One-Step Synthesis of Fully Functionalized Pyrazolo[3,4‑<i>b</i>]pyridines via Isobenzofuranone Ring Opening

A novel series of fully substituted pyrazolo­[3,4-<i>b</i>]­pyridines <b>4</b> has been prepared in a regioselective manner by the microwave-assisted reaction between <i>N</i>-substituted 5-aminopyrazoles <b>1</b> and 3-(3-oxo-2-benzofuran-1­(3<i>H</i>)-ylidene)­pentane-2,4-dione (<b>2</b>). This is the second reported example of a cyclocondensation reaction using substrate <b>2</b> as a 1,3-bis-electrophilic reagent. Remarkably, this synthesis offers functionalized products with acetyl and carboxyl groups in one step, in good yields, and with short reaction times. Additionally, the cyclization intermediate <b>3</b> was isolated, allowing us to postulate a mechanism for this reaction, which is initiated via isobenzofuranone ring opening of <b>2</b> in a Michael-type reaction. The structures of the products and regioselectivity of the reactions were determined on the basis of NMR measurements and X-ray diffraction. For this new reaction using substrate <b>2</b>, the optimal reaction conditions and its scope were investigated.