Expedient synthesis of 1,2,4-triazinyl substituted benzo[c]coumarins via double oxidation strategy

Herein, we report a convenient one-pot synthesis of 1,2,4-triazinyl derivatives of benzocoumarins. The proposed approach consists in the nucleophilic addition of tetrahydrobenzo annulated dimethoxycoumarin to 1,2,4-triazines followed by double oxidation of both dihydrotriazine and tetrahydrobenzo groups with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The nucleophilic addition of the dimethoxycoumarin to 1,2,4-triazines was carried out in the presence of three-fold excess of methanesulfonic acid in DCM at room temperature and take place between positions 4 and 5 of coumarin and 1,2,4-triazine, respectively. The double oxidation step was achieved with 3.6 equivalent of DDQ. Selective oxidation of dihydrotriazine moiety, without affecting the tetrahydrobenzo fragment, was achieved using 1.2 equivalent of tetrachlorobenzoquinone (TCQ). The differences in the oxidation with TCQ and DDQ appear to be related to the higher oxidative potential of DDQ in contrast to TCQ. The advantages of the method are the elimination of the use of transition metals, the availability of starting materials, and the simplicity of the procedure. The proposed approach provides a two-step one-pot protocol for the synthesis of triazinyl benzocoumarins, precursors for the preparation of push-pull pyridinyl chromophore

MnO2-Mediated Oxidative Cyclization of “Formal” Schiff’s Bases: Easy Access to Diverse Naphthofuro-Annulated Triazines

A different type of MnO2-induced oxidative cyclization of dihydrotriazines has been developed. These dihydrotriazines are considered as a “formal” Schiff’s base. This method provided easy access to naphthofuro-fused triazine via the C-C/C-O oxidative coupling reaction. The reaction sequence comprised the nucleophilic addition of 2-naphthol or phenol to 1,2,4-triazine, followed by oxidative cyclization. The scope and limitations of this novel coupling reaction have been investigated. Further application of the synthesized compound has been demonstrated by synthesizing carbazole-substituted benzofuro-fused triazines. The scalability of the reaction was demonstrated at a 40 mmol load. The mechanistic study strongly suggests that this reaction proceeds through the formation of an O-coordinated manganese complex