Mechanistic Studies of 2-(1-Hydroxyethyl)-2,4,5-trimethyl-3-oxazoline Formation under Low Temperature in 3-Hydroxy-2-butanone/Ammonium Acetate Model Systems

Article abstract of DOI:10.1021/jf960734o

Volatile compounds formed from the reaction of 3-hydroxy-2-butanone/ammonium acetate at 25, 55 and 85 °C were investigated. Six compounds were characterized by gas chromatography - mass spectrometry (EI and CI). Among the volatile compounds identified, an interesting intermediate compound, 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline, was found. ¹⁵N-Labeled ammonium acetate was used to confirm the structure of 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline. The formation pathway of these volatile compounds was proposed. In these model systems, 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline was formed at the reaction temperature below 25 °C. On the other hand, tetramethylpyrazine was the major component when the reaction temperature was higher than 85 °C. The amounts of 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline and tetramethylpyrazine increased linearly with the increasing heating time at 55 °C. Protic solvents did not promote 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline formation but did favor the formation of tetramethylpyrazine. A kinetic study of 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline formation was also performed, and the activation energy was found to be 16.5 kcal/mol.

Full text of DOI:10.1021/jf960734o

1878
J. Agric. Food Chem. 1997, 45, 1878−1882
Mech a n istic Stu d ies of 2-(1-Hyd r oxyeth yl)-2,4,5-tr im eth yl-3-
oxa zolin e F or m a tion u n d er Low Tem p er a tu r e in
3
-Hyd r oxy-2-bu ta n on e/Am m on iu m Aceta te Mod el System s
†
,‡
Hui-Yin Fu and Chi-Tang Ho*
Department of Food Sanitation, Ta J en Pharmaceutical J unior College, Pingtung 912, Taiwan, and
Department of Food Science, Rutgers University, New Brunswick, New J ersey 08903
Volatile compounds formed from the reaction of 3-hydroxy-2-butanone/ammonium acetate at 25,
5
5 and 85 °C were investigated. Six compounds were characterized by gas chromatography-mass
spectrometry (EI and CI). Among the volatile compounds identified, an interesting intermediate
compound, 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline, was found.
1
5
N-Labeled ammonium
acetate was used to confirm the structure of 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline. The
formation pathway of these volatile compounds was proposed. In these model systems, 2-(1-
hydroxyethyl)-2,4,5-trimethyl-3-oxazoline was formed at the reaction temperature below 25 °C. On
the other hand, tetramethylpyrazine was the major component when the reaction temperature was
higher than 85 °C. The amounts of 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline and tetrameth-
ylpyrazine increased linearly with the increasing heating time at 55 °C. Protic solvents did not
promote 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline formation but did favor the formation of
tetramethylpyrazine. A kinetic study of 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline formation
was also performed, and the activation energy was found to be 16.5 kcal/mol.
Keyw or d s: 2-(1-Hydroxyethyl)-2,4,5-trimethyl-3-oxazoline; GC-MS; kinetic study; activation energy
INTRODUCTION
tion of 2,4,5-trimethyl-3-oxazoline was higher than that
of 2,4,5-trimethyloxazole in the sample of canned beef
stew.
Oxazoles and oxazolines, which are oxygen- and
nitrogen-containing heterocyclics, have been identified
in many kinds of heated foods and have significant
sensory contribution (Maga, 1981). 5-Acetyl-2-methyl-
oxazole was first identified in coffee (Stoffelsma et al.,
The oxazoline as discussed in these reports was
formed in thermally generated aroma in food systems.
However, no detailed study has been reported on the
occurrence of oxazolines in heated foods. The purpose
of this study was to isolate and identify the important
flavor precursor, oxazoline, from the reaction of a
3-hydroxy-2-butanone/ammonium acetate model system
at low temperature. The kinetic and formation path-
ways among the important volatile compounds, oxazole,
oxazoline, and pyrazine, were also studied.
1
2
968). Later, Vitzthum and Werkhoff (1974) reported
0 oxazoles in the aroma of coffee flavor. Interestingly,
oxazoline, which has a structure similar to oxazole, was
not found in coffee. As in the case of cocoa, 2,5-
dimethyl-, 4,5-dimethyl-, 2,4,5-trimethyl-, and 5-methyl-
2
-propyloxazoles were identified, but oxazoline was not
found in cocoa aroma (Vitzthum et al., 1975). Other
vegetative foods, such as soy sauce (Nunomura et al.,
EXPERIMENTAL PROCEDURES
1
976) and wheat (Harding et al., 1978), have also been
Ma ter ia ls. 3-Hydroxy-2-butanone and dimethylpyrazine
were purchased from Aldrich Chemical Co. (Milwaukee, WI).
