One-pot synthesis and radical scavenging activity of novel polyhydroxylated 3-arylcoumarins.

An unexpected domino rearrangement brought about the development of a novel one-pot procedure for synthesis of coumarins. This protocol allowed the gram-scale synthesis of a variety of polyhydroxylated derivatives 3a-p, from readily available starting materials at a low cost. Based on two proven intermediates, a probable mechanism consisting of boron tribromide induced demethylation/lactone ring opening/elimination/isomerization/lactone ring closure reaction sequence of in situ formed 3-aryl-3,4-dihydroisocoumarin-4-carboxylic acids was deduced. Compared to the common methods, used for the synthesis of coumarins, the proposed herein possesses great advantages, such as mild conditions, good yields for short reaction time, simple work-up procedure and easy isolation of the final products. The structure of the newly synthesized compounds 3a-p was established by spectroscopic methods ((1)H NMR, (13)C NMR, IR, MS and HRMS) and their radical scavenging activity was evaluated in vitro against 1,1-diphenyl-2-picrylhydrazyl free radical (DPPH). The results obtained show that compounds 3g-p posses higher radical scavenging activity (3.16 ≤ SC50 [μM] ≤ 6.82) than well-known antioxidants such as trolox, protocatechuic acid, caffeic acid and gallic acid (SC50 [μM] = 9.34, 8.83, 9.48, 5.33, respectively), which is a precondition for promising antioxidant activity of these compounds to be expected.