SYNTHETIC AND SPECTRAL STUDIES OF GAMMA-PYRONES

The synthesis of 3-hydroxy-4H-pyran-4-one-2-carboxaldehyde has been accomplished by selectively blocking the enolic hydroxyl group of (alpha)-hydroxy-maltol with p-nitrobenzyl bromide, followed by oxidation of the 2-hydroxymethyl function with barium manganate to the aldehyde, and finally removal of the p-nitrobenzyl blocking group using a hydrobromic acid-acetic acid solution. 3-Hydroxy-4H-pyran-one-2-carboxaldehyde has also been prepared by direct air oxidation, under aqueous basic conditions, of (alpha)-hydroxymaltol using a palladium-on-carbon catalyst. The enolic hydroxyl group of (alpha)-hydroxymaltol has been selectively methylated with dimethyl sulfate in acetone. A study has been conducted in which Jones\u27 reagent, pyridinium dichromate and barium manganate were tested as oxidizing agents in the synthesis of 3-methoxy-4H-pyran-4-one-2-carboxaldehyde from 2-hydroxymethyl-3-methoxy-4H-pyran-4-one. Barium manganate has proven to be the reagent of choice for this transformation. Two carboxylic acid derivatives have been synthesized in this series. 2-Hydroxymethyl-3-benzyloxy-4H-pyran-4-one was oxidized directly to 2-carboxy-3-benzyloxy-4H-pyran-4-one using an excess of Jones\u27 reagent in acetone. Oxidation of 3-methoxy-4H-pyran-4-one-2-carboxaldehyde with silver oxide in aqueous base gave moderate yields of 2-carboxy-3-methoxy-4H-pyran-4-one. Kojic acid monomethyl ether has been reacted with several p-and m-substituted anilines, having substituents ranging from strong electron donating groups to strong electron withdrawing groups, in refluxing dilute hydrochloric acid, to give the corresponding N-aryl-pyridones in moderate to good yields. Reaction of the pyridones with concentrated hydrochloric acid gave pyridone hydrochloride salts. An alternative method has been developed for the synthesis of 1-phenyl-2-hydroxymethyl-3-methoxy-4-pyridone and 1-(p-nitrophenyl)-2-hydroxymethyl-3-methoxy-4-pyridone by reacting kojic acid monomethyl ether with the appropriate aniline hydrochloride in water at reflux temperature for 24 hours. This method has also been used for the synthesis of nitrogen-15 enriched 1-phenyl-2-hydroxymethyl-3-methoxy-4-pyridone using 95% nitrogen-15 enriched aniline hydrochloride. Nitrogen-15 nuclear magnetic resonance studies have been conducted on this enriched compound. Magnitudes of (\u271)JNC were determined to be approximately 14 Hz indicating the nitrogen to be almost fully sp(\u272). Studies have been conducted towards the derivatization of rutin, a glycoside of quercetin. Rutin decaacetate was synthesized by reacting rutin with acetic anhydride in pyridine using 4-(dimethylamino)pyridine as an acetylation catalyst. Rutin decaacetate has been determined to be labile on alumina and only moderately stable on silica gel. Several purification techniques involving column chromatography were investigated and are discussed. We also report the synthesis of rutin decamesylate and quercitrin heptamesylate under high dilution, long-term reaction conditions. Rutin decamesylate has been hydrolyzed in an acid solution yielding quercetin 5,7,3\u27,4\u27-tetra-O-mesylate. Finally, the carbon-13 NMR spectra of rutin, rutin decaacetate, rutin decamesylate, quercitrin heptamesylate, and quercetin 5,7,3\u27,4\u27-tetra-O-mesylate have been tentatively interpreted