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.
A new synthetic pathway toward the Maillard flavor compounds 6-acyl-1,2,3,4-tetrahydropyridines and 2-acetyl-1-pyrroline is presented. The reaction sequence involves deprotonation of a vicinal diimine and subsequent alkylation with an N,N-diprotected ω-bromoalkylamine, followed by deprotection and intramolecular transimination of the functionalized intermediate. Acidic workup affords the above-mentioned heterocycles, which are principal flavor constituents of bread and cooked rice, respectively. In addition, the synthesis of the more stable diethyl acetal of 2-acetyl-1-pyrroline is described.
Cyclic six-rnernbered imines, i.e. 2,3,4,5tetrahydropyridines, are efficiently converted under mild conditions into the corresponding pyridines by highly regioselective a, a-dichlorination with N-chlorosuccinimide (NCS) followed by double dehydrochlorination with methanolic bases.
The enantioselective total synthesis of two of the four possible stereoisomers of stenusine (1), the spreading agent of the beetle Stenus comma, is described. The silyl ether-substituted aldehyde SAMP hydrazone 2 was alkylated with (S)-1-bromo-2-methylbutane (3) yielding the hydrazone 4 in high diastereomeric purity (de >95 %). By several steps including the reduction of the hydrazone functionality and the cleavage of the N-N bond of the intermediates, 4 was converted into the BOC-protected amino alcohol 6. Subsequent cyclization of 6 afforded the (S,S) diastereomer of stenusine with 96.6 % de, >99.9 % ee, and in 11.3 % overall yield. Repetition of this synthesis using the aldehyde RAMP hydrazone (R)-2 as the starting material produced (SR)-1 with 95.0 % de, >99 % ee, and in 8.2 % overall yield. The synthetic samples of 1 were employed to investigate the stereochemistry of natural stenusine by means of GC analysis on both a chiral and an achiral, stationary phase. As a result of these studies natural stenusine was found to be a mixture of all four stereoisomers in a ratio of (S,S)/(SR)/(RR)/(R:S) = 43:40:13:4
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.