Synthesis and reactivity of enediyne–nucleobase hybrids: effect of intramolecular π-stacking

Anion Control of Lanthanoenediyne Cyclization

A suite of lanthanoenediyne complexes of the form Ln(macrocycle)X₃ (Ln = La³⁺, Ce³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Lu³⁺; X = NO₃^-, Cl^-, OTf^-) was prepared by utilizing an enediyne-containing [2 + 2] hexaaza-macrocycle (2). The solid-state Bergman cyclization temperatures, measured via DSC, decrease with the denticity of X (bidentate NO₃^-, T = 267-292 °C; monodentate Cl^-, T = 238-262 °C; noncoordinating OTf^-, T = 170-183 °C). ¹³C NMR characterization shows that the chemical shifts of the acetylenic carbon atoms also rely on the anion identity. The alkyne carbon closest to the metal binding site, C_A, exhibits a Δδ > 3 ppm downfield shift, while the more distal alkyne carbon, C_B, displays a concomitant Δδ ≤ 2.5 ppm upfield shift, reflecting a depolarization of the alkyne on metal inclusion. For all metals studied, the degree of perturbation follows the trend 2 < NO₃^- < Cl^- < OTf^-. This belies a greater degree of electronic rearrangement in the coordinated macrocycle as the denticity of X and its accompanying shielding of the metal's Lewis acidity decrease. Computationally modeled structures of LnX₃ show a systematic increase in the lanthanide-2 coordination number (CN_La-mc = 2 (NO₃^-), 4 (Cl^-), 5 (H₂O, model for OTf^-)) and a decrease in the mean Ln-N bond length (La-N_average = 2.91 Å (NO₃^-), 2.78 Å (Cl^-), 2.68 Å (H₂O)), further suggesting that a decrease in the anion coordination number correlates with an increase in the metal-macrocycle interaction. Taken together, these data illustrate a Bergman cyclization landscape that is influenced by the bonding of metal to an enediyne ligand but whose reaction barrier is ultimately dominated by the coordinating ability of the accompanying anion.

Enediynes bearing polyfluoroaryl sulfoxide as new antiproliferative agents with dual targeting of microtubules and DNA

A novel series of enediynes possessing pentafluorophenylsulfoxide have been developed. The innovative compounds possess antiproliferative activity against a broad panel of human cancer cells originating from breast, blood, lung, kidney, colon, prostate, pancreas or skin with IC₅₀ ranging from 0.6 to 3.4 μM. The antiproliferative activity of enediynes in darkness is associated to their ability to compromise microtubule network. In addition, exposure to UV leads to double-stranded DNA cleavage caused by the newly synthesized molecules reducing further their IC₅₀ in nanomolar range against human tumor cells, including chemo-resistant pancreatic cancer cells. Taken together, the examined data demonstrate that enediynes possessing pentafluorosulfoxide are promising molecules in the cancer therapy.