The mechanism of the reaction of nicotinic acid 1-oxide with acetic anhydride

In order to elucidate the mechanism of the 2-acetylation in the reaction of nicotinic acid 1-oxide (2a) with boiling acetic anhydride, thermal reactions and reactions with hot acetic anhydride have been explored with 3-X-pyridine 1-oxides (2). The former reactions of 2d (X =CONHAc), 2f (X = CONMeAc), 2h (X = CH2OAc) and 2j [X = CH (OAc) 2] result in recovery or decomposition. The latter reactions of 2c (X =CONH2), 2d, 2e (X =CONHMe), 2h and 2j bring about mainly deoxygenative α-acetoxylation, no 2-acetylation being noticed. However, the reaction of 2f with acetic anhydride affords 6, 7-dihydro-6-methyl-7-methylene-5H-pyrrolo [3, 4-b] pyridin-5-one 1-oxide (7) as an initial product, which further undergoes deoxygenative β-acetoxylation to give 7-acetoxy-7-acetoxymethyl-6, 7-dihydro-6-methyl-5H-pyrrolo [3, 4-b] pyridin-5-one (8) and 7-acetoxymethylene-6, 7-dihydro-6-methyl-5H-pyrrolo [3, 4-b] pyridin-5-one (9). On the basis of these results we propose a new electrophilic pathway for the 2-acetylation of 2a and 2f.