The present invention provides 4-substituted-2-azetidinone compounds, bicyclic 2-5-diketopiperazine compounds, and pharmaceutical compositions thereof that are potent, safe and effective neuroprotective agents. Due to their strong central nervous system (CNS) activity, the compounds can be used to enhance memory and to treat a variety of neurological disorders. The compounds are particularly useful for treating neurological disorders caused by, or associated with, CNS trauma.