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).