Asymmetric Michael addition and deracemization of enolate by chiral crown ether

Development of chiral potassium strong Brønsted base catalysts for enantioselective carbon-carbon bond-forming reactions

Chiral alkaline metal Brønsted bases are traditional and reliable promoters in enantioselective catalysis. Here, new chiral potassium strong base catalysts were developed for enantioselective carbon-carbon bond-forming reactions of weakly acidic carbon pronucleophiles. Chiral potassium amide or alkyl potassium catalyzed enantioselective addition reactions to imines or α,β-unsaturated amides with good to high enantioselectivities. The good potential of chiral potassium Brønsted bases to act as proton transfer catalysts has been shown.

Synthesis and Applications of Carbohydrate-Based Organocatalysts

Organocatalysis is a very useful tool for the asymmetric synthesis of biologically or pharmacologically active compounds because it avoids the use of noxious metals, which are difficult to eliminate from the target products. Moreover, in many cases, the organocatalysed reactions can be performed in benign solvents and do not require anhydrous conditions. It is well-known that most of the above-mentioned reactions are promoted by a simple aminoacid, l-proline, or, to a lesser extent, by the more complex cinchona alkaloids. However, during the past three decades, other enantiopure natural compounds, the carbohydrates, have been employed as organocatalysts. In the present exhaustive review, the detailed preparation of all the sugar-based organocatalysts as well as their catalytic properties are described.

The Synthesis of Hydrobenzoin-Based Monoaza Crown Ethers and Their Application as Recyclable Enantioselective Catalysts

Abstract

New recyclable monoaza-15-crown ethers have been synthesized starting from (R,R)-(+)- and (S,S)-(−)-hydrobenzoin. These macrocycles proved to be efficient and reusable phase transfer catalysts in a few asymmetric reactions under mild conditions. The asymmetric epoxidation of trans-chalcone took place with up to 81% ee, while using other chalcone derivatives, the products were formed with 68–88% ee. The hydrobenzoin-based lariat ethers were also tested in the cyclopropanation of a few electron deficient olefins using diethyl bromomalonate to afford the product with good enantioselectivities (54–75% ee). The catalysts were recovered by salt formation, followed by extraction, and were reused without the loss of the activity and effect on the enantioselectivity.

Graphic Abstract

The synthesis of hydrobenzoin-based monaza crown ethers and their application as recyclable enantioselective catalysts.

Enantioselective intramolecular cyclization of alkynyl esters catalyzed by a chiral Brønsted base

An enantioselective intramolecular cyclization reaction of alkynyl esters was developed, which employs a Brønsted base catalyst generated in situ from a chiral Schiff base and t-BuOK. This reaction is a rare example of the enantioselective intramolecular addition of simple ester enolates to alkynes under Brønsted base catalysis.

Enantioselective Alkylation of Amino Acid Derivatives Promoted by Cyclic Peptoids under Phase-Transfer Conditions

The effects of substituents and cavity size on catalytic efficiency of proline-rich cyclopeptoids under phase-transfer conditions were studied. High affinity constants (Ka) for the sodium and potassium cations, comparable to those reported for crown ethers, were observed for an alternated N-benzylglycine/L-proline hexameric cyclopeptoid. This compound was found to catalyze the alkylation of N-(diphenylmethylene)glycine cumyl ester in values of enantioselectivities comparable with those reported for the Cinchona alkaloid ammonium salts derivatives (83-96% ee), and with lower catalyst loading (1-2.5% mol), in the presence of a broad range of benzyl, allyl and alkyl halides.

Nitrogen-containing macrocycles having a carbohydrate scaffold

Nitrogen-containing macrocyclic compounds (amines, amides, and N-heterocyclic derivatives) are important targets in supramolecular chemistry. This chapter discusses the importance of aza-macrocycles in general and, in particular, those receptors containing sugar unit(s). The combination of a carbohydrate scaffold bearing nitrogen-containing functional groups in macrocyclic molecules opens a convenient route to chiral receptors having potentially useful properties. The carbohydrate-based macrocycles discussed are classified into several general groups: (1) aza-crown ethers containing a carbohydrate subunit, (2) cyclic homooligomers from amino sugars, (3) sugar-based cryptands, (4) cyclic peptides containing amino sugar units (including C2- and C3-symmetrical macrocyclic glycopeptides), (5) nitrogen- containing glycophanes, and (6) 1,2,3-triazoles containing synthetic cyclodextrin analogues. The general strategies employed, as well as specific ones leading to such complex derivatives, are surveyed. Applications of such carbohydrate receptors, pointing to their importance as hosts in supramolecular chemistry, are discussed.

Recent advances in asymmetric phase-transfer catalysis

The use of chiral nonracemic onium salts and crown ethers as effective phase-transfer catalysts have been studied intensively primarily for enantioselective carbon-carbon or carbon-heteroatom bond-forming reactions under mild biphasic conditions. An essential issue for optimal asymmetric catalysis is the rational design of catalysts for targeted reaction, which allows generation of a well-defined chiral ion pair that reacts with electrophiles in a highly efficient and stereoselective manner. This concept, together with the synthetic versatility of phase-transfer catalysis, provides a reliable and general strategy for the practical asymmetric synthesis of highly valuable organic compounds.

Helical Chirality in Donor-Acceptor Catenanes

A [2]catenane in which the macrocyclic polyether, bisparaphenylene[34]crown-10, is interlocked with the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), is shown by dynamic (1)H NMR spectroscopy, using (i). neutral and (ii). anionic chiral shift reagents (CSRs), to exist at low temperatures (197 K) in acetone-d(6) solutions as 1:1 and 2:1 mixtures of diastereoisomeric complexes and salts, respectively, as a consequence of the helical chirality associated with the [2]catenane interacting with the CSRs.

Design of N-spiro C2-symmetric chiral quaternary ammonium bromides as novel chiral phase-transfer catalysts: synthesis and application to practical asymmetric synthesis of alpha-amino acids

A series of C(2)-symmetric chiral quaternary ammonium bromides 10 and 11 have been designed as a new, purely synthetic chiral phase-transfer catalyst, and readily prepared from commercially available optically pure 1,1'-bi-2-naphthol as a basic chiral unit. The details of the synthetic procedures of each requisite chiral binaphthyl subunit have been disclosed, and the structures of the assembled N-spiro chiral quaternary ammonium bromides 11a and 11f were unequivocally determined by single-crystal X-ray diffraction analysis. The reactivity and selectivity of these chiral ammonium bromides as chiral phase-transfer catalysts have been evaluated in the asymmetric alkylation of the benzophenone Schiff base of glycine ester 7 under mild liquid-liquid phase-transfer conditions, and the optimization of the reaction variables (solvent, base, and temperature) has also been conducted. Further, the scope and limitations of this asymmetric alkylation have been thoroughly investigated with a variety of alkyl halides, in which the advantage of the unique N-spiro structure of 11 and dramatic effect of the steric as well as the electronic properties of the aromatic substituents on the 3,3'-position of one binaphthyl moiety have been particularly emphasized. Finally, the potential synthetic utility of the present method for the practical asymmetric synthesis of structurally diverse natural and unnatural alpha-amino acids has been demonstrated by its successful application to the facile asymmetric syntheses of (S)-N-acetylindoline-2-carboxylate, a key intermediate in the synthesis of the ACE inhibitor, and l-Dopa (l-3,4-dihydroxyphenylalanine) ester and its analogue.