Formation of the Meisenheimer spiro adduct of N-(2,4,6-trinitrophenyl)alanine methylamide and its rearrangement to 2-amino-N-methyl-N-(2,4,6-trinitrophenyl)propanamide

N-(2,4,6-trinitrophenyl)alanine methylamide (I) undergoes base-catalyzed cyclization in methanol to give the spiro adduct II. In aniline-anilinium chloride buffers, the spiro adduct is protonated at the oxygen atom of 2-nitro group to give the neutral compound III. In 4-bromoaniline buffers or by action of methanolic hydrogen chloride, the compound III is opened to E and Z isomers of 2-amino-N-methyl-N-(2,4,6-trinitrophenyl)propanamide hydrochloride (IV). The rate-limiting step of cyclization of compound Z-IV to compound III consists in the isomerization Z-IV → E-IV. At higher pH values (acetate buffers), the rate-limiting step is gradually changed to the isomerization of 2-amino-N-methyl-N-(2,4,6-trinitrophenyl)propanamide (Z-V → E-V).