The rice flavor compound.......2-Acetyl-1-pyrroline

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  2-acetyl-4,5-dihydro-3H-pyrrole
  
  US4522838
  
  2-Acetyl-1-pyrroline, abbreviated 2AP, with the IUPAC name 1-(3,4-dihydro-2H-pyrrol-5-yl)ethanone, is an aroma compound and flavor that gives white bread, jasmine rice and basmati rice, the spice pandan (Pandanus amaryllifolius), and bread flowers (Vallaris glabra) their customary smell.^[1] 2-Acetyl-1-pyrroline and its structural homolog, 6-acetyl-2,3,4,5-tetrahydropyridine of similar smell, can be formed by Maillard reactions during heating of food such as the baking of bread dough. Both compounds have odor thresholds below 0.06 ng/l.^[2]
  A pyrroline that is 1-pyrroline in which the hydrogen at position 2 is replaced by an acetyl group. It is an aroma and flavour compound present in jasmine rice and basmati rice. It is responsible for the 'popcorn' aroma in a large variety of cereal and food products. It is one of the key odourants of the crust of bread and considered to be responsible for the cracker-like odour properties. In bread, it is primarily generated during baking but amounts are influenced by ingredient composition and fermentation conditions.
  

  Structure and properties

  2-Acetyl-1-pyrroline is a substituted pyrroline and a cyclic imine as well as a ketone.
  
   
  The analogous 2-acetyl-1-pyrroline 6 has a similar potent cracker-like flavor and is considered as the most important flavor component of cooked rice. It has been identified and isolated from different varieties of cooked rice [R.G. Buttery, L.C. Ling, B.O. Juliano, Chem. Ind., 958 (1982); R.G. Buttery, L.C. Ling, B.O. Juliano, J.G. Turnbaugh, J. Agric. Food Chem., vol.31, 823 (1983); R.G. Buttery, L.C. Ling, T.R. Mon, J. Agric. Food Chem., vol.34, 112 (1986)] and the crust of wheat and rye bread [P. Schieberle, W. Grosch, J. Agric. Food Chem., vol.35, 252 (1987); P. Schieberle, w. Grosch, Z. Lebensm. Unters Forsch., vol. 180, 474 (1985)]. It is remarkable that 2-acetyl-1-pyrroline 6 has been found in pandam leaves (Pandanus amaryllifolius Roxb.)[R.G. Buttery, B.O. Juliano, L.C. Ling, Chem. Ind., 478 (1983)]. This fact explains that it has long been the practice in India and other parts of Asia to use leaves of Pandanus species in the cooking of common rices to impart a resemblance of the aroma of the more costly scented rice.
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  U.S. Pat. 4,522,838 discloses the sole known synthetic route to 2-acetyl-1-pyrroline.
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  The synthesis entails hydrogenation of 2-acetylpyrrole 4 with rhodium on alumina, followed by oxidation of the resulting aminoalcohol 5 by means of an excess of silver carbonate (absorbed on celite) in benzene.
  

  

  
  2-Acetyl-1-pyrroline 6 has been used in flavoring foods, particularly in imparting a scented rice flavor to foods. The drawback of this synthesis of the rice flavor component 6 is the use of the very expensive reagents, the low overall yield of 10%, the use of toxic chemicals (e.g. benzene) and the virtually inaccessibility of the compound on a larger scale. Indeed, according to the patented procedure mentioned above, 2-acetyl-1-pyrroline 6 was isolated and purified by preparative gas chromatography, which entails at best subgram quantities.
  
