Cyclic nucleotide phosphodiesterase type 4 inhibitors: Evaluation of pyrazolo[1,5-a]-1,3,5-triazine ring system as an adenine bioisostere

A series of 8-substituted pyrazolo[1,5-a]-1,3,5-triazines were considered as a bioisosteric replacement for the 9-substituted adenine derivatives resulting in the discovery of 8-(2-methoxybenzyl)-4-(N-methylamino)-2-n-propylpyrazolo[1,5-a]-1,3,5-triazine (14d) and 2-trifluoromethyl-8-(2-methoxybenzyl)-4-(N-methylamino)pyrazolo[1,5-a]-1,3,5-triazine (14e) as a new structural class of potent phosphodiesterase type 4 inhibitors (IC50 = 13 nM and 11 nM, respectively) with high isoenzyme selectivity. An original tandem of reactions involving a palladium-mediated cross-coupling reaction (PMCCR) of the readily available 8-iodo-2-methyl-4-(N-methyl-N-phenylamino)pyrazolo[1,5-a]-1,3,5-triazine (11a) and arylboronic acids or alkynes followed by the displacement of the N-methyl-N-phenylamino group constitute the key steps in a novel synthetic approach developed herein. The treatment of 11a–c with n-BuLi and selected aldehydes represents an interesting alternative to the PMCCR for the synthesis of benzylic derivatives 14a–i. Preliminary biological testing has shown that compounds 14d and 14e strongly inhibit LPS-induced TNFα release from human mononuclear cells from healthy subjects. These two compounds were selected for further biological evaluation.

Graphical abstractSome derivatives belonging to the series of pyrazolo[1,5-a]-1,3,5 triazines act as adenine bioisosteres and present potent and selective PDE4 inhibitory properties.Figure optionsDownload full-size imageDownload as PowerPoint slide