Liquid Chromatography on Triacetycellulose, 14 Chromatographic Separation of Enantiomers and Barriers to Enantiomerization of Axially Chiral Aromatic Carboxamides

Exploring the Versatility of the Amidation of Aryl Acid Fluorides using the Germylamines R3 GeNMe2

The formation of amide bonds is an important process since this linkage is an essential component in proteins, pharmaceuticals, and other medicinally and biologically significant molecules. Recently, it was demonstrated that germylamines R3 GeNR'2 were useful reagents for the conversion of acid fluorides to amides. This transformation occurs readily at room temperature and has a low activation energy. In the present study, the versatility of this amidation reaction with aryl acid fluorides is investigated. A series of thirteen acid fluorides with various substituents on the aromatic ring were reacted with the germylamine Ph3 GeNMe2 and twelve of these were converted to the corresponding amides in high yields, the exception being 1,4-benzenedicarbonyl difluoride. The germylamines Bun 3 GeNMe2 and Pri 3 GeNMe2 also could be used for this interconversion, and both of these species successfully converted 1,4-benzenedicarbonyl difluoride to the corresponding amide. In addition, the crystal structure of Ph3 GeNMe2 is reported. This represents one of only three crystallographically characterized germylamines. The synthesis and 19 F NMR characterization of three fluorogermanes R3 GeF (R=Bun , Pri , and Mes) are also reported herein.

Asymmetric Catalysis Mediated by Synthetic Peptides, Version 2.0: Expansion of Scope and Mechanisms

Low molecular weight synthetic peptides have been demonstrated to be effective catalysts for an increasingly wide array of asymmetric transformations. In many cases, these peptide-based catalysts have enabled novel multifunctional substrate activation modes and unprecedented selectivity manifolds. These features, along with their ease of preparation, modular and tunable structures, and often biomimetic attributes make peptides well-suited as chiral catalysts and of broad interest. Many examples of peptide-catalyzed asymmetric reactions have appeared in the literature since the last survey of this broad field in Chemical Reviews (Chem. Rev. 2007, 107, 5759-5812). The overarching goal of this new Review is to provide a comprehensive account of the numerous advances in the field. As a corollary to this goal, we survey the many different types of catalytic reactions, ranging from acylation to C-C bond formation, in which peptides have been successfully employed. In so doing, we devote significant discussion to the structural and mechanistic aspects of these reactions that are perhaps specific to peptide-based catalysts and their interactions with substrates and/or reagents.

Regioselective derivatizations of a tribrominated atropisomeric benzamide scaffold

The enantioselective synthesis of atropisomeric, tribrominated benzamides and subsequent regioselective transformations to afford derivatized, axially chiral molecules is reported. The enantioenriched tribromides were carried through sequential Pd-catalyzed cross-coupling and lithium-halogen exchange with high regioselectivity and enantioretention. A variety of complexity-generation functional group installations were performed to create a library of homochiral benzamides. The potential utility of these molecules is demonstrated by using a phosphino benzamide derivative as an asymmetric ligand in a Pd-catalyzed allylic alkylation.

Spontaneous transfer of chirality in an atropisomerically enriched two-axis system

One of the most well-recognized stereogenic elements in a chiral molecule is an sp(3)-hybridized carbon atom that is connected to four different substituents. Axes of chirality can also exist about bonds with hindered barriers of rotation; molecules containing such axes are known as atropisomers. Understanding the dynamics of these systems can be useful, for example, in the design of single-atropisomer drugs or molecular switches and motors. For molecules that exhibit a single axis of chirality, rotation about that axis leads to racemization as the system reaches equilibrium. Here we report a two-axis system for which an enantioselective reaction produces four stereoisomers (two enantiomeric pairs): following a catalytic asymmetric transformation, we observe a kinetically controlled product distribution that is perturbed from the system's equilibrium position. As the system undergoes isomerization, one of the diastereomeric pairs drifts spontaneously to a higher enantiomeric ratio. In a compensatory manner, the enantiomeric ratio of the other diastereomeric pair decreases. These observations are made for a class of unsymmetrical amides that exhibits two asymmetric axes--one axis is defined through a benzamide substructure, and the other axis is associated with differentially N,N-disubstituted amides. The stereodynamics of these substrates provides an opportunity to observe a curious interplay of kinetics and thermodynamics intrinsic to a system of stereoisomers that is constrained to a situation of partial equilibrium.

