Synthesis and antimalarial and antituberculosis activities of a series of natural and unnatural 4-methoxy-6-styryl-pyran-2-ones, dihydro analogues and photo-dimers

Previous studies have identified the 3,6-dialkyl-4-hydroxy-pyran-2-one marine microbial metabolites pseudopyronines A and B to be modest growth inhibitors of Mycobacterium tuberculosis and a range of tropical diseases including Plasmodium falciparum and Leishmania donovani. In an effort to expand the structure-activity relationship of this compound class towards infectious diseases, a library of natural product and natural product-like 4-methoxy-6-styryl-pyran-2-ones and a subset of catalytically reduced examples were synthesized. In addition, the photochemical reactivity of several of the 4-methoxy-6-styryl-pyran-2-ones were investigated yielding head-to-head and head-to-tail cyclobutane dimers as well as examples of asymmetric aniba-dimer A-type dimers. All compounds were evaluated for cytotoxicity and activity against M. tuberculosis, P. falciparum, L. donovani, Trypanosoma brucei rhodesiense and Trypanosoma cruzi. Of the styryl-pyranones, natural product 3 and non-natural styrene and naphthalene substituted examples 13, 18, 21, 22 and 23 exhibited antimalarial activity (IC50 <10 μM) with selectivity indices (SI) >10. Δ7 Dihydro analogues were typically less active or lacked selectivity. Head-to-head and head-to-tail photodimers 5 and 34 exhibited moderate IC50s of 2.3 to 17 μM towards several of the parasitic organisms, while the aniba-dimer-type asymmetric dimers 31 and 33 were identified as being moderately active towards P. falciparum (IC50 1.5 and 1.7 μM) with good selectivity (SI ∼80). The 4-tert-butyl aniba-dimer A analogue 33 also exhibited activity towards L. donovani (IC50 4.5 μM), suggesting further elaboration of this latter scaffold could lead to the identification of new leads for the dual treatment of malaria and leishmaniasis.