Water-Compatible Staudinger–Diels–Alder Ligation

The development of bioorthogonal reactions is expected to propel further advances in chemical biology. In this study, we demonstrate Staudinger–Diels–Alder (SDA) ligation as a candidate for a new bioorthogonal reaction. This reaction ligates two molecules via strong C–C bonds at room temperature. We found that the aryl substituent of azide-benzocyclobutene (azide-BCB) had a strong influence on the molecule's tolerance to water. In particular, Cl-substituted azide-BCBs generated the ligated product in high yield, even in the presence of water. Mechanistic investigations using DFT methods revealed that hydrophobic electron-withdrawing substituents suppressed the side reactions of SDA ligation.