Cyclizations of Benzo-Fused Substrates Involving Two Multiple Bonds, Including Heteroatom-Substituted Unsaturated Bonds

Abstract

Cyclization between two multiple bonds have been attractive processes that allow the discovery of novel reactions and the construction of carbo- and/or heterocycles. Among them, heteroatom-substituted unsaturated bonds show different reactivity from those with a heteroatom displaced on the α- or β-carbon, but surprisingly few reactions using them have been reported until the 2000s. In recent years, benzo-fused substrates cyclization between two multiple bonds, including heteroatom-substituted unsaturated bonds, have been reported, and this review describes them by reaction type.

Intermolecular Allene–Alkyne Coupling: A Significantly Useful Synthetic Transformation

“…Acyclic C-skeletons containing unsaturated C–C bonds significantly influence several scientific and industrial domains as they serve as essential building blocks for the synthesis of physiologically active carbocycles/heterocycles and are directly present in organic materials. − ,,, Accordingly, researchers in the field of organic synthesis have comprehensively investigated the synthetic routes to acyclic C-skeletons from easily accessible starting materials (for instance, alkynes and allenes) . Drawbacks of classical thermally induced AA coupling immediately gained the attention of synthetic organic chemists, leading them to develop activator-based synthetic approaches that are primarily focused on milder reaction conditions.…”

“…The state of the art in chemical synthesis is entirely related to the alteration of structural topologies of the substrates containing π-unsaturated sites. − Consequently, the discovery of breakthrough C-skeletons via the integration of two distinct π-carbogenic fragments in a selective fashion can be realized using revolutionary synthetic techniques, providing access to underexplored chemical space. , The finest demonstration of this connection is allene–alkyne (AA) coupling, which has received considerable research interest since Benson and Lindsey’s study in this field in 1959 . Widespread implementation of this disconnection, which can simplify approaches for the preparation of numerous new classes of structural motifs, can be attributed to the novel activation modes of the cross-coupling components toward metallic and often nonmetallic activators. , Despite this, efforts to improve intermolecular AA coupling as the basis of modern-day organic synthesis are frequently complicated by either one or all of the following issues: (1) challenges arise from the chemoselective activations of π-systems; (2) difficulty associated with regioselective interception of these systems; and (3) stereoselectivity, which is the most often encountered problem. The electronic nature of the substrates, the type of activating elements (catalytic or noncatalytic), and the assisting abilities of the neighboring functionalities present in the substrates demonstrate potentials toward addressing the aforementioned issues. − Nevertheless, most of the recent successful applications of AA coupling in the synthesis of diverse unsaturated C-skeletons, while typically demonstrating the utility and often leading to the discovery of new reactivity, exhibit the strategic significance of chelation-assisted functionalization.…”

Study of the Pauson–Khand reaction in flow over alkynylphenyl vinyl ethers: towards the synthesis of tricyclic multisubstituted benzofurans

“…Ohno and Arisawa recently reviewed the use of these, and other kinds of benzo-fused substrates to form heterocycles. 12 Therefore, extensive attention has been paid to develop versatile methods for the synthesis of these structurally diverse heterocycles. 13 Among tricyclic benzofurans that would be similar to our Pauson–Khand adducts, we highlight silvestrol, 14 a natural product which is a potent inhibitor of Ebola virus replication; beraprost, 15 a PGI analog; and natural sesquiterpenes, isolated from the red alga Laurencia and the sea hare Aplysia specie ( Fig.…”

Iridium-Catalyzed Isomerization/Cycloisomerization/Aromatization of N-Allyl-N-sulfonyl-o-(λ1-silylethynyl)aniline Derivatives to Give Substituted Indole Derivatives

“…Especially, researchers envisioned that transition-metal-catalyzed cyclization of enynes or dienes could be utilized as a useful indole synthesis approach. Indeed, transition metal (Ru, Fe, Pd)-catalyzed cyclization of 1,7-dienes and 1,6-enynes, which are reactions between an enamide and a multiple bond without a heteroatom, have been reported to yield the corresponding indole derivatives, and further chemical transformations of them are rather difficult (Scheme a). , However, transition-metal-catalyzed cyclization between an enamide and a multiple bond with a heteroatom, which gives indole derivatives with a heteroatom substituent, have not been developed except in our report of Ru-catalyzed cyclization of 1,7-enynes involving an aromatic enamide and a silylalkyne (Scheme b) . Although it is an atom-economical and efficient method to yield 2-vinyl-3-(silylmethyl)­indoles, which have chemical transferability, two kinds of Ru catalysts and one more extra camphorsulfonic acid for aromatization are necessary.…”