The present invention provides compounds of Formulae (I)-(V), pharmaceutically acceptable salts thereof, and pharmaceutical compositions thereof. In one aspect, compounds of the present invention are useful as glycosyltransferase inhibitors, in particular, O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) inhibitors. In another aspect, compounds of the present invention are useful as kinase inhibitors, in particular, PLK1 inhibitors, GSK3P inhibitors, or MAPKAPK2 inhibitors. The present invention further provides methods of using the inventive compounds, e.g., as biological probes to study the inhibition of OGT and/or kinase activity and as therapeutics, e.g., for the treatment of OGT-associated and/or kinase-associated conditions.