1H and 13C NMR spectral studies of some 4H-3,1-benzoxazin-4-ones and their 2-acylaminobenzoic acid precursors

Fischer indolisation ofN-(α-ketoacyl)anthranilic acids into 2-(indol-2-carboxamido)benzoic acids and 2-indolyl-3,1-benzoxazin-4-ones and their NMR study

“…High tendency of related 3,1-benzoxazin-4-ones for hydrolysis into the corresponding anthranilic acids has been documented. 29,30 With these results in mind it can be assumed that higher quantities of the 3,1-benzoxazin-4-one products 4 are actually formed during the Fischer indolisation of N-(α-ketoacyl)anthranilic acids 2 shown in Table 3 as they were actually isolated. During the isolation workup the latter most probably partly hydrolyse back into the 2-(indol-2-carboxamido)benzoic acids 1.…”

“…Our results are in agreement with the literature data. 29 Chemical shifts for the 3,1-benzoxazin-4-one nitrogen atoms in compounds 4 were in the range of δ 212.0-222.8 ppm (Table 7). Due to the lack of the 15 N NMR data no comparison with the literature could be done.…”

“…Systematic NMR investigations of 3,1-benzoxazin-4-ones are more scarce. 29,35 As pointed out by Osborne and Goolamali some early NMR data should be taken with care because these compounds often show high tendency for hydrolysis into the corresponding anthranilic acids, especially when measured in polar solvents such as DMSO-d 6 . 29 High susceptibility towards hydrolysis has been illustrated by 2-methyl-3,1-benzoxazin-4-one that reacts with water into 2-acetylaminobenzoic acid already in the solid state.…”

“…29,35 As pointed out by Osborne and Goolamali some early NMR data should be taken with care because these compounds often show high tendency for hydrolysis into the corresponding anthranilic acids, especially when measured in polar solvents such as DMSO-d 6 . 29 High susceptibility towards hydrolysis has been illustrated by 2-methyl-3,1-benzoxazin-4-one that reacts with water into 2-acetylaminobenzoic acid already in the solid state. 30 Osborne and Goolamali reported an unequivocal differentiation between anthranilic acids and 3,1-benzoxazin-4-ones that was achieved through determination of characteristic J CH coupling interactions in the carbonyl region of the proton coupled 13 C NMR spectra.…”

“…30 Osborne and Goolamali reported an unequivocal differentiation between anthranilic acids and 3,1-benzoxazin-4-ones that was achieved through determination of characteristic J CH coupling interactions in the carbonyl region of the proton coupled 13 C NMR spectra. 29 Herein, the proton, carbon and nitrogen resonances belonging to the anthranilic moiety in 1 and the 3,1-benzoxazin-4-one group in 4 were rapidly assigned using the gradientselected 2D NMR experiments. The results are collected in Tables 6 and 7.…”