Gold-Catalyzed Migration of Propargyl Acetate as an Entry into the Total Synthesis of Natural Products

Gold-catalyzed polyene cyclization and its application in the total synthesis of natural products

Polyene cyclization has emerged as a powerful tool for increasing molecular complexity. In recent years, gold-catalyzed polyene cyclization reactions have made significant progress, particularly in the total synthesis of complex polycyclic natural products with biological activity. Simultaneously constructing multiple stereogenic centers and core polycyclic ring structures, especially through the catalytic asymmetric reactions, greatly improves the efficiency of natural product synthesis. The alkynyl functional group can initiate polyene cyclization reactions, and the resulting vinyl moiety can facilitate subsequent functional group transformations, making it highly versatile. This review mainly summarizes the synthetic methodology of polyene cyclization reactions based on the underlying mechanisms and their applications in the total synthesis of polycyclic natural products.

Recent advances in site-selective transformations of β-enaminones via transition-metal-catalyzed C-H functionalization/annulation

β-Enaminone transformation strategies are widely employed in the synthesis of numerous biologically active drugs and natural products, highlighting their significance in medicinal chemistry. In recent years, various strategies have been developed for synthesizing several five- and six-membered heterocycles, as well as substituted polyaromatic scaffolds, which serve as crucial synthons in drug development, from β-enaminones. Among these approaches, site-selective transformations of β-enaminones via C-H activation and annulation have been particularly well explored. This review summarizes the most recent literature (over the past eight years) on β-enaminone transformations for developing bioactive scaffolds through site-selective C-H bond functionalization and annulation.

Gold(I)-Catalyzed Regioselective Synthesis of Indenylidene Derivatives via 1,5-Acryl Migration

A novel gold (I)-catalyzed synthetic strategy has been achieved for an efficient construction of indenylidene derivatives from substituted 1,6-diynes. This reaction describes the unique reactivity of gold catalysis in facilitating the intramolecular [3,3]-sigmatropic rearrangement, 5-exo dig cyclization followed by 1,5-migration of acryl group, resulting in the formation of substituted indenylidenes. Various substituted indenylidenes were successfully synthesized with up to 92 % yields. In this protocol, two new C-C bonds were sequentially formed atom economically in one pot.

β-Cyclodextrin-NHC-Au(I)-Catalyzed Enantioconvergent 1,5-Enyne Cycloisomerizations

Enantioconvergent transformations from racemic mixtures are attractive since they allow the generation of optically active products with full conversion despite the possibly adverse kinetic resolution process. When dealing with gold(I)-catalyzed cycloisomerizations, chirality transfer from the precursor is another possible diverting pathway, which renders enantioconvergence challenging. Not surprisingly, enantioconvergent Au(I)-catalyzed processes have remained extremely rare. Herein we show that cavity-driven catalysis using β-cyclodextrin-NHC-Au(I) complexes brings opportunities to conduct highly enantioconvergent cycloisomerizations of 1,5-enynes, -enynols, and, -enynyl esters.

Gold-catalyzed endo-selective cyclization of alkynylcyclobutanecarboxamides: synthesis of cyclobutane-fused dihydropyridones

Cyclobutane-fused dihydropyridones can be efficiently synthesized by a completely endo-selective gold-catalyzed cyclization of alkynylcyclobutanes bearing an appended amide, which proceeds under mild conditions. The observed selectivity, which is reversed from that previously observed for the cyclization of related alcohols and acids, is supported by DFT calculations.

Gold-Catalyzed Addition of Propargyl Acetates to Olefins via O-Acyl Migration/Cyclopropanation Sequence: Insight into the Diastereoselective Formation of the Alkene

This article discloses a study on the well-known addition of propargyl acetates to olefins via an O-acyl migration/cyclopropanation sequence. Herein, we show that the stereochemical outcome of the olefin is strongly dependent on the gold catalyst and reaction parameters (concentration, temperature, and alkene partner equivalents); the E- and Z-isomers can be selectively formed by the judicious choice of reaction conditions.

Palladium-catalyzed alkynylative [5 + 1] carboannulation of 1,3-diarylprop-2-yn-1-yl acetates with terminal alkynes enabled by C–H functionalization

Using 1,3-diarylprop-2-yn-1-yl acetates as the five-carbon components enables alkynylative[5 + 1] carboannulation involving C–H functionalization toward 3-ethynyl-1-methylene-1,2-dihydronaphthalenes.

Gold-Catalyzed Cycloisomerization of Propargyl Pyruvates Enabling Unified Access to Tricladolides C and D, Chaetomellic Anhydride A, and Tyromycin A

Gold-catalyzed cycloisomerization of propargyl pyruvates has been developed as a key reaction to prepare maleic anhydride-type natural products. By combining with chemoselective epoxidation of the formed γ-alkylidenebutenolides and oxidative cleavage of epoxides, the first synthesis of tricladolide D and racemic tricladolide C has been achieved in 52 and 16% overall yields with five to seven steps starting from commercially available compounds. Further catalytic hydrogenation of alkenylated maleic anhydrides derived from γ-alkylidenebutenolides produced chaetomellic anhydride A (19% yield for six steps) and tyromycin A (15% yield for six steps), which provides flexible synthetic approaches to these naturally occurring dialkylated maleic anhydrides distinct from the documented ones.

Stereoselective Gold(I)-Catalyzed Vinylcyclopropanation via Generation of a Sulfur-Substituted Vinyl Carbene Equivalent

A stereoselective gold(I)-catalyzed vinylcyclopropanation of alkenes has been developed. A gold-coordinated cationic vinyl carbene species, readily generated via a rearrangement of the ethylenedithioacetal of propargyl aldehyde, reacts with a wide range of alkenes to afford thio-substituted vinylcyclopropanes. The gold-catalyzed vinyl cyclopropanation proceeds under mild conditions at room temperature and is generally selective for the formation of cis-substituted cyclopropanes. The reaction allows the formal introduction of a "naked" vinyl carbene, by subsequent chemoselective hydrodesulfurisation of the ethylenedithio-bridge. The synthetic utility of the new method is demonstrated by a short, racemic formal synthesis of the alkaloid cephalotaxin, hinging on a key vinyl cyclopropane-cyclopentene rearrangement.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

α-Imino Gold Carbene Intermediates from Readily Accessible Sulfilimines: Intermolecular Access to Structural Diversity

Catalytic approaches to pharmaceutically important bioactive skeletons through gold carbene intermediates have experienced a dramatic development in the last decade. Although various carbene precursors continue to play an important role in heterocyclic syntheses, these reagents are associated with some drawbacks in terms of functional group tolerance, synthetic methods and safety limitations. A new generation of nitrene transfer reagents was established in 2019: the sulfilimines. These are safe, inexpensive and readily available. They can conveniently be stored and handled, and thus represent ideal reagents for the fast and modular modification of scaffolds and the preparation of libraries by intermolecular reactions of two components. Both the practical methods for synthesizing sulfilimines and the versatility of these ylidic species in gold-catalyzed preparation of structural diversity, for both heterocycles and carbocycles, will be outlined in this Concept article.