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DOI: 10.1055/s-0029-1219126
Ruthenium-Catalyzed Heteroannulation Route to Benzofurans and Isochromenes
A. Varela-Fernández, C. González-Rodrígues, L. A. Varela, L. Castedo, C. Saá*
Universidad de Santiago de Compostela, Spain
Publication History
Publication Date:
21 January 2010 (online)
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Significance
Reported is a ruthenium-catalyzed cycloisomerization of aromatic alkynols 1, 3, 5 and 7 to afford isochromenes 2 and benzofurans 6. The combination of CpRuCl(PPh3)2 and amine (crucial for the catalytic cycle) promotes the chemo- and regioselective cycloisomerization (6- and 5-endo cyclization). Both electron-rich and electron-poor substrates 1 and 5 give good yields of the corresponding isochromenes and benzofurans. However, while smooth 6-endo cyclization occurred with secondary and tertiary benzylic alcohol 1, 2-ethynylbenzoic acid 3 underwent only 5-exo cyclization to afford benzofuran 4. In an intelligent experiment with compound 7, the 5-endo regioselectivity of the cyclization to exclusively benzofuran 8 was demonstrated.