An Optimized Synthesis of Fmoc‑l‑Homopropargylglycine-OH

An efficient multigram synthesis of alkynyl amino acid Fmoc-l-homopropargylglycine-OH is described. A double Boc protection is optimized for high material throughput, and the key Seyferth–Gilbert homologation is optimized to avoid racemization. Eighteen grams of the enantiopure (>98% ee) noncanonical amino acid was readily generated for use in solid phase synthesis to make peptides that can be functionalized by copper-assisted alkyne–azide cycloaddition.