Synthesis and odor evaluation of stereoisomers of imine derivatives in roasted spotted shrimp

Article abstract of DOI:10.1002/hlca.200390028

All possible stereoisomers of imine derivatives 1-4. which have the characteristic roast odor of seafood, were synthesized. As a result of odor evaluation of all isomers, we found that each isomer has a different and characteristic odor of roasted seafood.

Full text of DOI:10.1002/hlca.200390028

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Helvetica Chimica Acta Vol. 86 (2003)
Synthesis and Odor Evaluation of Stereoisomers of Imine Derivatives in
Roasted Spotted Shrimp
by Toru Tachiharaa), Susumu Ishizakia), Yoshiko Kurobayashia), Hiroshi Tamuraa), Youichi Ikemotob),
Atsushi Onumab), Keisuke Yoshikawaa), Tetsuya Yanaia), and Takeshi Kitahara*c)
a) Technical Research Center, T. Hasegawa Co., Ltd., 335 Kariyado, Nakahara-ku, Kawasaki-shi,
Kanagawa 211-0022, Japan
b) Flavor Institute, Flavor Division, T. Hasegawa Co., Ltd., 335 Kariyado, Nakahara-ku, Kawasaki-shi,
Kanagawa 211-0022, Japan
c) Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences,
The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
All possible stereoisomers of imine derivatives 1 4, which have the characteristic roast odor of seafood,
were synthesized. As a result of odor evaluation of all isomers, we found that each isomer has a different and
characteristic odor of roasted seafood.
Introduction. Imine derivatives 1 4 have the characteristic roast odor of seafood.
In 1974, Lien and Nawar found that thermolysis of leucine gave imine 4, while
thermolysis of isoleucine gave imine 3 [1]. Imines 2, 3, and 4 were detected from
hydrolyzed corn protein by Flath and co-workers in 1989 [2]. Imines 2 and 4 were
detected in dried squid as volatile compounds by Kawai and co-workers in 1991 [3],
and, recently, we also found them in roasted spotted shrimp. There is, however, no
report on odor evaluation of those stereoisomers. We were interested in the
relationship of the configuration and the odor of the stereoisomers. Therefore, we
synthesized all possible stereoisomers and evaluated their odors.
Results and Discussion. The synthesis of both enantiomers of 2-methylbutanal
(10) and 2-methylbutanamine (11), key intermediates of imine derivatives, is shown in
Scheme 1.

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Scheme 1. Synthesis ofBoth Enantiomers of 2-Methylbutanal (10) and 2-Methylbutanamine (11)
DHP ˆ 3,4-Dihydro-2H-pyran, LAH ˆ LiAlH4, PPTS ˆ pyridinium p-toluenesulfonate, TEA ˆ Et3N,
TEMPO ˆ 2,2,6,6-tetramethylpiperidin-1-yloxy, THP ˆ tetrahydro-2H-pyran-2-yl.
Commercially available methyl (S)-3-hydroxy-2-methylpropionate (5) was em-
ployed as the starting material. Tosylate 7 was obtained from 5 in three steps by a
known route [4]. Exchange of the protecting group THP with a Bz group in two steps
gave benzoate 8 for easy purification at a later step. The replacement of the TsO group
with a Me group with Me2CuLi and then deprotection of the Bz group under hydrolytic
conditions gave (R)-2-methylbutan-1-ol (9). The conversion of 9 to (R)-2-methyl-
butanal (10) was achieved by TEMPO oxidation [5], while the conversion to (R)-2-
methylbutylamine 11 was achieved in three steps: tosylation, azidation, and reduction.
(S)-2-methylbutanal (10) and (S)-2-methylbutanamine were prepared in the same
manner from commercially available (S)-2-methylbutan-1-ol (9).
All stereoisomers of imine 1 3 and 4 were synthesized from combinations of
aldehydes and amines under neat conditions (Scheme 2).
Odor Evaluation. The odors of all synthesized stereoisomers of 1 3 and 4 were
evaluated by special panelists. The results of the odor evaluations are shown in
Tables 1 4.
In the case of the enantiomers of 1, we found that (S)-1 has amine- and butter-like
odor, while (R)-1 has esteric and fruity odor (Table 1).
Table 1. Odor Evaluation ofBoth Enantiomers of2-Methyl- N-(3'-methylbutylidene)butanamine (1)
Amine, internal organs of shrimp, cocoa-like element with sweet butter-like odor.
Heavier esteric odor than (S)-1. Fruity, cereal-like with slightly fishy scent.

