Dinucleotide cap analogs are disclosed, modified at different phosphate positions with a boranophosphate group or a phosphoroselenoate group. The analogs are useful as reagents in the preparation of capped mRNAs and have increased stability both in vitro and in vivo. They may be used as inhibitors of cap-dependent translation. Optionally, the boranophosphate or phosphoroselenoate group has a 2'-O or 3'-O-alkyl group, preferably a methyl group, producing analogs called BH3-ARCAs or Se-ARCAs. ARCAs may be modified with alpha-, beta-, or gamma-boranophosphate or phosphoroselenoate groups.
