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

Stable 2-Azaallyl Salts as a Bridge between 2H-Azirines and Densely Functionalized 2H-Pyrroles

An efficient protocol for the synthesis of stable 2-azaallyl anion salts by the reaction of alkyl 2-bromo-2H-azirine-2-carboxylates with trimethylsilyl cyanide/Bu4NF has been developed. The domino reaction proceeds in four steps via the cleavage of the azirine C-C bond to provide the tetrabutylammonium salts of stereochemically pure 2-azaallyl anions having U-configuration relative to the cyano groups. The anions with an ortho-substituted aryl group or styryl group exist as a mixture of two geometrical isomers across the N2-C3 bond. 2-Azaallyl anion salts have been shown to be convenient substrates for the one-pot synthesis of densely functionalized 2H-pyrroles by the alkylation-cyclization sequence.

Cinchona Alkaloids-Derivatives and Applications

Major Cinchona alkaloids quinine, quinidine, cinchonine, and cinchonidine are available chiral natural compounds (chiral pool). Unlike many other natural products, these alkaloids are available in multiple diastereomeric forms which are separated on an industrial scale. The introduction discusses in short conformational equilibria, traditional separation scheme, biosynthesis, and de novo chemical syntheses. The second section concerns useful chemical applications of the alkaloids as chiral recognition agents and effective chiral catalysts. Besides the Sharpless ethers and quaternary ammonium salts (chiral PTC), the most successful bifunctional organocatalysts are based on 9-amino derivatives: thioureas and squaramides. The third section reports the main transformations of Cinchona alkaloids. This covers reactions of the 9-hydroxyl group with the retention or inversion of configuration. Specific Cinchona rearrangements enlarging [2.2.2]bicycle of quinuclidine to [3.2.2] products are connected to the 9-OH substitution. The syntheses of numerous esterification and etherification products are described, including many examples of bi-Cinchona alkaloid ethers. Further derivatives comprise 9-N-substituted compounds. The amino group is introduced via an azido function with the inversion of configuration at the stereogenic center C9. The 9-epi-amino-alkaloids provide imines, amides, imides, thioureas, and squaramides. The syntheses of 9-carbon-, 9-sulfur-, and 9-selenium-substituted derivatives are discussed. Oxidation of the hydroxyl group of any alkaloid gives ketones, which can be selectively reduced, reacted with Grignard reagents, or subjected to the Corey-Chaykovsky reaction. The alkaloids were also partially degraded by splitting C4'-C9 or N1-C8 bonds. In order to immobilize Cinchona alkaloids the transformations of the 3-vinyl group were often exploited. Finally, miscellaneous functionalizations of quinuclidine, quinoline, and examples of various metal complexes of the alkaloids are considered.

Synthesis of novel quinine analogs and evaluation of their effects on Trypanosoma cruzi

AIM

Chagas disease is a tropical disease caused by the hemoflagellate protozoan Trypanosoma cruzi. There is no vaccine for Chagas disease and available drugs (e.g., benznidazole) are effective only during the acute phase, displaying a variable curative activity in the established chronic form of the disease. New leads with high efficacy and better toxicity profiles are urgently required. Materials & methods: A library of novel quinine derivatives was synthesized using Heck chemistry and evaluated against the various developmental forms of T. cruzi.

RESULTS AND CONCLUSION

Several novel quinine analogs with trypanocidal activity have been identified with the para-nitro-substituted derivative displaying a submicromolar IC₅₀, which is 83-times lower than quinine and three-times lower than benznidazole. Transmission electron microscopy analysis demonstrated that these compounds induced a marked vacuolization of the kinetoplast of intracellular amastigotes and cell-derived trypomastigotes.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Dimeric Cinchona alkaloids

Nature is full of dimeric alkaloids of various types from many plant families, some of them with interesting biological properties. However, dimeric Cinchona alkaloids were not isolated from any species but were products of designed partial chemical synthesis. Although the Cinchona bark is amongst the sources of oldest efficient medicines, the synthetic dimers found most use in the field of asymmetric synthesis. Prominent examples include the Sharpless dihydroxylation and aminohydroxylation ligands, and dimeric phase transfer catalysts. In this article the syntheses of Cinchona alkaloid dimers and oligomers are reviewed, and their structure and applications are outlined. Various synthetic routes exploit reactivity of the alkaloids at the central 9-hydroxyl group, quinuclidine, and quinoline rings, as well as 3-vinyl group. This availability of reactive sites, in combination with a plethora of linker molecules, contributes to the diversity of the products obtained.

Enantioselective synthesis of benzoindolizidine derivatives using chiral phase-transfer catalytic intramolecular domino aza-Michael addition/alkylation

An efficient and enantioselective strategy to synthesize benzoindolizidinesviadomino intramolecular aza-Michael addition/alkylation was developed.

Facile one-pot fabrication of a silica gel-supported chiral phase-transfer catalyst—N-(2-cyanobenzyl)-O(9)-allyl-cinchonidinium salt

A novel silica gel-supported cinchona alkaloid-based PTC catalyst was prepared by one-pot synthesis for the first time. Excellent enantioselectivities (72.0–96.9% ee) and 80–96% yields in enantioselective α-alkylation were achieved without significant loss in the catalytic performance for five runs.

d-Glucose based syntheses of β-hydroxy derivatives of l-glutamic acid, l-glutamine, l-proline and a dihydroxy pyrrolidine alkaloid

The β-hydroxy derivatives of l-glutamic acid, l-glutamine and l-proline, useful for peptide/protein studies, were synthesized starting from d-glucose.

