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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