Electrophile-Induced Reactions of Cross-Conjugated Enynones in the Synthesis of Heterocycles
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Electrophile-induced reactions of cross-conjugated enynones in the synthesis of heterocycles Alena S. Pankova1* 1
Institute of Chemistry, Saint Petersburg State University, 26 Universitetsky Ave., Saint Petersburg 198504, Russia; е-mail: [email protected] Submitted April 28, 2020 Accepted May 25, 2020
Published in Khimiya Geterotsiklicheskikh Soedinenii, 2020, 56(7), 829–836
Being compounds with multiple electrophilic centers, cross-conjugated enynones can be involved in various transformations upon treatment with electrophiles, in particular, Lewis and Brønsted acids, providing a convenient access to acyclic and cyclic compounds. This review covers literature data on electrophile-assisted reactions of substrates with cross-conjugated enynone scaffold toward the synthesis of heterocyclic compounds. The information is arranged according to the type of enynone motif, and chemical transformations of 2-(alkyn-1-yl)alk-2-en-1-ones and alk-1-en-4-yn-3-ones are discussed. The bibliography includes 52 sources. Keywords: enynone, furan, Brønsted acid, Lewis acid, cyclization, electrophilic activation, metal catalysis.
Conjugated alkenynones have found different applications in organic synthesis. Being primarily compounds with multiple electrophilic centers, enynones are readily involved in reactions with broad range of mono- and binucleophiles smoothly providing various acyclic compounds as well as carbo- and heterocycles.1,2 As extended π-systems, enynones are extremely reactive toward metal catalysts. This feature opens a plethora of potential modifications of these substrates, e.g., cyclization reactions,3 generation of metal carbenoids, and carbene transfer reactions.4 The big appeal of using enynones is the possibility to rapidly increase molecular complexity that has found applications also in medicinal chemistry.5 Herein we aim to present transformations of enynones induced by electrophiles (Lewis and Brønsted acids), since, to the best of our knowledge, this aspect of the enynone chemistry has not been specifically covered earlier. However, this issue is of great relevance to heterocyclic synthesis. Considering the progress demonstrated in this field over the recent years, the review covers original articles, which have been published mainly since 2010. Among four types of conjugated enynones, we have focused our attention on cross-conjugated ones, which can be represented by two types of structures: 2-(alkyn-1-yl)alk0009-3122/20/56(7)-0829©2020 Springer Science+Business Media, LLC
R3
O R1
O R
R2 A
1
R3
R2 B
Figure 1. Structures of cross-conjugated enynones A and B.
2-en-1-ones A and alk-1-en-4-yn-3-ones B (Fig. 1). Only genuine carbonyl compounds, namely aldehydes and ketones (not esters, amides, etc.), without inclusion of the double bond into a benzene ring are described. 2-(Alkyn-1-yl)alk-2-en-1-ones In 2004, Larock and coworkers presented AuCl3catalyzed synthesis of highly substituted furans from 2-(alkyn-1-yl)alk-2-en-1-ones in the presence of various nucleophiles.6 Several other π-acidic metal salt
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