Emtricitabine cis-Cyclic Impurity

: (3S,6R,11S)-11-fluorotetrahydro-2H-3,6-epoxypyrimido[6,1-b][1,6,3]oxathiazocine-8,10(3H,9H)-dione; (1S,6S,7R,10R)-6-Fluoro-8.13-dioxa-11-thia-2.4-diazatricyclo[8.2.1.02,7]tridecane-3,5-dione ;