Scheme 4. Regioselective Mono-Boc-Protection of Hydrodabcyl (2) due to Engagement of Two of the Three Hydroxyl Groups in Intramolecular H Bonds Leading to Stable Six-Membered Rings.

The reaction sequence 2a–2d is the suggested mechanism for the observed proton–deuteron exchange in ring B. It is drawn only once to avoid overfill; however, it concerns both aromatic protons of ring B. The exchange of the second aromatic proton occurs via the same mechanism after rotation of the ring in 2d.