Gold(I)-catalyzed synthesis of γ-vinylbutyrolactones by intramolecular oxaallylic alkylation with alcohols

Gold-Catalyzed Lactone Synthesis: Advancements and Insights

Lactones represent a class of fundamental structural motifs ubiquitous in nature, holding significance across diverse scientific domains such as pharmaceuticals, natural products, drug discovery, and industry. Despite their simplicity, the synthesis of lactones has garnered considerable interest due to their pivotal roles. Gold, traditionally regarded as a noble metal, has emerged as an efficient catalyst, challenging conventional perceptions. The utilization of gold in lactone synthesis has captivated researchers, leading to the development of numerous effective methodologies. Motivated by this, we present a comprehensive compilation of reports on the gold-catalyzed synthesis of lactones, encompassing literature till date.

Highly Efficient and Mild Gold (I) Catalyzed Synthesis of 3,8-Diarylidene-2,7-dioxaspiro[4.4]nonane-1,6-diones

The gold-catalyzed cyclization of 2,2-bis(3-arylprop-2-yn1-yl)malonic acid has been proposed as an efficient approach to substituted 3,8-dibenzyl-2,7-dioxaspiro[4.4]nonane-1,6-diones. The reaction proceeds smoothly in mild reaction conditions to give the desired products in quantitative yields in the presence of variously substituted starting materials. In addition, the synthesis of γ-arylidene spirobislactone bearing different substituents on the two aromatic rings has been achieved. This kind of compound could be of great interest in pharmaceutical science given the widespread presence of this scaffold in bioactive natural and synthetic products.

A Review of the Pharmacological Activities and Recent Synthetic Advances of γ-Butyrolactones

γ-Butyrolactone, a five-membered lactone moiety, is one of the privileged structures of diverse natural products and biologically active small molecules. Because of their broad spectrum of biological and pharmacological activities, synthetic methods for γ-butyrolactones have received significant attention from synthetic and medicinal chemists for decades. Recently, new developments and improvements in traditional methods have been reported by considering synthetic efficiency, feasibility, and green chemistry. In this review, the pharmacological activities of natural and synthetic γ-butyrolactones are described, including their structures and bioassay methods. Mainly, we summarize recent advances, occurring during the past decade, in the construction of γ-butyrolactone classified based on the bond formation in γ-butyrolactone between (i) C5-O1 bond, (ii) C4-C5 and C2-O1 bonds, (iii) C3-C4 and C2-O1 bonds, (iv) C3-C4 and C5-O1 bonds, (v) C2-C3 and C2-O1 bonds, (vi) C3-C4 bond, and (vii) C2-O1 bond. In addition, the application to the total synthesis of natural products bearing γ-butyrolactone scaffolds is described.

Catalytic Dehydrative Lactonization of Allylic Alcohols

A convenient strategy for the synthesis of phthalides and γ-butyrolactones is reported. The method utilizes readily prepared allylic alcohols in formal Au(I)- and Pd(II)-catalyzed S_N2' reactions. Using these catalysts, exclusive formation of the desired five-membered lactones is observed, completely avoiding the competing direct lactonization pathway that forms the undesired seven-membered ring with protic acids and alternative metal salts. This mild and operationally simple method notably tolerates exomethylene groups and should find use in both phthalide and terpene syntheses.

Chirality Transfer in Gold(I)-Catalysed Direct Allylic Etherifications of Unactivated Alcohols: Experimental and Computational Study

Gold(I)-catalysed direct allylic etherifications have been successfully carried out with chirality transfer to yield enantioenriched, γ-substituted secondary allylic ethers. Our investigations include a full substrate-scope screen to ascertain substituent effects on the regioselectivity, stereoselectivity and efficiency of chirality transfer, as well as control experiments to elucidate the mechanistic subtleties of the chirality-transfer process. Crucially, addition of molecular sieves was found to be necessary to ensure efficient and general chirality transfer. Computational studies suggest that the efficiency of chirality transfer is linked to the aggregation of the alcohol nucleophile around the reactive π-bound Au-allylic ether complex. With a single alcohol nucleophile, a high degree of chirality transfer is predicted. However, if three alcohols are present, alternative proton transfer chain mechanisms that erode the efficiency of chirality transfer become competitive.

