Scheme 2.
Preparation of target compounds 4a–j. Reagents and conditions: (i) 2-hydroxy-5-(methoxycarbonyl) benzoic acid, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, dichloromethane, and rt; (ii) LiOH, methanol: H2O, and rt; and (iii) 2-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate, N,N-diisopropylethylamine, dichloromethane, and rt.