Conformationally constrained compounds that mimic the secondary structure of reverse-turn regions of biologically active peptides and proteins having the following structure are disclosed: ##STR00001## wherein A, R.sub.1, R.sub.2, R.sub.2a and R.sub.3 are as defined herein. Such compounds have utility over a wide range of fields, including use as diagnostic and therapeutic agents. In particular, compounds of this invention are useful in pharmaceutical compositions as anti-inflammatory agents as well as inhibitors of central nervous disorders. Libraries containing the compounds of this invention are also disclosed, as well as methods for screening the same to identify biologically active members.