New and Convenient Syntheses of the Important Roasty, Popcorn-like Smelling Food Aroma Compounds 2-Acetyl-1-pyrroline and 2-Acetyltetrahydropyridine from Their Corresponding Cyclic α-Amino Acids

Article abstract of DOI:10.1021/jf9706437

Novel straightforward syntheses have been developed supplying the important food odorants 2-acetyl-1-pyrroline (AP) and 2-acetyltetrahydropyridine (ATHP) in high yields. The four-step reaction sequence starts from the N-shielded cyclic a-amino acids L-proline and pipecolinic acid, respectively, which, in the first step, are converted into the N-shielded 2-acetyl derivatives. Removing the shielding group with trifluoroacetic acid afforded the 2-acetylpyrrolidine and 2-acetylpiperidine trifluoroacetate, respectively, which, upon increasing the pH of their aqueous solutions to 7.0, are spontaneously oxidized in high yields into AP (43% based on L-proline) or ATHP (35% based on pipecolinic acid), respectively, by air oxygen. The latter step is an important hint at the last step in the yet unclear formation pathways of both odorants in foodstuffs.

Full text of DOI:10.1021/jf9706437

616
J. Agric. Food Chem. 1998, 46, 616−619
New a n d Con ven ien t Syn th eses of th e Im p or ta n t Roa sty,
P op cor n -lik e Sm ellin g F ood Ar om a Com p ou n d s 2-Acetyl-1-p yr r olin e
a n d 2-Acetyltetr a h yd r op yr id in e fr om Th eir Cor r esp on d in g Cyclic
r-Am in o Acid s
Thomas Hofmann and Peter Schieberle*
Deutsche Forschungsanstalt fu¨r Lebensmittelchemie, Lichtenbergstrasse 4, D-85748 Garching, Germany
Novel straightforward syntheses have been developed supplying the important food odorants 2-acetyl-
1-pyrroline (AP) and 2-acetyltetrahydropyridine (ATHP) in high yields. The four-step reaction
sequence starts from the N-shielded cyclic R-amino acids L-proline and pipecolinic acid, respectively,
which, in the first step, are converted into the N-shielded 2-acetyl derivatives. Removing the
shielding group with trifluoroacetic acid afforded the 2-acetylpyrrolidine and 2-acetylpiperidine
trifluoroacetate, respectively, which, upon increasing the pH of their aqueous solutions to 7.0, are
spontaneously oxidized in high yields into AP (43% based on L-proline) or ATHP (35% based on
pipecolinic acid), respectively, by air oxygen. The latter step is an important hint at the last step
in the yet unclear formation pathways of both odorants in foodstuffs.
Keyw or d s: Bread flavor; popcorn flavor; Basmati rice flavor; 2-acetyl-1-pyrroline; 2-acetyltetrahy-
dropyridine
INTRODUCTION
Sch em e 1
2-Acetyl-1-pyrroline (AP, 1 in Scheme 1) was identi-
fied for the first time among the food flavors as a
constituent of cooked rice by Buttery and Ling (1982).
Application of aroma extract dilution analyses or the
odor activity value concept later on established AP as
the key odorant in wheat bread crust (Schieberle and
Grosch, 1987), popcorn (Schieberle, 1991, 1995), sweet
corn (Buttery et al., 1994), and roasted sesame (Schie-
berle, 1996).
2-Acetyltetrahydropyridine which occurs in a tauto-
meric equilibrium (ATHP, 2a and 2b in Scheme 1) was
detected for the first time in a study on wheat bread
crust flavor by Hunter et al. (1969) and has recently
been established as the key odorant especially in
popcorn (Schieberle, 1991, 1995).
