Journal of Agricultural and Food Chemistry
Article
Germany). Dichloromethane, diethyl ether, and pentane were distilled
prior to use.
°C, and water (50 mL) was slowly added, followed by an aqueous
saturated ammonium chloride solution (40 mL) to dissolve the
precipitate. The organic layer was separated, and the aqueous layer was
extracted with diethyl ether (3 × 50 mL). The combined organic layers
were washed with saturated aqueous sodium bicarbonate solution (50
mL), followed by water (50 mL). After drying over anhydrous sodium
sulfate, the solvent was removed by rotary evaporation.
Bromination of the Tertiary Alcohol. Bromination was done as
previously reported.9 To a mixture of the tertiary alcohol (30 mmol)
and lithium bromide (3.91 g; 45 mmol) was slowly added aqueous
HBr (10.1 g; 60 mmol; 48%) at room temperature. The organic layer
was separated, washed with ethylene glycol (3 × 10 mL), and dried
over anhydrous sodium sulfate.
Thioacetylation of the Bromo Derivatives. Following the method
by Gurudutt et al.10 zinc sulfide (1.95 g; 20 mmol) was added to a
solution of thioacetic acid (3.04 g; 40 mmol) in dichloromethane (40
mL), and stirred for 2 h at room temperature. The tertiary bromo
derivative (20 mmol) was added, and the solution was refluxed for 10
h. The filtrate was washed with dichloromethane (50 mL), and the
combined organic solutions were washed with aqueous hydrochloric
acid (2 mol/L; 30 mL), followed by a saturated aqueous sodium
bicarbonate solution (30 mL). After drying over anhydrous sodium
sulfate, the solvent was removed by rotary evaporation. Purification of
the compound was performed by column chromatography on silica gel
using pentane/diethyl ether (99/1; v/v) as the eluent.
Reference Odorants. Propane-1-thiol, butane-1-thiol, pentane-1-
thiol, hexane-1-thiol, heptane-1-thiol, octane-1-thiol, nonane-1-thiol,
decane-1-thiol, butane-2-thiol, 2-methylbutane-1-thiol, 3-methylbu-
tane-2-thiol, propane-1,3-dithiol, butane-1,4-dithiol, hexane-1,6-dithiol,
octane-1,8-dithiol, and nonane-1,9-dithiol were obtained from Sigma-
Aldrich. 2-Methylbutane-2-thiol, decane-1,10-dithiol, 1-(methylthio)-
pentane, 1-(methylthio)heptane, 1-(methylthio)-octane, and 1-
(methylthio)decane were purchased from TCI Europe Laboratory
Chemicals (Eschborn, Germany). Pentane-1,5-dithiol, 1-(methylthio)-
butane, and 2-(methylthio)butane were supplied by Alpha-Aesar.
Syntheses. In total 40 sulfur compounds were newly synthesized.
All reactions were carried out in oven-dried glassware under argon
atmosphere unless otherwise noted.
Primary and Secondary Thiols. The primary and secondary
thiols were prepared from the corresponding alcohols in a three-step
synthesis following a previously described method8 with slight
modifications. If the respective alcohol was not commercially available
(e.g., 2-methylhexan-1-ol and 2-methylheptan-1-ol), these were
synthesized from the corresponding carboxylic acid by reduction
with LiAlH4.
Tosylation of Primary and Secondary Alcohols. To a solution of
the alcohol (30 mmol) in anhydrous pyridine (20 mL) was slowely
added p-toluenesulfonyl chloride (6.29 g; 33 mmol) at 0 °C. The
solution was allowed to reach room temperature, and the mixture was
stirred overnight. Hexane (30 mL) was added, the solution was
filtered, and the residue was washed with hexane (30 mL). The
combined organic layers were washed with aqueous hydrochloric acid
(5 mol/L; 2 × 30 mL). After drying over anhydrous sodium sulfate,
the solvent was removed by rotary evaporation.
Reduction with LiAlH4. Reduction with LiAlH4 was performed as
described above for the primary and secondary thiols.
Synthesis of Alkanedithiols. Alkanedithiols were prepared from
the respective dibromoalkanes in a two-step procedure, that is, by
thioacetylation with potassium thioacetate, followed by reduction with
LiAlH4 as described above.
Thioacetylation of the p-Tosylderivates. Potassium thioacetate
(5.71 g; 50 mmol) was dissolved in DMF (20 mL). The tosylated
alcohol (20 mmol) was added, and the solution was stirred at 80 °C
for 2 h. Then, a saturated aqueous sodium chloride solution (100 mL)
was added, the aqueous solution was extracted with diethyl ether (3 ×
100 mL), and the combined organic layers were washed with a
saturated aqueous sodium chloride solution (5 × 50 mL). After drying
over anhydrous sodium sulfate, the solvent was removed by rotary
evaporation. Purification of the compound was performed by column
chromatography on silica gel (silica 60, 0.040−0.063 mm) (Merck,
Darmstadt, Germany) using pentane/diethyl ether (99/1; v/v) as the
eluent.
