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■ The 4-Methoxybenzyl (PMB) Function as a Versatile Protecting Group in the Synthesis of N-Unsubstituted Pyrazolones

Gernot A. Eller and Wolfgang Holzer*

*Institute of Pharmaceutical Chemistry, University of Vienna, Pharmaziezentrum, Althanstraße 14, A-1090 Vienna, Austria

Abstract

Starting from diethyl ethoxymethylenemalonate and 4-methoxybenzylhydrazine (PMB-NHNH₂) 2-(4-methoxybenzyl)-2,4-dihydro-3H-pyrazol-3-one was prepared. Reaction of the latter with carboxylic acid chlorides / calcium hydroxide in 1,4-dioxane afforded 4-acyl-5-hydroxy-1-PMB-1H-pyrazoles, whereas with dimethylformamide diethyl acetal or benzaldehyde the corresponding (E)-4-dimethylaminomethylene or (E)-4-benzylidene products, respectively, were obtained. The PMB protecting group could be conveniently removed from the pyrazole nucleus by treatment with trifluoroacetic acid to give the N-unsubstituted pyrazolones. Detailed NMR spectroscopic investigations (¹H, ¹³C, ¹⁵N) with the obtained compounds are presented.