Readily Accessible Ambiphilic Cyclopentadienes for Bioorthogonal Labeling

A new class of bioorthogonal reagents based on the cyclopentadiene scaffold is described. The diene 6,7,8,9-tetrachloro-1,4-dioxo­spiro­[4,4]­nona-6,8-diene (a tetrachlorocyclopentadiene ketal, TCK) is ambiphilic and self-orthogonal with remarkable stability. The diene reacts rapidly with a trans-cyclo­octene and an endo-bicyclo­nonyne, but slowly with dibenzo­aza­cyclo­octyne (DIBAC), allowing for tandem labeling studies with mutually orthogonal azides that react rapidly with DIBAC. TCK analogues are synthesized in three steps from inexpensive, commercially available starting materials.