Identification of Dihydropyrazolo[1,5-a]pyrazin-4(5H)-ones as Cyclic Products of Beta-Amidomethyl Vinyl Sulfone Alphavirus Cysteine Protease Inhibitors

Optimized syntheses of (E)-5-(2-ethoxyphenyl)-N-(3-(methylsulfonyl)allyl)-1H-pyrazole-3-carboxamide (RA-0002034, 1), a promising antiviral covalent cysteine protease inhibitor lead, were developed. The syntheses avoid the contamination of 1 with the cyclic dihydropyrazolo[1,5-a]pyrazin-4(5H)-one 2, which formed by intramolecular aza-Michael reaction of the vinyl sulfone warhead under basic conditions and at pH 7.4 in phosphate buffer. 1 could be synthesized using either modified amide coupling conditions or through introduction of a MOM-protecting group and was stable as a TFA or HCl salt. Although acyclic 1 demonstrated poor pharmacokinetics with high in vivo clearance in mice, the cyclic 2 showed improved plasma exposure. The potential use of dihydropyrazolo[1,5-a]pyrazin-4(5H)-ones as prodrugs for the acyclic beta-amidomethyl-vinyl sulfone warhead was demonstrated by GSH capture experiments.