Recent Advances in Cinchona Alkaloid Chemistry

Edge-to-face CH/pi aromatic interaction and molecular self-recognition in epi-cinchona-based bifunctional thiourea organocatalysis

The impact of cooperativity between intermolecular interactions is demonstrated by the molecular self-recognition properties of highly enantioselective epi-cinchona bifunctional thiourea organocatalysts. Low-temperature NMR experiments in inert solvents have revealed two sets of nonequivalent resonances in equal population for thiourea-modified members of the epi-quinine and epi-quinidine families. In solution, the predominance of an asymmetric (C1) dimeric self-assembly with noteworthy structural motifs became evident: simultaneous intra- and intermolecular thiourea hydrogen bonding and a CH/pi interaction were observed. Both the stereochemical and the diverse conformational features of the system favor the observed quinoline T-shaped aromatic pi-pi stacking interaction. The structure findings are supported by quantitative proton-proton distance data that were available from NOE buildup curves. The 3D structure of the dimeric assembly has been modeled in agreement with the H-H distance restraints. Owing to the geometrical preference associated with the dimerization process, the self-assembled bifunctional system is interpreted as a charge-transfer complex with the potential for catalyst self-activation.

Asymmetric organocatalysed [1,3]-sigmatropic rearrangements

The first organocatalysed enantioselective [1,3]-sigmatropic O- to N-rearrangement reactions are presented. The reactions take place under regio- and enantioselective control, and are catalysed by cinchona alkaloids. Two reactions have been developed the first one is the rearrangement of imidates to amides, while the other rearrangement occurs from carbamates to amines via a decarboxylation. Both transformations give nitrogen protected beta-amino acid derivatives as the product. These novel asymmetric organocatalysed [1,3]-sigmatropic O- to N-rearrangement reactions provide a reliable and efficient synthetic method for obtaining enantioenriched beta-amino acid derivates in good yield from racemic starting materials.

Asymmetric catalysis with bifunctional cinchona alkaloid-based urea and thiourea organocatalysts

Cinchona alkaloid derivatives modified to include a (thio)urea component have emerged in the last three years as readily accessible, robust and tunable bifunctional organocatalysts for a range of synthetically useful transformations. The origins of these catalysts and the major developments in their application in enantioselective synthesis are reviewed.

An improved synthesis of 10,11-didehydroCinchona alkaloids

A revised procedure for the conversion of the four major Cinchona alkaloids (quinine, quinidine, cinchonidine, and cinchonine) into their respective 10,11-didehydro derivatives is described. The reported protocol offers several advantages over a recently published synthetic route. These include (i) enhanced robustness (ii) ready scalability (iii) reduced operational complexity and number of steps (iv) chromatography-free work-up. In addition, toxic solvents were replaced by environmentally less problematic alternatives.

The enantioselective Morita-Baylis-Hillman reaction and its aza counterpart

The development of asymmetric Morita-Baylis-Hillman (MBH) reactions has evolved dramatically over the past few years, parallel to the emerging concept of bifunctional organocatalysis. Whereas organocatalysis is starting to compete with metal-based catalysis in several important organic transformations, the MBH reaction belongs to a group of prototypical reactions in which organocatalysts already display superiority over their metal-based counterparts. This Minireview summarizes recent mechanistic insights and advances in the design and synthesis of small organic molecules for enantioselective MBH and aza-MBH reactions.

Temperature-dependent Time-resolved Fluorescence Study of Cinchonine Alkaloid Dication

Photo induced excited state dynamical processes of cinchonine alkaloid dication (C(++)) have been studied over a wide range of temperature using steady state and nanosecond time-resolved fluorescence spectroscopic techniques. The temperature-dependent fluorescence studies of C(++) clearly indicate the existence of two distinct emitting species having their own characteristic decay rates. The shorter-lived species shows a usual temperature dependence with increasing non-radiative deactivation at higher temperatures, while the longer-lived species show features resembling to the excited state solvent relaxation process with a large solvent relaxation time (tau(r) approximately 6 ns). The species emitting in the lower energy side, having longer decay time is found to be more sensitive towards chloride ion quenching and has a charge transfer character. Further, concentration quenching with decrease in tau(r) of long lived species shows the possibility of energy migration along with solvent relaxation in C(++).

Novel Cinchona alkaloid derived ammonium salts as catalysts for the asymmetric synthesis of beta-hydroxy alpha-amino acids via aldol reactions

Using the cinchonidine-derived phase-transfer catalyst de veloped by Park and Jew as a lead structure, we have prepared novel chiral ammonium salts and investigated their efficacy for the preparation of beta-hydroxy alpha-amino acids via asymmetric aldol reactions. The modifications were performed at C3 of the cinchonidine nucleus and include dimers as well as catalysts possessing electron-deficient alkyne and alkene moieties. Some of the new catalysts yielded improvements relative to the Park-Jew catalyst in the aldol reaction.

Cupreines and cupreidines: an emerging class of bifunctional cinchona organocatalysts

In the steadily expanding field of organocatalysis, cinchona alkaloids play a prominent role. Until the late 1990s, bifunctional catalysts based on this scaffold relied exclusively on the C9-hydroxy group as the hydrogen-bond donor. Recently, new cinchona catalysts have been developed that feature a phenolic OH group in the C6' position-a structural feature that allows a diverse set of reactions to be catalyzed in a highly stereoselective fashion. This Minireview describes the scope and modes of action of this new class of asymmetric bifunctional organocatalysts.

