Die Protonierung von 1‐Formyl‐ und 1‐Acetyl‐azulenen

NMR. data show that protonation of 1‐formyl‐azulene yields essentially a one‐to‐one mixture of conjugate acids in which the hydroxyl group assumes the syn ‐planar or anti ‐planar configuration relative to the tropylium nucleus. The presence of a minute amount of protonation in position 3 is demonstrated by the rapid hydrogendeuterium exchange in this position. Steric interference in 1‐formyl‐azulenes with a methyl group in the peri position 8 favours the anti ‐planar configuration. As shown by one example, the conjugate acids of 1‐acetyl‐azulenes without substituents in positions 2 or 8 assume the anti ‐planar configuration. In 1‐acetyl‐azulenes carrying a methyl group in position 8, addition of a proton to the carbon centre 1 is the preferred route of protonation, as a consequence of the accompanying strain release.