NMR assay of diastereoisomers of 7-chloro-3,3a-dihydro-2-methyl-2h, 9h- isoxazolo(3,2-b)(1,3)benzoxazin-9-one using deuterated tris(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionato)europium III shift reagent