| abstract |
A compound of formula (I): ** Formula ** or a pharmaceutically acceptable salt thereof, wherein R1 is selected from the group consisting of an H and a protecting group; and R2 is a substituted or unsubstituted residue selected from the group consisting of C1 to C6 alkyl, C2 to C6 alkenyl, C2 to C6 alkynyl, aryl or heteroaryl. |