A comprehensive comparative study, microwave (MW) versus conventional thermal heating (TH), for synthesis
of fused pyrrolopyridazine derivatives with dihydroxyacetophenone is described. The reaction pathway is efficient and
straight applicable, involving a Huisgen [3 + 2] dipolar cycloaddition of pyridazinium ylides to activated Z-alkynes symmetrical
or unsymmetrical substituted. Under MW irradiation the consumed energy decreases considerably, the amount of
used solvent also decreases (at least three-fold less comparative with conventional TH), and the reaction conditions are
milder. Consequently these reactions could be considered environmentally friendly. It is also to be pointed out that under
MW irradiation the yields are higher and the reaction time decreased substantially (from hours to minutes) comparative
with conventional TH. A feasible explication for the MW efficiency in Huisgen [3+2] dipolar cycloaddition reactions was
furnished. The regiochemistry studies indicate that the reactions occur highly regiospecific (a single regioisomer being
formed), under charge control.
Keywords: Environmentally friendly methods, cycloaddition, microwave, pyridazine, dihydroxyacetophenone, pyrrolopyridazine,
regiochemistry, regiospecific, ylides, Z-alkynes.
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