Reduction with LiAlH4. The acetylthio derivative (10 mmol)
obtained was dissolved in anhydrous diethyl ether (15 mL) and added
slowly to a suspension of lithium aluminum hydride (0.76 g; 20 mmol)
in anhydrous diethyl ether (20 mL) at 0 °C. The solution was stirred
for 2 h at room temperature, and a saturated aqueous ammonium
chloride solution (20 mL) was added slowly at 0 °C. Then, aqueous
hydrochloric acid (2 mol/L; 50 mL) was added to dissolve the
precipitate formed, the ethereal layer was separated, and the aqueous
layer was extracted with diethyl ether (3 × 70 mL). The combined
organic layers were washed with a saturated aqueous sodium
bicarbonate solution (50 mL), followed by water (50 mL). After
drying over anhydrous sodium sulfate, the solvent was distilled off by
means of a Vigreux column (50 × 1 cm). Purification of the compound
was performed by column chromatography on silica gel using pentane
as the eluent.
Synthesis of Tertiary Thiols. Tertiary thiols were prepared in a
three-step synthesis from the corresponding alcohols by bromination,9
followed by substitution of the bromine with zinc thioacetate10 and
reduction of the thioacetates with LiAlH4. If the respective alcohol was
not commercially available (e.g., 2-methyloctan-2-ol, 2-methylnonan-2-
ol, and 2-methyldecan-2-ol), these were synthesized from the
respective methyl ketone by a Grignard reaction with methylmagne-
sium bromide.
Tertiary Alcohols. To a solution of the respective methyl ketone
(40 mmol) in diethyl ether (100 mL) was added a methylmagnesium
bromide solution (3.0 mol/L in diethyl ether; 19 mL; 56 mmol) at 0
°C, and the mixture was refluxed for 2 h. The mixture was cooled to 0
Preparation of the Methylthioethers. Methylation of the thiols
was done as previously reported.11 To a solution of the thiol (4 mmol)
in anhydrous tetrahydrofuran (12 mL) was added n-butyllithium (2.5
mol/L in hexane; 2 mL, 5 mmol) at 0 °C. The solution was allowed to
warm to room temperature, and the mixture was stirred for 10 min.
Then, methyl iodide (568 mg; 4 mmol) was added slowly at 0 °C, and
the reaction mixture was stirred for 20 min at room temperature. After
the addition of water (20 mL), the mixture was extracted with pentane
(3 × 30 mL), and the combined organic layers were washed with water
and dried over anhydrous sodium sulfate. The solvent was distilled off
by means of a Vigreux column (50 × 1 cm). Purification of the
compound was performed by column chromatography on silica gel
using pentane as the eluent.
Gas Chromatography−Flame Ionization Detection (GC-FID)
and Gas Chromatography−Olfactometry (GC-O). GC-FID and
GC-O analyses were performed by means of a gas chromatograph
8160 (Fisons Instruments, Mainz, Germany) using helium as the
carrier gas. Samples were applied by the cold-on-column injection
technique. Capillaries used were DB-5 and DB-FFAP (each 30 m ×
0.32 mm i.d., 0.25 μm film thickness, 70 kPa head pressure) (J&W
Scientific, Chromatographie-Handel Mueller, Fridolfing, Germany).
The temperature programs were adapted to the respective volatility of
each analyte. For GC-O applications, the ends of the capillaries were
connected to a deactivated Y-shaped glass splitter (Chromatographie
Handel Mueller) dividing the effluent into two equal parts, which were
transferred via two deactivated fused silica capillaries (50 cm × 0.25
mm) to a sniffing port and an FID, respectively. The sniffing port
consisted of a cylindrically shaped aluminum device (80 mm × 25 mm
i.d.) with a beveled top and a central drill hole (2 mm) housing the
capillary. It was mounted on a detector base of the GC and heated to
200 °C. The FID was operated at 250 °C with hydrogen (20 mL/min)
and air (200 mL/min). Nitrogen (30 mL/min) was used as the
makeup gas. The injection volume was 1.0 μL. During a GC-O run,
the nose of the panelist was placed closely above the top of the sniffing
port, and the odor of the effluent was evaluated. If an odor was
recognized, the retention time was marked in the chromatogram, and
the odor quality was assigned. The evaluation was performed by three
panelists, and the results were averaged. Panelists were trained on a
B
J. Agric. Food Chem. XXXX, XXX, XXX−XXX