Ammonium acetate, methanol, ethanol, propanol, butanol, and
the solvents for GC were chemical grade and obtained from
Fisher Chemical Co. [ N]Ammonium acetate was purchased
from Isotec, Inc. (Miamisburg, OH).
Rea ction Con d ition s. Reaction mixtures were composed
of 0.0025 mol (0.625 M) of 3-hydroxy-2-butanone and 0.0075
mol of ammonium salts dissolved in 4 mL of deionized water
or solvents. The vials were shaken regularly to assure that
all the reactants were dissolved. The reactions were run in a
water bath at the required constant temperatures.
Ch a r a cter iza tion a n d Qu a n tita tion of Vola tile Com -
p ou n d . Gas Chromatography. An HP 5890 gas chromato-
graph equipped with a fused silica gel column (60 m × 0.32
mm i.d., film thickness 0.25 µm, DB-1; J & W Scientific) and
a flame ionization detector was used to analyze the volatile
compounds. The operating conditions were as follows: injector
and detector temperatures, 270 and 300 °C, respectively;
reported to contain only oxazoles.
The first oxazoline, 2,4,5-trimethyl-3-oxazoline, was
reported by Chang et al. in the flavor of meat (1968).
2
reaction of ammonia, acetaldehyde, and 3-hydroxy-2-
butanone has been reported to be a constituent of cooked
meat at room temperature (Mussinan et al., 1976).
1
5
,4,5-Trimethyl-3-oxazoline isolated from the model
2
,4,5-Trimethyl-3-oxazoline has been characterized as
having woody, musty, and green aromas. Other oxazo-
lines such as 2,5-dimethyl-3-oxazoline, 2,4-dimethyl-5-
ethyl-3-oxazoline, and 2,5-dimethyl-4-ethyl-3-oxazoline
were also identified, and their aromas were character-
ized as nutty, vegetable-like, and sweet (Mussinan et
al., 1976). 2,4,5-Trimethyl-3-oxazoline was reported to
be the major compound in boiled beef (Hirai, 1973).
Peterson et al. (1975) also reported that the concentra-
helium carrier flow rate, 1.0 mL/min; temperature program,
o
4
0-260 °C at 2 °C/min and then isothermally held at 260
C
*
To whom correspondence should be addressed (fax,
for 10 min. Dimethylpyrazine (150 mg/mL) was prepared and
used as the internal standard; 10 µL of dimethylpyrazine was
added after reaction. Then the mixture was saturated with
sodium chloride and extracted by 1 mL of methylene chloride;
(
908) 932-6776; e-mail, HO@AESOP.Rutgers.Edu).
†
Ta J en Pharmaceutical J unior College.
Rutgers University.
‡
S0021-8561(96)00734-0 CCC: $14.00
© 1997 American Chemical Society

2-(Hydroxyethyl)-2,4,5-trimethyloxazoline Formation
J. Agric. Food Chem., Vol. 45, No. 5, 1997 1879
Ta ble 1. Ten ta tively Id en tified Vola tile Com p ou n d s in th e 3-Hyd r oxy-2-bu ta n on e/Am m on iu m Aceta te Mod el System a t
Low Tem p er a tu r e (25 °C)
peak no.
compound identified
2,4,5-trimethyl-3-oxazoline
MS m/ z (relative intensity)
1
2
3
4
5
6
113(23), 98(30), 72(100), 71(24), 70(12), 69(21), 68(13), 60(8), 54(6)
111(100), 96(1), 82(16), 70(3), 68(33), 55(50), 43(94), 42(74)
114(47), 113(8), 87(4), 72(100), 57(12)
157(1), 113(16), 112(100), 98(7), 72(10), 71(62)
136(42), 121(1), 95(3), 80(2), 54(94), 53(23), 52(11), 51(7)
157(14), 131(7), 115(7), 114(5), 89(2), 72(100), 57(15)
2,4,5-trimethyloxazole
isomers, aldol condensation of 3-hydroxy-2-butanone
2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline
tetramethylpyrazine
Schiff base
Ta ble 2. Qu a n tita tion of Ten ta tively Id en tified Vola tile Com p ou n d s fr om th e 3-Hyd r oxy-2-bu ta n on e/Am m on iu m Aceta te
Mod el System (Rea ction Tim e: 4 h )
peak no.
