Patent ID: 8431591

Claim:
A compound of formula I: or a pharmaceutically acceptable salt thereof, wherein Y 1 is H, C(O)—R 3 , C(O)—O—R 3 , P(O)(OH)—O—R 3 , C(S)—R 3 , C(S)—O—R 3 , C(S)—NR 3 R 4 , or fatty acid acyl; R 1 is H, C 1-4 alkyl, C 2-4 alkenyl, or C 2-4 alkynyl; and each of R 3 and R 4 is, independently, selected from H, C 1-12 alkyl, C 2-12 alkenyl, C 2-12 alkynyl, C 2-6 heterocyclyl, C 6-12 aryl, C 7-14 alkaryl, C 3-10 alkheterocyclyl, and C 1-7 heteroalkyl, or R 3 and R 4 together form a heterocyclic ring containing at least one nitrogen atom.