Heteroatom-Directed Metalation. Lithiation of N-Propenylbenzamides and N-Propenyl-o-toluamides. Novel Routes to Ortho-Substituted Primary Benzamide Derivatives and N-Unsubstituted Isoquinolin-1(2H)-ones

Article abstract of DOI:10.1021/jo00035a030

Reaction of N-propenylbenzamides 4 and 9, obtained by LDA-induced isomerization of the corresponding N-allylbenzamides, 1, 8, and 14, with 2 equiv of sec-butyllithium or tert-butyllithium at low temperature regiospecifically generates the highly reactive N,ortho-dilithiated species (e.g., 5 and 17).These dilithio species react avidly with a wide spectrum of electophilic reagents, including alky halides, giving adducts which on hydrolysis with warm 50percent aqueous acetic acid are converted into ortho-substituted primary benzamides in excellent yields.Ortho-lithiation of N-propenylbenzamides is thus formally equivalent to ortho-lithiation of primary benzamides themselves.The utility of this important, previously unknown, synthetic operation is enhanced by the well-known facility with which the primary amide moiety can be transformed into other useful functional groups, as exemplified by the synthesis of 2-methoxy-6-methylbenzoic acid (12) and 2-methoxy-6-methylbenzonitrile (13) from N-propenyl-2-methoxybenzamide (9).N-Propenyl-o-toluamide (7) undergoes regiospecific dilithiation on nitrogen and on the methyl group under conditions analogous to those used for the N-propenylbenzamides.These dilithio species react with DMF or "Weinreb type" amides to give condensation products which cyclize to N-propenylisoquinolin-1(2H)-ones under mildly acidic conditions.Removal of the N-propenyl moiety under more strongly acidic conditions provides N-unsubstituted isoquinolin-1(2H)-ones with high overall efficiency.This process is exemplified by the synthesis of isoquinolin-1(2H)-one (23) and its 3-n-butyl congener 26 from N-propenyl-2-methylbenzamide (7).