Electronic structure of pi systems: XIX. Keto-enol tautomerism of dihydrofurocoumarinones

Tautomeric transformations of 4-methyldihydrofuro[2,3-h]coumarin-9-one and its 8-substituted derivatives were studied by H-1 NMR, electronic absorptio n spectroscopy, and quantum chemistry. The 1H NMR spectra of these compound s in CDCl3 show that they exist in the ketone form, and in more polar solve nts they can pass into the enol form. By electronic absorption spectroscopy it was established that the derivatives containing electron-acceptor subst ituents in the 8 position of the furanone ring undergo tautomeric transform ations as the composition of the solvent is varied from 100% methanol to 10 0% CCl4. At the same time, the derivatives with electron-donor substituents in the same position do not show any specific alterations in the absorptio n spectra with solvent. Analogous pattern was observed in the enolization O r Substituted dihydrofurocoumarinones by acetylation: In presence of electr on-donor substituents in the 8 position, no acetylation occurred, while wit h the compounds containing electron-acceptor substituents, the correspondin g 9-acetoxy-4-methylangelicins were prepared in high yields. Calculations b y the PPP/CI method of the electronic absorption spectra 4-methyldihydrofur o[2,3-h]coumarin-9-one showed that in polar solvents (methanol) it prefers the enol form. Data of spectral measurements were compared with results of semiempirical (MNDO, AM1, and PM3) and nonempirical quantum-chemical calcul ations (with 3-21G, 6-31G*, and 31G** basis sets).