configuration of chiral 2-alkyl-substituted carboxylic acids and primary amines by [α]_D measurements has been developed. The method requires the conversion of the chiral acids or amines into the corresponding 4,4'-disubstituted biphenylamides or biphenylazepines, respectively. In these derivatives a central-to-axial chirality transfer induces a preferred torsion in the biphenyl moiety revealed by the sign of the biphenyl A band in the ECD spectrum. By 4,4'-substitution on the biphenyl moiety a redshift of the A band is obtained, leading to an increase of its relative contribution to optical rotation. This allows to reliably establish a direct correlation between the [α]_D sign, the biphenyl twist and, then, the substrate absolute configuration. This approach thus constitutes a really practical and reliable method to assign the absolute configuration of chiral carboxylic acids and primary amines by simple and straightforward [α]_D measurement, readily obtainable by a routine instrumentation like the polarimeter.

Central-to-axial chirality induction in biphenyl chiroptical probes for the stereochemical characterization of chiral primary amines

Flexible biphenyls have been applied as efficient and practical chiroptical probes for absolute configuration assignment to chiral primary amines. The mechanism of the central-to-axial chirality transfer from the amine moiety to the conformationally flexible biphenyl system has been determined by NMR and computational studies. This allowed proposing a general non-empirical rule in order to establish, simply by looking at the sign of the 250 nm A band in the ECD spectrum of the biphenyl derivative, the torsion of the biphenyl and thus the absolute configuration of the amine. The method proved to be very reliable and sensitive, allowing treatment of samples on the μmol scale and permitting the simultaneous determination of the amine sample's absolute configuration and enantiopurity.

Crystal structure of (Z)-3-(4-meth-oxy-benzyl-idene)-2,3-di-hydro-benzo[b][1,4]thia-zepin-4(5H)-one

In the title compound, C₁₇H₁₅NO₂S, the two C atoms linking the S and carbonyl C atoms of the seven-membered thia­zepine ring are disordered over two sites, with occupancies of 0.511 (4) and 0.489 (4); both disorder components adopt distorted twist-boat conformations. In the crystal, N—H⋯O and C—H⋯O hydrogen bonds link inverted-related mol­ecules into dimers, incorporating R₁²(6) and R₂²(8) ring motifs; the acceptor carbonyl O atom is bifurcated. These dimers are further linked by C—H⋯O hydrogen bonds, forming supra­molecular tapes running along the a axis.

(Z)-3-(3,4-Dimeth-oxy-benzyl-idene)-2,3-dihydro-1,5-benzothia-zepin-4(5H)-one

In the title compound, C₁₈H₁₇NO₃S, the seven-membered thia­zepine ring adopts a slightly distorted sofa conformation. The dihedral angle between the mean plane of the benzothia­zepine ring system and the benzene ring is 5.9 (1)°. The mol­ecular conformation is stabilized by an intra­molecular C—H⋯S hydrogen bond, which generates an S(7) ring motif. In the crystal, N—H⋯O and C—H⋯O hydrogen bonds link inversion-related mol­ecules into dimers, incorporating R₁²(6) and R₂²(8) ring motifs; the acceptor O atom is bifurcated. These dimers are further linked by C—H⋯O hydrogen bonds, forming supra­molecular tapes running along the a axis. These are connected into the three-dimensional architecture by C—H⋯π inter­actions.

(Z)-3-Benzyl-1,5-benzothia-zepin-4(5H)-one

In the crystal structure of the title compound, C(16)H(13)NOS, mol-ecules are linked into cyclic centrosymmetric R(2) (2)(8) dimers via pairs of N-H⋯O hydrogen bonds. The seven-membered ring adopts a boat conformation.

Asymmetric epoxidation using iminium salt organocatalysts featuring dynamically controlled atropoisomerism

Introduction of a pseudoaxial substituent at a stereogenic center adjacent to the nitrogen atom in binaphthyl- and biphenyl-derived azepinium salt organocatalysts affords improved enantioselectivities and yields in the epoxidation of unfunctionalized alkenes. In the biphenyl-derived catalysts, the atropoisomerism at the biphenyl axis is controlled by the interaction of this substituent with the chiral substituent at nitrogen.

