title: Prediction by (13)C NMR of regioselectivity in 1,3-dipolar cycloadditions of acridin-9-yl dipolarophiles.
abstract: Strong correlation was found between (13)C NMR chemical shifts of dipolarophilic CH=CH carbons and regioselectivity in 1,3-dipolar cycloadditions of new acridin-9-yl dipolarophiles with stable benzonitrile oxides (BNO). Accordingly, two starting dipolarophiles, (acridin-9-yl)-CH=CH-R (R = COOCH3 or Ph), reacted with three BNOs (2,4,6-trimethoxy, 2,4,6-trimethyl, and 2,6-dichloro) to give a mixture of two target isoxazoline regioisomers in which the acridine was bound either to isoxazoline C-4 carbon (4-Acr) or C-5 one (5-Acr). Methyl 3-(acridin-9-yl)propenoate afforded major 4-(acridin-9-yl)-isoxazoline-5-carboxylates (4-Acr) and minor 5-(acridin-9-yl)-4-carboxylates (5-Acr). 9-(2-Styryl)acridine regiospecifically afforded only 4-Acr cycloadducts. The ratios of regioisomers were compared with analogous reactions of acridin-4-yl dipolarophiles. Regioselectivity was dependent on a polarity of the CH=CH bond, donor effects in BNO, and stabilization by stacking of aromatic substituents in the products.
authors.affiliation: P. J. Šafárik University, Faculty of Science, Institute of Chemistry, NMR Laboratory, Moyzesova 11, 040 01, Košice, Slovakia.
authors.forename: Mária
authors.initials: M
authors.lastname: Vilková
authors.affiliation: P. J. Šafárik University, Faculty of Science, Institute of Chemistry, NMR Laboratory, Moyzesova 11, 040 01, Košice, Slovakia.
authors.forename: Lucia
authors.initials: L
authors.lastname: Ungvarská Maľučká
authors.affiliation: P. J. Šafárik University, Faculty of Science, Institute of Chemistry, NMR Laboratory, Moyzesova 11, 040 01, Košice, Slovakia.
authors.forename: Ján
authors.initials: J
authors.lastname: Imrich
keywords: 1,3-dipolar cycloadditions; 13C; 1H; NMR; acridine; benzonitrile oxide; correlation; isoxazoline; regioselectivity