Azepine synthesis from alkyl azide and propargylic ester via gold catalysis

An efficient new method was developed to synthesize multisubstituted 4,5-dihydro-1H-azepine derivatives through the gold-catalyzed reaction of two molecules of propargylic esters with one molecule of alkyl azide. It was proposed that vinyl gold carbenoid, in situ generated from propargylic ester through gold-catalyzed 1,2-rearrangement, was trapped by alkyl azide to give vinyl imine intermediate. These, in turn, could undergo a formal [4 + 3] cycloaddition with another molecule of vinyl gold carbenoid to afford the desired azepine product.     

Room temperature Au(I)-catalyzed exo-selective cycloisomerization of acetylenic acids: an entry to functionalized gamma-lactones

A highly efficient gold-catalyzed cyclization reaction of various functionalized acetylenic acids is described. The cyclizations are conducted in the presence of Au(I) catalyst in acetonitrile at room temperature in a short reaction time. The reaction conditions are compatible with several functional groups, such as ester, alkene, alkyne, chloro, and free or protected alcohol, and lead to original gamma-lactones in good to excellent yields.     

Gold-catalyzed cyclizations of alkynol-based compounds: synthesis of natural products and derivatives

The last decade has witnessed dramatic growth in the number of reactions catalyzed by gold complexes because of their powerful soft Lewis acid nature. In particular, the gold-catalyzed activation of propargylic compounds has progressively emerged in recent years. Some of these gold-catalyzed reactions in alkynes have been optimized and show significant utility in organic synthesis. Thus, apart from significant methodology work, in the meantime gold-catalyzed cyclizations in alkynol derivatives have become an efficient tool in total synthesis. However, there is a lack of specific review articles covering the joined importance of both gold salts and alkynol-based compounds for the synthesis of natural products and derivatives. The aim of this Review is to survey the chemistry of alkynol derivatives under gold-catalyzed cyclization conditions and its utility in total synthesis, concentrating on the advances that have been made in the last decade, and in particular in the last quinquennium.     

Ligand effects and ligand design in homogeneous gold(I) catalysis

Gold catalysis is considered one of the most important breakthroughs in organic synthesis during the past decade, but a rational understanding of ligand effects in gold catalysis is lacking. Most gold-catalyzed reactions go through three major stages: (i) electronic activation of alkyne (or allene) to generate a vinyl gold intermediate; (ii) protodeauration to generate the product and regenerate the cationic gold catalyst; (iii) decay of the active gold catalyst. Our research provides a clearer understanding of how ligands influence each of the three stages in the gold catalytic cycle. What is even more important, by not focusing on a particular gold-catalyzed reaction, we have been able to categorize most gold-catalyzed reactions and propose a ligand design protocol for each category of gold-catalyzed reactions.     

Gold-catalyzed intramolecular hydroamination of α-amino allenamides as a route to enantiopure 2-vinylimidazolidinones

An efficient gold-catalyzed procedure for the preparation of 2-vinylimidazolidinones has been developed. The starting materials for the synthesis of these compounds are α-amino allenamides which undergo heterocyclization by means of nucleophilic attack of the amino group on the inside double bond of the 1,2-diene moiety. This is the first example of a gold-catalyzed cyclization on allene substrates bearing an amido group which, however, resulted inactive.     

Recent applications of gold-catalyzed cascade reactions in total synthesis of natural product

From the point of the view of the synthetic efficiency, the concept ‘step economy’ was required in total synthesis of natural product. Recently, versatile gold-catalyzed cascade reactions have been developed, and relating reports on practical applications in total synthesis has increased. This digest focuses and summarizes the gold-catalyzed reaction cascades in natural product synthesis during last five years with brief discussion on the reaction mechanism of the key gold-catalyzed cascade transformations.     

Gold-catalyzed reactions of 2-alkynyl-phenylamines with alpha,beta-enones

[reaction: see text] The gold-catalyzed reaction of 2-alkynyl-phenylamines with alpha,beta-enones represents a new general one-pot entry into C-3-alkyl-indoles by sequential reactions. Gold-catalyzed sequential cyclization/alkylation, N-alkylation/cyclization, or N-alkylation/cyclization/alkylation reactions leading to different indoles can be directed by changing the 2-alkynyl-phenylamine 1/alpha,beta-enone 3 ratio and the reaction temperature. Unusual gold-catalyzed rearrangement reaction of indoles are observed at 140 degrees C. New gold-catalyzed formation of propargyl-alkyl ether under mild conditions and the hydration reaction of N-acetyl-2-ethynyl-phenylamine are reported.     

Ligand-controlled gold catalyzed highly site-selective carbene transfer in C-H bond formation

A ligand-controlled gold-catalyzed highly site-selective insertion of a carbene into carbonhydrogen bonds has been achieved.This new gold-catalyzed direct C-H bond functionalization opens up new exciting opportunities for the functionalization of C-H bond.     

Heterogeneous gold-catalyzed oxidative cross-coupling of propargylic acetates with arylboronic acids leading to (E)-α-arylenones

An efficient heterogeneous gold-catalyzed oxidative cross-coupling of propargylic acetates with arylboronic acids has been developed that proceeds smoothly in the presence of Selectfluor and provides a general and powerful tool for the preparation of various valuable α-arylenones with moderate to good yields, excellent E-selectivity, and recyclability of the gold catalyst. The reaction is the first example of heterogeneous gold-catalyzed arylative rearrangement of propargylic acetates for construction of complex enones.     

