alternatively, when two Rc groups are attached to the same carbon atom, they form a 5–6 membered carbocyclic or heterocyclic spiro ring C consisting of carbon atoms, 0–4 ring heteroatoms selected from O, N, and Sp, and 0–2 double bonds, provided that {HEAD ~ ring C} contains other than a S—S, O—O, or {TAIL ~ S}—O bond 