Aristeromycin and neplanocin A are two naturally occurring carbocyclic nucleosides isolated from Streptomyces citricolor. They belong to a small family of cyclopentane-containing natural products that are derived from carbohydrates, in this case D-glucose, as the ultimate biosynthetic precursor. This review outlines our current understanding of the biosynthesis of aristeromycin and neplanocin and proposes possible mechanisms for the conversion of carbohydrates into 5-membered ring carbocycles.