A solution of rpm281 (0.149 g, 0.586 mmol) and methyl 2-formylbenzoate (0.105 g, 0.645 mmol, 1.1 eq) in ethanol (10 ml) was stirred in presence of piperidine (1 drop) at 80° C., under Ar for 1 h. After cooling to room temperature, the solvent was removed under reduced pressure. The residue was dissolved in ethyl acetate (15 ml) and washed with HCl 1M (10 ml). The organic extracts were collected, dried over Na2SO4 and the solvent removed under reduced pressure to afford a orange solid. The pure compound rpm347 was obtained after trituration with hexane/ethyl acetate mixture (6:4) (10 ml) as a off-white solid (0.135 g, 0.337 mmol, 60%), mp 271-273° C. MS (API-ES): m/z 401 (M+H)+; HRMS (API-ES) m/z Found: 401.1173 (M+H)+.
