Phosphine-catalyzed [4 + 2] annulation of γ-benzyl allenoates: facile synthesis of benzothieno[3,2-b]pyran derivatives

Abstract

An efficient tris(4-methoxyphenyl)phosphine-catalyzed domino reaction between γ-benzyl allenoates and ethyl (Z)-2-(3-oxobenzo[b]thiophen-2(3H)-ylidene)acetate has been developed, which produces a series of 2H-benzo[4,5]thieno[3,2-b]pyran derivatives in high yields. The substrate scope includes both electron-withdrawing (e.g., halogen) and electron-donating (e.g., methoxy) groups on both the benzothiophene and allenoate moieties. The reaction can also be performed on the gram scale with good yield (e.g., 77%). In this reaction, γ-substituted allenoate acts as a two-carbon synthon, in a manner rarely reported in the literature to date.

Construction of [6-5-5-6-6] Pentacyclic Skeleton via a Phosphine-Catalyzed Domino Reaction and Mechanism Study

“…A combination of privileged frameworks is a powerful way to improve molecular diversity and thus discover novel biological properties . Therefore, efficient construction of polyheterocyclic skeletons from readily available starting materials in a minimum number of steps is a highly desired but challenging theme in organic synthesis. − One of the ways to fulfill this goal is the development and use of domino processes, which allow the sequential construction of several bonds by a single-step manipulation. − During our ongoing investigation of domino reactions of thioaurones, − we are interested in the effect on the reaction mode of replacing a substrate oxygen atom with a sulfur atom. Due to the different atomic properties, the sulfur atom may play a different role in controlling the reactivity/selectivity as compared to the oxygen atom.…”

Synthesis of Benzothiophene-Fused Pyran Derivatives via Piperidine Promoted Domino Reaction

“…Many benzothiophene-fused heterocycles were synthesized (Scheme 2(a)) [18][19][20][21][22]. Herein, we will report another new domino reaction between thioaurone and malononitrile.…”

Substrate-Controlled Domino Reactions of Crotonate-Derived Sulfur Ylides: Synthesis of Benzothiophene Derivatives

“…As seen here, the crotonate-derived sulfur ylides usually act as one- or three-carbon synthons. − From a mechanistic perspective, it is challenging for crotonate-derived sulfur ylides to act as a two-carbon synthon (Figure III and IV ), − as dimethylsulfane cannot be released in such a reaction. During our ongoing investigation of domino reactions, − we have successfully developed the first [4 + 3] annulation reaction of a crotonate-derived sulfur ylide . We think that the sulfur atom in thioaurones may play a critical role, which works to stabilize the adjacent carbon anion by its vacant 3d orbital .…”