Design and Discovery of Pyrazolo[1,5- <i>a</i> ]pyrimidinyl Oxazoline as a Promising Antifungal Model

Our previous work demonstrated the values of 2-(2-oxazolinyl)aniline (<b>NHPhOx</b>) in achieving scaffold-distinct antifungal amides. Herein, amide-free antifungal leads are disclosed, featuring the replacement of <i>N</i>-phenylamide by pyrazolo[1,5-<i>a</i>]pyrimidine. Pyrazolo[1,5-<i>a</i>]pyrimidine oxazolines and analogs were designed and synthesized via <i>N</i>-distribution inspired hybridization of pharmaceutically important pyrazolo[1,5-<i>a</i>]pyrimidine and oxazolines. <i>In vitro</i> evaluation showed that compound <b>D10</b> stands out as a novel antifungal candidate against agriculturally important <i>Colletotrichum capsici</i> and <i>Pyricularia oryzae</i>. Specifically, this candidate (EC₅₀ value of 1.6 μM) is more promising against <i>C. capsici</i>, which is 1.6- and 3.5-fold more potent than carbendazim and azoxystrobin, respectively. At a concentration of 100 μM, it achieved 58% preventative and 60% curative efficacy against <i>C. capsici</i> in pepper fruits, which are marginally better than the positive control carbendazim. Mechanistic investigations confirmed that it could disrupt the cell membrane integrity of <i>C. capsici</i>. The biosafety assessment showed that no adverse effects of this candidate were observed for rice and earthworms.