Dehydrative thiolation of allenols: indium vs gold catalysis

Intermolecular additions of thiols to allenols via formal S(N)2' selectivity to produce functionalized dienes are described. Although this dehydrative reaction was initially developed using gold(I) catalysis, indium(III) proves to be a far superior catalyst in terms of selectivity and substrate scope.

Gold(I)-catalysed direct thioetherifications using allylic alcohols: an experimental and computational study

A gold(I)-catalysed direct thioetherification reaction between allylic alcohols and thiols is presented. The reaction is generally highly regioselective (S(N)2'). This dehydrative allylation procedure is very mild and atom economical, producing only water as the by-product and avoiding any unnecessary waste/steps associated with installing a leaving or activating group on the substrate. Computational studies are presented to gain insight into the mechanism of the reaction. Calculations indicate that the regioselectivity is under equilibrium control and is ultimately dictated by the thermodynamic stability of the products.

Gold(I) and Palladium(II) Complexes of 1,3,4-Trisubstituted 1,2,3-Triazol-5-ylidene "Click" Carbenes: Systematic Study of the Electronic and Steric Influence on Catalytic Activity

The synthesis of a small family of six electronically and sterically modified 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene gold(I) chloride complexes is described. Additionally, the corresponding trans-[PdBr2(iPr2-bimy)(1,3,4-trisubstituted 1,2,3-triazol-5-ylidene)] complexes are also generated and used to examine the donor strength of the 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene ligands. All compounds have been characterized by 1H and 13C NMR and IR spectroscopy, high-resolution electrospray mass spectrometry (HR-ESI-MS), and elemental analysis. The molecular structures of four of the gold(I) and four of the palladium(II) complexes were determined using X-ray crystallography. Finally, it is demonstrated that these 1,2,3-triazol-5-ylidene gold(I) chloride complexes (Au(trz)Cl) are able to catalyze the cycloisomerization of 1,6-enynes, in high yield and regioselectivity, as well as the intermolecular direct etherification of allylic alcohols. Exploiting the Au(trz)Cl precatalysts allowed the etherification of allylic alcohols to be carried out under milder conditions, with better yield and regioselectivity than selected commercially available gold(I) catalysts.

The regio- and stereospecific intermolecular dehydrative alkoxylation of allylic alcohols catalyzed by a gold(I) N-heterocyclic carbene complex

A 1:1 mixture of [AuCl(IPr)] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) and AgClO(4) catalyzes the intermolecular dehydrative alkoxylation of primary and secondary allylic alcohols with aliphatic primary and secondary alcohols to form allylic ethers. These transformations are regio- and stereospecific with preferential addition of the alcohol nucleophile at the γ-position of the allylic alcohol syn to the departing hydroxyl group and with predominant formation of the E stereoisomer. The minor α regioisomer is formed predominantly through a secondary reaction manifold involving regioselective γ-alkoxylation of the initially formed allylic ether rather than by the direct α-alkoxylation of the allylic alcohol.

Mechanistic insights into enantioselective gold-catalyzed allylation of indoles with alcohols: the counterion effect

Enantioselective gold-catalysis is emerging as a powerful tool in organic synthesis for the stereoselective manipulation of unfunctionalized unsaturated hydrocarbons. Despite the exponential growth, the molecular complexity of common chiral gold complexes generally prevents a complete description of the mechanism steps and activation modes being documented. In this study, we present the results of a combined experimental-computational (DFT) investigation of the mechanism of the enantioselective gold-catalyzed allylic alkylation of indoles with alcohols. A stepwise S(N)2'-process (i.e. anti-auroindolination of the olefin, proton-transfer, and subsequent anti-elimination [Au]-OH) is disclosed, leading to a library of tricyclic-fused indole derivatives. The pivotal role played by the gold counterion, in terms of molecular arrangement (i.e. "folding effect") and proton-shuttling in restoring the catalytic species, is finally documented.