Enantioselective synthesis of atropisomeric benzamides through peptide-catalyzed bromination

We report the enantioselective synthesis of atropisomeric benzamides employing catalytic electrophilic aromatic substitution reactions involving bromination. The catalyst is a simple tetrapeptide bearing a tertiary amine that may function as a Brønsted base. A series of tri- and dibrominations were accomplished for a range of compounds bearing differential substitution patterns. Tertiary benzamides represent appropriate substrates for the reaction since they exhibit sufficiently high barriers to racemization after ortho functionalization. Mechanism-driven experiments provided some insight into the basis for selectivity. Examination of the observed products at low conversion suggested that the initial catalytic bromination may be regioselective and stereochemistry-determining. A complex between the catalyst and substrate was observed by NMR spectroscopy, revealing a specific association. Finally, the products of these reactions may be subjected to regioselective metal-halogen exchange and trapping with I(2), setting the stage for utility.

Triazoloacridin-6-ones as novel inhibitors of the quinone oxidoreductases NQO1 and NQO2

A range of triazoloacridin-6-ones functionalized at C5 and C8 have been synthesized and evaluated for ability to inhibit NQO1 and NQO2. The compounds were computationally docked into the active site of NQO1 and NQO2, and calculated binding affinities were compared with IC(50) values for enzyme inhibition. Excellent correlation coefficients were demonstrated suggesting a predictive QSAR model for this series of structurally similar analogues. From this we have identified some of these triazoloacridin-6-ones to be the most potent NQO2 inhibitors so far reported.

Stereoselective Synthesis of Both Enantiomers ofN-Aryl Indoles with Axially Chiral N−C Bonds¶

N-Aryl indoles with axially chiral N-C bonds were synthesized by stereoselective nucleophilic aromatic substitution reactions of planar chiral tricarbonyl(2,6-disubstituted-1-fluorobenzene)chromium complexes. The stereochemistry of the products is highly dependent on the position of the substituent in the indole. When indoles devoid of a substituent at the 2-position were used, N-aryl indole chromium complexes having anti orientation with respect to the tricarbonylchromium fragment were obtained diastereoselectively. In contrast, 2-substituted indoles gave the N-aryl indoles with syn orientation between the tricarbonylchromium fragment and the benzene ring of the indole. These results demonstrate that we have succeeded in synthesizing both enantiomers of N-aryl indoles utilizing an identical planar chiral arene chromium complex.

Non-Biaryl Atropisomers in Organocatalysis

A new class of 6'-hydroxy cinchona alkaloids, with a non-biaryl atropisomeric functionalisation at position 5' of the quinoline core can be prepared by an easy amination procedure. These are the first derivatives for which the principle of atropisomerism is engrafted in the classical core of the cinchona alkaloids. The aminated cinchona alkaloids are effective organocatalysts for the Michael addition of beta-keto esters to acrolein and methyl vinyl ketone, in up to 93 % ee (ee=enantiomeric excess), as well as for the asymmetric Friedel-Crafts amination of a variety of 2-naphthols, permitting the preparation of the latter in up to 98 % ee. The aminated 8-amino-2-naphthol itself is the first chiral organocatalyst based on non-biaryl atropisomerism. The two enantiomers of this chiral primary amine can be used for the direct alpha-fluorination of alpha-branched aldehydes. The fluorinated compounds can thereby be accessed in up to 90 % ee.

