主权项 |
1. A compound of Formula I: or a pharmaceutically acceptable salt thereof, wherein
R1 is cyclobutyl or R2 is H or methoxy; andR3 and R4 are each, independently, H, hydroxyl, or NR5R6 wherein R5 and R6 are each independently H, alkyl, or substituted acyl, or alternatively, R3 and R4, together with the carbon atom to which they are attached, form C═O or C═CH2. |