3-[(Z)-Benzyl-idene]-2,3-dihydro-1,5-benzothia-zepin-4(5H)-one

In the title compound, C₁₆H₁₃NOS, the seven-membered ring adopts a distorted half-chair conformation. In the crystal, mol­ecules are linked by N—H⋯O hydrogen bonds, forming chains running along the b axis. The crystal packing is further stabilized by C—H⋯O inter­actions.

11-[3-(Dimethylamino)propyl]-6,11-dihydrodibenzo[b,e]thiepin-11-ol

There are two independent mol­ecules (A and B) in the asymmetric unit of the title compound, C₁₉H₂₃NOS. In each mol­ecule, the seven-membered thiepine ring is bent into a slightly twisted V-shape. The dihedral angles between the mean planes of the two benzene rings fused to the thiepine ring are 75.7 (5) in mol­ecule A and 73.8 (4)° in mol­ecule B. In both mol­ecules, an intra­molecular O—H⋯N hydrogen bond occurs. In the crystal, weak inter­molecular C—H⋯O and C—H⋯π-ring inter­actions are observed.

Dibenzo[b,e]thiepin-11(6H)-one

In the title compound, C₁₄H₁₀OS, the seven-membered thiepin ring adopts a distorted boat conformation with the dihedral angle between the mean planes of the two fused benzene rings being 56.5 (1)°.

6,11-Dihydro-dibenz[b,e]oxepin-11-one

In the title compound, C₁₄H₁₀O₂, the seven-membered oxepine ring adopts a twist-boat conformation with a dihedral angle between the mean planes of the two fused benzene rings of 42.0 (1)°. In the crystal, mol­ecules are linked into chains propagating along the c axis by inter­molecular C—H⋯O hydrogen bonds and the chains are arranged in layers parallel to (100).

3,9-Dibromo-6,7-dihydro-5H-dibenzo[c,e]thiepine

In the title mol­ecule, C₁₄H₁₀Br₂S, the two benzene rings form a dihedral angle of 48.35 (14)°. The seven-membered ring adopts a boat conformation. In the crystal structure, mol­ecules are related by translation along the b axis and exhibit C—H⋯π inter­actions.

The Bip method, based on the induced circular dichroism of a flexible biphenyl probe in terminally protected -Bip-Xaa*- dipeptides, for assignment of the absolute configuration of beta-amino acids

An induced axial chirality of the biphenyl core of the Bip (2',1':1,2;1'',2'':3,4-dibenzcyclohepta-1,3-diene-6-amino-6-carboxylic acid) residue in the terminally protected dipeptides Boc-Bip-beta-Xaa*-OMe (beta-Xaa* = L-beta(3)-HAla, L-beta(3)-HVal, L-beta(3)-HLeu, L-beta(3)-HPro, trans-(1S,2S)-ACHC, trans-(1R,2R)-ACHC, trans-(1S,2S)-ACPC, trans-(1R,2R)-ACPC) resulted in an induced circular dichroism, revealing the usefulness of the Bip method for a reliable and fast assignment of the absolute configuration of chiral beta-amino acids. Remarkably, the Bip method was also applied to the unique spin-labeled, cyclic, beta-amino acids cis/trans-beta-TOAC and trans-POAC. In particular, this study allowed the assignment of the unknown absolute configurations of the enantiomers of the latter compound.

