Chloroformate-mediated ring cleavage of indole alkaloids leads to re-engineered antiplasmodial agents.

Natural product ring distortion strategies have enabled rapid access to unique libraries of stereochemically complex compounds to explore new chemical space and increase our understanding of biological processes related to human disease. Herein is described the development of a ring-cleavage strategy using the indole alkaloids yohimbine, apovincamine, vinburnine, and reserpine that were reacted with a diversity of chloroformates paired with various alcohol/thiol nucleophiles to enable the rapid synthesis of 47 novel small molecules. Ring cleavage reactions of yohimbine and reserpine produced two diastereomeric products in moderate to excellent yields, whereas apovincamine and vinburnine produced a single diastereomeric product in significantly lower yields. Free energy calculations indicated that diastereoselectivity regarding select ring cleavage reactions from yohimbine and apovincamine is dictated by the geometry and three-dimensional structure of reactive cationic intermediates. These compounds were screened for antiplasmodial activity due to the need for novel antimalarial agents. Reserpine derivative 41 was found to exhibit interesting antiplasmodial activities against Plasmodium falciparum parasites (EC50 = 0.50 µM against Dd2 cultures), while its diastereomer 40 was found to be three-fold less active (EC50 = 1.78 µM). Overall, these studies demonstrate that the ring distortion of available indole alkaloids can lead to unique compound collections with re-engineered biological activities for exploring and potentially treating human disease.

Ring Distortion of Vincamine Leads to the Identification of Re-Engineered Antiplasmodial Agents.

There is a significant need for new agents to combat malaria, which resulted in ∼409,000 deaths globally in 2019. We utilized a ring distortion strategy to create complex and diverse compounds from vincamine with the goal of discovering molecules with re-engineered biological activities. We found compound 8 (V3b) to target chloroquine-resistant Plasmodium falciparum Dd2 parasites (EC50 = 1.81 ± 0.09 µM against Dd2 parasites; EC50 > 40 µM against HepG2 cells) and established structure-activity relationships for 25 related analogues. New analogue 30 (V3ss, Dd2, EC50 = 0.25 ± 0.004 µM; HepG2, EC50 > 25 µM) was found to demonstrate the most potent activity, which prevents exit on the parasite from the schizont stage of intraerythrocytic development and requires >24 h to kill P. falciparum Dd2 cells. These findings demonstrate the potential that vincamine ring distortion has toward the discovery of novel antimalarial agents and other therapies significant to human health.