Equipping 1,4,7-Triazacyclononane with Substituents via Solid-Phase Synthesis

Metal ion complexes frequently show substituted 1,4,7-triazacyclononane (tacn) as the ligand. Besides providing donor atoms for complex formation, tacn serves as a scaffold for equipping the complex with further functional units that are needed for the complementation and electronic tuning of the metal ion coordination sphere and/or add other features, e.g., light-absorbing antennas and groups for bioconjugation. To exploit the full potential of substituted tacn, strategies for directed syntheses of NO(R1,R1,R2) and NO(R1,R2,R3), i.e., tacn with two and even three different substituents R, are needed. Herein, we report a strategy that takes advantage of solid-phase synthesis in the assembly of the precursors NO(R1,R1,H) and NO(R1,R2,H). The assembly of NO(R1,R2,H) is based on a highly selective formation of NO(Cbz,tfAc,H), with Cbz being the link between tacn and solid phase. For this, tacn was loaded onto (4-nitrophenyl carbonate)-resin, thereby forming resin-bound (rb)-tacn, which corresponds to NO(Cbz,H,H) bound to the solid phase. Treatment of rb-tacn with ethyl trifluoroacetate gave rb-NO(tfAc,H), which corresponds to NO(Cbz,tfAc,H). With rb-tacn and rb-NO(tfAc,H) in hand, a variety of NO(R1,R1,H) and NO(R1,R2,H) were prepared, showing the broad applicability of the strategy with respect to the type of substituents and of reactions (nucleophilic substitution, reductive amination, aza-Michael addition, addition to epoxides, acylation). The study also identified limitations and points for improvement.

From Cyclic Peptoids to Peraza-macrocycles: A General Reductive Approach

Peraza-macrocycles form chelates of high thermodynamic and kinetic stability useful in diagnostic imaging (MRI, SPECT, PET), in coordination chemistry, and as catalysts. In this letter, we report an advantageous method to prepare these compounds via BH₃-induced reduction of cyclic peptoids. Using this procedure, 10 homo- and heterosubstituted aza-coronands, with different sizes and side chains, have been synthesized from the corresponding cyclic oligoamides. Solid structures of free, protonated, and Na⁺ coordinated polyaza-derivatives have been disclosed by single-crystal X-ray diffraction analysis.

Synthesis of chiral nine and twelve-membered cyclic polyamines from natural building blocks

A rational strategy for the facile and efficient cyclization of amino acid-based linear precursors forming nine and twelve-membered cyclic peptidomimetics is reported.