Ring-closing metathesis as a key step to construct the 2, 6-dihydropyrano[2, 3-c]pyrazole ring system

Abstract

A simple and efficient synthetic route to the 2, 6-dihydropyrano[2, 3-c]pyrazole ring system was developed by employing ring-closing metathesis (RCM) as a key step. The required diene substrate for the RCM reaction was prepared by a three-step procedure starting form 1-phenyl-1H-pyrazol-3-ol. Treatment of the obtained 4-ethenyl-1-phenyl-3-[(prop-2-en-1-yl)oxy]-1H-pyrazole with Grubbs' first-generation catalyst afforded the target 2-phenyl-2, 6-dihydropyrano[2, 3-c]pyrazole. 2-(4-Fluorophenyl)-and 2-(4-bromophenyl)-2, 6-dihydropyrano[2, 3-c]pyrazole were synthesized by an analogous way. The structures of the obtained heterocyclic products were unequivocally confirmed by detailed 1H, 13C, 15N and 19F NMR spectroscopic experiments and HRMS measurements. The optical properties of 2-phenyl-2, 6-dihydropyrano[2, 3-c]pyrazole were studied by UV-Vis and fluorescence spectroscopy.