Compounds which mimic the secondary structure of helical regions of biologically active peptides and proteins having the following structure: 1 including pharmaceutically acceptable salts and stereoisomers thereof, wherein Y, A, B, R.sub.1 and R.sub.2 are as defined herein. Such compounds have utility over a wide range of applications, including use as diagnostic and therapeutic agents. In particular, compounds of this invention, and pharmaceutical compositions containing the same, are neurokinin (tachykinin) antagonists. Libraries containing the compounds of this invention are also disclosed, as well as methods for screening such libraries to identify biologically active members.