Synthesis of pyrazolone fused benzodiazepines via Rh(iii)-catalyzed [4 + 3] annulation of 1-phenylpyrazolidinones with propargyl alcohols

Abstract

In this paper, a highly efficient and regioselective synthesis of pyrazolone fused benzodiazepine derivatives via the reaction of 1-phenylpyrazolidinones with propargyl alcohols is presented. Mechanistically, the formation of the title products involves Rh(III)-catalyzed inert C(sp²)–H functionalization followed by [4 + 3] annulation through cascade C–H/N–H/C–O bonds cleavage. To our knowledge, this should be the first example in which free propargyl alcohol directly acts as a C3 rather than C2 or C1 coupling partner to react with 1-phenylpyrazolidinone to afford the otherwise difficult-to-obtain fused(spiro) seven-membered heterocyclic scaffold. Easily obtainable substrates, high atom-economy, and excellent regioselectivity are the notable features of this novel method.