Puberulic acid

Novel derivatives of puberulic acid were synthesized and their antimalarial properties were evaluated in vitro against the Plasmodium falciparum K1 parasite strain, cytotoxicity against a human diploid embryonic cell line MRC-5, and in vivo efficacy using a Plasmodium berghei-infected mouse model. From previous information that three hydroxy groups on the tropone framework were essential for antimalarial activity, we converted the carboxylic acid moiety into the corresponding esters, amides, and ketones. These derivatives showed antimalarial activity against chloroquine-resistant Plasmodium in vitro equivalent to puberulic acid. We identified that the pentane-3-yl ester, cyclohexyl ester, iso-butyl ketone, cyclohexyl methyl ketone all show an especially potent antiparasitic effect in vivo at an oral dose of 15 mg/kg without any apparent toxicity. These esters were more effective than the existing commonly used antimalarial drug, artesunate.

Efficient and practical total synthesis of puberulic acid has been accomplished via 8 steps, with 54% overall yield, and only two C-C bond formations, without the introduction of oxygen atoms into the core skeleton. Construction of the tropolone framework as the key transformation was achieved by multi-tandem oxidation of the aliphatic-triol, from D-(+)-galactose as the starting material.

Divergent synthesis of antimalarial troponoids, including naturally occurring compounds, some of which were identified and isolated by our group, has been achieved utilizing the total synthetic route of puberulic acid. Structure-activity relationships of natural products and simple troponoids inspired us to explore more detailed properties of this class of compounds. Access to new derivatives was facilitated through intermediate compounds generated during the total synthesis of puberulic acid by a stepwise oxidation-aromatization sequence to provide 7-hydroxytropolones and bromination for conversion of the carboxylic acid moiety. The first total synthesis of viticolin A, as well as the synthesis of different methyl-substituted derivatives, has also been achieved. In vitro antimalarial activity and cytotoxicity of novel derivatives were evaluated and fundamental information to facilitate the discovery of more promising antimalarials was obtained.