compound identified
2,4,5-trimethyl-3-oxazoline
2,4,5-trimethyloxazole
isomers, aldol condensation of 3-hydroxy-2-butanone
25 °C (mg)
55 °C (mg)
85 °C (mg)
1
2
3
0.078 (0.18%)a
0.059 (0.11%)
0.027 (0.15%)
.042 (0.10%)
.009 (0.05%)
12.32 (97.9%)
0.043 (0.10%)
0.008 (0.05%)
0.068 (0.35%)
0.056 (0.29%)
0.025 (0.10%)
0.036 (0.19%)
0.008 (0.04%)
14.28 (74.9%)
4.84 (25.0%)
0.014 (0.07%)
0.025 (0.04%)
0.025 (0.04%)
0
0
4
5
6
2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline
tetramethylpyrazine
Schiff base
0.062 (0.11%)
56.18 (99.6%)
0.136 (0.24%)
a
(Amount of compound)/(total amounts of volatile compounds) × 100%.
1
5
14
F igu r e 2. CI mass spectra in [ N]- and [ N]ammonium
F igu r e 1. GC profile of volatile compounds formed at 25 °C
acetate/3-hydroxy-2-butanone model systems.
(A), 55 °C (B), and 85 °C (C).
determined using the basic equation for the rate of change of
A with time: dA ) KA dt, where A ) concentration, t ) time,
K ) rate constant, and n ) reaction order. Slopes and
intercepts were calculated by the linear least-squares method.
The temperature dependence of the reaction rate constant
could be determined by the following Arrhenius equation:
1
µL of extract was injected into the GC after drying by
n
anhydrous sodium sulfate.
Gas Chromatography-Mass Spectrometry Analysis. GC-
MS was accomplished by using a Hewlett-Packard 5890A gas
chromatograph coupled to a Hewlett-Packard 5985B mass
spectrometer equipped with a direct split interface. EI mass
spectra were obtained using electron ionization at 70 eV and
an ion source temperature of 250 °C. For CI-MS analyses,
-Ea/RT
K ) K e
o
reactant gas (methane, CH
4
) was utilized. The operating
conditions were the same as those used in the GC analysis
described above. An [ N]ammonium acetate/3-hydroxy-2-
butanone system was set up to confirm the structure of the
where K ) rate constant, E ) activation energy in kcal/mol,
a
1
5
R ) gas constant (1.987 cal/mol K), and T ) temperature in
K. Thus, the activation energy for formation of the 2-(1-
hydroxyethyl)-2,4,5-trimethyl-3-oxazoline was calculated from
the slope of the line generated by plotting the natural log value
of the rate constant versus the reciprocal of the absolute
temperature (Labuza, 1983).
2
-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline.
Kinetic Studies on 2-(1-Hydroxyethyl)-2,4,5-trimethyl-3-ox-
azoline Formation at Low Temperature. The rate of oxazoline
formation in 3-hydroxy-2-butanone/ammonium acetate in a
water system at the three reaction temperatures (5, 15, 25 °C)
was analyzed by GC in the reaction mixture. All kinetic
studies were carried out in duplicate. The kinetics of forma-
tion of 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline were
Effect of Solvents on the Formation of the 2-(1-Hydroxyethyl)-
2,4,5-trimethyl-3-oxazoline and Tetramethylpyrazine. The re-
action products of the intermediate and tetramethylpyrazine
from 3-hydroxy-2-butanone/ammonium acetate in water, metha-

1880 J. Agric. Food Chem., Vol. 45, No. 5, 1997
Fu and Ho
form C2H5+ (Reg and Martin, 1991). Peak 186 which
+
was shown on the CI spectrum is (M + C2H5) ions
which have a mass 29 units higher than the relative
molecular mass of the molecular ion.
