A Phytosphingosine-Based O-Cyclic Phosphate Derivative as a Novel Antifungal Agent Against the Phytopathogenic Fungus Fusarium asiaticum

Phytosphingosine, a naturally occurring long-chain fatty alcohol present in the upper layers of the skin, is a key building block for ceramides essential to skin structure. In this study, three phytosphingosine derivatives, (2S,3S,4R)-3,4-dihydroxy-1-(phosphonooxy)octadecan-2-aminium chloride (I), (4S,5S)-2-hydroxy-4-((R)-1-hydroxypentadecyl)-1,3,2-dioxaphosphinan-5-aminium 2-oxide chloride (II), and (2S,3R)-3-hydroxy-1-(phosphonooxy)octadecan-2-aminium chloride (III), were synthesized to evaluate their potential as fungicides. Phytosphingosine-1-phosphate salt I was prepared via a novel four-step phosphorylation process using dimethyl chlorophosphate. O-Cyclic phytosphingosine-1-phosphate salt II was obtained by cyclizing Boc-protected phytosphingosine with POCl3, while Boc-protected sphinganine was phosphorylated under mild conditions using dimethyl chlorophosphate to yield sphinganine-1-phosphate salt III. The antifungal activities of compounds I–III were assessed in vitro against four phytopathogenic fungi: Botrytis cinerea, Fusarium asiaticum, Fusarium solani, and Rhizoctonia solani. Among these, compound II exhibited the most potent growth inhibitory activity against Fusarium asiaticum (IC50 = 27.5 μg/mL). In vivo evaluations using infected apples further confirmed the concentration-dependent inhibitory activity of compound II against F. asiaticum (IC50 = 108 μg/mL). Morphological analyses revealed shortened hyphal structures and abnormal branching in F. asiaticum, while compound II treatment was associated with a concentration-dependent shift toward predominantly yeast-form morphology in adherent Candida albicans populations. In silico docking against five selected fungal enzymes suggested a potential interaction between compound II and N-myristoyltransferase (NMT), characterized by predicted hydrophobic interactions within the acyl-chain binding pocket and a possible hydrogen bond involving LEU171. Molecular dynamics simulations further supported the relative stability of the predicted NMT–compound II complex. Together, these findings highlight the O-cyclic phytosphingosine-1-phosphate salt II as a promising antifungal candidate and suggest NMT as a potential molecular target warranting further experimental validation.