Maize seeds were germinated in an atmosphere of O-16(2)/O-18(2). 2,4-D
ihydroxy-7-methoxy-1,4-benzoxazine-3-one (DIMBOA) was isolated and ana
lysed by C-13 NMR spectroscopy. Heavy isotope shifts of C-13 signals i
ndicated that all four oxygen atoms bonded to carbon atoms of DIMBOA a
re derived from molecular oxygen. (C) 1997 Elsevier Science Ltd.