Accelerating effect of meta substituents in the ester-mediated nucleophilic aromatic substitution reaction

Abstract

The ester-mediated nucleophilic aromatic substitution (S_NAr) reaction of 2-methoxybenzoic ester 1 with Grignard reagents 11 is greatly accelerated by introduction of a methoxy or halo substituent at the 3-position of the benzoate ring (7–10). The substituent effects of these groups at the 3-position are compared with those at the 5-position to suggest that the activation mechanism of the methoxy substituent is different from that of the halo substituent; the ligating ability of the 3-methoxy group plays a crucial role in enhancing the reactivity of the 2-methoxy moiety, while the electron-withdrawing ability is more important in the case of the halo groups. It has also been found that introduction of an additional methoxy substituent at the meta-position (33, 34) enables the S_NAr methoxy-displacement reaction even at the para-position to the ester activator. The accelerating effect of the 3-bromo substituent is advantageously utilized for regioselective allylation of 3-bromo-2,6-dimethoxybenzoic ester 55 at the 2-position to provide an easy access to a multisubstituted naphthol 59, which is a key compound for the syntheses of michellamines A–C and the related naphthylisoquinoline alkaloids.