Both aroma compounds exhibit a characteristic roasty
popcorn-like odor at the extremely low odor thresholds
of 0.02 ng/L (AP) or 0.06 ng/L (ATHP) in air, respectively
(Schieberle, 1995). Due to their unique flavor proper-
ties, both are interesting food flavorings. However, only
a limited number of syntheses have been published for
either AP (Buttery et al., 1983), ATHP (Bu¨chi and
Wuest, 1971), or both (Rewicki et al., 1993; De Kimpe
et al., 1993; De Kimpe and Keppens, 1996).
AP and ATHP are known to be relatively unstable
when stored, and significant losses may also occur
during the preparation and/or purification procedures
used in their synthesis. Therefore, there is also a need
for a stable intermediate from which both odorants can
easily be liberated before use. The purpose of our study
was, therefore, to develop a synthetic procedure for the
preparation of stable educts yielding straightforward AP
and ATHP upon careful treatment.
EXPERIMENTAL PROCEDURES
Ch em ica ls. The following compounds were obtained com-
mercially. Triethylamine, N-(tert-butoxycarbonyl)-L-proline,
pipecolinic acid, 2,2′-dipyridyl disulfide, triphenylphosphine,
methylmagnesium bromide, and trifuloroacetic acid were from
Aldrich (Steinheim, Germany), and 2-[(tert-butoxycarbonyl)-
oximino]-2-phenylacetonitril and 1,4-dioxane were from Lan-
caster (Muehlheim, Germany).
Syn th esis of 2-Acetyl-1-p yr r olin e (1). As outlined in
Figure 1, 1 was synthesized in a four-step sequence starting
from N-(tert-butoxycarbonyl)-L-proline.
N-(tert-Butoxycarbonyl)-2-[(2-pyridylthio)carbonyl]pyrro-
line (4 in Figure 1). The preparation of the thioester was
performed on the basis of procedures published by Endo et al.
(1970) and Aoyagi et al. (1993). A mixture of N-(tert-butoxy-
carbonyl)-L-proline (10.0 mmol), 2,2′-dipyridyl disulfide (15
mmol), and triphenylphosphine (15 mmol) in dry acetonitrile
(35 mL) was refluxed for 1 h. After the mixture was cooled to
room temperature, the solvent was distilled off in vacuo (100
mbar/40 °C), and the residue was purified by flash chroma-
tography (35.9 × 1.9 cm; J . T. Baker BV, Deventen, The
Netherlands; model 7022-01). The column was filled with a
slurry of silica gel (15 cm high, 30-60 µm, Baker analyzed
reagent) in n-pentane under a slight pressure of nitrogen. After
application of an aliquot of the crude reaction mixture (0.5 mL),
chromatography was performed using n-pentane (A, 150 mL)
* Author to whom correspondence should be addressed
(phone +49-89-289 141 70; fax +49-89-289 141 83; e-mail
LEBCHEM.Schieberle@lrz.tu-muenchen.de).
S0021-8561(97)00643-2 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/22/1998

Syntheses of Roast Aroma Compounds
J. Agric. Food Chem., Vol. 46, No. 2, 1998 617
F igu r e 2. Mass spectrum (MS/EI) of 2-acetylpyrrolidine.
F igu r e 1. Synthetic route leading to 2-acetyl-1-pyrroline (1).
followed by n-pentane/diethyl ether mixtures (150 mL each):
B (95:5 by volume), C (90:10 by volume), D (80:20 by volume),
E (70:30 by volume), and F (50:50 by volume). 4 (9.0 mmol,
yield of 90%) which was eluted in fractions E and F was
obtained after concentration in vacuo and storage for 1 h at
-30 °C as white crystals.
The target compound was characterized by mass spectrom-
1
etry (D-Cl) and H NMR: MS (EI) m/z (relative intensity) 168
(100), 170 (80), 253 (80), 253 (80), 309 (55), 198 (18); 1H NMR
(360 MHz, CDCl3, numbering of carbon atoms refers to 4 in
t
Figure 1) δ 1.47 (s, 9H, Bu), 1.81-2.43 (m, 4H, H at C-2 and
C-3), 3.39-3.62 (m, 2H, H at C-4), 4.41 and 4.49 (m, total 1H,
H at C-1), 7.21 (br d, 1H, H at C-10), 7.47-7.53 (m, 1H, H at
C-11), 7.60-7.65 (m, 1H, H at C-12), 8.55 (br s, 1H, H at C-13).
