Identification and Synthesis of 2-Heptanethiol, a New Flavor Compound Found in Bell Peppers ‡ HERVE Ä SIMIAN, † FABIEN ROBERT, AND IMRE BLANK* Nestle ´ Research Center, P.O. Box 44, 1000 Lausanne 26, Switzerland 2-Heptanethiol was identified for the first time as a constituent of red and green bell pepper extracts. The chemical structure of this new aroma compound was proposed on the basis of mass spectra and retention indices and confirmed by chemical synthesis and nuclear magnetic resonance spectroscopy measurements. Its aroma properties were described as sulfury, onion-like, and vegetable-like, reminiscent of bell pepper at lower concentrations, with an orthonasal detection threshold of 10 µg/L of water. No differences in odor note and threshold value were observed for the enantiomeric forms, which were prepared from enantiopure 2-heptanol by tosylation, followed by thioacetylation and reduction, giving the target thiol enantiomers. KEYWORDS: Bell peppers; 2-heptanethiol; identification; GC-MS; NMR; synthesis; odor thresholds INTRODUCTION The study of bell pepper’s volatile constituents has been the subject of numerous publications. Up to now almost 200 volatiles have been reported (1), of which only a few are reminiscent of bell pepper aroma, such as 3-isobutyl-2-meth- oxypyrazine first identified by Buttery and co-workers (2). This odorant has a characteristic green, bell pepper-like note and a very low detection threshold of 2 ng/L water (2). Additional odor-active compounds have been reported in fresh green bell pepper on the basis of gas chromatography/olfactometry (GC/ O), e.g. 2-methylpropanal, 2- and 3-methylbutanal, 2,3-butane- dione, 1-penten-3-one, hexanal, heptanal, -ocimene, trans-3- hepten-2-one, dimethyl trisulfide, and -cyclocitral (3). More recently, a systematic work characterized odor-active compounds of sweet bell pepper powder of different origins (4). The authors stressed the importance of -ionone, 4-hydroxy-2,5-dimethyl- 3(2H)-furanone, 3-hydroxy-4,5-dimethyl-2(5H)-furanone, and 2- and 3-methylbutanoic acid among more than 30 odorants identified. Changes during drying of bell pepper have been studied by Luning and co-workers (5). These authors reported on additional bell pepper-like smelling compounds such as the monoterpene Δ-3-carene, which was particularly abundant in red bell pepper. This typical bell pepper note had an increasing impact in turning and mature samples, particularly in fresh and dried red, yellow, and white bell peppers (6). In addition, 2-heptanone/heptanal has been suggested to elicit bell pepper and cooked vegetable notes in rehydrated bell peppers (7). Moreover, correlation of GC and sensory data revealed that green bell peppers were characterized by the sensory attributes grassy, green bell pepper, and fruity, which were closely related to 1-penten-3-one, (Z)- 3-hexanal, (Z)-3-hexanol, 3-isobutyl-2-methoxypyrazine, and linalool (8). Volatile organic sulfur compounds obtained by enzymatic and chemical reactions contribute to the aroma of many vegetables, fruits, and food products (9, 10). In general, thiols and sulfides belong to the most intense and characteristic aroma substances with sulfury, vegetable-like, fruity notes perceived at low concentrations. For example, (2R,3S)-3-mercapto-2-methyl-1- pentanol found in fresh onions and dimethyl sulfide found in cooked tomatoes show low odor thresholds of 0.03 and 0.3 µg/ kg of water, respectively (11, 12), and are character-impact constituents of the respective vegetable. In bell peppers, only a few odor-active sulfur compounds have been identified, such as methional (4) and dimethyl trisulfide (3, 5). However, no thiols have so far been reported as odorants of bell peppers. In our work on the natural constituents of vegetables, we investigated the odor-active compounds of red bell pepper. This paper deals with the identification of a secondary alkanethiol, a new odorant found in red bell pepper extracts, as well as its sensory properties and synthesis. EXPERIMENTAL PROCEDURES Materials. The following chemicals were commercially available: 2-heptanol (>99%), p-toluenesulfonyl chloride (>99%), sodium hydrogen sulfide monohydrate (NaSH, 97%), potassium thioacetate (97%), lithium aluminum hydride (>99%) (Fluka, Buchs, Switzerland); (R)-2-heptanol (>99%), (S)-2-heptanol (>99%), deuterochloroform (C 2 - HCl3, >99%) (Aldrich, Buchs, Switzerland); ammonium chloride (98%), hydrochloric acid (HCl, 37%), magnesium sulfate (98%), sodium sulfate, silica gel 60 (Merck, Darmstadt, Germany). The solvents pentane, ethyl acetate (EtOAc), methylene chloride, ethanol (EtOH), toluene, pyridine, dimethylformamide (DMF), and diethyl ether (Et 2O) were from Merck and were freshly distilled prior to use. ‡ Dedicated to Professor Dr. Werner Grosch on the occasion of his 70th birthday. * To whom correspondence should be addressed. Telephone: +21/785- 8607. Fax: +21/785-8554. E-mail: imre.blank@rdls.nestle.com. † Present address: Product Technology Center Orbe, 1350 Orbe, Switzerland. 306 J. Agric. Food Chem. 2004, 52, 306-310 10.1021/jf035008h CCC: $27.50 © 2004 American Chemical Society Published on Web 12/03/2003