Abstract
The past chemistry of the pyrrocoline ring system has been reviewed and a study of the readily-obtainable 2-methyl derivative undertaken. The mono-aceto derivative of 2-methylprrocoline has been prepared and the aceto-group shown to be in the 3-position by reduction to 2-methyl-3-ethyl-pyrrocoline using a modification of the Clemmensen method and comparison with the product obtained by direct synthesis form a-picoline and 3-bromo-2-pentanone. Evidence was obtained that 2-methyl-3-a-hydroxyethylpyrrocline was also formed in this reduction. Catalytic reduction of the aceto-compound in the presence of Adamâ€(TM)s catalyst gave 3-aceto-2-methyl-5:6:y:8-tetrahydropyrrocoline as the main product, while reduction of 2-methyl-3-ethylpyrrocoline under the same conditions gave the octahydro-compound. This same compound was obtained from the 3-aceto-derivative by reduction at high temperature and pressure in the presence of Raney nickel. Attempts to synthesise 3-aceto-2-methylpyrrocoline direct from 3-chloracetyl-acetone and a-picoline were unsuccessful. The di-aceto derivative of 2-methylpyrrocoline was also prepared and the positions of the aceto-groups established. Nitration of 2-methylpyrrocoline in sulphuric acid solution at 0 yielded 1-nitro-2-methylpyrrocoline as the major product, together with a small quantity of the 3-isomeride. Nitration of both the mono-aceto- and di-aceto-derivatives in acetic acid solution led to the 1-nitro-3-aceto- and the 1:3 dinitro compounds, and the former compound was shown to be identical with the product of the acetylation of 1-nitro-2-methylpyrrocoline. The 1:3-dinitro-compound was also obtained in small yield by the nitration of 2-methylpyrrocoline in acetic acid solution. Nitrosation of 2-methylpyrrocoline gave 3-nitroso-2-methylypyrrocoline identified by oxidation to 3-nitro-2-methylpyrrocoline. Nitrosation of 3-aceto-2-methylpyrrocoline and 2-methyl-3-ethylpyrrocoline gave the 1-nitroso-derivatives. As a new route to pyrrocoline itself pyrrocoline-2-carboxylic acid was synthesised from a-picoline and ethyl bromopyruvate, and shown to be identical with the pyrrocoline mono-carboxylic acid obtained by the mild decarboxylation of pyrrocoline-1:2:3-tricarboxylic acid. Some theroretical discussion of the results obtained in this work has been presented.