Asymmetric Alkylation of Malonic Diester Under Phase-Transfer Conditions

Constructing spirooxindoles from non-oxindole precursors: a one-pot nitro-reduction/double lactamization approach to spiro[indoline-3,3'-quinoline]-2,2'-diones

2-(2-Nitrobenzyl)-2-(2-nitrophenyl)malonates, readily prepared by SNAr-arylation of diethyl malonate with o-fluoronitrobenzenes, followed by SN2-alkylatioin of the resulting products with o-nitrobenzyl bromides, undergo a tandem Fe/AcOH-promoted nitro-reduction and double lactamization sequence to afford spiro[indoline-3,3'-quinoline]-2,2'-diones in high overall yields. The method is operationally simple, economical, and has a broad substrate scope. The synthetic practicality of this methodology was illustrated by performing the reaction on a gram scale with the same efficiency.

Synthesis of chiral malonates by α-alkylation of 2,2-diphenylethyl tert-butyl malonates via enantioselective phase-transfer catalysis

An efficient synthetic approach for chiral malonates was established via enantioselective phase transfer catalysis. The α-alkylation of 2,2-diphenylethyl tert-butyl α-methylmalonates with (S,S)-3,4,5-trifluorophenyl-NAS bromide as a phase-transfer catalyst under phase-transfer catalytic conditions successfully produced corresponding α-methyl-α-alkylmalonates; these compounds are versatile chiral building blocks containing a quaternary carbon center in high chemical yields (up to 99%) with excellent enantioselectivities (up to 98% ee). α,α-Dialkylmalonates were selectively hydrolyzed to the corresponding chiral malonic monoacids under basic (KOH/MeOH) and acidic conditions (TFA/CH₂Cl₂), showing the practicality of the method.

Asymmetric Total Synthesis of Shizukaol J, Trichloranoid C and Trishizukaol A

The asymmetric total synthesis of three lindenane sesquiterpenoid oligomers, shizukaol J, trichloranoid C and trishizukaol A, has been accomplished concisely in 15, 16 and 18 longest linear steps, respectively. The expeditious construction of molecular architectures was facilitated by Nelson's catalytic asymmetric ketene-aldehyde cycloaddition, a sequence of allylic alkylation/reduction/acidic cyclization to forge a lactone, and a double aldol condensation cascade to construct the 5/6 bicyclic system. Diastereoselective nucleophilic substitution promoted by a phase transfer catalyst constructed the C₁₁ quaternary stereogenic center, thus prompting synthetic efficacy toward shizukaol J. The synthesis of trichloranoid C and trishizukaol A was achieved after a cascade involving furanyl diene formation and a Diels-Alder reaction, as well as a one-pot sequence involving furan oxidation and global deprotection. Furthermore, our biological evaluation revealed that two compounds exhibited unexpected toxicity against tumor cell lines.

Synthesis of Novel Crown Ether-Squaramides and Their Application as Phase-Transfer Catalysts

This work presents the synthesis of six new phase-transfer organocatalysts in which the squaramide unit is directly linked to the nitrogen atom of an aza-crown ether. Four chiral skeletons, namely hydroquinine, quinine, cinchonine (cinchonas), and α-d-glucopyranoside were responsible for the asymmetric construction of an all-carbon quaternary stereogenic center in α-alkylation and Michael addition reactions of malonic esters. We investigated the effects of these different chiral units and that of crown ethers with different sizes on catalytic activity and enantioselectivity. During extensive parameter investigations, both conventional and emerging green solvents were screened, providing valuable α,α-disubstituted malonic ester derivatives with excellent yields (up to 98%).

Enantioselective Synthesis of Acyclic Orthogonally Functionalized Compounds Bearing a Quaternary Stereocenter Using Chiral Ammonium Salt Catalysis

We herein report an asymmetric protocol to access a series of orthogonally functionalized acyclic chiral target molecules containing a quaternary stereogenic center by carrying out the enantioselective α-alkylation of novel orthogonally functionalized dioxolane-containing cyanoacetates under chiral ammonium salt catalysis. By using just 1 mol % of Maruoka's spirocyclic ammonium salt catalysts enantioselectivities up to e.r.=97.5 : 2.5 could be achieved and further functional group manipulations of the products were carried out as well.

Construction of Quaternary Carbon Center by Catalytic Asymmetric Alkylation of 3-Arylpiperidin-2-ones Under Phase-Transfer Conditions

A highly enantioselective synthesis of δ-lactams having a chiral quaternary carbon center at the α-position has been developed through an asymmetric alkylation of 3-arylpiperidin-2-ones under phase-transfer conditions. In this transformation, a 2,2-diarylvinyl group on the δ-lactam nitrogen atom plays a crucial role as a novel protecting group and an achiral auxiliary for improving both yield and enantioselectivity of the reaction.

Origin of and a Solution for Uneven Efficiency by Cinchona Alkaloid-Derived, Pseudoenantiomeric Catalysts for Asymmetric Reactions

Cinchona alkaloid-derived chiral catalysts represent one of the most widely applied classes of organocatalysts, which have been successfully utilized in the promotion of a wide variety of asymmetric reactions. Cinchona alkaloids exist in nature as pseudoenantiomers, which allow cinchona alkaloid-catalyzed reactions to provide high enantioselectivities and yields toward both enantiomers of interest in many reactions. On the other hand, the subtle structural difference between pseudoenantiomeric cinchona alkaloids could also lead to uneven efficiency that severely limits the applicability of some cinchona alkaloid-catalyzed reactions. We describe here the elucidation of the origin of and the consequent development of novel modified cinchona alkaloids to address such a problem in asymmetric imine umpolung reactions by cinchonium salts.

Enantioselective Intermolecular Pd-Catalyzed Hydroalkylation of Acyclic 1,3-Dienes with Activated Pronucleophiles

We report a highly enantioselective Pd-PHOX-catalyzed intermolecular hydroalkylation of acyclic 1,3-dienes. Meldrum's acid derivatives and other activated C-pronucleophiles, such as β-diketones and malononitriles, react with a variety of aryl- and alkyl-substituted dienes in ≤20 h at room temperature. The coupled products, obtained in up to 96% yield and 97.5:2.5 er, are easily transformed into useful chemical building blocks for downstream synthesis.

Highly enantioselective synthesis of α-tertiary chiral amino acid derivatives through rhodium-catalyzed asymmetric arylation of cyclic N-sulfonyl α-ketimino esters

A simple sulfur-olefin ligand promoted Rh-catalyzed highly enantioselective arylation of cyclic α-ketimino esters with arylboronic acids is described.

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.