TETRAHEDRON LETTERS Tetrahedron Letters 42 (2001) 3283–3286 Pergamon An unexpected ring-opening of a 2-pyrone ring at low temperatures. A mild and expeditious synthesis of novel coumarins Ossama Saleh Darwish, Kirsten A. Granum, Quang Tan † and Richard P. Hsung* Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, USA Received 5 February 2001; accepted 12 March 2001 Abstract—When the iminium salts of ortho -hydroxy arylaldehydes were condensed with a 4-hydroxy-2-pyrone at room temperature or well below, a series of unique coumarins were isolated via a translactonization at the expense of an unexpected ring-opening of the 2-pyrone nucleus. © 2001 Elsevier Science Ltd. All rights reserved. Coumarin is an important structural motif present in a variety of natural and unnatural products that possess significant biological activities. 1–3 Given its immense medicinal value, new protocols for the preparation of coumarins have been continuously sought after, rang- ing from transition metal-catalyzed processes 4 to classi- cal lactonizations. 5 We have been studying reactions of ,-unsaturated iminium salts with various 1,3-diketo equivalents. 6–9 These reactions proceed in a tandem process consisting of a Knoevenagel condensation, 10,11 followed by a 6-electron electrocyclic ring-closure, 12 leading to unique approaches for constructing complex heterocycles. These studies led us to also examine the use of iminium salts generated from aromatic aldehydes. 13 As shown in Scheme 1, we envisioned that reactions of 4-hydroxy-2-pyrones, such as 1, with the iminium salt 2 or 3 could lead to Knoevenagel condensation products 4 or 5, and subsequent ring-closure and/or lactol for- mation could lead to intermediates such as 6, 7, or 8. Aromatization and/or nucleophilic additions could lead Scheme 1. * Corresponding author. † 1999 McNair Undergraduate Summer Research Fellow from St. Olaf College. 0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII:S0040-4039(01)00456-7