A new straightforward synthesis of 2-acetyl-l-pyrroline, the principal rice flavor component with a cracker-like flavor, is reported. The reaction sequence involves the conversion of pyrrolidine into tripyrroline, subsequent hydrocyanation of the latter into 2-cyanopyrrolidine, oxidation into 2-cyano-1-pyrroline, and Grignard addition of methylmagnesium iodide, affording an overall yield of 16-19% from pyrrolidine. In similar way, 2-propionyl-l-pyrroline, a recently discovered flavor component of popcorn, was prepared in addition to several higher analogues, i.e., 2-acyl-1-pyrrolines. Also, the synthesis of 2-(acetyl-da)-1-pyrroline, a deuterated derivative of the rice flavor compound which is useful for the stable isotope dilution assay, is described.
Two new synthetic pathways toward the new Maillard flavor compound 5-acetyl-2,3-dihydro-1,4thiazine are disclosed. 1-Bromo-3,3-dimethoxy-2-butanone and N-protected 2-mercaptoethylamine are the key components in both synthetic routes. The first approach involves a one-step synthesis via nucleophilic substitution, followed by cyclization and hydrolysis. The second route entails a nucleophilic substitution, followed by TFA-deprotection of the primary amino function, which led to a spontaneous intramolecular transimination and hydrolysis of the acetal moiety to afford the desired flavor compound in very good yield.
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