Zinc-Catalyzed Cycloisomerizations. Synthesis of Substituted Furans and Furopyrimidine Nucleosides
datasetposted on 01.08.2008 by Adam Sniady, Audrey Durham, Marco S. Morreale, Andrzej Marcinek, Slawomir Szafert, Tadeusz Lis, Krystyna R. Brzezinska, Takanori Iwasaki, Takashi Ohshima, Kazushi Mashima, Roman Dembinski
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5-Endo-dig cycloisomerization of 1,4- and 1,2,4- mostly aryl-substituted but-3-yn-1-ones in the presence of a catalytic amount of zinc chloride etherate (10 mol %) in dichloromethane at room temperature gave 2,5-di- and 2,3,5-trisubstituted furans in high yields (85−97%). DSC studies confirmed that a solely thermal process does not take place. A relevant catalytic process, employing μ-oxo-tetranuclear zinc cluster Zn4(OCOCF3)6O, yielded bicyclic furopyrimidine nucleosides, when starting from acetyl-protected 5-alkynyl-2′-deoxyuridines (85−86%). Furopyrimidine was deprotected or simultaneously converted into pyrrolopyrimidine nucleoside. The time/concentration dependence for the reaction of 1-phenyl-4-(4-methylphenyl)butynone to 2-(4-methylphenyl)-5-phenylfuran displayed first-order kinetics with the rate dependent on catalyst concentration. The plot of ln kobs versus ln[ZnCl2] indicated first-order cycloisomerization, as referred to ZnCl2 concentration, using both NMR and UV−vis reaction monitoring. The crystal structure of propyl furopyrimidine nucleoside (orthorhombic, P212121, a/b/c = 5.684(2)/6.682(2)/36.02(2) Å, Z = 4) shows C2′-endo deoxyribose puckering, and the base is found in the anti position in crystalline form.