The present invention is directed to compounds of the structural Formula: wherein: R1 is H or —C1-C3 alkyl; R2 is selected from the group consisting of —H, —C1-C4 alkyl, —C1-C3 alkyl-CF3, phenyl, and pyridinyl; and R3 is selected from the group consisting of —H, —C1-C4 alkyl, —C1-C3 alkyl-O—CH3, —CH2-cyclopropyl, CH2—C═CH2, —CH2CH2-(2-F-phenyl), and phenyl substituted with from 1 to 2 fluorines; provided that when R1 and R2 are each H, then R3 is selected from the group consisting of —C1-C4 alkyl, —C1-C3 alkyl-O—CH3, —CH2-cyclopropyl, —CH2—C═CH2, —CH2CH2-(2-F-phenyl), and phenyl substituted with from 1 to 2 fluorines; or stereoisomers and pharmaceutically acceptable salts thereof.