1) The reaction of N-methyl-N-benzylamine (I) with isobutyraldehyde (II) and formaldehyde (III) in refluxing isopropanol, followed by hydrogenation with NaBH4 gives 3-(N-benzyl-N-methylamino)-2,2-dimethylpropanol (IV), which is condensed with diketene (V) in refluxing benzene to yield 3-(N-benzyl-N-methylamino)-2,2-dimethylpropyl acetoacetate (VI). Finally, this compound is submitted to cyclization with 2-fluoro-5-nitrobenzeldehyde (VII) (prepared by oxidation of 2-fluoro-5-nitrotoluene (VIII) with CrO3 in acetic ahydride-H2SO4) and methyl 3-aminocrotonate (IX) in refluxing isopropanol.
2) The condensation of acetoacetic ester (VI) with aldehyde (VII) gives the benzylidene derivative (X), which is then submitted to cyclization with crotonate (IX) as before. 3) Finally, the title product can also be obtained by cyclization of benzylidene derivative (X) with methyl acetoacetate (XI) and concentrated aqueous ammonia in refluxing isopropanol.
A new synthesis for TC-81 has been described: By condensation of 2-(2-fluoro-5-nitrobenzylidene)acetoacetic acid methyl ester (I) with 3-aminocrotonic acid 3-(N-benzyl-N-methylamino)-2,2-dimethylpropyl ester (II) by means of triethylamine in refluxing isopropanol. The starting products (I) and (II) are prepared as follows: The condensation of 2-fluoro-5-nitrobenzaldehyde (III) with methylacetoacetate (IV) by means of HCl in toluene gives the benzylidene derivative (I). The condensation of 3-(N-benzyl-N-methylamino)-2,2-dimethylpropanol (V) with diketene (VI) in hot benzene gives the acetoacetic ester (VII), which is treated with gaseous ammonia in ethanol, yielding crotonate (III).