Gold-Catalyzed Facile Synthesis and Crystal Structures of Benzene-/Naphthalene-Based Bispentalenes as Organic Semiconductors

The gold-catalyzed facile synthesis of U-shaped and S-shaped bispentalenes is described from easily available tetra(arylethynyl)-benzenes and -naphthalenes. The optoelectronic and transistor properties were also investigated. The selectivity between the U-shaped and S-shaped bispentalene isomers can be tuned by the bulkiness of the ligand and the substrates. The S-shaped naphthalene-based bispentalene shows a one-dimensional face-to-face packing pattern in solid state and a good hole mobility, indicating that the S-shaped bispentalene core is highly suitable for transistor applications. The gold-catalyzed annulation of tetraynes provides a useful protocol in the synthesis of bispentalenes for organic semiconductors.     

Unlocking the Chain-Walking Process in Gold Catalysis**

The successful realization of gold-catalyzed chain-walking reactions, facilitated by ligand-enabled Au(I)/Au(III) redox catalysis, has been reported for the first time. This breakthrough has led to the development of gold-catalyzed annulation reaction of alkenes with iodoarenes by leveraging the interplay of chain-walking and π-activation reactivity mode. The reaction mechanism has been elucidated through comprehensive experimental and computational studies.     

Gold(I)-catalyzed regioselective inter-/intramolecular addition cascade of di- and triynes for direct construction of substituted naphthalenes

The gold-catalyzed cascade intermolecular addition-intramolecular carbocyclization reaction of dialkynylbenzenes was developed. In this reaction, regioselective addition of an external nucleophile toward the terminal alkyne and subsequent 6-endo-dig cyclization proceeded to give the 1,3-disubstituted naphthalenes in good yields. The direct synthesis of disubstituted chrysenes via a gold-catalyzed addition and double cyclization cascade using a triyne-type substrate was also achieved.     

A concise route to indoloazocines via a sequential Ugi-gold-catalyzed intramolecular hydroarylation

A diversity oriented approach for the synthesis of indoloazocines is reported employing an Ugi reaction followed by a gold-catalyzed intramolecular hydroarylation.     

Sequential catalytic process: synthesis of quinoline derivatives by AuCl3/CuBr-catalyzed three-component reaction of aldehydes, amines, and alkynes

A sequential catalytic process has been developed based on gold-catalyzed nucleophilic addition of terminal alkynes to imines, and gold-catalyzed intramolecular reaction of aromatic ring to alkynes. This one-pot reaction of aldehydes, amines, and alkynes gives quinoline derivatives in good yields.     

Stereospecific access to bridged [n.2.1] skeletons through gold-catalyzed tandem reaction of indolyl homopropargyl amides

An efficient gold-catalyzed anti-Markovnikov cycloisomerization-initiated tandem reaction of Boc-protected indole tethered homopropargyl amides has been achieved. This method delivers a wide range of valuable bridged aza-[n.2.1] skeletons (n = 3–7) at room temperature with high diastereoselectivity and enantioselectivity by a chirality-transfer strategy. Moreover, the gold-catalyzed tandem reaction of homopropargyl alcohol is also achieved to produce the bridged oxa-[3.2.1] skeleton.     

Gold-catalyzed cascade cyclizations of 1,6-diynyl carbonates to benzo[b]fluorenes involving arylation of oxocarbenium ion intermediates and decarboxylative etherification

Rearranged: The described gold-catalyzed cycloisomerizations give access to highly substituted benzo[b]fluorenes under mild reaction conditions (see scheme). Experimental results indicate that the in?situ formed oxocarbenium ion intermediates, derived from gold-catalyzed 3,3-rearrangement and 6-endo-dig cyclization, undergo intramolecular arylation and subsequent decarboxylative etherification to furnish the final ether products.     

Gold(I)-Catalyzed Bis-Cyclization of Allenynes for the Synthesis of Strained and Planar Polycyclic Compounds

A gold-catalyzed cascade cyclization of naphthalene-tethered allenynes gave strained fused phenanthrene derivatives. The reaction proceeds through the nucleophilic reaction of an alkyne with the activated allene to generate a vinyl cation intermediate, followed by arylation with a tethered naphthalene ring to form the 4H-cyclopenta[def]phenanthrene (CPP) scaffold. When using aryl-substituted substrates on the alkyne terminus, the gold-catalyzed reaction produced dibenzofluorene derivatives along with the CPP derivatives. Selective formation of CPP and dibenzofluorene derivatives depending on the reaction conditions is also presented.     

Gold-catalyzed simultaneous formation of C-C, C═O, and C-F bonds in the presence of Selectfluor: a synthesis of fluoroindenes from allene esters

An approach for the synthesis of fluorinated indene derivatives has been developed via a gold-catalyzed three-component tandem reaction between allene esters, Selectfluor, and water.     

Enantiospecific synthesis of pyridinones as versatile intermediates toward asymmetric piperidines

The enantiospecific syntheses of pyridinones from amino acids via a gold-catalyzed strategy are reported. Excellent stereocontrol was observed during the cyclization. This approach provides a straightforward tool for further synthetic applications toward piperidines.     

Au-catalyzed tandem cyclization/[1,2]-alkyl migration reaction of epoxy alkynes: synthesis of spiropyranones

A novel gold-catalyzed tandem cyclization/[1,2]-alkyl migration process of epoxy alkynes to spiropyranones has been discovered. From this process, the construction of adjacent multiple stereocenters with a new quaternary carbon atom is achieved. The gold-catalyzed domino process is stereospecific with respect to the migrating carbon atom. A type of unusual C-C bond cleavage of epoxide systems has also been discovered, which can lead to the formation of two Z alkenes and a carbonyl functional group in one step with excellent stereoselectivity. Furthermore, this efficient domino process could be achieved in the presence of the simplest and least expensive gold catalyst [NaAuCl(4)].2H(2)O with a low catalyst loading.