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 tetrachlorocyclo-pentadiene ketal, TCK) is ambiphilic and self-orthogonal with remarkable stability. The diene reacts rapidly with a transcyclooctene 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. [ABSTRACT FROM AUTHOR]