Synthesis of new chiral pyrylium salts, the corresponding phosphinine and pyridine derivatives and the kinetic studies of the epimerization of pyrylium salts.
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Van der Velde, Nelson A.
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Despite the versatility of pyrylium salts as precursors to many heteroaromatic systems, chiral pyrylium salts are almost unknown in the literature. One reason for this scarcity is that pyrylium salts are often involved as intermediates rather than as isolated and characterized materials. Another is that many pyrylium salts preparations tend to result in non-characterizable black solid due to polymerization reactions. We have developed the synthesis of several new chiral pyrylium salts and their conversion to the corresponding pyridines and phosphinines. This work almost triples the number of reported chiral pyrylium salts, and also represents the first racemizable/epimerizable pyrylium salts. The derived phosphinines and pyridines represent rare alpha-chiral ligands for transition metals. Interestingly, only a few examples of chiral phosphinines have been reported in the literature. Incorporation of chirality directly (i.e., alpha to aromatic ring) onto these planar ring systems has proven to be difficult. From our pyrylium salts we have synthesized new phosphinines with the chirality as close as possible to the phosphorus center. Two known pyridinium salts were also prepared with the thiosemicarbazone moiety. The cytotoxicity and inhibition of cruzain were evaluated and found to be non-actives. Our interest in chiral pyrylium salts led us to investigate the configurational stability of chiral centers alpha to the pyrylium ring. Although no epimerizable (or even racemizable) pyrylium salts have been reported, deuterium exchange at ortho and especially para benzylic positions is well-known, suggesting that epimerization is possible. Described here is the first study of the base-catalyzed epimerization of chiral pyrylium salts. In one case, this required identifying all components of a complex mixture of diastereomers. It was found that the base-catalyzed epimerization mechanism of the pyrylium salts studied is first order on the pyrylium and first order on the pseudobase formed.
DepartmentChemistry and Biochemistry.
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