Indium-Catalyzed Synthesis of Furans and Pyrroles via Cyclization of α-Propargyl-β-keto Esters

Abstract

Abstract: In(OTf) 3 or In(NTf 2 ) 3 effectively catalyze the cyclo-isomerization reaction of a-propargyl-b-keto esters and their imineanalogues to afford trisubstituted furans and pyrroles, respectively.Both terminal and internal alkynes take part in the reaction withgood functional-group compatibility in the presence of only a smallamount of the catalyst. Key words: indium catalyst, keto ester, cyclization, furan, pyrrole The utility of indium reagents and catalysts has beenwidely proven by organic reactions involving carbonylgroups, 1 such as the Barbier reaction, 2 aldol reaction, 3 andHosomi–Sakurai reaction. 4 Indium can also activate C–Ctriple bonds to form new C–C bonds, because of its soft-ness, as reported by us 5 and others. 6 Formation of carbon–heteroatom bonds via activation of triple bonds has alsobeen reported. For instance, Tan et al. 7a described that 10mol% of InCl 3 promotes one-pot synthesis of furan deriv-atives via tandem reaction that involves intermolecularcondensation of a propargylic alcohol and a 1,3-dicarbon-yl compound, mainly 1,3-diketones, in refluxing chlo-robenzene. Herein we report that In(OTf)