Identification of Dihydropyrazolo[1,5-]pyrazin-4(5)-ones As Cyclic Products of β-Amidomethyl Vinyl Sulfone Alphavirus Cysteine Protease Inhibitors

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