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Scheme 2. Synthesis ofall Stereoisomers ofImine Derivatives
1 3 and 4
In the case of the enantiomers of 2, we found that both have a heavy, strong seafood
element, but (S)-2 has stronger odor than (R)-2. Also, we found that (S)-2 has a fruity
element along with the seafood element, while (R)-2 has a sweet butter element along
with the seafood element (Table 2).
Table 2. Odor Evaluation ofBoth Enantiomers of3-Methyl- N-(2'-methylbutylidene)butanamine (2)
Light and strong seafood element with fruity element including apple, pineapple,
chinese quince, pear.
Heavy and strong seafood element. Image of seaweed (wakame), laver with sweet
butter-like odor.
In the case of four stereoisomers of 3, it was found that the (2S)-isomer has a
metallic element, while (2R)-isomer has a phenolic element. All stereoisomers had a
fruity element (Table 3).
Odor evaluation of 4 is presented in Table 4.
As a result of odor evaluation, we found that all imine derivatives can be used not
only as seafood flavors, but also as fruit flavors, for instance, mango flavor.
The mechanism of generation of these odor compounds is thought to be as follows.
When spotted shrimp is roasted, leucine and isoleucine are thermolyzed to give the
corresponding aldehydes via decarboxylation, dehydrogenation, and then hydrolysis.
On the other hand, amines were generated by decarboxylation of these amino acids.
Condensation of these aldehydes and amines afford imine derivatives 1 4 [6].
Therefore, we predict that the absolute configuration of all these natural imines must

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Table 3. Odor Evaluation ofBoth Enantiomers of2-Methyl- N-(2'-methylbutylidene)butanamine (3)
Light, metallic, oily character. Seafood (internal organs of shrimp) with sweet
butter-like and esteric scent.
Mild, sweet, fruity, slightly phenolic element, including the peel of mango,
apple, citrus.
Seafood, including fish odor with phenolic, esteric, fruity element.
Metallic, slightly heavy, cocoa-like odor with esteric, fruity element.
Sweeter odor than (2S,2'S)-3. Image of burned fish, shrimp.
Table 4. Odor Evaluation ofBoth Enantiomers of3-Methyl- N-(3'-methylbutylidene)butanamine (4)
Oily element with aldehyde-like stimulus scent. Slightly sweet atmosphere. Image of
crab, internal organs of shrimp, burned fish, fish guts pickled in salt. Bacon-like element
with sweet chocolate, cocoa, jam.
be (S). To prove this hypothesis, separation of racemic imines 1 3 by GLC analysis
with proper chiral stationary phase is now being investigated.
In conclusion, we have succeeded in the stereoselective synthesis of imine
derivatives found in spotted shrimp that have the characteristic roast odor of seafood.
By odor evaluation, it was revealed that each stereoisomer has a different and
characteristic odor.
We thank Mitsubishi Rayon Co., Ltd. for the generous gift of methyl (S)-3-hydroxy-2-methylpropionate (5).
Experimental Part
General. M.p.: Yazawa BY-2. Optical rotations: HORIBA SEPA-200. Neutral column chromatography
(CC): Kanto Chemical silica gel 60N (spherical, neutral). GC: HP5890 gas chromatograph; ee values were
determined on a 50% heptakis(2,3-di-O-acetyl-6-O-TBDMS)-b-CD (OV-1701) column (0.25 mm I.D. Â 30 ml)
with the following temp. program 408 to 1408 (0.78/min); analysis of (R)-2-methylbutanal ((R)-10): tR 33.4 min,
analysis of (S)-2-methylbutanal ((S)-10): tR 34.9 min. IR Spectra: Jasco FT/IR-300E spectrometer, in nÄ [cmÀ1].
1H- and 13C-NMR spectra: Jeol JNM-LA400 spectrometer (400 and 100 MHz). HR-MS: Hitachi M-80B mass
spectrometer.
(R)-2-Methyl-3-[(4-methylphenylsulfonyl)oxy]propyl Benzoate (8). To a soln. of 2-methyl-3-[(tetrahydro-
2H-pyran-2-yl)oxy]propyl 4-methylbenzenesulfonate (7; 41.5 g, 126.4 mmol) in MeOH (250 ml) was added cat.
amount of TsOH ¥ H2O (2.0 g) at r.t. The mixture was stirred for 1 h at r.t. Na2CO3 (2g), was added to the
mixture and the mixture was concentrated. The residue was added to sat. Na2CO3 soln. and extracted with Et2O.
The extract was washed with brine, dried (MgSO4), and concentrated in vacuo to give crude oil (32.96 g), which
was employed in the next step without further purification.
To a soln. of this crude alcohol (32.9 g) and Et2N (17.7 g, 175 mmol) in Et2O (350 ml) was added BzCl
(20.8 g, 148 mmol) over 10 min at 08. The mixture was stirred for 3 h at r.t. The mixture was added to H2O. The
Et2O layer was separated, washed with sat. NaHCO3 soln. and brine, dried (MgSO4), and concentrated in vacuo.
The residue was obtained as a white solid. Recrystallization from hexane gave 8 (38.7 g; 88% from 7). Colorless