Conformational preference in aromatic amides bearing chiral ortho substituents: its origin and application to relayed stereocontrol

Tertiary aromatic amides bearing stereogenic centres ortho to the amide group may adopt two diastereoisomeric conformations which interconvert slowly on the NMR timescale at ambient temperature, and are therefore detectable by NMR. Certain classes of stereogenic centre--particularly sulfoxides, ephedrine-derived oxazolidines, and proline-derived imidazolidines--strongly bias the population of the two conformers. We propose a model, supported by molecular mechanics calculations, which rationalises the sense and magnitude of the conformational selectivity attained in terms of the steric and electronic properties of the controlling centre. The control over conformation may be exploited either by trapping the favoured conformer as an atropisomer, or by using it to relay information about the stereochemistry of the controlling centre.

Stereodynamics of bond rotation in tertiary 1-naphthoic acid amides: a computational study

Computations sho that independent N-CO rotation is not possible in N,N-diethyl-1-naphthamide, and that the coalescence signal corresponding to methyl equivalence observed in the VT NMR spectrum of this system is caused by a complex process whose rate-determining step implies concerted N-CO, Ar-CO, and ethyl rotations. The calculated Gibbs energy barriers for these processes in solution are in good agreement with the experimental values.

Kinetic and thermodynamic stereocontrol in the atroposelective formation of sulfoxides by oxidation of 2-sulfanyl-1-naphthamides

Atropisomeric aromatic amides bearing 2-sulfanyl groups are oxidised by m-CPBA to the corresponding sulfoxides apparently with very high diastereoselectivity. NMR studies and oxidations of chiral benzamides however indicate that the kinetic selectivity of the oxidation is in fact relatively poor, and that the final diastereoisomeric ratio (typically >99:1) is under thermodynamic control, with relatively unhindered Ar-CO rotation readily converting the less stable to the more stable product diastereoisomer. Molecular mechanics indicates that the thermodynamic diastereoselectivity results principally from electrostatic repulsion between the C=O and S-O dipoles.

Ultra-remote stereocontrol by conformational communication of information along a carbon chain

Many receptors and allosteric proteins function through binding of a molecule to induce a conformational change, which then influences a remote active site. In synthetic systems, comparable intramolecular information transfer can be effected by using the shape of one part of a molecule to control the stereoselectivity of reactions occurring some distance away. However, the need for direct communication with the reaction site usually limits such remote stereocontrol to distances of not more than about five bond lengths. Cyclic structures overcome this problem by allowing the controlling centre and the reaction site to approach each other, but the information transfer spans only short absolute distances. Truly remote stereocontrol can, however, be achieved with rigid compounds containing amide groups: the conformation of the amides can be controlled by stereogenic centres and responds to that of neighbouring amide groups and in turn influences stereoselective reactions. This strategy has allowed remote stereocontrol spanning 8 (ref. 11) or 9 (ref. 12) bonds. Here we demonstrate stereocontrol over a reaction taking place more than 20 bond lengths from the controlling centre, corresponding to a linear distance of over 2.5 nm. This transmission of information, achieved by conformational changes relayed through the molecule, provides a chemical model of allostery and might serve as a molecular mechanism for communicating and processing information.

Internal Motions in a Fulleropyrrolidine Tertiary Amide with Axial Chirality

The kinetic parameters for topomerization around the N-CO bond and enantiomerization around the C-CO bond in N-1-naphthoyl fulleropyrrolidine 1 and N-1-naphthoyl pyrrolidine 2 have been determined by dynamic NMR (line shape simulation and selective inversion transfer). The DeltaS(not =) values are negligible. The DeltaH# value for topomerization of 1 is smaller with respect to that of 2 by 4.3 kcal mol(-1) (explained by the electron-withdrawing effect of fullerene) and the value for enantiomerization is greater by 1.4 kcal mol(-1) (explained by the greater rigidity of the fulleropyrrolidine ring, as confirmed by ab initio analyses).

Determination of the interconversion energy barrier of enantiomers by separation methods

Separation methods have become versatile tools for the determination of kinetic activation parameters and energy barriers to interconversion of isomers and enantiomers in the last 20 years. New computer-aided evaluation systems allow the on-line determination of these data after separating minute amount of pure compounds or mixture of isomers or enantiomers, respectively. Both dynamic interconversion during the separation process as well as static stopped-flow techniques have been applied to determine the kinetic activation parameters and interconversion energy barriers by separation methods. The use of (1) combinations of batchwise kinetic studies with enantioselective separations, (2) a continuous flow model, (3) a comparison of real chromatograms with simulated ones, (4) stopped-flow techniques, (5) stochastic methods, (6) approximation functions and (7) deconvolution methods, for the determination of interconversion energy barriers by separation methods is summarized in detail.

