Synthesis of Lipophilic Paramagnetic Contrast Agents

The facile, high-yielding synthesis of a series of macrocycles 7ak in 75−100% yield is reported. The transformation of these compounds to their carboxymethylated analogues 8ak in 75−90% yield and subsequent gadolinium complexes 9ak provides a series of homologous neutral paramagnetic contrast agents (PCAs) with tunable lipophilicity. Alkylated cationic intermediates 6ak are prepared in yields of 72−94% from glyoxal adduct of cyclen (5) and slight excesses of alkyl iodides. The methodology is selective for monoalkylation and amenable to large-scale synthesis.