Synthesis of m,n‑Diaza[n]helicenes via Skeletal Editing of Indeno[2,1‑c]fluorene-5,8-diols

Skeletal editing of 5-membered carbocycles to 6-membered heteroaromatic compounds represents an attractive concept that would enable late-stage heteroarene construction. However, the current state of the art does not offer many synthetic strategies on how to achieve such a goal. One of those is based on a reaction of aromatic tertiary alcohols with sodium azide under acidic conditions, i.e., the Schmidt reaction. Herein, we present a hitherto unexplored double Schmidt reaction toward aromatic compounds possessing two pyridine rings. It is the first example of a conversion of 5,8-disubstituted indeno-[2,1-c]-fluorene-5,8-diols to m,n-diaza-[5]-helicenes in high yields. We also demonstrate that the regioisomeric ratio can be controlled, to a certain extent, by tuning conditions. Mechanistic investigation encompassing experimental as well as DFT calculations sheds light on the course of the reaction and provides a rationale for the observed regioselectivity and its control. In addition, structures of several diaza[5]-helicenes and intermediates were unequivocally confirmed by single crystal X-ray diffraction analyses. The double Schmidt rearrangement strategy was also applied for the transformation of enantioenriched [7]-helical indeno-[2,1-c]-fluorene-5,8-diols into the corresponding enantioenriched m,n-diaza-[7]-helicenes without significant loss of enantiopurity. Their structures were also unequivocally confirmed by single crystal X-ray diffraction analyses.