The peak at m/ z 112 was due to the loss of CH3-
+
CHOH from the parent ion (m/ z 157). The peak at
m/ z 71 represented a loss of the CH3CN fragment. One
unit higher of mass to charge (m/ z 159) was obtained
1
5
from the [ N]ammonium acetate model system. It
showed that only one nitrogen atom was present in the
molecular structure of this compound. No nitrogen was
found in the fragment of m/ z 71. This compound (peak
4
2
) was therefore characterized as 2-(1-hydroxyethyl)-
,4,5-trimethyl-3-oxazoline. These results agree with
those of Shu and Lawrence (1975).
F or m a tion of 2-(1-Hyd r oxyeth yl)-2,4,5-tr im eth yl-
-oxa zolin e a n d Tetr a m eth ylp yr a zin e fr om th e
3
F igu r e 3. 2-(1-Hydroxyethyl)-2,4,5-trimethyl-3-oxazoline for-
mation at different temperatures.
3-Hyd r oxy-2-bu ta n on e/Am m on iu m Aceta te Mod el
System a t Differ en t Tem p er a tu r es. Three temper-
atures (25, 55, 85 °C) were investigated in this model
system. Quantitation of volatile compounds is sum-
marized in Table 2. The data showed 2-(1-hydroxy-
ethyl)-2,4,5-trimethyl-3-oxazoline formed predominately
below 25 °C, whereas tetramethylpyrazine was the
major product at reaction temperatures higher than 85
°
2
C. At the temperature of 25 °C, 2-(1-hydroxyethyl)-
,4,5-trimethyl-3-oxazoline (97.9% peak area of the total
volatiles) was the major volatile compound formed in
the model system. Kinetic analysis for the formation
mechanism of 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-ox-
azoline was conducted at three temperatures (5, 15, 25
°
3
C). The amount of 2-(1-hydroxyethyl)-2,4,5-trimethyl-
-oxazoline increased linearly in all three temperatures.
The rates of 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazo-
line formation at each temperature followed pseudo-
zero-order reaction kinetics as shown by the linear plot
of 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline con-
centration versus time (Figure 3). Rate constants were
calculated from the slope using the least-squares fit
method. The natural log of the rate constant as a
function of the reciprocal of temperature is shown in
the Arrhenius plot (Figure 4). Linear regression data
summarizing the effects of temperature on 2-(1-hy-
droxyethyl)-2,4,5-trimethyl-3-oxazoline formation are
shown in Table 4. The activation energy was found to
be 16.5 kcal/mol.
When the reaction temperature was elevated to 55
°C, tetramethylpyrazine (25.0%) started to form along
with 2-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline
(74.9%) in the same system. Both major compounds,
tetramethylpyrazine and 2-(1-hydroxyethyl)-2,4,5-tri-
methyl-3-oxazoline, increased linearly with increasing
times. 2-(1-Hydroxyethyl)-2,4,5-trimethyl-3-oxazoline
showed a higher formation rate than tetramethylpyra-
zine. At a reaction temperature higher than 85 °C,
tetramethylpyrazine was the major compound (99.6%)
generated in the model system. It was found that 2-(1-
hydroxyethyl)-2,4,5-trimethyl-3-oxazoline and tetra-
methylpyrazine were well separated by using GC analy-
sis. The formation of tetramethylpyrazine was evi-
denced as the major component above 85 °C in this
experiment.
F igu r e 4. Arrhenius plot of 2-(1-hydroxyethyl)-2,4,5-tri-
methyl-3-oxazoline formation.
nol, ethanol, propanol, and butanol at 25 and 55 °C for 4 h
were quantified by gas chromatography. Dimethylpyrazine
was used as an internal standard to quantify the amount of
2
-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline and tetrameth-
ylpyrazine formation in the model system.
RESULTS AND DISCUSSION
Vola t ile Com p ou n d s Id en t ified in t h e 3-Hy-
d r oxy-2-bu ta n on e/Am m on iu m Aceta te Mod el Sys-
tem . The gas chromatograms of volatile compounds
formed at various temperatures (25, 55, 85 °C) are
shown in Figure 1. Six compounds were tentatively
identified by GC-MS(EI) to be 2,4,5-trimethyl-3-oxazo-
line (peak 1), 2,4,5-trimethyloxazole (2), isomers of aldol
condensation product of acetoin (3), 2-(1-hydroxyethyl)-
2
,4,5-trimethyl-3-oxazoline (4), tetramethylpyrazine (5),
and a Schiff base (6). Table 1 shows the major frag-
mentation data of these identified compounds. Among
these compounds, we found an interesting oxazoline.
