Ascamycin 1 and dealanylascamycin 2 are adenosine containing nucleoside antibiotics carrying the rare 5’‐O‐sulfamoyl ribose motif and a chlorine atom attached to C2 of the adenine ring. Gene knockouts (ΔacmK and ΔacmJ) in the producing strain, Streptomyces sp. JCM9888, generated mutants that were disabled in the assembly of the sulfamoyl moiety. This resulted in the production of 2‐chloroadenosine 3 and 2‐chloroadenine 4, indicating that purine halogenation can occur independently of sulfamoylation. Incubations in media enriched in bromide content resulted also in the identification of 2‐bromoadenine 5, 2‐bromoadenosine 6 and 5′‐O‐sulfamoyl‐2‐bromoadenosine 7.