The dimeric diazofluorenes known as the lomaiviticins are produced by the marine bacterium Salinispora pacifica DPJ-0019. Investigation of the fermentation broth of DPJ-0019 has yielded the first monomeric benzo[b]fluorene isolated from this species, (-)-homoseongomycin (13). (-)-Homoseongomycin (13) is related to the known natural product seongomycin (10), which is co-produced with the monomeric diazofluorenes known as the kinamycins. We describe the synthesis of the isotopically labeled derivative homoseongomycin-d5 (14), via the intermediacy of the diazofluorene "prelomaiviticin-d5" (12). Our studies establish that (-)-homoseongomycin (13) may be derived from prelomaiviticin (11) and suggest that 13 and 10 are shunt or detoxification metabolites in lomaiviticin and kinamycin biosynthesis, respectively.