Macrocyclic compounds, bridging the gap between small molecules and biomacromolecules, represent a class of promising ligands that are advantageous in interacting with protein targets [
49,
50]. Among the strategies that have been developed to discover new macrocycles, DEL technology is featured the rapid synthesis of macrocycle library by means of combinatorial chemistry. The crucial step in the construction of a macrocycle library is macrocyclization [
51]. Currently, prevalent methods in DNA-compatible macrocyclization are amide bond formation [
52], Cu-catalyzed azide/alkyne cycloaddition [
53-
56], Wittig olefination [
57], ring-closing metathesis [
58,
59], and others [
60-
64]. Although these approaches demonstrate robustness and efficacy, seeking more chemical diversity during macrocyclization remains unexplored. We envisioned that DHQ formation has several merits. First, it would accomplish a privileged scaffold into the rigid macrocycle to consequently yield druglike properties, advantageous over most used amide bond formation. Second, an increase in molecular weight after the reaction eases analytical monitoring, in contrast to the CuAAC method. As shown in
Scheme 4, we implemented our method onto DNA-conjugated linear peptides carrying a free amine and a distant aldehyde, by using isatoic anhydride
via a "one-pot" synthesis. To our delight, macrocycles (
m1-
m5) with different ring sizes and chemical compositions were achieved with moderate-to-good conversions (40%−80%). DNA-tethered oligopeptidic
S1-
S5, regardless of amino acid replacement influence including alpha-amino acid (
m3), beta-turn unit like proline (
m4), and adjacent hindrance (
m5), all gave desired macrocycles. The product structures, unlike ordinary macrocycles generated by methods discussed above, are featured a medium macrocycle fused with a small DHQ cycle which could serve as a pharmacophore. Moreover, we would gain more structural diversity after the cyclization reaction if more decorations are installed on the ring of isatoic anhydride. Since reductive amination reaction is also feasible in cyclization when there is a free amine group and a free aldehyde group in the substrate structure [
65], we conducted macrocyclization of
m1 precursor
via reductive amination to make a comparison between these two methods. The desired macrocycle was obtained with an inferior 40% conversion, and by deconvoluted mass analysis, the main by-product was probably the product of aldehyde reduction (Fig. S6 in Supporting information) [
66]. This implies that to some extent our antimony trichloride-promoted DHQ formation reaction improved macrocyclization efficiency. Overall, antimony trichloride-promoted DHQ formation reaction can be readily employed in DNA-compatible macrocyclization, with merits of high efficiency and simultaneous chemical diversity introduction.