Synthesis and chemistry of N-arylated pyrano[2,3-c]pyrazoles

Novel N2-arylated pyrano[2,3-c]pyrazol-6-ones 2 can be prepared in a selective manner by generating the anion of 1 (R=H) with lithium hexamethyldisilazide in DMF and quenching with activated aryl halides. Sterically demanding groups such as phenyl as in 5 reduce reactivity significantly while electron-withdrawing substituents such as trifluoromethyl and phenyl at C4 of the pyranone ring as in 10 and 15 render the pyranone carbonyl particularly susceptible to attack by nucleophiles resulting in ring-opening to give novel crotonyl derivatives. Proof of structure required a variety of nmr methods involving proton, carbon, and nitrogen nuclei.