Asymmetric synthesis of axially chiral benzamides and anilides by enantiotopic lithiation of prochiral arene chromium complexes

Axially chiral benzamides and anilides were prepared by enantiotopic lithiation at the distinguished benzylic methyl of prochiral tricarbonylchromium complexes of N,N-diethyl 2,6-dimethylbenzamide (1) and N-methyl-N-acyl 2,6-dimethylaniline (14 and 21) with a chiral lithium amide base followed by electrophilic substitution in good yields with high optical purity. The resulting axially chiral chromium-complexed benzamides and anilides were oxidized under air to give chromium-free axially chiral benzamides and anilides in an enantiomerically active form without axial bond rotation at room temperature.

Stereodynamics of bond rotation in tertiary aromatic amides

The degree to which the rotations about the C-N and Ar-CO bonds of aromatic amides occur in a concerted manner was investigated by a variety of NMR and kinetic techniques. Otherwise complex kinetic analyses were simplified by exploiting symmetry and asymmetry in the N-substituents of amides. In 2-unsubstituted 1-naphthamides bearing branched N-substituents, most conformational changes about the amide group were by correlated rotation, though uncorrelated Ar-CO rotation also occurred to some extent. In 2-substituted 1-naphthamides, correlated rotation accounted for all of the Ar-CO rotations, though a significant amount of uncorrelated C-N rotation also occurred. Naphthamides bearing branched N-substituents thus turn out to be efficient molecular gears: Compound 12 showed almost no gear slippage.

Stereospecificity and stereoselectivity in electrophilic substitution reactions of non-alpha-heterosubstituted organolithiums and stannanes: a rotationally restricted amide as an internal stereochemical marker

The complete stereochemical course of a tin-lithium exchange/electrophilic quench sequence has been unambiguously determined by stereochemical characterization (using X-ray crystallography or NOE studies) at every step. Pairs of diastereoisomeric stannanes of known stereochemistry bearing atropisomeric amide substituents undergo tin-lithium exchange with alkyllithiums to give diastereoisomeric benzylic organolithiums whose stereochemistry can be assigned by NMR. For one atropisomer of the stannanes, the tin-lithium exchange is fully stereospecific and proceeds with retention of stereochemistry. The other atropisomer undergoes nonstereospecific tin-lithium exchange: the first reported example of a lack of stereospecificity in electrophilic substitution of tin for lithium. One of the diastereoisomeric atropisomeric organolithiums produced by the tin-lithium exchange is deuterated and alkylated with retention but stannylated with inversion of stereochemistry. The other is alkylated nonstereospecifically but stannylated with retention.

Total synthesis of (+/-)-lennoxamine and (+/-)-aphanorphine by intramolecular electrophilic aromatic substitution reactions of 2-amidoacroleins

Intramolecular electrophilic aromatic substitution reactions of 2-amidoacroleins constitute the key steps in the total syntheses of lennoxamine and aphanorphine. The aldehyde moiety of one cyclization product was transformed to a double bond, which was then engaged in a radical cyclization to produce the complete ring system of lennoxamine. The aldehyde functionality of the other cyclization product was converted to the corresponding mesylate, which underwent intramolecular displacement by a lactam enolate to furnish the ring system of aphanorphine. [reaction: see text]

Atropisomeric amides as chiral ligands: using (-)-sparteine-directed enantioselective silylation to control the conformation of a stereogenic axis

