Preparation of long-chain β-enaminones and β-diketones from long-chain 3,5-disubstituted isoxazole compounds

摘要

A series of long-chain compounds containing the β-enaminone functionality were prepared in yields ranging from 71 to 88% from their corresponding long-chain 3,5-disubstituted isoxazole precursors utilizing a Raney nickel-catalyzed reductive ring-opening procedure. These newly prepared multifunctional compounds were subsequently hydrolyzed under mild acidic conditions (pH 4–5) to give their corresponding long-chain β-diketones in yields ranging from 79 to 98%. Both the β-enaminone and β-diketone functionalized compounds were characterized by NMR, IR spectroscopy, GC-MS, and m.p. The mass spectra for these two classes of compounds, derived by utilizing electron impact ionization, gave distinctive McLafferty rearrangement fragmentation ions that clearly established the newly introduced functionality to reside at the C-2 and C-4 positions of the lipid’s alkyl chain. 1H NMR spectra of the pure β-diketone compounds were complex owing to the keto enol tautomerism displayed by the β-diketone moiety. In solution the β-diketone compounds were shown to exist mainly in the enolic tautomeric form. Long-chain β-diketone compounds are known to be relatively common constituents of some plant waxes, and the overall procedure starting from soybean methyl esters provides a complementary approach to prepare these types of compounds.