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needles. M.p. 59 608. [a]2D0 ˆ ‡4.22 (c ˆ 0.99, CHCl3). IR (KBr): 2927, 1717, 1455, 1351, 1278, 1174, 1116, 939,
841, 810, 720, 668, 592, 556. 1H-NMR (400 MHz, CDCl3): 1.05 (d, J ˆ 6.8, 3 H); 2.33 (s, 3 H); 4.06 (d, J ˆ 5.9,
2H); 4.14 ( dd, J ˆ 6.8, 11.2, 1 H); 4.22 (dd, J ˆ 5.0, 11.2, 1 H); 7.24 7.26 (m, 2H); 7.40 7.44 ( m, 2H); 7.54 7.58
‡
(m, 1 H); 7.77 (d, J ˆ 8.3, 2H); 7.91 ( d, J ˆ 7.1, 2H). HR-MS: 348.1065 ( M , C18H20O5S; calc. 348.1031).
(R)-2-Methylbutan-1-ol (9). To a suspension of CuI (19.8 g, 104 mmol) in dry Et2O (200 ml) was added
1.04m MeLi (200 ml, 208 mmol) over 30 min under Ar at À308, and the mixture was stirred for 2h at 0 8. To the
mixture was added a soln. of 8 (30.3 g, 87 mmol) in dry Et2O (50 ml) over 20 min at À108, and the mixture was
stirred for 2h at 0 8. The mixture was added to sat. NH4Cl soln., filtered, and separated. The Et2O layer was
washed with sat. NaHCO3 soln. and brine, dried (MgSO4), and concentrated in vacuo. The oily residue was
purified by distillation (b.p. 958/0.4 kPa) to give 12.4 g (74%) of (R)-2-methylbutyl benzoate. [a]2D0 ˆ À4.958
(c ˆ 1.29, CHCl3). IR (film): 2963, 2877, 1720, 1451, 1314, 1273, 1113, 711. 1H-NMR (400 MHz, CDCl3): 0.96
(t, J ˆ 7.2, 3 H); 1.02 (d, J ˆ 6.8, 3 H); 1.26 1.32 (m, 1 H); 1.51 1.57 (m, 1 H); 1.84 1.89 (m, 1 H); 4.13 (dd, J ˆ
6.8, 10.8, 1 H); 4.21 (dd, J ˆ 6.0, 10.8, 1 H); 7.42 7.46 ( m, 2H); 7.53 7.58 ( m, 1 H); 8.04 8.06 (m, 2H). HR-
‡
MS: 192.1103 (M , C12H16O2; calc. 192.1150).
To a soln. of NaOH (2.67 g, 66.8 mmol) in H2O (20 ml) and MeOH (5 ml) was added (R)-2-methylbutyl
benzoate (10.7 g, 55.7 mmol) at r.t. The mixture was stirred for 2h at reflux temp. The mixture was added to
NaCl and extracted with Et2O. The extract was washed with brine, dried (MgSO4), and concentrated in vacuo.
The oily residue was purified by distillation (b.p. 828/15.7 kPa) to give 4.1 g (84%) of 9. [a]2D0 ˆ ‡5.85 (c ˆ 0.90,
1
MeOH). IR (film): 3339, 2961, 2876, 1463, 1380, 1046, 1015, 648. H-NMR (400 MHz, CDCl3): 0.91 (t, J ˆ 7.3,
3 H); 0.91 (d, J ˆ 6.8, 3 H); 1.05 1.20 (m, 1 H); 1.38 1.59 (m, 2H); 1.70 ( s, 1 H); 3.42( dd, J ˆ 6.6, 10.5, 1 H);
‡
3.51 (dd, J ˆ 5.8, 10.5, 1 H). 13C-NMR (100 MHz, CDCl3): 11.5; 16.2; 25.9; 37.5; 68.2. HR-MS: 88.0917 (M ,
C5H12O; calc. 88.0888).
(R)-2-Methylbutanal (10). To a soln. of 9 (2.0 g, 22.7 mmol) in CH2Cl2 (28 ml) were added TEMPO
(36 mg, 0.23 mmol) in CH2Cl2 (28 ml) and KBr (270 mg, 2.3 mmol) in H2O (4.8 ml) at 08. The mixture was
added to a mixture of NaClO (43 ml), H2O (55.44 ml), and NaHCO3 (4.92g) at 0 8. The mixture was stirred for
15 min at 08. The mixture was separated, and the org. layer was washed with brine, dried (MgSO4), and
concentrated at low temperature in vacuo. The oily residue was purified by CC (neutral silica gel (50 g); hexane/
