Development of a Multigram Asymmetric Synthesis of 2-(R)-2-(4,7,10-Tris tert-Butylcarboxymethyl-1,4,7,10-tetraazacyclododec-1-yl)-pentanedioic Acid, 1-tert-Butyl Ester, (R)-tert-Bu4-DOTAGA

A process for the multigram asymmetric synthesis of the chiral tetraazamacrocycle 2-(R)-2-(4,7,10-tris tert-butylcarboxymethyl-1,4,7,10-tetraazacyclododec-1-yl)-pentanedioic acid, 1-tert-butyl ester ((R)-tert-Bu4-DOTAGA, 4) has been devised and demonstrated. The nine-step synthesis features an improved synthesis of 2-(S)-5-oxotetrahydrofuran-2-carboxylic acid, tert-butyl ester 8, the precursor to the novel alkylating agent (S)-5-benzyl 1-tert-butyl 2-(methylsulfonyloxy)pentanedioate 12, which was used to introduce an orthogonally protected chiral glutarate arm to the 1,4,7,10-tetraazacyclododecane (cyclen) nucleus in high optical purity. Cyclen derivative (R)-t-Bu4-DOTAGA, 4, a key intermediate for the manufacture of a magnetic resonance imaging (MRI) candidate, was produced with high chemical (≥95%) and optical (ee ≥ 97%) purity. The process developed was successfully applied to the kilogram-scale cGMP synthesis of (R)-t-Bu4-DOTAGA.