Although oxazolines are usually formed at high tem-
perature, we have found that 2-(1-hydroxyethyl)-2,4,5-
trimethyl-3-oxazoline formed abundantly in our model
1
5
system at low temperature. [ N]Ammonium acetate
was used to confirm the structure of this oxazoline
compound. Figure 2A,B shows the results of GC-MS-
1
4
15
(CI) in the [ N]- and [ N]ammonium acetate model
Rizzi (1988) showed that tetramethylpyrazine could
be derived from acetoin and ammonium acetate at
temperatures as low as 22 °C. An HPLC method for
tetramethylpyrazine determined developed by Rizzi was
utilized in our previous study (Huang et al., 1996). In
that study, it was determined that the activation energy
of tetramethylpyrazine was to be 18.84 kcal/mol. The
1
4
systems, respectively. In the [ N]ammonium acetate
model system, m/ z at 158 is interpreted as the M + 1
•
+
peak. Since the reactant gas was methane, the CH4
ion may undergo fragmentation before it collides with
+
a CH4 molecule and produces CH3 . The fragment ion
+
CH3 may itself undergo an ion-molecule reaction and

2-(Hydroxyethyl)-2,4,5-trimethyloxazoline Formation
J. Agric. Food Chem., Vol. 45, No. 5, 1997 1881
F igu r e 5. Overview of reaction pathways of 3-hydroxy-2-butanone with ammonium acetate.
Ta ble 3. Effect of Solven ts on 2-(1-Hyd r oxyeth yl)-2,4,5-tr im eth yl-3-oxa zolin e a n d Tetr a m eth ylp yr a zin e F or m a tion a t 25
a
a n d 55 °C
quantification (mg)
solvent
compound
water
methanol
ethanol
propanol
butanol
2
5
5 °C
5 °C
2
-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline
12.32
0.04
12.99
2.64
9.92
2.34
12.00
2.64
15.42
2.90
tetramethylpyrazine
2
-(1-hydroxyethyl)-2,4,5-trimethyl-3-oxazoline
14.28
4.84
7.54
8.16
7.80
8.28
7.30
8.22
6.56
7.88
tetramethylpyrazine
a
Reaction time: 4 h.
Ta ble 4. Ra te Con sta n t (K) a n d Cor r esp on d in g
Regr ession Coefficien t of 2-(1-Hyd r oxyeth yl)-2,4,5-
tr im eth yl-3-oxa zolin e F or m a tion fr om th e
of tetramethylpyrazine formation. More water needs
to be removed for one molecule of tetramethylpyrazine
formation than for 2-(1-hydroxyethyl)-2,4,5-trimethyl-
3
-Hyd r oxy-2-bu ta n on e/Am m on iu m Aceta te System
3
-oxazoline formation. It was proposed that protic
T (°C)
K (mg/mL h)
R2
solvents could promote Schiff base formation in our
previous study (Huang et al., 1996). These data indi-
cated that protic solvents did not promote 2-(1-hydroxy-
ethyl)-2,4,5-trimethyl-3-oxazoline formation but in-
creased the amount of tetramethylpyrazine at both 25
and 55 °C. These results are in good agreement with
our previous studies. Therefore, protic solvent was more
effective in tetramethylpyrazine formation than in 2-(1-
hydroxyethyl)-2,4,5-trimethyl-3-oxazoline formation.
5
5
5
10.54
66.09
81.64
0.99
0.98
0.99
1
2
amounts of tetramethylpyrazine were overestimated
due to the close elution of tetramethylpyrazine and 2-(1-
hydroxyethyl)-2,4,5-trimethyl-3-oxazoline by HPLC analy-
sis. Low activation energy for tetramethylpyrazine
formation at a level similar to that of 2-(1-hydroxyethyl)-
2
,4,5-trimethyl-3-oxazoline could be expected.
