General synthesis of polysubstituted benzo[b]furans

Abstract

2-(Benzotriazol-1-ylmethyl)furans 4a-c, on treatment with n-BuLi followed by various α,β-unsaturated ketones and aldehydes, gave the adducts 6a-f which underwent intramolecular cyclization to form substituted benzo[b]furans 10a-f. Lithiation and subsequent alkylation of 4a-c afforded intermediates 8a-c, which underwent similar transformations with α,β- unsaturated ketones or aldehydes to give the corresponding polysubstituted benzo[b]furans 7a-c. o-Iodophenol (11) with 1-propargylbenzotriazole (1) in the presence of (PPh3)2PdCl2, CuI, and Et3N gave 2-(benzotriazol-1- ylmethyl)benzo[b]furan (12). The benzotriazolyl-CH2 side chain of 12 was further elaborated by (i) alkylation with trans-cinnamaldehyde to give the adduct 14, followed by intramolecular cyclization to give dibenzofuran 17; (ii) alkylation with benzyl bromide followed by elimination or nucleophilic displacement of the benzotriazolyl moiety with Grignard reagent and subsequent dehydrogenation to give 2-alkenylbenzofurans 15 or 20, respectively; (iii) alkylation with (CH3)3CCHO followed by low-valent titanium-promoted olefination to give olefin 19; and (iv) alkylation with benzaldehyde followed by Lewis acid catalyzed pinacol type reaction to give 18.