Strong deshielding in aromatic isoxazolines

Very strong proton deshielding was found in di/tri-aromatic isoxazoline regioisomers prepared from acridin-4-yl dipolarophiles and stable benzonitrile oxides (BNO). Three alkenes, (acridin-4-yl)-CH=CH-R (R = COOCH3, Ph, and CONH2), reacted with three BNO dipoles (2,4,6-trimethoxy, 2,4,6-trimethyl, 2,6-dichloro) to give pairs of target isoxazolines with acridine bound to C-4 or C-5 carbon of the isoxazoline (denoted as 4-Acr or 5-Acr). Regioselectivity was dependent on both the dipolarophile and dipole character. The ester and amide dipolarophile displayed variable regioselectivity in cycloadditions whereas the styrene one afforded prevailing 4-Acr regioisomers. 2,4,6-Trimethoxy-BNO was most prone to form 5-Acr isoxazolines while mesitonitrile oxide gave major 4-Acr isoxazolines. Basic hydrolysis of the amide cycloadduct led to an unexpected isoxazolone product. The structure of the target compounds was studied by NMR, MS, and X-ray crystallography.