A base-dependent reactivity switch in Baylis–Hillman ketones: access to <i>N’</i>-alkyl benzohydrazides and fluorescent dihydropyrazoles

A switch in the reactivity of Morita-Baylis-Hillman (MBH) ketones was observed upon interaction with hydrazines in the presence/absence of a base. The addition of NEt₃ brought about an insertion of hydrazine into the MBH ketone skeleton, resulting in the formation of benzohydrazides; whereas in the absence of a base, a concomitant aza-Michael addition/condensation led to the formation of strongly fluorescent dihydropyrazoles, which could be further oxidized to aromatic pyrazoles.