10 :1) furnishing pure 10 as an oil (1.0 g, 52%; 98% ee). [a]2D0 ˆ À35.88 (c ˆ 1.21, MeOH). IR (film): 2971, 2935,
2878, 1713, 1463, 1191, 1103, 970. 1H-NMR (400 MHz, CDCl3): 0.95 (t, J ˆ 7.6, 3 H); 1.09 (d, J ˆ 7.1, 3 H); 1.39
1.50 (m, 1 H); 1.70 1.81 (m, 1 H); 2.23 2.32 (m, 1 H); 9.63 (d, J ˆ 1.7, 1 H). 13C-NMR (100 MHz, CDCl3): 11.3;
‡
12.7; 23.4; 47.6; 205.2. HR-MS: 86.0742 (M , C5H10O; calc. 86.0732).
(R)-2-Methylbutanamine (11). To a soln. of 9 (2.0 g, 22.7 mmol) in pyridine (10 ml) was added TsCl (5.2 g,
27.3 mmol) at 08, and the mixture was allowed to stand overnight. The mixture was poured into ice-water and
extracted with Et2O. The Et2O soln. was washed with H2O, CuSO4 soln., NaHCO3 soln., and brine, dried
(MgSO4), and concentrated in vacuo to give 5.5 g of tosylate as oil. This was employed in the next step without
further purification.
To a soln. of crude tosylate (5.5 g) in DMF (25 ml) was added NaN3 (3.0 g, 45.4 mmol) at r.t. The mixture
was stirred for 3 h at 1008. The mixture was poured into H2O and extracted with Et2O. The Et2O soln. was
washed with brine, dried (MgSO4), and concentrated in vacuo to give 2.2 g of azide as oil. This was employed in
the next step without further purification.
A soln. of crude azide (2.2 g) in dry Et2O (10 ml) was added dropwise to a stirred suspension of LiAlH4
(1.5 g, 40 mmol) in dry Et2O (50 ml) at 08. The mixture was stirred for 4 h at r.t. The excess LiAlH4 was
decomposed by the successive addition of H2O (1.5 ml), 15% aq. NaOH (1.5 ml), and H2O (4.5 ml). The
mixture was filtered, and the solid was washed with Et2O. The combined filtrate and washings were dried
(MgSO4) and concentrated in vacuo. The oily residue was purified by distillation (b.p. 42 44 8/1.6 kPa) to give
1.2g (61% from 9) of 11. [a]2D0 ˆ ‡5.9 (c ˆ 1.05, MeOH). IR (film): 3373, 3298, 2960, 2926, 2874, 1603, 1462,
1379, 923, 840. 1H-NMR (400 MHz, CDCl3): 0.90 (t, J ˆ 7.4, 3 H); 0.90 (d, J ˆ 6.4, 3 H); 1.10 1.17 (m, 1 H);
1.34 1.44 (m, 2H); 1.61 ( s, 2H); 2.49 ( dd, J ˆ 6.8, 12.4, 1 H); 2.63 (dd, J ˆ 5.4, 12.4, 1 H). 13C-NMR (100 MHz,
‡
CDCl3): 11.3; 17.0; 26.8; 37.8; 48.0. HR-MS: 87.0987 (M , C5H13N: calc. 87.1048).
Imine Derivatives 1 4. A soln. of aldehyde (1.0 equiv.) and amine (1.0 equiv.) was stirred for 3 h at r.t. The
mixture was diluted with Et2O and was washed with sat. NaHCO3 soln. and brine, dried (MgSO4), and
concentrated in vacuo. The oily residue was purified by distillation to give the corresponding imine.
(S)-2-Methyl-N-(3'-methylbutylidene)butanamine ((S)-1)): yield 59%. B.p. 628/2.0 kPa. [a]2D0 ˆ À7.10 (c ˆ
0.17, MeOH). IR (film): 1167, 1380, 1463, 1670, 2826, 2873, 2929, 2957. 1H-NMR (300 MHz, CDCl3): 0.86 (d, J ˆ
6.8, 3 H); 0.90 (t, J ˆ 5.1, 3 H); 0.96 (d, J ˆ 6.6, 6 H); 1.09 1.19 (m, 1 H); 1.35 1.45 (m, 1 H); 1.66 1.74 (m,
1 H); 1.90 1.97 (m, 1 H); 2.15 (dd, J ˆ 5.6, 12.2, 2 H); 3.12 (dd, J ˆ 7.3, 10.8, 1 H); 3.34 (dd, J ˆ 5.8, 10.8, 1 H);

