SOLID STATE SYNTHESIS OF SUBSTITUTED COUMARIN-3-CARBOXYLIC ACIDS VIA THE KNOEVENAGEL CONDENSATION UNDER MICROWAVE IRRADIATION

Synthesis of 4-(2-fluorophenyl)-7-methoxycoumarin: experimental and computational evidence for intramolecular and intermolecular C-F···H-C bonds

4-(2-Fluorophenyl)-7-methoxycoumarin (6) was synthesized by Pechmann reaction under mild conditions via a three-step reaction. The solution-state ¹H NMR spectra of 6 showed a strong intramolecular interaction between F and H5 (J_FH = 2.6 Hz) and ¹³C NMR suggested that this C–F···H–C coupling is a through-space interaction. The 2D ¹⁹F-{¹H} HOESY and ¹H-{¹⁹F} 1D experiments were done to confirm this F···H interaction. The single crystal X-ray structure and the DFT-optimized structure showed that the fluorinated phenyl ring favors the orientation with the fluorine atom closer to H5 than H3. The X-ray structure also showed the existence of the intermolecular C–F···H–C interaction.

Solvent-Free L-Proline Catalysed Condensation of Ethyl Cyanoacetate with Aldehydes

Solvent-free Knoevenagel condensation of ethyl cyanoacetate with aromatic aldehydes, catalysed by environmentally friendly silica gel supported L-proline under microwave irradiation is studied.

Regio-Controlled Michaelis-Arbuzov Reactions of 3-(halomethyl)-coumarins

3-(Iodomethyl)coumarins and 3-(chloromethyl)coumarins, obtained chemoselectively via Baylis–Hillman reactions of salicylaldehyde derivatives with t-butyl acrylate, can be reacted with triethyl phosphite to afford regioisomeric Michaelis-Arbuzov products. Under nitrogen, the 3-(iodomethyl)coumarins undergo direct displacement of iodide to afford the expected 1′-phosphonated derivatives. The reactions with 3-(chloromethyl)coumarins in air, however, proceed with overall allylic rearrangement to afford the regioisomeric 3-methyl-4-phosphonated derivatives.