Selective functionalization of the 1H-imidazo[1,2-b]pyrazole scaffold. A new potential non-classical isostere of indole and a precursor of push–pull dyes

Abstract

We report the full selective functionalization of the 1H-imidazo[1,2-b]pyrazole scaffold using a Br/Mg-exchange, as well as regioselective magnesiations and zincations with TMP-bases (TMP = 2,2,6,6-tetramethylpiperidyl), followed by trapping reactions with...

Recent Advances in Synthesis and Properties of Pyrazoles

“…In the final part of this review, we describe four additional examples that are part of two rings with a fusion of type 5:5 and 5:7. Two of the works correspond to the imidazo[1,2b]pyrazole ring [75,76] and the other ones to the pyrazolo [3,4-b]azepine ring [77,78]. In these examples, the 5-or 7-membered ring is built on the respective pyrazole derivative, and, similar to the two previous examples, they are scarce fused rings to find.…”

“…In these examples, the 5-or 7-membered ring is built on the respective pyrazole derivative, and, similar to the two previous examples, they are scarce fused rings to find. Regarding imidazo[1,2-b]pyrazoles, Schwärzer et al [75] reported a selective functionalization of the fused ring to obtain products 146-148 (Scheme 23a), beginning with the exchanging Br for Mg (to form 146) from the SEM-protected fused pyrazole 148 (SEM = trimethylsilylethoxymethyl), and continuing with a regioselective magnesiations (to form 147) or zinc cation (to form 148) using TMP-bases (TMP = 2,2,6,6-tetramethylpiperidyl). Precursor 144 was selectively brominated with NBS producing the 7-bromo subs tuted derivative 145, which was used as a substrate in the reaction with i-PrMgCl•LiC forming the magnesiated intermediate 145′ that then reacts with varios electrophiles (i. S-methyl sulfonothioate, tosyl cyanide, and TESCl) and by transmetallations to gi 146a-i.…”