Reaction of thallium(III) salts with homoallylic alcohols: ring contraction vs. dimethoxylation.

The oxidation of 2-(3,4-dihydronaphthalen-1-yl)-ethanol (1) with a variety of thallium(III) salts was investigated. An indan, formed by a ring contraction reaction, was obtained in good to moderate yields under a variety of reaction conditions: i) thallium triacetate (TTA) in aqueous AcOH; ii) thallium tris-trifluoroacetate (TTFA) in aqueous TFA; iii) TTFA in CH(2)Cl(2); iv) thallium tripropionate (TTP) in aqueous propionic acid and v) thallium tris-[(S)-(-)-triacetoxypropionate] in aqueous (S)-(-)-2-acetoxypropionic acid. On the other hand, the reaction of compound 1 with TTA in methanol led to a 2:1 mixture of the corresponding cis- and trans-dimethoxylated compounds, respectively. These compounds were formed by a thallium-promoted addition of methanol to the double bond.