2-Acetyl-N-(tert-butoxycarbonyl)pyrrolidine (5 in Figure 1).
The conversion of the thioester into the 2-alkanone was
performed using a method described by Mukaiyama et al.
(1973). Methylmagnesium bromide (9.9 mmol or 3.3 mL of a
3.0 M solution in diethyl ether) in dry ethyl ether (6.5 mL)
was added dropwise to an ice-cooled, stirred solution of N-(tert-
butoxycarbonyl)-2-[(2-pyridylthio)carbonyl]pyrrolidine (9.0 mmol)
in dry diethyl ether (25 mL) maintained under an atmosphere
of pure argon. After the cooled mixture was stirred for 1.5 h,
water (15 mL) was added and the suspension was extracted
three times with diethyl ether (total volume of 100 mL). The
organic layer was dried over Na2SO4 and then freed from the
solvent by evaporation in vacuo (100 mbar/30 °C). The target
compound was purified by flash chromatography on silica gel
using the solvent mixtures described above. 2-Acetyl-N-(tert-
butoxycarbonyl)pyrrolidine (6.4 mmol, yield of 71%), which was
eluted in fractions E and F, was obtained as a pale yellow oil
after distilling off the solvent: MS (EI) m/z (relative intensity)
70 (100), 57 (88), 114 (47), 41 (27), 43 (21), 170 (12), 96 (11),
140 (8); 1H NMR (360 MHz, CDCl3, numbering of carbon atoms
F igu r e 3. Synthetic route leading to 2-acetyltetrahydropyri-
dine (2a and 2b).
2-Acetyl-1-pyrroline. The crude 2-acetylpyrrolidine trifluo-
roacetate was suspended in water (20 mL), and the pH of the
mixture was adjusted to 7.0. The solution was then stirred
for 1 h at 50 °C under an atmosphere of pure oxygen and
finally extracted with diethyl ether (4 × 15 mL). The organic
layer was dried over Na2SO4 and finally concentrated by
distilling off the solvent at 35 °C using a Vigreux column and
yielding 2-acetyl-1-pyrroline (4.3 mmol, yield of 67%). The MS
and 1H NMR data were identical with those reported in the
literature (Buttery et al., 1982; De Kimpe and Stevens, 1993).
t
Syn th esis of 2-Acetyltetr a h yd r op yr id in e (2a a n d 2b
in Sch em e 1). The synthesis was performed following the
route detailed in Figure 3 starting from pipecolinic acid and
N-(tert-butoxycarbonyl)pipecolinic acid (7), respectively.
refers to 5 in Figure 1) 1.42 (s, 9H, Bu) 1.65-2.29 (m, 4H, H
at C-2 and C-3), 2.09 (s, 3H, H at C-9), 3.21-3.45 (m, 2H, H
at C-4), 4.15 (m, 1H, H at C-1).
2-Acetylpyrrolidine Trifluoroacetate (6 in Figure 1). Trif-
luoroacetic acid was added dropwise to a stirred solution of
2-acetyl-N-(tert-butoxycarbonyl)pyrrolidine (6.4 mmol) in me-
thylene chloride (3 mL) at room temperature. After the
mixture was stirred for 1.5 h, the solvent was blown off at 20
°C under a stream of nitrogen, affording crude 2-acetylpiperi-
dine trifluoroacetate (4.1 mmol, yield of 71%). For mass
spectral measurements, the 2-acetylpyrrolidine was freed from
its salt by the following procedure. 6 (0.1 mmol) was sus-
pended in methylene chloride under an atmosphere of pure
argon. After addition of dicyclohexylamine (0.1 mmol), an
aliquot of the solution was immediately analyzed by HRGC/
MS using the DB-5 column. The mass spectrum of the free
2-acetylpyrrolidine liberated from its salt is displayed in Figure
2.