  

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  B.3. Synthesis of 2-Acetyl-1-pyrroline 6
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    The rice flavor compound 6 was prepared in exactly the same way as described in detail for the synthesis of the bread flavor component 3 (see A.3.). Compound 6 was obtained as a clear light-yellow oil (purity ⋟ 96%) which darkened rapidly on standing at room temperature in neat form (yield 40%). Compound 6 was characterized by the usual spectrometric methods (¹H NMR,¹³C NMR, IR, MS). It should be stressed that, contrary to compound 3, the rice flavor component 6 exclusively occurs as the imine form. The compound is preferably kept in dilute solution (pentane, dichloromethane) at -20°C. After an inital decantation from a small amount of dark viscous liquid (one week at -20°C), the clear solution is stable for several months at -20°C (up to now, we observed a good stability over a period of two years).
  http://www.google.com/patents/EP0436481A1?cl=en 
  
   
  
  
  
  
  
  
  
  
  
  

  References

  1. S. Wongpornchai, T. Sriseadka, S. Choonvisase (2003). "Identification and quantitation of the rice aroma compound, 2-acetyl-1-pyrroline, in bread flowers (Vallaris glabra Ktze)". J. Agric. Food. Chem. 51 (2): 457–462. doi:10.1021/jf025856x. PMID 12517110.
  2. T. J. Harrison, G. R. Dake (2005). "An expeditious, high-yielding construction of the food aroma compounds 6-acetyl-1,2,3,4-tetrahydropyridine and 2-acetyl-1-pyrroline". J. Org. Chem. 70 (26): 10872–10874. doi:10.1021/jo051940a. PMID 16356012.

  
  http://pubs.acs.org/doi/pdf/10.1021/jf00118a036
  
  

  Yang DongSik, Lee Kyu‐Seong, Kays StanleyJ (2010) Characterization and discrimination of premium‐quality, waxy, and black‐pigmented rice based on odor‐active compounds Journal of the science of food and agriculture 90, 2595-2601 [Agricola:IND44456134] [show Abstract]

  Wongpornchai S, Sriseadka T, Choonvisase S (2003) Identification and quantitation of the rice aroma compound, 2-acetyl-1-pyrroline, in bread flowers (Vallaris glabra Ktze). Journal of agricultural and food chemistry 51, 457-462 [PubMed:12517110] [show Abstract]

  Costello PJ, Henschke PA (2002) Mousy off-flavor of wine: precursors and biosynthesis of the causative N-heterocycles 2-ethyltetrahydropyridine, 2-acetyltetrahydropyridine, and 2-acetyl-1-pyrroline by Lactobacillus hilgardii DSM 20176. Journal of agricultural and food chemistry 50, 7079-7087 [PubMed:12428963] [show Abstract]

  Maraval I, Sen K, Agrebi A, Menut C, Morere A, Boulanger R, Gay F, Mestres C, Gunata Z (2010) Quantification of 2-acetyl-1-pyrroline in rice by stable isotope dilution assay through headspace solid-phase microextraction coupled to gas chromatography-tandem mass spectrometry. Analytica chimica acta 675, 148-155 [PubMed:20800726] [show Abstract]

  Arikit S, Yoshihashi T, Wanchana S, Uyen TT, Huong NT, Wongpornchai S, Vanavichit A (2011) Deficiency in the amino aldehyde dehydrogenase encoded by GmAMADH2, the homologue of rice Os2AP, enhances 2-acetyl-1-pyrroline biosynthesis in soybeans (Glycine max L.). Plant biotechnology journal 9, 75-87 [PubMed:20497370] [show Abstract]

  Poonlaphdecha J, Maraval I, Roques S, Audebert A, Boulanger R, Bry X, Gunata Z (2012) Effect of timing and duration of salt treatment during growth of a fragrant rice variety on yield and 2-acetyl-1-pyrroline, proline, and GABA Levels. Journal of agricultural and food chemistry 60, 3824-3830 [PubMed:22404867] [show Abstract]

  Harrison TJ, Dake GR (2005) An expeditious, high-yielding construction of the food aroma compounds 6-acetyl-1,2,3,4-tetrahydropyridine and 2-acetyl-1-pyrroline. The Journal of organic chemistry 70, 10872-10874 [PubMed:16356012] [show Abstract]

  Adams A, De Kimpe N (2006) Chemistry of 2-acetyl-1-pyrroline, 6-acetyl-1,2,3,4-tetrahydropyridine, 2-acetyl-2-thiazoline, and 5-acetyl-2,3-dihydro-4H-thiazine: extraordinary Maillard flavor compounds. Chemical reviews 106, 2299-2319 [PubMed:16771451]