Strategic atom replacement enables regiocontrol in pyrazole alkylation

Abstract

Pyrazoles are heterocycles commonly found as key substructures in agrochemicals and medicinally active compounds alike1,2. Despite their pervasiveness, established methods fall notably short in delivering complex pyrazoles selectively due to issues of differentiation during either assembly or N-functionalization3. This is a direct consequence of a dominant synthetic strategy that attempts to control selectivity-determining bonds between poorly differentiated starting materials. To overcome this longstanding challenge, we here describe a prototypical example of an alternative conceptual approach, ‘strategic atom replacement’, in which we synthesize N-alkyl pyrazoles from isothiazoles. The net forward transformation is a ‘swap’ of the isothiazole sulfur atom with a nitrogen atom and its associated alkyl fragment to deliver the alkylated pyrazole4,5. Linking the two azoles is an orphaned heterocycle class, 1,2,3-thiadiazine-S-oxides, whose synthetic potential has yet to be tapped6. By proceeding through these unusual heterocycles, the typical selectivity and separation challenges associated with exclusively bond-based pyrazole preparations are circumvented, and even minimally differentiated peripheral substituents can be discriminated to afford isomerically pure products. A prototypical example of a ‘strategic atom replacement’ approach enables synthesis of N-alkyl pyrazoles from isothiazoles by swapping the sulfur atom with a nitrogen atom and its associated alkyl fragment to deliver the alkylated pyrazole.