A convenient way of 3-fluoro-1,8-naphthalimide synthesis
The syntheses of 3-fluoro-1,8-naphthalimide and 3-fluoro-1,8-naphthalic anhydride from 3-nitro-1,8-naphthalimide have been described. 3-Nitro-1,8-naphthalimide can be obtained by nitration of naphthalimide with sodium nitrate in sulfuric acid. Stability of naphthalimides under acidic and alkaline conditions makes 3-substituted naphthalimides more suitable synthones for further functionalization compared to the corresponding 3-substituted naphthalic anhydrides. The best yield of 3-fluoro-1,8-naphthalimide was achieved through the following sequence: 1) nitration of naphthalic anhydride to 3-nitronaphthalic anhydride by sodium nitrate in the concentrated sulfuric acid; 2) ammonolysis of 3-nitronaphthalic anhydride to 3-nitronaphthalimide with aqueous ammonia; 3) reduction of 3-nitronaphthalimide to 3-amino-naphthalimide with sodium dithionite solution in aqueous ethanol; 4) preparation of 3-fluoronaphthalimide by diazotization of 3-aminonaphthalimide and subsequent thermal decomposition of the corresponding tetrafluoroborate. Nitration of naphthalic anhydride makes it possible to avoid the problem of poor solubility of naphthalimide in sulfuric acid. Subsequent reactions are conveniently carried out with naphthalimide derivatives, thus avoiding the difficulties encountered in isolating of 3-aminonaphthalic anhydride that can undergo a polymerization reaction. The new protocol is more efficient than the preparation route previously described leading to 3-substituted naphthalic anhydrides and naphthalimides by virtue of the substitution reaction at C-4 of acenaphthene followed by oxidation.
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