Strategies for the De Novo Synthesis of Highly Substituted Pyridine Scaffolds: Unified Total Synthesis of the Limonoid Alkaloids

Highly substituted pyridine scaffolds are found in many biologically active natural products and therapeutics. Accordingly, numerous complementary de novo approaches to obtain differentially substituted pyridines have been disclosed. This article delineates the evolution of the synthetic strategies designed to assemble the demanding tetrasubstituted pyridine core present in the limonoid alkaloids isolated from Xylocarpus granatum, including xylogranatopyridine B, granatumine A and related congeners. The most efficient and convergent construction of the core framework present in xylogranatopyridine B involved a Liebeskind pyridine synthesis and late-stage benzylic oxidation. By contrast, the synthesis of the bislactone limonoid alkaloids, such as granatumine A which exhibited moderate PTP1B-inhibitory activities, necessitated the development of a novel pyran-to-pyridine conversion. In addition, NMR calculations suggested structural misassignment of several limonoid alkaloids, and predicted their C3-epimers as the correct structures, which was further validated unequivocally through chemical synthesis. While preliminary results of the pNPP assays showed that these bislactone limonoid alkaloids were only weakly inhibitory against PTP1B, C3-deoxy-xylogranatin F, an unnatural synthetic analog, was demonstrated to be more potent than the other congeners.