Das Elektronenspektrum eines vinylogen Tropons (Die photochemische darstellung des 2‐Hydroxy‐guaj‐azulens)

Irradiation of guaj‐azulene‐2‐sulfonic acid (VII) in 40 percent aqueous or methanolic sulfuric acid with light of 370 nm wave‐length yields 2‐hydroxy‐guaj‐azulene (V) or 2‐methoxy‐guaj‐azulene (X) respectively. The former compound (V) exhibits a keto‐enol tautomerism V ⇋ VI, the equilibrium constant of which is strongly dependent on solvent. The keto form VI contains the π‐electron system of a vinylogue tropone IV. The theoretical analysis of the electronic spectrum of VI, using a CI‐model of the PARISER‐PARR‐POPLE type, supports the previously proposed assignement of the electronic transitions observed in tropone (I), benzotropone (II) and naphthotropone (III).