N-(tert-Butoxycarbonyl)pipecolinic Acid. The target com-
pound was prepared following closely the procedure described
by Itho et al. (1975). A mixture of pipecolinic acid (12 mmol),
triethylamine (18 mmol), and 2-[(tert-butoxycarbonyl)oximino]-
2-phenylacetonitrile (13.2 mmol) in 1,4-dioxane/water (15 mL,
1:1 by volume) was stirred for 2 h at room temperature. After
the solvent was distilled, water (20 mL) was added and the
solution was shaken with ethyl acetate (4 × 20 mL). The
aqueous layer was then acidified to pH 3.0 with hydrochloric
acid (1 mol/L), and the target compound was isolated by
extraction with diethyl ether (four times, total volume of 100
mL). After drying over Na2SO4, the solvent was distilled off
(100 mbar/30 °C) and the residue was taken up in n-pentane/
diethyl ether (3 mL, 1:1 by volume). N-(tert-Butoxycarbonyl)-

618 J. Agric. Food Chem., Vol. 46, No. 2, 1998
Hofmann and Schieberle
Sch em e 2
F igu r e 4. Mass spectrum (MS/EI) of 2-acetylpiperidine.
pipecolinic acid (10.0 mmol, yield of 84%) was obtained by
storing the solution at -30 °C as white crystals: MS (D-CI)
m/z (relative intensity) 175 (100), 230 (90); 1H NMR (360 MHz,
CDCl3, numbering of carbon atoms refers to 7 in Figure 3)
spectrometer (Bruker, Karlsruhe, Germany). The signals were
assigned using tetramethylsilane as the internal standard.
t
1.23-1.73 (m, 5H, H at C-2, C-3, and C-4), 1.49 (s, 9H, Bu),
2.27 (br s, 1H, H at C-2), 3.00 (m, 1H, H at C-5), 4.04 (m, 1H,
H at C-5), 4.80 and 4.98 (each br s, total 1H, H at C-1), 9.58
(br s, 1H, OH at C-9).
RESULTS AND DISCUSSION
In a previous investigation, we observed that 2-(1-
hydroxyethyl)-4,5-dihydrothiazol is easily oxidized into
2-acetylthiazoline simply when heated in water for 10
min in the presence of oxygen (Hofmann and Schieberle,
1995). It might be concluded that the oxidation runs
via the tautomeric forms shown in Scheme 2. This
result gave us the idea that probably 2-acetylpyrrolidine
and 2-acetylpiperidine, having the same structural
element of a cyclic R-aminoketone, may also be key
intermediates in the generation of the food flavorants
2-acetyl-1-pyrroline (AP) and 2-acetyltetrahydropyridine
(ATHP). A literature search, however, revealed that
neither the 2-acetylpyrrolidine nor the 2-acetylpiperi-
dine had yet been characterized by analytical data.
Therefore, we have developed a concept for the synthesis
of cyclic R-aminoketones starting from the N-tert-
butoxycarbonyl-shielded L-proline and pipecolinic acid.
