A compound, or a pharmaceutically acceptable salt or ester thereof, having a structure of: wherein A, B and D are each oxygen or sulfur, provided that least one of A, B and D is sulfur; and R1-R8 are each independently hydrogen, hydroxyl, acyl, substituted acyl, acyloxy, substituted acyloxy, alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, alkoxy, substituted alkoxy, aryl, substituted aryl, amino, substituted amino, halogen, heterocycloalkyl, substituted heterocycloalkyl, heteroaryl, substituted heteroaryl, or a thio-containing group.