https://chemistry.mdma.ch/hiveboard/rhodium/pdf/indole.synth.review.1994-99.pdf

Recent developments in indole ring synthesis—methodology and applications

Advances in the chemistry of tetrahydroquinolines.

Pyrazole containing natural products: Synthetic preview and biological significance

The chemistry of cyclopropanols

1,n-Dicarbanionic Titanium Intermediates from Monocarbanionic Organometallics and Their Application in Organic Synthesis

A new straightforward synthesis of 2-acetyl-1-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-1-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-d 3 )-1-pyrroline, a deuterated derivative of the rice flavor compound which is useful for the stable isotope dilution assay, is described

Synthesis of 2-acetyl-1-pyrroline, the principal rice flavor component

A new efficient synthesis of pyridines

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.

Enantioselective synthesis and determination of the configuration of stenusine, the spreading agent of the beetle Stenus comma

Novel routes to indoles, indolines, quinolines and tetrahydroquinolines from N-(cyclohexylidene)amines.

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. Keywords: Rice flavor; bread flavor; 2-acetyl-1-pyrroline; 6-acetyl-1,2,3,4-tetrahydropyridine; Maillard

M. Keppens

Papers

Synthesis of 2-acetyl-1-pyrroline, the principal rice flavor component

A new efficient synthesis of pyridines

Enantioselective synthesis and determination of the configuration of stenusine, the spreading agent of the beetle Stenus comma

Novel routes to indoles, indolines, quinolines and tetrahydroquinolines from N-(cyclohexylidene)amines.

Novel syntheses of the major flavor components of bread and cooked rice.