The synthetic concept is discussed below for the syn-
thesis of 2-acetyl-1-pyrroline. In the first step, the
carboxy function of the N-(tert-butoxycarbonyl)-L-proline
(3 in Figure 1) is converted into the pyridyl thioester in
nearly 90% yield by treatment with 2,2′-dipyridyl di-
sulfide and triphenylphosphine. The thioester 4 ob-
tained was then exposed to methylmagnesium bromide,
yielding 2-acetyl-N-(tert-butoxycarbonyl)pyrrolidine 5
after hydrolysis with water. The structure of 5 was
established by the disappearance of the aromatic pro-
After treatment with 2,2′-dipyridyl disulfide, 7 was then
converted into the corresponding thio ester 8 N-(tert-butoxy-
carbonyl)-2-[(2-pyridylthio)carbonyl]piperidine in an overall
87% yield: MS (D-CI) m/z (relative intensity) 184 (100), 168
1
(98), 212 (70), 267 (62), 157 (55), 169 (29), 323 (11); H NMR
(360 MHz, CDCl3, numbering of carbon atoms refers to 8 in
Figure 3) δ 1.31-1.80 (m, 5H, H at C-2, C-3, and C-4), 1.44 (s,
9H, tBu), 2.27 (br s, H at C-2), 2.97 (m, 1H, H at C-5), 4.06 (m,
1H, H at C-5), 4.84 and 5.05 (each m, total 1H, H at C-1), 7.22
(br d, 1H, H at C-11), 7.48-7.52 (m, 1H, h at C-12), 7.61-
7.65 (m, 1H, H at C-13), 8.54 (br s, 1H, H at C-14).
Treatment of 8 with CH3MgBr then yielded 2-acetyl-N-(tert-
butoxycarbonyl)piperidine (9 in Figure 3): MS(EI) m/z (relative
intensity) 84 (100), 57 (83), 128 (58), 184 (13), 155 (7), 110 (6),
227 (1); 1H NMR (360 MHz, CDCl3, numbering of carbon atoms
refers to 9 in Figure 3) δ 1.19-1.70 (m, 5H, H at C-2, C-3, and
t
C-4), 1.46 (s, 9H, Bu), 2.14 (s, 3H, H at C-10), 2.17 (m, 1H, H
at C-2), 2.85 (m, 1H, H at C-5), 3.97 (m, 1H, H at C-5), 4.65
(m, 1H, H at C-1).
After treatment of 9 with trifluoroacetic acid, 2-acetylpip-
eridine trifluoroacetate (10) was obtained in 67% yield. The
mass spectrum of the free 2-acetylpiperidine liberated from
its salt at pH 7.0 is shown in Figure 4. By MS/CI, the expected
molecular weight of 127 could by established. Oxidation of
10 finally yielded 2a and 2b (61% yield) which were character-
ized by their mass spectra, in good agreement with data
published earlier (Schieberle, 1995).
High -Resolu tion Ga s Ch r om a togr a p h y (HRGC)/Ma ss
Sp ectr om etr y (MS). High-resolution gas chromatography/
mass spectrometry (HRGC/MS) was performed using a DB-5
capillary column (30 m × 0.32 mm; J + W Scientific, Fisons
Instruments, Mainz, Germany) which was installed in a type
5300 gas chromatograph (Fisons) coupled to a type 8230 mass
spectrometer (Finigan, Bremen, Germany). The mass spec-
trometer was operated either in the electron impact mode (MS/
EI) at 70 eV or in the chemical ionization mode (MS/CI) at
115 eV with isobutane as the reagent gas. The samples were
applied by the cold on column injection technique at 40 °C.
After 2 min, the oven temperature was raised by 40 °C/min to
50 °C, held for 2 min isothermally, then raised at 6 °C/min to
230 °C, and held for 5 min isothermally.
1
tons in the H NMR spectra and the new signal of the
methyl group at δ 2.09 (C-9 in Figure 1). The yield of
this Grignard reaction was relatively high (71%). Depro-
tection of 5 with trifluoroacetic acid then yielded
2-acetylpyrrolidine trifluoroacetate 6.
In the first experiment, we tried to liberate 2-acetylpyr-
rolidine from an aliquot of 6 by treatment with a weak
base. However, only the oxidized homologue 2-acetyl-
1-pyrroline was detectable after HRGC/MS. To estab-
lish that we really had the 2-acetylpyrrolidine at hand,
an aliquot of 6 was suspended in dichloromethane in a
closed vessel under an atmosphere of pure helium.