Quinoline, quinazoline and acridone alkaloids

This review covers the isolation, structure determination, synthesis and biological activity of quinoline, quinazoline and acridone alkaloids from plant, microbial and animal sources: 134 references are cited.

Effect of Protonation on the Conformation of Cinchonidine

The protonation of cinchona alkaloids in solution leads to the severe restriction of their internal rotational degrees of freedom and to the locking of the molecule around a specific conformation held in place by a bridging counterion of the acid used for protonation. For HF, direct interactions were detected by NMR between the fluorine anion and not only with the acidic hydroxo group but also with non-acidic hydrogen atoms in the quinoline ring.

Dienamine Catalysis: Organocatalytic Asymmetric γ-Amination of α,β-Unsaturated Aldehydes

A new concept in organocatalysis is presented, the direct asymmetric gamma-functionalization of alpha,beta-unsaturated aldehydes. We disclose that secondary amines can invert the usual reactivity of alpha,beta-unsaturated aldehydes, enabling a direct gamma-amination of the carbonyl compound using azodicarboxylates as the electrophilic nitrogen-source. The scope of the reaction is demonstrated for the enantioselective gamma-amination of different alpha,beta-unsaturated aldehydes, giving the products in moderate to good yields and with high enantioselectivities up to 93% ee. Experimental investigations and DFT calculations indicate that the reaction might proceed as a hetero-Diels-Alder cycloaddition reaction. Such a mechanism can explain the "unexpected" stereochemical outcome of the reaction.

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.

Novel Oxygen Chirality Induced by Asymmetric Coordination of an Ether Oxygen Atom to a Metal Center in a Series of Sugar-Pendant Dipicolylamine Copper(II) Complexes

Six sugar-pendant 2,2'-dipicolylamine (DPA) ligands (L1-3 and L'1-3) have been prepared. OH-protected and unprotected D-glucose, D-mannose, and D-xylose were attached to a DPA moiety via an O-glycoside linkage. X-ray crystallography of the copper(II) complexes (1-5) with these ligands revealed that the anomeric oxygen atom is coordinated to the metal center in the solid state, generating a chiral center at the oxygen atom. The CD spectra of these copper complexes in methanol or aqueous solution exhibit Cotton effects in the d-d transition region, which indicates that the ether oxygen atoms remain coordinated to the metal center and the oxygen-atom chirality is preserved even in solution. For complexes 1 and 2, the inverted oxygen-atom chirality and chelate-ring conformation in the solid state are well correlated with the mirror-image CD spectra in methanol solution. The concomitant inversion of the asymmetric configuration around the copper center was also observed in a methanol solution of complex 3 and a pyridine solution of complex 2. The square-pyramidal/octahedral copper(II) centers also exhibited characteristic absorption and CD spectra.

Crystal and molecular structures of trichloro-cobalt(II) complexes of epiquinine, epiquinidine, and epidihydrocinchonine

Cinchona alkaloids are very well known antimalarials but the mechanism of their biological action still remains to be elucidated. The structural studies of active erythro and inactive threo alkaloid complexes are an important step to this aim. In this paper results of crystal structure analysis of three cobalt complexes of threo alkaloids are presented: (epiquininium)trichlorocobalt(II) (EpiQnCoCl3), (epiquinidinium)trichlorocobalt(II) (EpiQdCoCl3) and (epidihydrocinchoninium)trichlorocobalt(II) (EpiCnCoCl3). The complexes are zwitterions in which trichlorocobalt substituents are coordinated to quinoline nitrogen atoms and quinuclidine nitrogen atoms are protonated. EpiQnCoCl3 adopts uncommon conformation with quinoline moiety oriented in the opposite direction in comparison to the analogous uncomplexed alkaloid. The packing in the crystal structures is determined mainly by the hydrogen bonds, in which the chlorine atoms of substituents and solvent molecules contribute. Atoms participating in hydrogen bonds in EpiQnCoCl3 and EpiQdCoCl3 form large rings, while in EpiCnCoCl3 only chains are present. Solvent molecules are very important for the packing mode. In contrast to most erythro alkaloids, the hydroxyl oxygen atom in the title complexes forms weak or not well defined hydrogen bonds. The contribution of very weak intramolecular interactions N1--H1...O12 cannot be excluded. Such "trace" interactions can be considered a relic of the unprotonated status of an epi alkaloid.

Cinchona alkaloid-based polymer-bound phase-transfer catalysts: Efficient enantioselective alkylation of benzophenone imine of glycine esters

Cross-linked polystyrene-bound and poly(ethylene glycol)-bound phase-transfer catalysts as well as homopolymers of cinchona alkaloid derivatives have been synthesised. Both soluble and insoluble polymers have been investigated. The enantioselective alkylation of N-diphenyl methylene glycine t-butyl ester has been successfully carried out in heterogeneous and homogeneous systems. High enantioselectivities (up to 96%) have been obtained. The polymer-bound catalysts have been easily recovered and conditions for efficient recycling have been studied.