Mixed Liquid and Solid Phase Synthesis of Isopeptidic Desferrioxamine Analogues for Complexation of Zirconium

Abstract

Abstract89Zr has a number of advantageous properties for PET‐imaging, but optimal chelators for its widespread clinical application still need to be found. Tetrahydroxamates derived from the natural siderophore DFOB are promising candidates in this context. This study describes a solid‐phase approach to isopeptide analogs of DFOB (termed ipDFO*) with four hydroxamate groups for stable Zr(IV)‐chelation. The route uses either commercial or easily available Fmoc‐protected precursors and allows the synthesis of ipDFO* and clickable AZA−ipDFO* derivatives in only a few hours. The solid‐phase synthesis reduces the workload for the synthesis of ipDFO* and AZA−ipDFO* derivatives thus significantly compared to a previous multistep solution synthesis. The route is also modular and allows the synthesis of various ipDFO* derivatives with variation in the spacer length between the hydroxamate groups. This spacing is a critical parameter for finding the optimal chelator for hard metal cations like Zr(IV). It might thus help to speed up the search for an ideal Zr‐chelator for PET‐imaging and might also allow the extension to other metal cations of different size.