Synthesis of Pyrroles and Pyrazoles by Mukai-yama-Michael Addition/Heterocyclization

Significance

By means of addition of 1,2-diaza-1,3-butadienes 1 on Danishefsky-type dienes 2, followed by intramolecular cyclization, a number of pyrrole (4, 5) and pyrazole (6) derivatives were prepared in good to excellent preparative yields. The mechanism of all transformations was discussed in detail and includes first Michael-type addition resulting in 3, which undergoes cyclization in presence of acid catalysts. The key factor which directs regioselectivity appears to be the acidity of the α-hydrogen to the C=N function, determined by the nature of R^¹. Under aqueous conditions, Danishefsky’s diene 2 acts as a synthetical analogue of 3-oxobutanal resulting in the formation of 4; however, only a few examples were presented.