Intramolecular asymmetric Pummerer reactions as a key step in the synthesis of bicyclic precursors of anthracyclinones

Highly stereoselective Pummerer reactions were observed on reaction of β-hydroxysulfoxides 2a–2d and 2′b with TMSOTf. Sulfenium intermediates are captured intramolecularly by the electrophilic aromatic ring, thus yielding bicyclic structures with a p-tolylsulfenyl group at the benzylic position in a cis arrangement with respect to the hydroxyl group. The stereogenicity transfer seems to be mainly controlled by the hydroxylated chiral carbon. Resulting compounds 3a, 3c and 3d can be used as bicyclic precursors of different anthracyclinones.