Synthesis and Evaluations of “1,4‐Triazolyl Combretacoumarins” and Desmethoxy Analogs

Abstract

1,4‐Triazolyl combretacoumarins have been prepared by linking the trimethoxyarene unit of combretastatin A4 with coumarins, via a 1,2,3‐triazole. For this, 4‐azidocoumarins were accessed by a sequential two‐step, one‐pot reaction of 4‐hydroxycoumarins with (benzotriazol‐1‐yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP), followed by reaction with NaN ₃. In the reaction with BOP, a coumarin‐derived phosphonium ion intermediate seems to form, leading to an O ⁴‐(benzotriazolyl)coumarin derivative. For the CuAAC reaction of azidocoumarins with 5‐ethynyl‐1,2,3‐trimethoxybenzene, catalytic [(MeCN) ₄Cu]PF ₆ in CH ₂Cl ₂/MeOH with 2,6‐lutidine, at 50 ^oC, was suitable. The 4‐azidocoumarins were less reactive as compared to PhN ₃ and the NBO coefficients of the azido groups were compared by DFT analysis. Compound solubility was a problem in biological assays. On the basis of the biological and solubility data of one 1,4‐triazolyl combretacoumarin, four analogs lacking one or two methoxy groups were synthesized. Reactivity differences among the phenylacetylenes were noted and the NBO coefficients of the alkynes were compared by DFT analysis. In cytotoxicity assays, 1‐phenyl‐4‐(3,4,5‐trimethoxyphenyl)‐1 H‐1,2,3‐triazole showed activity in CEM and MDA‐MB‐231 cell lines by apoptosis. The desmethoxy 6‐bromo‐4‐(4‐(4‐methoxyphenyl)‐1 H‐1,2,3‐triazol‐1‐yl)‐2 H‐chromen‐2‐one also showed cytotoxicity against the two cell lines, but this did not appear to be consistent with apoptosis. The antiviral activity of the compounds was unremarkable.

Abstract

A facile, one‐pot synthesis of 4‐azidocoumarins from 4‐hydroxycoumarins has been developed. Cu‐catalyzed azide‐alkyne cycloaddition of these compounds with 5‐ethynyl‐1,2,3‐trimethoxybenzene or mono and dimethoxy phenylacetylenes gave a series of 1,4‐triazolyl combretacoumarins and desmethoxy analogs. These were analyzed for their antiproliferative and antiviral properties.