The present invention provides a convenient and cost-effective process for preparation of 4-carboxyl-5,8,11-tri(carboxymethyl)-l-phenyl-2-oxa-5,8,11-triazatridecan-13-oic acid (BOPTA), which is the key intermediate in the synthesis of Gadobenate dimeglumine. The process comprises reaction of tertiary-butylcarboxymethyl substituted diethylenetriamines with tertiary butyl (3-benzylbxy-2-bromo)propionate, alkylation of the resulting amine with tertiary butyl bromoacetate, followed by deprotection of the tertiary butyl group and isolation to give BOPTA (I) having desired purity.
