Catalytic asymmetric alkylation of α-cyanocarboxylates and acetoacetates using a phase-transfer catalyst

The catalytic asymmetric alkylation of α-cyanocarboxylates and acetoacetates with an alkyl halide was performed under phase-transfer conditions to afford compounds which have a chiral quaternary carbon with up to 97% and 94% ee, respectively. As applications of this method, chiral 2-oxindole derivatives and a β-lactam derivative were synthesized.     

Catalytic asymmetric phase-transfer reactions using tartrate-derived asymmetric two-center organocatalysts

A new highly versatile asymmetric two-center catalyst, tartrate-derived diammonium salt (TaDiAS), was designed and a catalyst library containing more than 70 new two-center catalysts was constructed. A variety of (S,S)- and (R,R)-TaDiAS were easily synthesized from diethyl l- and d-tartrate, respectively, using common and inexpensive reagents under operationally simple reaction conditions. TaDiAS was used in phase-transfer alkylations and Michael additions to afford various optically active α-amino acid equivalents in up to 93% yield. Moreover, dramatic counter anion effects were observed in phase-transfer catalysis (PTC) for the first time, making it possible to further improve reactivity and selectivity. These findings validate the usefulness of three-dimensional fine-tuning of the catalyst (acetal, Ar, and counter anion) for optimization. Recovery and reuse of the catalyst was also possible using simple procedures. The present asymmetric PTC was successfully applied to enantioselective syntheses of serine protease inhibitor aeruginosin 298-A and its analogues.     

Pd-catalyzed asymmetric allylic alkylation of glycine imino ester using a chiral phase-transfer catalyst

Pd-catalyzed asymmetric allylic alkylation of the glycine imino ester 1a has been developed using a chiral quaternary ammonium salt 3d without chiral phosphine ligands. The proper choice of the achiral Pd ligand, P(OPh)3, is important to achieve high enantioselectivity. By this method with the dual catalysts, numerous enantiomerically enriched alpha-allylic amino acids 4a-h could be prepared with comparable to higher enantioselectivity than that of the conventional asymmetric alkylation of 1a. In addition, the Pd-catalyzed reaction of 1a with 1-phenyl-2-propenyl acetate 2i afforded the branch product 6 with high enantio- and diastereoselectivity (>95% de, 85% ee).     

Catalytic asymmetric synthesis of spirooxindoles by a mannich-type reaction of isothiocyanato oxindoles

Easy access: a strontium/Schiff base complex as catalyst for the title reaction provided straightforward access to enantiomerically enriched spiro[imidazolidine-4,3'-oxindole] compounds, as well as a spiro[imidazoline-4,3'-oxindole] through a two-step conversion from the Mannich adduct.     

Catalytic asymmetric sulfenylation of unprotected 3-substituted oxindoles

Catalytic asymmetric sulfenylation of unprotected 3-substituted oxindoles has been developed via cooperative catalysis of a chiral N,N'-dioxide-Sc(OTf)(3) complex and a Br?nsted base. Utilizing readily available N-(phenylthio)phthalimide as the sulfur source, a wide range of optically active 3-phenylthiooxindoles were obtained in excellent yields with excellent enantioselectivities under mild reaction conditions.     

Catalytic asymmetric syntheses of isoxazoline-N-oxides under phase-transfer conditions

Catalytic asymmetric syntheses of various isoxazoline-N-oxides have been accomplished by asymmetric phase-transfer conjugate addition of bromomalonate to nitroolefins and subsequent ring-closing O-alkylation.     

Chiral iminophosphoranes organocatalyzed asymmetric hydroxylation of 3-substituted oxindoles with oxaziridines

Enantioselective hydroxylation of N-protected 3-substituted oxindoles has been developed via chiral iminophosphorane catalysis with oxaziridines as oxidants. As such, a variety of optically active 3-substituted-3-hydroxy-2-oxindoles were obtained in excellent yields (91–99%) and moderate to excellent level of enantiomeric excess (up to 94% ee).     

Catalytic asymmetric synthesis of axially chiral o-iodoanilides by phase-transfer catalyzed alkylations

Catalytic asymmetric synthesis of axially chiral o-iodoacrylanilides and N-allyl-o-iodoanilides as useful chiral building blocks was achieved via chiral quaternary ammonium salt-catalyzed N-alkylations under phase-transfer conditions. The transition-state structure for the present reaction is discussed on the basis of the X-ray crystal structure of ammonium anilide.     

Identification of a highly effective asymmetric phase-transfer catalyst derived from α-methylnaphthylamine

A library of quaternary ammonium salts has been generated via reaction of simple chiral amines with a series of conformationally dynamic biphenyl units. Screening of this library against the alkylation of a glycine imine has led to the identification of a highly effective asymmetric phase-transfer catalyst derived from α-methylnaphthylamine.     

Catalytic asymmetric aza Diels-Alder reactions of hydrazones using a chiral zirconium catalyst

Catalytic asymmetric aza Diels-Alder reactions of acylhydrazones with Danishefsky’s dienes have been developed. A chiral zirconium complex derived from zirconium propoxide and 3,3′,6,6′-I₄BINOL was found to be effective in this reaction, and the desired optically active 2,3-dihydro-4-pyridone derivatives were obtained with high enantioselectivities. Asymmetric formal synthesis of a natural product, coniine, was conducted using this catalytic asymmetric reaction as a key step.     

Combining chiral elements: a novel approach to asymmetric phase-transfer catalyst design

A new dicationic asymmetric phase-transfer catalyst, designed by combining chiral elements, is described. Catalytic testing using standard glycine imino ester alkylations shows good yields and moderate enantioselectivities.     

