AK-2243GR_published_mainmanuscript

AK-2243GR_published_mainmanuscript - Issue in Honor of...

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Issue in Honor of Prof. Alain Krief ARKIVOC 2007 (x) 121-141 Metal-mediated synthesis of furans and pyrroles Nitin T. Patil and Yoshinori Yamamoto* Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan E-mail: [email protected] Dedicated to Professor Alan Krief on the Occasion of his 65 th Birthday Abstract The transition metal catalyzed synthesis of furans and pyrroles is becoming an important and highly rewarding protocol in organic synthesis. This review focuses on the use of transition metal as a catalyst for the synthesis of furans and pyrroles along with their interesting mechanistic aspects. The synthetic efforts in this area will clearly culminate in tackling the major issues pertaining to atom economy and environmentally acceptable technologies for future. Keywords: Metal catalysis, furans, pyrroles, cascade cyclization Introduction Highly substituted furans 1 and pyrroles 2 are a structural component of a vast number of biologically active natural and unnatural compounds. Synthesis of them has been the objective of research for over a century, and a variety of well-established classical methods are now available in the literature. 3 Development of newer approaches for heterocycle syntheses employing efficient and atom economical routes is popular research area nowadays. Among a variety of new synthetic transformations, transition metal catalyzed reactions are the most attractive methodologies, since those reactions can directly construct multiply substituted molecules from readily accessible starting materials under mild conditions. 4 The formation of furans and pyrroles by using various transition metals has been investigated in the literature. Since the synthesis of these heterocycles is thoroughly reviewed up to 1999 and 2001, respectively, this review will discuss recent advances for the synthesis of furans and pyrroles focusing on the use of transition metals. 5 It should be noted that the reactions in which a heterocyclic ring is essentially generated are only described. Other reactions in which a heterocyclic ring already exists in the molecule and a transition metal catalyzes the further structural manipulations are not described here. ISSN 1424-6376 Page 121 © ARKAT-USA, Inc.
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Issue in Honor of Prof. Alain Krief ARKIVOC 2007 (x) 121-141 It is reported that cyclization of allenyl ketone 1 provided furan 2 in the presence of catalytic amounts of AuCl 3 . The reaction was extended also to one pot selective cross cycloisomerization/dimerization of 3 with α , β -unsaturated ketones 4 to give C-2 substituted furans 5 (Scheme 1). 6 The mechanism of the reaction is shown in Scheme 2. The authors believed the intermediacy of 7 and proposed two possibilities for the formation of it. Either the AuCl 3 activates the enones 4 to form 8 , which then create the new C-C bonds by an electrophilic aromatic substitution at the 5-position of the furan to provide 7 (path A), or a cyclization of 3 under gold catalysis forms a furyl–gold species 6
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This note was uploaded on 02/17/2011 for the course CHEMISTRY 101 taught by Professor Csr during the Spring '11 term at University of Louisville.

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AK-2243GR_published_mainmanuscript - Issue in Honor of...

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