A class of polycyclic compounds of general formula (I), wherein Base1, Base2, Y, Za, Xa, Xa1, Xb, Xb1, Xc, Xc1, Xd, Xd1, R1, R1a, R2, R2a, R3, R4, R4a, R5, R6, R6a, R7, R7a, R8, R8a, and R9 are defined herein, that may be useful as inductors of type I interferon production, specifically as STING active agents, are provided. Also provided are processes for the synthesis and use of compounds.