Recent Advances of the Combination of Au/Acid Catalysis

Biomimetic approach to the catalytic enantioselective synthesis of tetracyclic isochroman

Polyketide oligomers containing the structure of tetracyclic isochroman comprise a large class of natural products with diverse activity. However, a general and stereoselective method towards the rapid construction of this structure remains challenging due to the inherent instability and complex stereochemistry of polyketide. By mimicking the biosynthetic pathway of this structurally diverse set of natural products, we herein develop an asymmetric hetero-Diels–Alder reaction of in-situ generated isochromene and ortho-quinonemethide. A broad range of tetracyclic isochroman frameworks are prepared in good yields and excellent stereoinduction (up to 95% ee) from readily available α-propargyl benzyl alcohols and 2-(hydroxylmethyl) phenols under mild conditions. This direct enantioselective cascade reaction is achieved by a Au(I)/chiral Sc(III) bimetallic catalytic system. Experimental studies indicate that the key hetero-Diels-Alder reaction involves a stepwise pathway, and the steric hindrance between in-situ generated isochromene and t-Bu group of Sc(III)/N,N’-dioxide complex is responsible for the enantioselectivity in the hetero-Diels–Alder reaction step.

General and stereoselective synthesis of tetracyclic isochroman-containing polyketide oligomers is challenging. Here, the authors report on an Au(I)/chiral Sc(III) bimetallic catalyst for a biomimetic asymmetric hetero-Diels–Alder reaction for in-situ generation of isochromene and ortho-quinonemethide.

An Approach for the Generation of γ-Propenylidene-γ-butenolides and Application to the Total Synthesis of Rubrolides

Design and synthesis of a new class of γ-butenolides, viz. β-aryl-γ-propenylidene-γ-butenolides, have been reported from β-aryl-Z-enoate propargylic alcohols in the presence of acid. Isolation of β-aryl-γ-propenylidene-γ-butenolides and their O¹⁸-isomer confirmed the intermediacy of the allenyl-lactonium ion as well as the cyclic-hemiacetal during the proposed mechanism. By utilizing the β-aryl-γ-methylenecyclohexenylidene-γ-butenolides as starting materials, a highly stereoselective and efficient approach has been developed for the syntheses of frameworks of rubrolide natural products. This strategy was further extended for the total synthesis of rubrolide E.

Synergistic Effects of Bimetallic AuPd and La2O3 in the Catalytic Reduction of NO with CO

Bimetallic AuPd nanoparticles supported on TiO2 are known to catalyze the reduction of NO with CO. Here, we investigated the effects of the addition of lanthanum oxide to a AuPd/TiO2 catalyst with a AuPd particle size of 2.1–2.2 nm. The addition of La2O3 enhanced the catalytic activity; for example, at 250 °C, there was 40.9% NO conversion and 49.3% N2-selectivity for AuPd/TiO2, and 100% NO conversion and 100% N2-selectivity for AuPd-La (1:1)/TiO2. The temperature requiring 100% NO conversion dropped from 400 °C to 200 °C by the simple post-impregnation of La2O3 onto AuPd/TiO2. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) combined with modulation excitation spectroscopy (MES) demonstrated that CO adsorption occurs first on Au atoms and then, within 80 s, moves onto Pd atoms. This transformation between two adsorption sites was facilitated by the addition of La2O3.

Gold-Catalyzed Intramolecular Dearomatization Reactions of Indoles for the Synthesis of Spiroindolenines and Spiroindolines

A gold-catalyzed dearomatization reaction of indole derivatives was realized in the presence of JohnPhosAuCl/AgOMs to afford a series of spiroindolenines in excellent yields (≤99%). In addition, when the Hantzsch ester was used as the hydrogen transfer reagent, various spiroindolines were obtained in a cascade fashion starting from readily available indole derivatives in modest to good yields (≤79%). Both reactions feature readily available substrates, mild conditions, and good functional group tolerance.

Gold-catalyzed domino reactions of alkynol and p-quinone methides: divergent synthesis of fused- and spiro-ketals

A gold-catalyzed domino reaction of alkynyl alcohols and p-quinone methides to obtain divergent fused- and spiro-ketals has been developed.

Progress in the Chemistry of Tetrahydroquinolines

Tetrahydroquinoline is one of the most important simple nitrogen heterocycles, being widespread in nature and present in a broad variety of pharmacologically active compounds. This Review summarizes the progress achieved in the chemistry of tetrahydroquinolines, with emphasis on their synthesis, during the period from mid-2010 to early 2018.

The roles of Lewis acidic additives in organotransition metal catalysis

We present recent advances in prominent organotransition metal-catalysed reactions in which Lewis acid cocatalysts are employed to increase catalyst activity or selectivity. The roles of Lewis acids are discussed.

Recent Advances in the Cycloisomerizations of Methylenecyclopropanes using Gold Catalysis

During the past decades, cycloisomerizations of methylenecyclopropanes using gold catalysis have attracted much attention from organic chemists. The different patterns of gold(I)-catalyzed cycloisomerizations of methylenecyclopropanes have been developed, and they can be classified as ring-opening and ring-expansion reaction patterns. This synthetic methodology provides a new approach to novel cyclic- or polycyclic compounds. The corresponding products have great potential for material science, medicinal chemistry and total synthesis. This Concept article will mainly focus on the recent advances in the cycloisomerizations of methylenecyclopropanes in the past four years.

Synthesis of benzannulated spiroketals with gold-catalyzed cycloisomerization/spiroketalization cascade

An unprecedented gold-catalyzed cascade reaction of alkynyl alcohol and quinone monoimines to diverse 5,5-benzannulated spiroketals has been successfully developed.

Breaking aziridines to construct morpholines with a gold(i)-catalyzed tandem ring-opening and cycloisomerization reaction

A convenient synthetic method for the construction of morpholine derivatives from easily available aziridines and propargyl alcohols has been successfully developed. A tandem nucleophilic ring-opening of aziridine/6-exo-dig cyclization/double bond isomerization sequence was achieved by using a single gold(i) catalyst under mild conditions. The gold(i) catalyst served as a π acid and also a σ acid to realize the dual activation of both reactants in this reaction. The obtained unsaturated morpholine products could be easily hydrogenated to achieve target morpholine derivatives with good diastereoselectivities in high yields.

Construction of spirocarbocycles via gold-catalyzed intramolecular dearomatization of naphthols

A highly efficient, gold-catalyzed intramolecular dearomatization reaction of naphthols via 5-endo-dig cyclization is described. This facile and direct approach furnishes spirocarbocycles in excellent yields under mild conditions.

Zn/Sc bimetallic relay catalysis: one pot cycloisomerization/carbonyl–ene reaction toward oxazole derivatives

A novel tandem metal relay catalytic system combining Zn(ii)-catalyzed cycloisomerization and a Sc(iii)-catalyzed carbonyl–ene reaction has been successfully developed.

An efficient synthesis of gem-diiodoolefins and (E)-iodoalkenes from propargylic amides with a Cu(i)/Cu(iii) cycle

A copper-catalyzed cyclization/iodination reaction from propargylic amides to access gem-diiodoolefins and (E)-iodoalkenes has been developed.