A Flexible Strategy for the Regiocontrolled Synthesis of Pyrazolo[1,5-a]pyrazines.

A four-step protocol for the synthesis of pyrazolo[1,5-a]pyrazines has been developed. Commercially available pyrazoles were alkylated and formylated in a regiocontrolled manner to give pyrazole-5-aldehydes bearing 2,2-dialkoxyethyl substitution on N-1. Efficient conditions for the subsequent deprotection and cyclization of these intermediates allowed access to pyrazolo[1,5-a]pyrazines with multiple substitution patterns. The versatility of the pyrazole-5-aldehyde intermediates was further demonstrated through a deprotection and double-reductive amination sequence to give 4,5,6,7-tetrahydropyrazolo[1,5-a]pyrazines.