Anti-toxoplasma activity of a semisynthetic azasteroid derived from β-sitosterol

β-Sitosterol from Hylotelephium erythrostictum (Miq.) H. Ohba, an abundant plant sterol, was converted into eight A-ring-modified derivatives, introducing a seven-membered ring bearing an exocyclic formyl group-a motif associated with antiparasitic activity. Evaluation against Toxoplasma gondii revealed that A-ring expansion with C-3 formylation and 2-aza substitution correlated with increased activity, while the parent compound and 4-aza analogues were inactive (IC₅₀ values exceeded 500 µM). The most active derivative, N-formyl-2-aza-A-homo-3-chlorostigmast-5-ene, exhibited an IC₅₀ of 206.22 µM (SI = 1.20), compared with spiramycin (IC₅₀ = 236.14 µM, SI = 0.99). Despite modest potency, this work introduces a steroidal scaffold combining A-ring expansion with an exocyclic aldehyde for anti-Toxoplasma activity. The observed trends provide a preliminary foundation for further optimisation of this scaffold.