Flexible biphenyl chromophore as a circular dichroism probe for assignment of the absolute configuration of carboxylic acids

A general and nonempirical approach to determine the absolute configuration (AC) of 2-substituted chiral carboxylic acids by circular dichroism (CD) spectroscopy has been developed. In this protocol, the chiral acids are converted to the corresponding biphenyl amides, in which a flexible biphenyl probe gives rise to a Cotton effect at 250 nm (A band) in the CD spectrum, the sign of which is related to the acid AC. Two different mechanisms of transfer of chirality from the acid stereogenic center to the biphenyl moiety are operative in amides derived from 2-alkyl- and 2-aryl-substituted acids, respectively. For both classes of compounds, a model has been defined which allows one to predict, for a given acid AC, the preferred twist of the biphenyl moiety and thus the sign of the A band in the CD spectrum, related to the biphenyl torsion. Interestingly, while in alkyl-substituted substrates the preferred biphenyl twist is determined only by steric interactions, in the aryl-substituted ones the structure of the prevalent conformer and thus the biphenyl twist are dictated by arene-arene edge-to-face stabilizing interactions. Following this protocol, the AC of a 2-substituted chiral acid can be established simply by preparing its biphenyl amides, recording the CD spectrum, and looking at the sign of the A band. From the sign of such a band, the torsion of the biphenyl can be deduced and then the acid AC. Substrates having different structures and functionalities have been investigated, always obtaining reliable AC assignments by this simple protocol.

On the enantioselective olefinepoxidation by doubly bridged biphenyl azepine derivatives – mixed tropos/atropos chiral biaryls

Diastereomeric doubly bridged biphenyl azepines, atropos at 20 degrees C and tropos at 80 degrees C, are precursors to effective iminium organocatalysts that are employed in the enantioselective epoxidation of prochiral olefins (up to 85% ee).

Induced Axial Chirality in the Biphenyl Core of the Proatropoisomeric, Cα-Tetrasubstituted α-Amino Acid Residue Bip in Peptides

An induced axial chirality in the biphenyl core of the 2',1':1,2;1'',2'':3,4-dibenzcyclohepta-1,3-diene-6-amino-6-carboxylic acid (Bip) residue, a conformationally labile, atropoisomeric, C(alpha)-tetrasubstituted alpha-amino acid, was observed by CD and (1)H NMR spectroscopic techniques in the linear dipeptides Boc-Bip-Xaa*-OMe where Boc=tert-butoxycarbonyl, OMe=methoxy, and Xaa*=D- and/or L-Ala, -Val, -Leu, -Phe, -(alphaMe)Val and -(alphaMe)Leu. Chiral induction was significantly lower in the isomeric dipeptides Boc-Xaa*-Bip-OMe, with the Xaa* residue located at the N-terminus of Bip, as well as in the cyclic dipeptide cyclo-[Bip-L-Ala]. The results obtained in solution were confirmed by X-ray diffraction analysis of a crystalline sample of Boc-(R)-Bip-D-Ala-OMe.

Molecular design of DABNTf as a highly efficient resolving reagent for racemic Pd complex with Tropos biphenylphosphine (BIPHEP) ligand: circular dichroism (CD) spectra of enantiopure BIPHEP-Pd complex

The racemic Pd complexes with chirally flexible (tropos) biphenylphosphine (BIPHEP) ligands can be resolved but transformed into the enantio- and diastereo-pure complex. The enantiopure metal complex of BIPHEP ligand is thus obtained through enantiomer-selective complexation of a racemic BIPHEP-Pd complex with enantiopure 1,1'-binaphthyl-2,2'-di(triflyl)amide, DABNTf. The differential CD spectra of the enantiopure BIPHEP-Pd complex is also reported.

Determination of absolute configurations and conformations of organic compounds by theoretical calculations of CD spectra

The usefulness and limitations of the CNDO/S method for calculations of rotational strengths of inherently chiral chromophores are illustrated with examples from bridged biphenyls. The more empirical Schellman matrix method is employed to calculate CD spectra of compounds containing two chirally disposed chromophores. Analysis of the CD spectra of a compound containing two interacting thioamide groups gives detailed conformational information. Analysis of the CD spectra of two closely analogous 1,2,3, 4-tetrahydro-2-aryl-N-formyl-pyridines with spiro structure shows how a small structural change can lead to the appearance of a nearly mirror image CD spectrum with retained configuration at the spiro atom. Copyright 2000 Wiley-Liss, Inc.