Concise Chemoenzymatic Synthesis of 4,5-Dihydroxyisoleucine Fragment of α-Amanitin

The ability of α-amanitin to potently inhibit RNA polymerase II (RNAP II) has elicited further research into its use as a novel payload for antibody-drug conjugates. Despite this promise, the de novo synthesis of α-amanitin is still a major chal-lenge, as it possesses an unusual bicyclic octapeptide structure that contains several oxidized amino acids, most notably 4,5-dihydroxy-L-isoleucine. Here, we report a concise chemoenzymatic synthesis of this key amino acid residue, which features two regio- and diastereoselective enzymatic C–H oxidations on L-isoleucine.