5392 J. Agric. Food Chem., Vol. 50, No. 19, 2002
Engel and Schieberle
Figure 3. Possible structures suggested for 1 as derived from the mass
spectral data.
Table 2. Odor Thresholds of Roasty, Popcorn-like Smelling Odorants
Detected in Foods or Maillard Reaction Flavors, Respectively
odor threshold
odorant
(ng/L in air)
N-(2-mercaptoethyl)-1,3-thiazolidine
2-acetyl-1-pyrroline
2-propionyl-1-pyrroline
2-acetyltetrahydropyridine
5-acetyl-2,3-dihydro-1,4-thiazine
2-acetyl-2-thiazoline
2-propionyl-2-thiazoline
2-propionyl-2,3-dihydro-1,4-thiazine
2-acetylpyrazine
0.005
0.02a
0.02a
0.06a
0.06a
0.05a
0.02
0.1a
Figure 2. Mass spectrum (MS/EI) of compound 1.
Table 1. Key Ions and Sum Formulas Obtained by High-Resolution
Mass Spectrometry of 1
ion (m/z)
molecular formula
C H NS •+
possible fragment
149a
102
88
61
56
(M)•+
5
11
2
+
0.4a
• +
C H NS
(M•+ − CH S )
4
8
2
4a
+
• +
C H NS
(M•+ − C H S )
2-acetylthiazole
3
6
2
5
+
• +
C H S
(M•+ − C H NS )
2
5
3 6
+
C H N
(m/z 102 − HCHS)+
(m/z 88 − HCHS)+
a Data are taken from ref 8.
3
6
+
42
C H N
2 4
the loss of either a mercapto methylene or a thio methyl group.
On the basis of these data, four structures were proposed for
compound 1 (Figure 3). However, the elimination of a C2H5S
radical from the molecular ion, yielding m/z 88 (Table 1), seems
to be possible preferably from structure IV.
a The theoretical mass was calculated as 149.0333, found 149.0337.
13:1 effluent splitter. During condensation the traps were maintained
at -50 °C. After about four runs, the material was flushed with diethyl
ether (0.5 mL) into a small vessel. To avoid condensation of water,
the traps were connected to CaCl2 tubes between the single GC runs.
It should be stressed that this method of preparation leads to consider-
able losses of 1 due to its thermal degradation in the hot injector; a
cold on-column injection would probably result in higher yields.
High-Resolution Gas Chromatography)Mass Spectrometry
(HRGC-MS). The material isolated by preparative gas chromatography
was chromatographed on a fused silica column (30 m × 0.32 mm DB-
5; 0.5 µm; Fisons Instruments, Mainz, Germany). Mass spectral analysis
was done as previously reported (5).
Nuclear Magnetic Resonance (NMR) Spectrometry. NMR spectra
were recorded using an AM 360 spectrometer (Bruker, Karlsruhe,
Germany) in CDCl3 at 298 K and at 360 MHz in a Wilmad 535 PP
tube. The program WIN NMR (version 4.0) was used for spectra
calculation based on either TMS or CHCl3 as the internal standard.
13C DEPT spectra were measured as reported previously (4).
Following this suggestion and starting from formaldehyde
and cysteamine, N-(2-mercaptoethyl)-1,3-thiazolidine was syn-
thesized by a reaction of 1,3-thiazolidine with ethylene sulfide
(thiirane) according to the scheme given in Figure 1. The mass
spectra (MS/EI and MS/CI) obtained for the synthetic compound
completely agreed with the spectra obtained for compound 1
isolated from the reaction of cysteamine with fructose.
1
The H NMR measurement gave six signals, among which
in particular the singlet of two H atoms at δ 4.03 and the two
triplets of two H atoms at δ 2.85 and 3.03 with identical
coupling constants confirmed the unsubstituted positions at
carbon atoms 2, 4, and 5 in the thiazolidine ring. The results of
the 13C NMR experiments, in particular, of the DEPT experi-
ments indicating five methylene groups finally proved the
structure of the N-substituted 1,3-thiazolidine.
Analytical and Sensory Attributes of 1. A determination
of the retention indices of 1 on two stationary GC phases gave
an RI of 1325 on DB-5 and an RI of 2035 on FFAP. The latter
index is very close to that of 4-hydroxy-2,5-dimethyl-3(2H)-
furanone (RI ) 2030), which is commonly formed in significant
amounts in Maillard-type reactions and might be why 1 has
not yet been reported in the literature. The retention indices of
1 were identical with the data obtained for N-(2-mercaptoethyl)-
1,3-thiazolidine.
A determination of the odor threshold of N-(2-mercaptoethyl)-
1,3-thiazolidine and of 1 in air using a GC-O method recently
described (6) gave a value of 0.005 ng/L in air. Compared to
several other compounds known in the literature to elicit roasty,
popcorn-like aromas (Table 2), 1 proved to be by far the most
potent aroma compound in this group. However, compared to
the other compounds given in Table 2, 1 does not contain the
same structural element of a cyclic amine with an adjacent R-oxo
group, which is believed to cause roasty, popcorn-like aromas
(7). Obviously, different receptors may exist in the human
olfactory system causing similar odor qualities.
RESULTS AND DISCUSSION
Characterization of 1. In Figure 2 is displayed the mass
spectrum (MS/EI) of 1 isolated by preparative GC from ∼40
model reactions. Because the molecular mass of m/z 149 could
clearly be established by MS/CI (data not shown), the presence
of one nitrogen atom was suggested by the uneven molecular
mass. Furthermore, because the isotopic distribution in the
molecular ion (data not shown) suggested the presence of two
sulfur atoms, the very intense base fragment at m/z 102 hints at
either a morpholine ring or a methyl-substituted thiazolidine
ring, which was in good agreement with the isotopic pattern in
this fragment.
Application of high-resolution mass spectrometry revealed a
sum formula of C5H11NS2 for the molecular ion (m/z 149; Table
1). From the ratio of carbon to hydrogen atoms one double bond
or ring equivalent can be calculated. Because a ring structure
was very probable due to the intense m/z 102 ion, no additional
double bond was expected. Furthermore, the elimination of 47
from the molecular mass (m/z 149 - 47 ) m/z 102) suggested