Masaki Koshimizu, Masashi Suzuki, Saori Miya, Hiroyuki Miyachi

Organic Chemistry Frontiers

DOI: 10.1039/d6qo00903d

Abstract

DNA-encoded library (DEL) technology enables efficient exploration of chemical space, but its synthetic scope remains constrained by the limited repertoire of DNA-compatible transformations. Pyrazoles are privileged motifs in medicinal chemistry, yet they remain underrepresented in DEL synthesis because their preparation often requires conditions that are poorly compatible with DNA. Here we report a mild, DNA-compatible three-component reaction for on-DNA pyrazole construction from DNA-conjugated aldehydes, hydrazines, and nitroolefins. The reaction proceeds in aqueous media at room temperature without added oxidants or heating, while preserving DNA integrity and amplifiability under the tested conditions. Systematic optimisation identified buffer composition and pH as key parameters, enabling broad substrate tolerance across all three reaction partners. A streamlined preparation workflow further makes a practical subset of nitroolefins accessible as DEL building blocks. Mechanistic experiments are consistent with a hydrazone-controlled [3 + 2] cycloaddition pathway under on-DNA conditions. Multistep DEL synthesis and chemical-space analysis show that this chemistry enables access to drug-like pyrazole-centred libraries that complement regions of ChEMBL chemical space with relatively low compound density, including areas that are not efficiently populated by a corresponding amide-based DEL design. More broadly, this study illustrates how reaction design, combined with scaffold-focused chemical-space analysis, can guide the introduction of underrepresented heterocyclic motifs into DEL discovery collections.