NMR spectroscopy of naturally occurring phloroglucinol derivatives.

Carbon-13 NMR spectra of eight 2-acyl- and -6-methyl-substituted filicinic acid (4,4-dimethyl-1,3,5-cyclohexantrione) derivatives were recorded in hexadeuterioacetone and in deuteriochloroform and the signals assigned on basis of chemical shifts and J (CH) coupling considerations. The data prove that the prevailing tautomeric structure of these derivatives is the monoketonic one with the carbonyl function in position 3 and hydroxyl groups in positions 1 and 5, the former being hydrogen bonded to the acyl side chain carbonyl. The 2-acyl-6,6-dimethyl compounds have the diketonic structure and a hydrogen-bonded hydroxyl group in position 3. The skeletal structure of 2-butyryl-4-hydroxy-4,6-dimethyl-1,3,5-cyclohexantrione is identical with that of humulone. This compound and its homologues also exist in the monoketonic tautomeric form, where C-3 carries the carbonyl function. In the solvents used the 6-methyl-2,3-dihydropyrane-2,4-dione exists predominantly as a single tautomer having two ring double bonds; the remaining carbonyl group is a part of an alpha, beta-unsaturated lactone grouping.