Radical SAM enzyme that catalyzes the cyclization of dehypoxanthine futalosine (DHFL) into cyclic dehypoxanthine futalosine (CDHFL), a step in the biosynthesis of menaquinone (MK, vitamin K2). dehypoxanthine futalosine + S-adenosyl-L-methionine = 5'-deoxyadenosine + cyclic dehypoxanthinylfutalosinate + H(+) + L-methionine Binds 1 [4Fe-4S] cluster. The cluster is likely coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. Quinol/quinone metabolism; menaquinone biosynthesis. Belongs to the radical SAM superfamily. MqnC family.