Regioselective acylation of 1-hydroxypyrazoles via metalated intermediates

Abstract

A range of C-4 and C-5 acylated 1-benzyloxypyrazoles (7a–e) and (4) have been prepared via Pd(0) catalysed cross-coupling between acid chlorides and 1-benzyloxy-4-(tributylstannyl)pyrazole (8) or 1-benzyloxypyrazol-5-ylzinc chloride. 3-Acylated 2-(4-methoxybenzyl)-2H-pyrazole 1-oxides (15a–f) were formed by reaction between the 3-magnesiated 2H-pyrazole 1-oxide (14) and acid chlorides. The benzyl group of 4 and 7a and the 4-methoxybenzyl (PMB) group of 15a were removed by treatment with conc. HCl or TFA in the presence of water, furnishing the corresponding C-acylated 1-hydroxypyrazoles.