A novel and efficient synthesis of phenanthridines and aza analogues is reported. The key step is a microwave-mediated intramolecular Diels-Alder cyclization of o-furyl(allylamino)arenes. In the presence of a catalytic amount of acid, the DA-adduct reacts further to give the dihydrophenanthridines, which easily can be oxidized to fully aromatic compounds by air in the presence of UV light or by DDQ.
The structure of the title compound, C6H6O3, has been redetermined at low temperature [room-temperature structure: Maartmann-Moe (1965 ▶). Acta Cryst. 19, 155–157]. The molecule is planar with approximate D
3h point symmetry, yet it crystallizes in the chiral orthorhombic space group P212121 with a three-dimensional hydrogen-bonding network containing infinite O—H⋯O—H⋯O—H chains.
8‐Hydroxyphenathridines have been synthesized efficiently from N‐propargyl‐ortho‐furylanilines employing microwave‐mediated one‐pot intramolecular Diels–Alder reaction on furans (IMDAF) reaction and subsequent aromatization. The 8‐hydroxyphenathridines were also subjected to further functionalization; O‐alkylation and conversion to the corresponding triflate followed by Suzuki coupling. N‐Propargyl‐7‐furylindole and the corresponding indoline also underwent smooth IMDAF cyclization to give, after rearomatization, 7H‐pyrrolo[3,2,1‐de]phenanthridin‐9‐ol or the 4,5‐dihydro analog. These cyclization products could be further functionalized, i.e. O‐ or N‐alkylation and oxidation at C‐6. The oxidation could lead to 6‐oxo derivatives or pyrrolo[3,2,1‐de]phenanthridin‐6‐ium salts depending on the reaction conditions and structure of the substrate. The 9‐oxy‐pyrrolophenanthridines synthesized are structurally closely related to bioactive lycorine alkaloids.
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