Chemical and Antiplasmodial Investigations on Carapa-Derived Gedunin Derivatives and Semisynthetic 6- and 7-Substituted Gedunin Analogues from the Brazilian Amazon

The aim of this work was to explore structure-antiplasmodial activity relationships among 6- and 7-acyloxy and hydroxy substituted gedunin derivatives. 7-Deacetyl-7-oxogedunin (cedrolide, 1) and 6α-acetoxygedunin (12)─known antiplasmodial limonoids from the seeds of Carapa guianensis Aublet (Meliaceae)─were targeted for isolation at scale and used as starting materials for the preparation of a small, semisynthetic compound library that included gedunin (4), 7α- or 7β-acyloxy substituted 7-deacetylgedunins (5–9), and 6α-acyloxy substituted 7-deacetylgedunin derivatives (13–15). The concentrations of these compounds that inhibit 50% of the in vitro growth (IC₅₀) of the multidrug-resistant K1 strain of the human malaria parasite, Plasmodium falciparum, were determined. Also, these compounds were screened for toxicity to MRC-5 human fibroblasts. The most antiplasmodial compounds featured a 7α-acetoxy or a 7β-acetoxy moiety (IC₅₀ = 2.3–4.4 μM). Lower antiplasmodial activity was observed for gedunin derivatives exhibiting a 7α- or 7β-hydroxy, O-butanoyl, or O-pentanoyl moiety (and a C6 substituent). This study highlights the antiplasmodial effects, low toxicity to fibroblasts, and good selectivity (SI > 37) of 7-epi-gedunin (5), a compound that is available only through semisynthesis and whose antiplasmodial activity is reported for the first time.