An enantiomerically pure (1-trimethylsilyl)ethyl group, constructed by a (-)-sparteine-directed enantioselective quench of a laterally lithiated tertiary aromatic amide, exerts powerful thermodynamic control over the conformation of the adjacent tertiary amide substituent. Ortholithiation and functionalization of the amide in the 6-position allows the single amide conformer to be trapped as an enantiomerically and diastereoisomerically pure amide atropisomer. Protodesilylation of the amide gives functionalized atropisomeric amides with a stereogenic axis of single absolute configuration, whose barriers to racemization have been determined by polarimetry. Enantiomerically pure amides bearing phosphine substituents are effective ligands in a Pd-catalyzed allylic substitution reaction-the first use of a nonbiaryl atropisomer as a chiral ligand-and give products with 90% ee. The rate of racemization of the phosphine-substituted amide is powerfully influenced by the presence of palladium.

Asymmetric Synthesis of Axially Chiral Anilides by Enantiotopic Lithiation of Tricarbonyl(N-methyl-N-acyl-2,6-dimethylanilide)chromium Complex

Axially chiral N-methylanilides were synthesized by enantioselective lithiation of prochiral tricarbonyl(N-methyl-N-pivaloyl-2,6-dimethylaniline)chromium (1) with the lithium amide of the 4-methylpiperazinylethylamine derivative 13 followed by electrophilic quenching up to 97% ee in good yields. The resulting axially chiral chromium-complexd anilides 2 were oxidized under air to give the axially chiral anilides 14 in enantiomerically active form without axial bond rotation at room temperature.

Synthesis of axially chiral benzamides utilizing tricarbonyl(arene)chromium complexes

Axially chiral N,N-diethyl 2,6-disubstituted benzamides were stereo-selectively prepared utilizing planar chiral (arene)chromium complexes as an enantiomerically active form by following two methods. Ortho-lithiation of the enantiomerically pure planar chiral tricarbonyl(N,N-diethyl 2-methylbenzamide)chromium complex followed by electrophilic quenching gave axially chiral 2-methyl-6-substituted N,N-diethyl benzamide chromium complexes. Photo-oxidative demetalation produced the chromium-free axially chiral benzamides as optically active compounds. An alternative method for the preparation of axial chiral benzamides is an enantioselective lithiation at the benzylic methyl of meso tricarbonyl(N,N-diethyl 2,6-dimethylbenzamide)chromium with appropriate chiral lithium amide base followed by quenching with alkyl halides.

Axially chiral N-benzyl-N,7-dimethyl-5-phenyl-1, 7-naphthyridine-6-carboxamide derivatives as tachykinin NK1 receptor antagonists: determination of the absolute stereochemical requirements

A potent and orally active NK1 antagonist, trans-N-[3, 5-bis(trifluoromethyl)benzyl]-7,8-dihydro-N, 7-dimethyl-5-(4-methylphenyl)-8-oxo-1,7-naphthyridine-6-carboxamide (1t), was shown to exist as a mixture of separable and stable (R)- and (S)-atropisomers (1t-A and 1t-B) originating from the restricted rotation around the -C(6)-C(=O)- bond; the antagonistic activities of 1t-A were ca. 6-13-fold higher than those of 1t-B. Analogues of 1t (3), which have (S)- and (R)-methyl groups at the benzylic methylene portion of 1t, were prepared and separated into the diastereomeric atropisomers, 3a-A, 3a-B and 3b-A, 3b-B, in enantiomerically pure forms. Among the four isomers of 3, the (aR, S)-enantiomer (3a-A) exhibited the most potent antagonistic activities with an IC50 value of 0.80 nM (in vitro inhibition of [125I]BH-SP binding in human IM-9 cells) and ED50 values of 9.3 micrograms/kg (iv) and 67.7 micrograms/kg (po) (in vivo inhibition of capsaicin-induced plasma extravasation in guinea pig trachea), while the activity of the (aS,R)-enantiomer (3b-B) was the weakest with an IC50 value of 620 nM. The structure-activity relationships in this series of antagonists indicate that the (R)-configuration at the axial bond and the stacking (or stacking-like) conformation between the two phenyl rings as shown in 1t-A and 3a-A are essential for high-affinity binding and suggest that the amide moiety functions as a hydrogen bond acceptor in the interaction with the receptor.