Issue in Honor of Prof. Binne Zwanenburg ARKIVOC 2004 (ii) 109-121 ISSN 1424-6376 Page 109 © ARKAT USA, Inc Synthesis of 2,4-disubstituted furans and 4,6-diaryl-substituted 2,3- benzo-1,3a,6a-triazapentalenes Alan R. Katritzky,* a Deniz Hür, a Kostyantyn Kirichenko, a Yu Ji, a and Peter J. Steel b a Center for Heterocyclic Compounds, University of Florida, Department of Chemistry, Gainesville, Florida 32611-7200, USA E-mail: Katritzky@chem.ufl.edu b Department of Chemistry, University of Canterbury, Christchurch, New Zealand E-mail: p.steel@chem.canterbury.ac.nz Dedicated to our long-time friend Binne Zwanenburg (received 15 Oct 03; accepted 17 Dec 03; published on the web 28 Dec 03) Abstract Reactions of acylacetylenes 1a–h with benzotriazole 2 give intermediates 3a–h. The treatment of 3a–h with trimethylsulfonium iodide in the presence of base give intermediate oxiranes 4a–h and 2,3-benzo-1,3a,6a-triazapentalenes 7d–g depending on substituent. Acid-catalyzed rearrangement of crude 4a–h give 2,4-disubstituted furans 5a–h. Keywords: 2,4-Disubstituted furans, heteropentalenes, 2,3-benzo-1,3a,6a-triazapentalenes, oxiranes Introduction The furan ring is an important structural unit in many biologically active and naturally occurring compounds. 1 Furan syntheses and their applications have been reviewed in detail. 1d,2 The available strategies for the preparation of 2,4-disubstituted furans (Scheme 1) utilize ring construction via: (i) acid-catalyzed rearrangement of oxiranyl ketones, 1c,3 and (ii) their enol (or thioenol) ethers; 1a,1e–g,4 (iii) thermal annulation of 3,4-dihaloketones; 5 (iv) radical cyclization – reductive demethoxylation of bromoketals; 6 palladium-catalyzed cyclization of (v) 2-methoxy- or 2-unsaturated pent-4-yn-1-ol, 7 and (vi) allenyl aldehydes; 8 (vii) base-catalyzed cyclization of γ-alkynyl ketones, 9 (viii) carbon insertion into cyclic enol – metal complexes; 10 and (ix) an oxidative ring transformation of pyrylium salts. 11