Synthesis, Herbicidal Activity, and Structure–Activity Relationships of O‑Alkyl Analogues of Khellin and Visnagin

Khellin and visnagin furanochromones were recently reported as potential new bioherbicides with phytotoxic activities comparable to those of some commercially available herbicides. In this study, we examined the effect of O-alkylation and O-arylalkylation of both khellin and visnagin on its effect on herbicidal and antifungal activity. Synthetic analogues included O-demethyl khellin and visnagin, acetylated O-demethyl khellin and visnagin, O-benzylated demethyl khellin and visnagin, four O-demethyl alkylated khellin analogues, and six O-demethyl alkylated visnagin analogues, many of which are reported here for the first time. Both acetate analogues of khellin and visnagin indicated more activity as herbicides on Lemna pausicostata than visnagin, with IC₅₀ values of 71.7 and 77.6 μM, respectively. Complete loss of activity for all O-alkyl analogues with a carbon chain length of greater than 14 carbons was observed. The O-demethyl butylated visnagin analogue was the most active compound with an IC₅₀ of 47.2 μM against L. pausicostata. O-Demethyl ethylated analogues of both khellin and visnagin were as effective as khellin. In the antifungal bioautography bioassay against Colletotrichum fragariae at 100 μg, the only active O-alkyl and O-arylalkyl analogues were O-ethylated, O-butylated, and O-benzylated visnagin analogues with zones of inhibition of 10, 9, and 9 mm, respectively, an effect comparable to that of visnagin and khellin.