HETEROATOM-DIRECTED METALATION - LITHIATION OF N-PROPENYLBENZAMIDES AND N-PROPENYL-ORTHO-TOLUAMIDES - NOVEL ROUTES TO ORTHO-SUBSTITUTED PRIMARY BENZAMIDE DERIVATIVES AND N-UNSUBSTITUTED ISOQUINOLIN-1(2H)-ONES

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 electrophilic reagents, including alkyl halides, giving adducts which on hydrolysis with warm 50% 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 midly 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).