Synthesis and conformational features of topographically constrained designer chimeric amino adds: The β-isopropyl phenylalanines

All four optically pure isomers of the highly conformationally constrained novel chimeric amino acid, β-isopropylphenylalanine or β- phenylleucine, were asymmetrically synthesized in five to six steps in 20 - 25% overall yield. Computer-assisted molecular modeling revealed that the β- isopropyl group in these chimeric amino acids plays the dominant role in determining the most favorable side chain conformations.