P r op osed Mech a n ism of Vola tile Com p ou n d s
F or m ed in th e 3-Hyd r oxy-2-bu ta n on e/Am m on iu m
Aceta te Mod el System a t Low Tem p er a tu r e. Fig-
ure 5 summarizes the reaction scheme for the six major
compounds. Two molecules of 3-amino-2-butanone,
which was derived from the reaction of ammonia and
3-hydroxy-2-butanone, were condensed to form a Schiff
base that can then be rearranged to form tetrameth-
ylpyrazine (peak 5). Three isomers (peak 3) were
formed from the aldol condensation of 3-hydroxy-2-
E ffect of P r ot ic Solven t s on t h e F or m a t ion of
-(1-Hydr oxyeth yl)-2,4,5-tr im eth yl-3-oxazolin e. Table
shows the effect of protic solvents, methanol, ethanol,
2
3
propanol, and butanol, on the formation of 2-(1-hydroxy-
ethyl)-2,4,5-trimethyl-3-oxazoline and tetramethylpyra-
zine. Water systems seem to favor the formation of 2-(1-
hydroxyethyl)-2,4,5-trimethyl-3-oxazoline which needed
two Schiff base reactions. On the other hand, four Schiff
base formation reactions are involved in one molecule

1882 J. Agric. Food Chem., Vol. 45, No. 5, 1997
Fu and Ho
butanone. The reaction also led to the formation of the
Schiff base (peak 6). The major pathway involved the
reaction of 3-amino-2-butanone with 3-hydroxy-2-bu-
tanone. The reaction formed a Schiff base which was
rearranged to form 2-(1-hydroxyethyl)-2,4,5-trimethyl-
and high hydrostatic pressure. J . Agric. Food Chem. 1996,
4
4, 240-246.
Labuza, T. P. In Computer-aided techniques in food technology;
Saguy, I., Ed.; Marcel Dekker, Inc.: New York, 1983; p 71.
Maga, J . A. Oxazoles and oxazolines in foods. CRC Crit. Rev.
Food Sci. Nutr. 1981, 14, 295-307.
Mussinan, C. J .; Wilson, R. A.; Katz, I.; Hruza, A.; Vock, M.
H. Identification and flavor properties of some 3-oxazolines
and 3-thiazolines isolated from cooked beef. In Phenolic,
Sulfur, and Nitrogen Compounds in Food Flavors; Charalam-
bous, G., Katz, I., Eds.; ACS Symp. Ser. 26, American
Chemical Society: Washington, DC, 1976; pp 133-145.
Numomura, N.; Sasaki, M.; Asao, Y.; Yokotsuka, T. Shoyu (soy
sauce) volatile flavor components: basic fraction. Agric. Biol.
Chem. 1978, 42, 2123-2128.
Peterson, R. J .; Izzo, H. J .; J ungermann, E.; Chang, S. S.
Changes in volatile flavor compounds during the retorting
of canned beef stew. J . Food Sci. 1975, 40, 948-954.
Piloty, M.; Baltes, W. Z. Investigations on the reaction of amino
acids with R-dicarbonyl compounds. Lebensm. Unters. For-
sch. 1979, 168, 374-380.
3
(
-oxazoline (peak 4) and then 2,4,5-trimethyloxazole
peak 2). 2,4,5-Trimethyl-3-oxazoline (peak 1) was also
formed as another rearrangement product. A similar
,4,5-trimethyl-3-oxazoline formation mechanism has
2
been proposed by Piloty and Baltes (1979). They found
that oxazoles, pyrazines, pyrroles, and pyridines were
formed in the heated model system of amino acids and
diacetyl (2,3-butanedione). We concluded that 2-(1-
hydroxyethyl)-2,4,5-trimethyl-3-oxazoline (peak 4) was
formed in the greatest amount indicating that this was
the major pathway below 25 °C. Only small amounts
of 2,4,5-trimethyl-3-oxazoline (peak 1), 2,4,5-trimethyl-
oxazole (2), tetramethylpyrazine (5) and the compound
from peak 6 were produced at low temperatures in this
model system. However, tetramethylpyrazine was
formed at temperatures higher than 85 °C.
Rizzi, G. P. Formation of pyrazine from acyloin precursors
under mild conditions. J . Agric. Food Chem. 1988, 36, 349-
3
52.
ACKNOWLEDGMENT
Shu, C. K.; Lawrence, B. M. Formation of 2-(1-hydroxyalkyl)-
-oxazolines from the reaction of acyloins and ammonia
precursors under mild conditions. J . Agric. Food Chem.
995, 43, 2922-2924.
3
This work was supported by a grant from the National
Science Foundation, Taiwan, R.O.C. (CNS 85-2321-B
1
1
27-001).
Stofflesma, J .; Sipma, G.; Kettenes, D. K.; Pypker, J . New
volatile compounds of roasted coffee. J . Agric. Food Chem.
1968, 16, 1000-1004.
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