One-step polymerization of polytrithiocarbonate using phase-transfer catalyst

A series of aliphatic and aromatic polytrithiocarbonates was prepared using a novel “one-pot” synthesis procedure employing a phase-transfer catalyst. The starting reagents were either an aliphatic or an aromatic dihalide and an excessive amount of carbon disulfide. The effects of the phase-transfer catalyst and reaction conditions on yield were studied. The structure and composition of the polymers and reaction side-products were determined from infrared, ultraviolet, ¹H-NMR spectra, and elemental analyses. The polymers were further characterized by viscosity measurement and thermal analysis. © 1993 John Wiley & Sons, Inc.     

Chiral phosphine-free Pd-mediated asymmetric allylation of prochiral enolate with a chiral phase-transfer catalyst

[reaction: see text]. A chiral phase-transfer catalyst has been applied to the asymmetric allylation of the tert-butyl glycinate-benzophenone Schiff base with various allylic acetates for the first time to give the allylated products in good yields and with comparable to higher enantioselectivity than for asymmetric alkylation at the same temperature (91-96% ee) without any chiral ligands for coordinating to the palladium.     

Catalytic enantioselective Meerwein-Eschenmoser Claisen rearrangement: asymmetric synthesis of allyl oxindoles

The first catalytic, enantioselective Meerwein-Eschenmoser Claisen rearrangement has been achieved. Palladium(II) BINAP or phosphinooxazoline catalysts were employed to generate oxindole products with 100% conversion and up to 92% ee.     

Catalytic asymmetric synthesis of a nitrogen analogue of dialkyl tartrate by direct mannich reaction under phase-transfer conditions

[reaction: see text] Phase-transfer-catalyzed direct Mannich reaction of glycinate Schiff base 3 with alpha-imino ester 4 has been accomplished with high enantioselectivity by the utilization of N-spiro C(2)-symmetric chiral quaternary ammonium bromide 2 as a catalyst. This methodology enables the catalytic asymmetric synthesis of differentially protected 3-aminoaspartate, a nitrogen analogue of dialkyl tartrate. The product (syn-5) was converted into a precursor (6) of streptolidine lactam.     

Catalytic asymmetric synthesis of 3,3-disubstituted oxindoles: diazooxindole joins the field

The catalytic asymmetric synthesis of 3,3-disubstituted oxindoles, a big family of privileged scaffolds in natural products and drugs, is of current interest. Recently, the catalytic asymmetric functionalization of diazooxindoles emerges as a potentially general and flexible strategy for this purpose, with several notable examples coming out in 2013. In this digest, synthetic applications of diazooxindoles have been summarized and discussed, which might be helpful for readers to understand the special properties of this type of donor/acceptor cyclic diazo reagent and to develop new catalytic asymmetric reactions.     

Triethylamine-modified Keggin heteropolyacid: a novel phase-transfer catalyst for hydroxylation of benzene with H2O2

The Keggin-structured heteropolyacid H₄PMo₁₁VO₄₀ and its triethylamine-modified derivative [(C₂H₅)₃NH]₄PMo₁₁VO₄₀ were prepared and characterized by FT-IR and UV–visible spectrometry, and elemental analysis. Direct hydroxylation of benzene to phenol by H₂O₂ over these two catalysts was then compared. The effect of the triethylamine in [(C₂H₅)₃NH]₄PMo₁₁VO₄₀ on the catalytic hydroxylation of benzene was investigated in detail. The results showed that although the triethylamine-modified catalyst is not significantly more active than the parent heteropolyacid catalyst, its reaction-controlled phase-transfer characteristics enable easy separation of the catalyst from the reaction medium, and its reuse, suggesting its potential for application to the hydroxylation of benzene to phenol with H₂O₂.     

Catalytic oxidation of fluorobenzene by molecular oxygen

Kinetic and thermal-desorption methods have been applied to the oxidation of fluorobenzene by molecular oxygen. The catalytic oxidation mechanism for fluorobenzene is similar to that for benzene. A suggestion is made on why there is no fluoromaleic anhydride in the oxidation products from fluorobenzene.Translated from, Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 1, pp. 116–119, January–February, 1989.     

Catalytic asymmetric epoxidation of enones using La-BINOL-triphenylarsine oxide complex: structural determination of the asymmetric catalyst.

The catalytic asymmetric epoxidation of enones using the La-BINOL-Ph(3)As=O complex generated from La(O-i-Pr)(3), BINOL, and Ph(3)As=O in a ratio of 1:1:1 is described herein. Using 1-5 mol % of the asymmetric catalyst, a variety of enones, including a dienone and a cis-enone, were found to be epoxidized in a reasonable reaction time, providing the corresponding epoxy ketones in up to 99% yield and with more than 99% ee. The possible structure of the actual asymmetric catalyst has been clarified by various methods, including X-ray crystal structure analysis. This is the first X-ray analysis of an alkali-metal free lanthanoid-BINOL complex. Although La(binaphthoxide)(2)(Ph(3)As=O)(2) (7) was observed as the major complex in the complexes' solution, generated from La(O-i-Pr)(3), BINOL, and Ph(3)As=O in a ratio of 1:1:1, the possible active species turned out to be the La-BINOL-Ph(3)As=O complex in a ratio of 1:1:1. A probable reaction mechanism of the catalytic asymmetric epoxidation of enones is also proposed, suggesting that preferential formation of a heterochiral complex is the reason for asymmetric amplification. Moreover, the interesting role of La(O-i-Pr)(3) for accelerating the epoxidations while maintaining high ee's is discussed.