1,4,7,10,13,16-Hexanoxacyclooctadecane 1,4,7,10,13,16-Hexaoxacyclooctadecane 17455-13-9 18-Crown-6 18-Crown-6 ether Ethylene oxide cyclic hexamer NSC159836 WLN: T-18-O DO GO JO MO POTJ