Construction of functionalized indans by thallium(III) promoted ring contraction of 3-alkenols

Rearrangement of beta,gamma-unsaturated esters with thallium trinitrate: synthesis of indans bearing a beta-keto ester moiety

“…4,6 Thus, the first step would be the formation of the oxythallated adduct 12 that is produced by the electrophilic addition of thallium(III) to the double bond. Such an addition would be assisted by the oxygen of the carbonyl group through coordination with the thallium atom.…”

“…Considering this mechanism, one could expect that the formation of the indans 9a-d would be diastereoselective, similarly to the observed in the ring contraction of 7. 4,6 However, the indans 9a-d were obviously isolated as a 1:1 mixture of diastereomers, because the estereocenter flanked by the two carbonyl groups readily epimerizes under the acidic conditions of the reaction medium. Furthermore, the epimerization could also take place by abstraction of the α-carbonyl hydrogen of the cyclopentane ring of indans such as 9.…”

“…2,3 Our research group has investigated approaches to obtain indans from 1,2-dihydronaphthalenes, using a thallium(III)-promoted ring contraction reaction. [4][5][6][7][8][9] During these studies, we found that a side chain at the double bond has a strong influence in the reaction pathway. 5 The treatment of 1,2-dihydronaphthalenes bearing a β,γ-unsaturated carboxylic acid unit, such as 1, with thallium triacetate (TTA) gave a mixture of the isomeric allylic acetates 2 and 3, which are produced through an oxidative decarboxylation process (Scheme 1).…”

“…5 In contrast, the presence of a hydroxyl group at the side chain makes the rearrangement the preferable reaction, as exemplified in the formation of 8 from the alkenol 7. 4 This subtle modification has been attributed to the coordination of the hydroxyl group with the thallium(III), which would help the electrophilic addition step. 4,6 The homoallylic alcohols 7 were efficiently obtained in three steps from 1-tetralones, through the β,γ-unsaturated esters 6.…”

“…4 This subtle modification has been attributed to the coordination of the hydroxyl group with the thallium(III), which would help the electrophilic addition step. 4,6 The homoallylic alcohols 7 were efficiently obtained in three steps from 1-tetralones, through the β,γ-unsaturated esters 6. We then realized that these esters would also be interesting substrates for thallium(III)-promoted ring contraction reactions, because the indans products of this transformation (9), would bear a β-keto ester unit, which is a valuable moiety for further functionalization 11 (Scheme 3).…”