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

A simple and efficient method was developed for the synthesis of 2,​6-​dihydropyrano[2,​3-​c]​pyrazoles I [R = H, F, Br] via ring-​closing metathesis (RCM) of 4-​ethenyl-​1-​aryl-​3-​[(prop-​2-​en-​1-​yl)​oxy]​-​1H-​pyrazoles II using Grubbs' first generation catalyst. The required diene substrates II for the RCM reaction were prepd. by a three-​step procedure starting from 1-​aryl-​1H-​pyrazol-​3-​ols. The structures of the obtained heterocyclic products were unequivocally confirmed by detailed 1H, 13C, 15N and 19F NMR spectroscopic expts. and HRMS measurements. The optical properties of compd. I [R = H] were studied by UV-​Vis and fluorescence spectroscopy.