After addition of an aquimolar amount of a weak base
(dicyclohexylamine), the 2-acetylpyrrolidine formed was
immediately analyzed by HRGC/MS, yielding one main
product showing the spectrum displayed in Figure 2.
The molecular weight (M+ + 1 ) 114) obtained by
Direct inlet mass spectra (D-CI) were recorded with the type
8230 mass spectrometer (Finnigan) running in the chemical
ionization mode with isobutane as the reagent gas.
NMR Sp ectr oscop y. 1H NMR spectra were recorded in
CDCl3 (MSD isotopes, Montreal, Canada) with an AM 360

Syntheses of Roast Aroma Compounds
J. Agric. Food Chem., Vol. 46, No. 2, 1998 619
Ta ble 1. Tim e Cou r se of th e Gen er a tion of
2-Acetyl-1-p yr r olin e (AP ) fr om 2-Acetylp yr r olid in e
(AP D)a
Buttery, R. G.; Ling, L. C.; J uliano, B. O. 2-Acetyl-1-pyrro-
line: an important component of cooked rice. Chem. Ind.
1982, 958-959.
Buttery, R. G.; Ling, L. C.; J uliano, B. O.; Turnbaugh, J . G.
Cooked rice aroma and 2-acetyl-1-pyrroline. J . Agric. Food
Chem. 1983, 31, 823-826.
De Kimpe, N. G.; Stevens, C. A convenient synthesis of
6-acetyl-1,2,3,4-tetrahydropyridine, the principle bread fla-
vor component. J . Org. Chem. 1993, 58, 2904-2906.
De Kimpe, N. G.; Keppens, M. Novel syntheses of the major
flavor components of bread and cooked rice. J . Agric. Food
Chem. 1996, 44, 1515-1519.
reaction time (min)
AP (µg)
conversion of APD (%)
5
30
60
5b
12.0
32.5
42.1
2.0
27
72
94
4
a
2-Acetylpyrrolidine trifluoroacetate (0.4 µmol, 45 µg) was
dissolved in tap water (1 mL) which had been saturated with
oxygen and contained dicyclohexylamine (0.5 µmol) and was
b
allowed to stand at 25 °C in a brown glass vial. The oxygen had
been removed by flushing with helium.
De Kimpe, N. G.; Stevens, C.; Keppens, M. A. Synthesis of
2-acetyl-1-pyrroline, the principle rice flavor component. J .
Agric. Food Chem. 1993, 41, 1458-1461.
De Kimpe, N. G.; Dhooge, W. S.; Shi, Y.; Keppens, M. A.;
Boelens, M. M. On the Hodge mechanism of the formation
of the bread flavor component 6-acetyl-1,2,3,4-tetrahydro-
pyridine from proline and sugars. J . Agric. Food Chem.
1994, 42, 1739-1742.
Endo, T.; Ikenaga, S.; Mukaiyama, T. A convenient method
for preparation of thiolesters. Bull. Chem. Soc. J pn. 1970,
43, 2632-2633.
running a chemical ionization spectrum confirmed the
structure of the 2-acetylpyrrolidine.
In a further experiment, the helium was substituted
by pure oxygen while adding the base, which led to a
spontaneous oxidation of the 2-acetylpyrrolidine into the
2-acetyl-1-pyrroline in a nearly quantitative yield after
60 min (Table 1).
The same concept was then successfully used in the
synthesis of the homologous 2-acetyltetrahydropyridine,
indicating 2-acetylpiperidine as an important educt for
yielding ATHP simply by treatment of the piperidine
with air oxygen.
These results corroborate our previous findings on the
formation of the homologuos 2-acetylthiazoline from
2-(1-hydroxyethyl)-4,5-dihydrothiazol or 2-acetylthiazo-
lidine by air oxidation and indicated that obviously
R-cycloalkylaminoketones are very susceptible to oxida-
tion.
