The best yield of 61% was achieved using 2.5 equiv. of TfOH (entry 1-3). Next, the influence of the solvent was investigated. A 1:1 mixture of DCM and TFE gave an increased yield of 71% (entry 4), while in pure DCM or chloroform only incomplete conversion of the oxidized intermediate was observed (entries 5-6). Finally, MeCN was found to be the best solvent for this transformation, giving 1a in 76% yield (entry 7). Even in this solvent the amount of added TfOH could not be decreased (entry 8). On a 1 mmol scale, 3 equiv. of TfOH were slightly superior (entries 9-10).