Effect of methoxyl groups on the NMR spectra: configuration and conformation of natural and synthetic indanic and tetralinic structures.

Abstract

Here, we studied the influence of the methoxyl groups attached at C-7 and C-2' of natural and synthetic 1-arylindanes on the chemical shift of the signal of bibenzylic hydrogen and carbon atoms and J1,2 coupling constants. This influence was also analysed in natural 1-aryltetralins and related compounds that possess methoxyl and/or hydroxyl groups bound at C-8 and C-2'. The methoxyl groups attached at C-7 in indanes or at C-8 in tetralins produce a deshielding signal at H-1 and shield at C-1 and a strong decrease in the value of J1,2 due to the pseudoequatorial location adopted by the aryl group bound at C-1, avoiding an 'A1,3 strain'. Furthermore, compounds with hydroxyl or methoxyl groups in C-2', in the absence of substituents of C-7 or C-8, present a strong deshielding signal at H-1, strong shield of the C-1 signal and a decrease in the value of J1,2 . This is attributed to the stereoelectronic effects of the methoxyl or hydroxyl groups, which we have called 'Asarone effect'. NOESY experiments were conducted to confirm the configuration and conformation of some of the compounds included in this work. This study shows that both effects, A1,3 strain and Asarone effect, must be taken into account when the structure of natural indanes and tetralins is analysed by using 1 H-NMR and 13 C-NMR spectra. Copyright © 2016 John Wiley & Sons, Ltd.