Hofmann, T.; Schieberle, P. Studies on the formation and
stability of the roast-flavor compound 2-acetyl-2-thiazoline.
J . Agric. Food Chem. 1995, 43, 2946-2950.
Hunter, I. R.; Walden, M. K.; Scherer, J . R.; Lunden, R. E.
Preparation and properties of 1,4,5,6-tetrahydro-2-acetopy-
ridine, a cracker-odor constituent of bread aroma. Cereal
Chem. 1969, 46, 189-196.
Itho, M.; Hagiwara, D.; Kamiya, T. A new tert-butylcarbony-
lating reagent, 2-tert-butyloxycarbonyloxyimino-2-phen-
ylacetaldehyde. Tetrahedron Lett. 1975, 49, 4393-4394.
Mukaiyama, T.; Araki, M.; Takei, H. Reaction of S-(2-pyridyl)-
thioates with Grignard reagents. A convenient method for
the preparation of ketons. J . Am. Chem. Soc. 1973, 95,
4763-4765.
CONCLUSIONS
Rewicki, D.; Tressl, R.; Ellerbeck, U.; Kersten, E.; Burgert, W.;
Gorzynski, M.; Hauck, R. S.; Helak, B. Formation and
synthesis of some Maillard generated aroma compounds. In
Progress in Flavour Precursors Studies; Schreier, P., Win-
terhalter, P., Eds.; Proceedings of the International Confer-
ence, Wu¨rzburg, Germany; Allured Publishing Corp.: Carol
Stream, IL, 1983; pp 301-314.
Schieberle, P. Formation of 2-acetyl-1-pyrroline and other
important flavor compounds in wheat bread crust. In
Thermal Generation of Aromas; ACS Symposium Series 409;
American Chemical Society, Washington, DC, 1989; pp 268-
275.
Schieberle, P. Primary odorants of popcorn. J . Agric. Food
Chem. 1991, 39, 1141-1144.
Schieberle, P. Quantitation of important roast-smelling odor-
ants in popcorn by stable isotope dilution assays and model
studies on flavor formation during popping. J . Agric. Food
Chem. 1995, 43, 2442-2448.
Straightforward syntheses of 2-acetylpyrrolidine and
2-acetylpiperidine are presented which allow the general
preparation of R-cycloalkylaminoketones from their
corresponding cyclic R-amino acids. As their salts, the
cyclic amines are attractive stable intermediates for
quickly and quantitatively generating the corresponding
cyclic imines 2-acetyl-1-pyrroline and 2-acetyltetrahy-
dropyridine simply by air oxidation of the free base in
either an aqueous or an organic medium. 2-Acetylpyr-
rolidine and 2-acetylpiperidine may also be formed
during the Maillard reaction of carbohydrates and
amino acids. Both can, therefore, also be regarded as
possible intermediates in the formation of AP and ATHP
during food processing. Further studies on this type of
oxidation will soon be published in more detail.
Schieberle, P.; Grosch, W. Quantitative analysis of aroma
compounds in wheat and rye bread crusts using a stable
isotope dilution assay. J . Agric. Food Chem. 1987, 35, 252-
257.
LITERATURE CITED
Aoyagi, S.; Wang, T.-C.; Kibayashi, C. Highly stereoselective
total syntheses of (+)-Allopumiliotoxons 267A and 339A via
intramolecular nickel(II)/chromium(II)-mediated cyclization.
J . Am. Chem. Soc. 1993, 115, 11393-11409.
Bu¨chi, G.; Wu¨st, H. Synthesis of 2-acetyl-1,4,5,6-tetrahydro-
pyridine, a constituent of bread aroma. J . Org. Chem. 1971,
36, 609-610.
Received for review J uly 25, 1997. Revised manuscript
received November 21, 1997. Accepted November 25, 1997.
J F9706437