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Synthesis 2007(11): 1733-1737  
DOI: 10.1055/s-2007-965965
PSP
© Georg Thieme Verlag Stuttgart · New York

Preparation of Cyclopenta-Fused N-, O-, and S-Heterocycles by Lewis Acid Catalyzed Nazarov Reaction

Laura Bartali, Paolo Larini, Antonio Guarna, Ernesto G. Occhiato*
Department of Organic Chemistry ‘U. Schiff’ and Laboratory of Design, Synthesis and Study of Biologically Active Heterocycles (HeteroBioLab), Università di Firenze, Via della Lastruccia 13, 50019 Sesto Fiorentino, Italy
Fax: +39(055)4573531; e-Mail: ernesto.occhiato@unifi.it;
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Publication History

Received 7 February 2007
Publication Date:
28 February 2007 (online)

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Abstract

The products of carbonylative coupling between lactam-, lactone- and thiolactone-derived vinyl triflates and alkenylboronic acids are suitable substrates for the Lewis acid catalyzed Nazarov reaction. The most efficient Lewis acids for the Nazarov reaction are scandium(III) and indium(III) triflates (3-15 mol%) in 1,2-dichloroethane, which provide the Nazarov products in moderate to excellent yield (53-86%). The electrocyclization rate depends also on the heteroatom (N, O, S) and the N-protecting group. On the whole, the entire procedure is an expeditious synthesis of hexahydro[1]pyrindenes, -cyclopenta[b]pyrans, and -cyclopenta[b]thiopyrans of noteworthy interest as they form the structural core of several natural molecules.

Key words

carbonylative couplings - electrocyclic reactions - Nazarov reaction - Lewis acids - bicyclic compounds

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Enol triflates from thiolactones can be prepared as reported in ref. 5 for N-methoxycarbonyl-protected δ-valerolactam 1a.