The Synthesis of (S)‐(+)‐2‐Amino‐3‐(1‐adamantyl)‐propionic Acid (L‐(+)‐Adamantylalanine, Ada) as a ‘Fat’ or ‘Super’ Analogue of Leucine and Phenylalanine

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Abstract
Because of its overall steric resemblance to the phenylalanine analogue, carboranylalanine, the title compound was prepared by the modified Strecker synthesis of Patel & Worsley. The use of (S)‐(−)‐α‐methylbenzylamine in the synthesis, the positive trend of [α]D with increasing protonation, and the thin‐layer chromatographic behaviour of synthetic diastereomeric dipeptides are strong indications that the configuration at the asymmetric carbon atom is S (natural L). Its optical purity was ascertained by purification via the quinine and ephedrine salts of t‐butoxy‐carbonyl‐adamantylalanine. The new amino acid shall be used for studies of structure‐activity relationships of phenylalanine and leucine residues in biologically active peptides. In addition, a convenient synthesis in excellent yield of the starting material, 2‐(1‐adamantyl)‐ethanal, by Pfitzner‐Moffat oxidation of 2‐(1‐adamantyl)‐ethanol is described.