Preparation of N,N-bis[2-[N ',N '-bis[(tert-butoxycarbonyl) methyl]-amino]ethyl-L-aspartic acid: An intermediate in the synthesis of MRI contrast agents

A preparation of DTPA carboxylic acid 1, a suitable intermediate in the synthesis of MRI contrast agents 2, is described starting from alpha-tert-butyl-beta-benzyl-L-aspartate hydrochloride. In addition, an alternative procedure for the synthesis of bromide 3b is presented.