Design, Synthesis\nand Bioactivity of Benzyl Propiolates\nwith Broad-Spectrum Inhibition Activity on Phytopathogenic Fungi

As part of our continuing research on propiolic acid\nderivatives,\na series of benzyl propiolate derivatives and analogues were designed,\nsynthesized, and investigated for inhibition activity <i>in vitro</i> and <i>in vivo</i> on phytopathogenic fungi, structure–activity\nrelationship (SAR) and action mechanism. The results showed that most\nof the compounds had potent and broad-spectrum antifungal activity <i>in vitro</i> at 50 μg/mL. Among the compounds, 2-bromobenzyl\npropiolate (<b>3h</b>) displayed the highest comprehensive activity\nwith total activity index (TAI) of 4.57 against all the eight test\nfungi followed by 3-fluorobenzyl propiolate (<b>3c</b>) (TAI\n= 4.53), superior to positive fungicides thiabendazole (TAI < 4.12)\nand/or azoxystrobin (TAI < 2.90). Relative to the other fungi, <i>Fusarium solani</i> and <i>Fusarium graminearum</i> revealed higher total susceptibility indexes (TSI) of 11.1 and\n8.98, respectively, for all compounds. <b>3h</b> and <b>3g</b> (4-chlorobenzyl propiolate) gave the smallest EC<sub>50</sub> values\nof 0.86 and 0.67 μg/mL against <i>F. solani</i> and <i>F. graminearum</i>, respectively, superior or comparable with\nthiabendazole and azoxystrobin. As a representative, <b>3h</b> at 200 μg/mL displayed 100% protection on <i>F. solani</i> infections on potatoes over 7 days and high safety for plant growth.\nIn antifungal mechanism, <b>3h</b> was able to change mycelial\nmorphology, destroy the structures of both hypha and cell membrane,\nincrease the intracellular ROS level, decrease the intracellular mitochondrial\nmembrane potential level, and change the permeability of the cell\nmembrane. The SAR showed that ethynyl is a key group for the activity,\nand the presence of halogen atoms at the 2-site of the benzene ring\ncan significantly improve the activity in most cases. Thus, benzyl\npropiolate can be considered a novel antifungal lead compound, while <b>3h</b> can be considered a promising fungicide candidate to develop\nnew agricultural fungicides.