Synthesis of orthogonally protected (2S)-2-amino-adipic acid (α-AAA) and (2S,4R)-2-amino-4-hydroxyadipic acid (Ahad).

(2S,4R)-2-amino-4-hydroxyadipic acid (Ahad) building block 45 was synthesized in 11 steps and 6.5% overall yield from commercially available materials. Key steps in stereocontrol were an asymmetric conjugate addition employing a proline-based catalyst and a syn-selective intramolecular-conjugate addition of an oxygen nucleophile to an α,β-unsaturated ester. To enable incorporation of α-amino-adipic acid (α-AAA) and Ahad into peptides, a truly orthogonal protecting group scheme was developed, encompassing an allyloxycarbonyl (Alloc) carbamate for Nα, a tert-butyl ester for the δ-COOH, an acetol ester for the α-COOH, and a tert-butyldimethylsilyl ether for the γ-hydroxy group of Ahad.