This invention provides compounds having the structure: ##STR00001## wherein R.sub.1 is methyl and R.sub.2 is H, or R.sub.1 is ethyl and R.sub.2 is hydroxymethyl; R.sub.3 is H or propargyl; and R.sub.4 is either H or t-butyloxycarbonyl; or an enantiomer or a pharmaceutically acceptable salt thereof. This invention further provides the use of a therapeutically effective amount of such compounds to treat a subject afflicted with a psychiatric disorder, Parkinson's disease, Alzheimer's disease, dementia, or a neurological disorder.