Bildung von Pyrazolen aus heterocyclischen Hydrazinen

Robust preparation of novel imidazo[5,1-b][1,3,4]oxadiazoles

“…16 Scenario B would proceed as initially envisioned, namely through formation of the oxadiazole amide 3a which could then proceed to form a novel imidazo-oxadiazole 1a. Scenario A was ruled out by exposing valine-derived hydrazide 2b (R 2 = isopropyl), which would be prohibited from undergoing the second dehydration, to POCl 3 providing exclusively imidazo[5,1b] [1,3,4]oxadiazole 1b in 73% isolated yield. In addition, when 2-methylalanine was employed as the core subunit in acyclic precursor 8, only the oxadiazole amide 9 was obtained in 65% yield as cyclodehydration to the imidazo-oxadiazole was precluded by the presence of a quaternary carbon atom (Scheme 2).…”