Novel open‐chain and cyclic conformationally constrained ( R )‐ and ( S )‐α,α‐disubstituted tyrosine analogues

A series of novel open‐chain and cyclic conformationally constrained ( R )‐ and ( S )‐α,α‐disubstituted tyrosine analogues 1a–e were synthesized in good yields and high optical purities ( Schemes 1 and 2 ). The absolute configurations of these tyrosine analogues were unambiguously determined based on the X‐ray structures of the precursor diastereoisomeric peptides of type 4 and 5 . Four of these structures are described ( Figs. 1–4 ), showing β‐turn type‐I geometries for dipeptides 4a, 5b , and 4c and an extended conformation for peptide 5c ( Table 3 ). The conversion of the free amino acids 1a–c into suitably protected building blocks 11a–d and 15d,e for peptide synthesis is discussed ( Schemes 3 and 4 ).