Methanol → Methyl DL-prolinate

Participants

Reaction Steps

To a solution of L-proline (2.23 g) in dry methanol (15 ml) cooled to −5° C., thionyl chloride (4.52 g) was added drop wise, while stirring and maintaining the temperature of the reactants below 0° C. The reaction mixture was allowed to attain room temperature (25° C.) and further stirred for 2 h. The solvent was distilled off from it and the residue stored in ice-chest for 12 h. The solid mass that separated was added to an aqueous potassium carbonate solution (50%, 20 ml) at 0° C. and the separated oily layer extracted into ether (15 ml×3). The combined ethereal layers were dried over anhydrous sodium sulfate and distilled to afford 2.3 g of 2-(carbomethoxy)-pyrrolidine [85%, MS (m/z) M+129 (1%), 114 (0.5), 70(100), 68(50), 43(80), 41(90)]. To an ice-cooled solution of the compound in dry diethyl ether (15 ml), t-butylhypochlorite (2.16 g) was added drop-wise. The reaction was monitored on thin layer of silica gel using chloroform as the eluant. After 15 min. anhydrous potassium acetate (2.76 g) was added gradually added to it over 10 min., and the mixture stirred at 25° C., while monitoring the reaction by TLC. At the end of the reaction (3 h), the solution was filtered and the solvent distilled under reduced pressure to afford 2-carbomethoxy)-1-pyrrroline [Yield: 1.9 g, MS (m/z) M+ 127(1%), 112(0.5), 97 (15), 69,(100), 54 (50), 41 (75)]. To a suspension of magnesium turnings (0.43 g) in dry diethylether, iodine (0.01 g) was added followed by a solution of methyl iodide (3.08 g) in ether (5 ml) dropwise at 35° C. The mixture was stirred until the disappearance of magnesium (15 min) and then after cooling to 0° C., a solution of 2-(carbomethoxy)-1-pyrrroline (1.9 g) in dry diethylether (5 ml) was added over 5 min. On completion of the reaction (2 h) as found by GC, the product was worked up by addition of dilute hydrochloric acid (5%, 10 ml) and separation of the organic layer. The aqueous layer was further extracted with diethylether (10 ml×2) and the combined organic layers dried over anhydrous sodium sulphate. Removal of the solvent afforded pure 2-acetyl-1-pyrroline [1.2 g, 95% purity, MS (m/z) M+ 111 (5%), 96 (0.1), 83 (15), 69(8), 68 (10), 55(2), 52 (0.2), 43 (100), 42 (25), 41 (50)].

Provenance & Evidence