Synthesis of 11,12-dihydro benzo[c]phenanthridines via a Pd-catalyzed unusual construction of isocoumarin ring/FeCl3-mediated intramolecular arene-allyl cyclization: First identification of a benzo[c]phenanthridine based PDE4 inhibitor

In spite of their various pharmacological properties the anti-inflammatory potential of benzo[c]phenanthridines remained underexplored. Thus, for the first time PDE4 inhibitory potential of 11,12-dihydro benzo[c]phenanthridine/benzo[c]phenanthridine was assessed in vitro. Elegant synthesis of these compounds was performed via a multi-step sequence consisting of a Pd-catalyzed unusual construction of 4-allyl isocoumarin ring and FeCl₃-mediated intramolecular regio- as well as site-selective arene-allyl cyclization as key steps. The overall strategy involved Sonogashira coupling followed by isocoumarin and isoquinolone synthesis, then chlorination and subsequent cyclization to afford a range of 11,12-dihydro derivatives. One of these dihydro compounds was converted to the corresponding benzo[c]phenanthridine that showed concentration dependent inhibition of PDE4B affording an initial hit molecule. The SAR study suggested that 11,12-dihydro analogs were less potent than the compound having unsaturation at the same part of the ring.

Theoretical Insight into the Mechanism and Origin of Divergent Reactivity in the Synthesis of Benzo-Heterocycles from o-Alkynylbenzamides Catalyzed by Gold and Platinum Complexes

This work presents a DFT study on the mechanism and origin of catalyst-controlled divergent reactivity in the synthesis of benzo-heterocycles from o-alkynylbenzamides by Au(I)/Pt(IV) catalysis. The results indicate that the transformations proceed via a nucleophilic cyclization process. In the Au(I) catalysis, the preferred O-attack mode mainly originates from the symmetry match in the dominant bond-forming interaction between the lone-pair orbital of carbonyl-O and the in-plane alkyne π* orbital, and the electronic property of the ligand controls the O-5-exo-dig/O-6-endo-dig selectivity. The preference for the N-attack mode in Pt(IV) catalysis is attributed to the stronger coordinate capability of carbonyl-O than amino-N in the substrate to PtCl₄, and the regioselective N-6-endo-dig or N-5-exo-dig cyclization depends on the stronger electrostatic interaction between the amino-N and alkynyl-C_β atoms. The theoretical results provide a fundamental understanding of why and how gold and platinum complexes catalyze the cyclization of o-alkynylbenzamides with different chemo- and regioselectivities.