Spirans. Part 10. Acid-catalysed rearrangement of oxygen heterocyclic spirans

Abstract

Hot trifluoroacetic acid isomerises the spiran (1) to the phenolic phenalene derivative (2), the structure of which is fully established. The isomerisation had previously been reported and a biradical intermediate suggested for it, but an ionic or an acid-catalysed [3,3] sigmatropic shift now offer better explanations.

Trifluoroacetic acid converts the spiran (10) into the dibenzoxanthylium cation (11a), a [3,3] sigmatropic shift now being prevented for steric reasons. The reactions probably involves heterocyclic ring opening to give a phenolic unsaturated ketone that cyclises to the cation in a conventional manner. This spiran is also sensitive to light which isomerises it to a phenolic oxepin derivative (14) that is readily oxidised by air to a quinone methide (15).

A similar xanthylium cation cannot be formed easily from the phenyl substituted spiran (18) because of the crowding caused by the substituent. A trace of the relatively unhindered cation (19) is formed, but the main reaction is diverted to a phenolic oxepin derivative (21) related to (14). It appears that a benzylic carbocation is formed and substitutes into the adjacent peri-position in the naphthalene ring.

In order to establish the structure of the oxepin (21) a detailed analysis of the ¹H n.m.r. spectrum was undertaken and the hydroxy-group located by the local shifts generated in the methanesulphonate, a method preferable to dimethyl sulphoxide solvent shifts because of its much greater specificity.