Stereocontrolled synthesis of syn-β-Hydroxy-α-amino acids by direct aldolization of pseudoephenamine glycinamide

β-Hydroxy-α-amino acids figure prominently as chiral building blocks in chemical synthesis and serve as precursors to numerous important medicines. Reported herein is a method for the synthesis of β-hydroxy-α-amino acid derivatives by aldolization of pseudoephenamine glycinamide, which can be prepared from pseudoephenamine in a one-flask protocol. Enolization of (R,R)- or (S,S)-pseudoephenamine glycinamide with lithium hexamethyldisilazide in the presence of LiCl followed by addition of an aldehyde or ketone substrate affords aldol addition products that are stereochemically homologous with L- or D-threonine, respectively. These products, which are typically solids, can be obtained in stereoisomerically pure form in yields of 55-98 %, and are readily transformed into β-hydroxy-α-amino acids by mild hydrolysis or into 2-amino-1,3-diols by reduction with sodium borohydride. This new chemistry greatly facilitates the construction of novel antibiotics of several different classes.

Development of ProPhenol ligands for the diastereo- and enantioselective synthesis of β-hydroxy-α-amino esters

A zinc-ProPhenol-catalyzed direct asymmetric aldol reaction between glycine Schiff bases and aldehydes is reported. The design and synthesis of new ProPhenol ligands bearing 2,5-trans-disubstituted pyrrolidines was essential for the success of this process. The transformation operates at room temperature and affords syn β-hydroxy-α-amino esters in high yields with good to excellent levels of diastereo- and enantioselectivity.

Recent developments in asymmetric phase-transfer reactions

Phase-transfer catalysis has been recognized as a powerful method for establishing practical protocols for organic synthesis, because it offers several advantages, such as operational simplicity, mild reaction conditions, suitability for large-scale synthesis, and the environmentally benign nature of the reaction system. Since the pioneering studies on highly enantioselective alkylations promoted by chiral phase-transfer catalysts, this research field has served as an attractive area for the pursuit of "green" sustainable chemistry. A wide variety of asymmetric transformations catalyzed by chiral onium salts and crown ethers have been developed for the synthesis of valuable organic compounds in the past several decades, especially in recent years.

Investigating the activity of quinine analogues versus chloroquine resistant Plasmodium falciparum

Plasmodium falciparum, the deadliest malarial parasite species, has developed resistance against nearly all man-made antimalarial drugs within the past century. However, quinine (QN), the first antimalarial drug, remains efficacious worldwide. Some chloroquine resistant (CQR) P. falciparum strains or isolates show mild cross resistance to QN, but many do not. Further optimization of QN may provide a well-tolerated therapy with improved activity versus CQR malaria. Thus, using the Heck reaction, we have pursued a structure-activity relationship study, including vinyl group modifications of QN. Certain derivatives show good antiplasmodial activity in QN-resistant and QN-sensitive strains, with lower IC(50) values relative to QN.

Regioselective base-free intermolecular aminohydroxylations of hindered and functionalized alkenes

Regioselective base-free intermolecular aminohydroxylations of functionalized trisubstituted and 1,1-disubstituted alkenes employing benzoyloxycarbamate 3a and catalytic OsO(4) are described. In all cases, the more substituted alcohol isomer is favored. Sluggish reactions could be promoted by gentle heating, the use of amine ligands, or increased catalyst loadings. A competitive rearrangement was observed with a secondary allylic alcohol substrate. The adducts serve as useful precursors to dehydroamino acids.

The direct catalytic asymmetric aldol reaction

Asymmetric aldol reactions are a powerful method for the construction of carbon-carbon bonds in an enantioselective fashion. Historically this reaction has been performed in a stoichiometric fashion to control the various aspects of chemo-, diastereo-, regio- and enantioselectivity, however, a more atom economical approach would unite high selectivity with the use of only a catalytic amount of a chiral promoter. This critical review documents the development of direct catalytic asymmetric aldol methodologies, including organocatalytic and metal-based strategies. New methods have improved the reactivity, selectivity and substrate scope of the direct aldol reaction and enabled the synthesis of complex molecular targets (357 references).

A diastereo- and enantioselective synthesis of alpha-substituted anti-alpha,beta-diaminophosphonic acid derivatives

Highly diastereo- and enantioselective additions of alpha-nitrophosphonates to imines catalyzed by a chiral Brønsted acid are described.

A diastereo- and enantioselective synthesis of alpha-substituted syn-alpha,beta-diamino acids

Highly diastereo- and enantioselective additions of substituted alpha-nitroesters to imines have been developed. High diastereoselection relies on the finding that the combination of chiral proton catalyst 2b and alpha-nitro aryl esters bearing 2,6-disubstitution combine to raise substrate-controlled diastereoselection to >20:1 in favor of the syn diastereomer. Furthermore, the chiral catalyst provides enantioselection to the 99% level through control of the addition step in which the azomethine pi-faces are differentiated. The bifunctional chiral protic acid catalyst enables these reactions to proceed without separate preactivation of either substrate, leading to a straightforward synthetic protocol for the formation of alpha,beta-diamino phenyl alanine derivatives.

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.