and the pharmaceutically acceptable salts thereof, wherein R1, R2, R2′, and X are as defined as set forth in the specification, for use in treating cognitive deficits and/or psychiatric disorders, such as those associated with neurological or neurodegenerative disorders, psychiatric or mood disorders, and HIV-associated neurocognitive disorders (HAND). Compounds having formula (I) are prodrugs that release glutamine analogs, e.g., 6-diazo-5-oxo-L-norleucine (DON).
