The macrocyclic core of the cytotoxic marine natural product callyspongiolide (1) was forged by ring‐closing alkyne metathesis (RCAM) of an ynoate precursor using a molybdenum alkylidyne complex endowed with triarylsilanolate ligands as the catalyst. This result is remarkable in view of the failed attempts documented in the literature at converting electron deficient alkynes with the aid of more classical catalysts. The subsequent Z‐selective semi‐reduction of the resulting cycloalkyne by hydrogenation over nickel boride required careful optimization in order to minimize overreduction and competing dehalogenation of the compound's alkenyl iodide terminus as needed for final attachment of the side chain of 1 by Sonogashira coupling. The required cyclization precursor itself was prepared via Kocienski olefination.

A path‐scouting investigation into the highly cytotoxic marine macrolide callyspongiolide is reported that capitalizes on the selective formation of the C10−C11 alkene site. While the closure of the macrocycle by ring closing alkyne metathesis (RCAM) with the aid of a molybdenum alkylidyne complex was high yielding, the envisaged semi‐reduction of the cycloalkyne to the corresponding E‐alkene proved challenging. The reasons are likely steric in origin, in that the methyl branches on either side of the alkyne seem to prevent effective coordination of the substrate to the ruthenium catalyst, which must carry a bulky Cp* ligand to ensure high trans‐selectivity. This notion is supported by the preparation of a callyspongiolide analogue, in which the two methyl groups in question are excised; its formation by RCAM followed by trans‐hydrostannation/proto‐destannation was straightforward. In parallel work the formation of the fully functional building block 54 showed that the presence of an unprotected ‐OH group allows even hindered substrates to be processed: the protic group adjacent to the triple bond engages with a chloride ligand on the ruthenium catalyst in hydrogen bonding and hence assists in substrate binding. Moreover, the preparation of an alkynylogous callyspongiolide analogue is described.

Teraryl‐based α‐helix mimetics have proven to be useful compounds for the inhibition of protein‐protein interactions (PPI). We have developed a modular and flexible approach for the synthesis of teraryl‐based α‐helix mimetics using pyridine containing boronic acid building blocks to increase the water solubility. Following our initial publication in which we have introduced the methodology in combination with sequential Pd‐catalyzed cross‐coupling for teraryl assembly, we can now report a complete set of pyridine based boronic acid building blocks decorated with side chains of all proteinogenic amino acids relevant for PPI (Ala, Arg, Asn, Asp, Cys, Gln, Glu, His, Ile, Leu, Lys, Met, Phe, Ser, Thr, Trp, Tyr, Val) to complement the core fragment set. For a representative set of teraryls we have studied the influence of the pyridine rings on the solubility of the assembled oligoarenes.

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