We prepared the key selenenylsulfide building blocks in a straightforward, divergent manner from bis-mesylate 1 (Fig. ), which can easily be accessed from aspartic acid. Reaction with one equivalent of potassium thioacetate afforded a mixture of mono-substitution products 2 and 3, with a small preference for 2. These regioisomers were separated by chromatography. Treatment with potassium selenocyanate followed by thioester cleavage directly resulted in the formation of the cyclic dichalcogenides 5 and 8 by expulsion of cyanide. Boc-deprotection then gave the amine hydrochlorides 6 and 9. In parallel to our work, Iwaoka elegantly reported the synthesis of compounds 6 and 9 via a similar approach, but using non-commercial PMBSeH as the selenium source. In comparison, our synthesis offers more atom economy, reduced step count, fewer chromatographic separations and more rapid access to the final compounds, although these differences are not crucial. To obtain the control Se and S series dichalcogenides (Fig. ), bis-mesylate 1 was either treated with excess potassium thioacetate and KOH/air, or excess KSeCN and KOH, followed by Boc-deprotection to form disulfide 10 or diselenide 11, respectively. The symmetrical linear disulfide SS00 was accessed from commercial cystamine. We diversified these trigger building blocks by installing the sidechains, giving secondary amines. Finally, deprotection and coupling with chloroformate PQ-OCOCl (Fig. ) yielded a panel of twelve PQ probes (Fig. and Fig. ): the A-type and G-type cyclic selenenylsulfides RX1, A2-A3 and G1-G3 as candidates/controls for TrxR-selective probes; the cyclic diselenide Se3 and cyclic disulfides S1-S3 as well as the linear disulfide SS00 as reduction-responsive controls; and the non-reducible cyclohexyl probe C1 as a control for non-reductive activation. All probes were nonfluorescent as solids and in solution (see Supporting Information). report by Melnyk 53 with halflife ca. 4.5 h), followed by fast E1cb, could also be postulated as a mechanism. We also wished to test whether the triggers resist parasitic reactions such as TCEP deselenisation which would otherwise complicate cell-free benchmarking.