NOVEL OPEN-CHAIN AND CYCLIC CONFORMATIONALLY CONSTRAINED (R)-ALPHA,ALPHA-DISUBSTITUTED AND (S)-ALPHA,ALPHA-DISUBSTITUTED TYROSINE ANALOGS

A series of novel open-chain and cyclic conformationally constrained ( R)- and (S)-alpha,alpha-disubstituted tyrosine analogues 1a-e were syn thesized in good yields and high optical purities (Schemes 1 and 2). T he absolute configurations of these tyrosine analogues were unambiguou sly determined based on the X-ray structures of the precursor diastere oisomeric peptides of type 4 and 5. Four of these structures are descr ibed (Figs. 1-4), showing beta-turn type-I geometries for dipeptides 4 b, 5b, and 4c and an extended conformation for peptide 5c (Table 3). T he conversion of the free amino acids 1a-c into suitably protected bui lding blocks 11a-d and 15d,e for peptide synthesis is discussed (Schem es 3 and 4).