Many fungi, lichens, and bacteria produce xanthones (derivatives of 9H-xanthen-9-one, “xanthone” from the Greek “xanthos”, for “yellow”) as secondary metabolites. Xanthones are typically polysubstituted and occur as either fully aromatized, dihydro-, tetrahydro-, or, more rarely, hexahydro-derivatives. This family of compounds appeals to medicinal chemists because of their pronounced biological activity within a notably broad spectrum of disease states, a result of their interaction with a correspondingly diverse range of target biomolecules. This has led to the description of xanthones as “privileged structures”.(1) Historically, the total synthesis of the natural products has mostly been limited to fully aromatized targets. Syntheses of the more challenging partially saturated xanthones have less frequently been reported, although the development in recent times of novel and reliable methods for the construction of the (polysubstituted) unsaturated xanthone core holds promise for future endeavors. In particular, the fascinating structural and biological properties of xanthone dimers and heterodimers may excite the synthetic or natural product chemist.

Xanthones from fungi, lichens, and bacteria : the natural products and their synthesis

Ritter reaction is an atom economy reaction which produces an amide through the reaction of a nitrile with an alcohol or alkene in the presence of an acid. A number of important advances have been achieved in recent years with respect to substrates, the variety of catalysts, the reaction media and the diversity of products. This paper reviews recent findings and assesses the Ritter reaction.

Recent developments in Ritter reaction

The organocatalytic reduction of CC and CN double bonds with biomimetic reductants, e.g. Hantzsch 1,4-dihydropyridine esters and benzothiazolines, is reviewed. Very high yields and stereoselectivities have been achieved with a variety of catalysts, including chiral amines, thioureas and phosphoric acids, even with loadings equivalent to those of transition metal-catalyzed reactions in some cases. Reductive amination reactions and the dearomatization of heteroaromatic substrates are the subject of more than one half of the contributions. Of lately, methodologies based on kinetic resolution, cascade reactions involving transfer hydrogenation and the development of novel reductants have become prominent in an area which brings great prospects for the future of target oriented-synthesis.

Recent advances in organocatalytic enantioselective transfer hydrogenation

An efficient method for palladium-catalyzed cross-coupling of aryl chlorides and triflates with sodium cyanate is reported. The protocol allows for the synthesis of unsymmetrical N,N′-di- and N,N,N′-trisubstituted ureas in one pot and is tolerant of a wide range of functional groups. Insight into the mechanism of aryl isocyanate formation was gleaned through studies of the transmetalation and reductive elimination steps of the reaction, including the first demonstration of reductive elimination from an arylpalladium isocyanate complex to produce an aryl isocyanate.

Palladium-Catalyzed Cross-Coupling of Aryl Chlorides and Triflates with Sodium Cyanate: A Practical Synthesis of Unsymmetrical Ureas

SBA-15/PrN(CH2PO3H2)2 as a novel and efficient mesoporous solid acid catalyst with phosphorous acid tags and its application on the synthesis of new pyrimido[4,5-b]quinolones and pyrido[2,3-d]pyrimidines via anomeric based oxidation

A cascade Knoevenagel condensation–reduction approach, which was carried out in water, has been reported. Using Hantzsch esters as reducing agent, under the promotion of base, a variety of reduced Knoevenagel adducts could be easily prepared by direct alkyl­ation of malononitrile, ethyl 2-cyanoacetate, and 2-(4-nitrophenyl)acetonitrile, respectively. Meanwhile, a gram-scale synthesis of the protocol was also realized with excellent isolated yield.

Base-Promoted Cascade Approach for the Preparation of Reduced Knoevenagel Adducts Using Hantzsch Esters as Reducing Agent in Water

Novel N-doped carbon dots prepared via citric acid and benzoylurea by green synthesis for high selectivity Fe(III) sensing and imaging in living cells

A series of N-formamides derived from pipecolinic acid have been synthesized and tested as Lewis base catalysts for the enantioselective reduction of N-aryl imines by trichlorosilane. Through the investigation of the structure–efficacy relationship between the side amide group and catalytic performance, several highly effective catalysts were discovered. In particular, arylamido-type catalyst 5i and non-arylamido-type catalyst 6c exhibited high reactivity and enantioselectivity, furnishing the reduction of a wide variety of N-aryl imines with high isolated yields (up to 98%) and ee values (up to 96%) under mild conditions. Moreover, these two catalysts complement each other in terms of their tolerances to nonaromatic ketimines and non-methyl ketimines.

L-Pipecolinic acid derived Lewis base organocatalyst for asymmetric reduction of N-aryl imines by trichlorosilane: effects of the side amide group on catalytic performances.

Pentatomic heterobiaryl performs as a key structural motif in various natural products and bioactive compounds. With the rapid growth of asymmetric catalysis, five‐membered heterobiaryl‐based catalysts and ligands have emerged as an efficient complementary toolbox for asymmetric catalysis. Therefore, the development of enantioselective construction of such pentatomic atropisomers has received significant attention in recent decade. Various catalytic asymmetric strategies have been established, including central to axial chirality conversion, direct generation of heteroaryl, direct assembly of aryl‐heteroaryls, functionalization of racemic or prochiral biaryls, and chirality transfer from atropisomeric alkenes. Hundreds of unprecedented pentatomic atropisomers have proliferated. Importantly, a few promising axially chiral catalysts and ligands have been obtained from the prepared heterobiaryls after simple transformations. Hence, recent advances of catalytic asymmetric construction of axially chiral pentatomic heterobiaryls through asymmetric catalysis are summarized in this review, involving their scope, mechanism, transformations, and applications.

Zhouyu Wang

Papers

Base-Promoted Cascade Approach for the Preparation of Reduced Knoevenagel Adducts Using Hantzsch Esters as Reducing Agent in Water

Novel N-doped carbon dots prepared via citric acid and benzoylurea by green synthesis for high selectivity Fe(III) sensing and imaging in living cells

L-Pipecolinic acid derived Lewis base organocatalyst for asymmetric reduction of N-aryl imines by trichlorosilane: effects of the side amide group on catalytic performances.

Recent Advances in Catalytic Atroposelective Construction of Pentatomic Heterobiaryl Scaffolds

Synthesis of osthol-based botanical fungicides and their antifungal application in crop protection.