Helvetica Chimica Acta Vol. 86 (2003)
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7.61 (tt, J ˆ 1.2, 5.4, 1 H). 13C-NMR (75 MHz, CDCl3): 11.3; 17.6; 22.5; 26.4; 27.4; 35.6; 44.7; 67.8; 164.6. GC/MS:
‡
‡
154 ([M À 1] ), 140, 126, 113, 98, 84, 70, 56, 42, 39, 29. HR-MS: 155.1596 (M , C10H21N; calc. 155.1674).
Compound (R)-1: yield 62%. [a]2D0 ˆ 7.12( c ˆ 0.15, MeOH). HR-MS: 155.1726 (M , C10H21N; calc.
‡
155.1674).
(S)-3-Methyl-N-(2'-methylbutylidene)butanamine ((S)-2): yield: 56%. 798/3.3 kPa. [a]2D0 ˆ 17.7 (c ˆ 0.19,
MeOH). IR (film): 1383, 1462, 1670, 2871, 2928, 2959. 1H-NMR (300 MHz, CDCl3): 0.90 (d, J ˆ 6.6, 6 H); 0.91
(t, J ˆ 5.1, 3 H); 1.04 (d, J ˆ 6.8, 3 H); 1.31 1.66 (m, 5 H); 2.14 2.26 (m, 1 H); 3.36 (t, J ˆ 7.3, 2H); 7.45 ( d, J ˆ
5.8, 1 H). 13C-NMR (75 MHz, CDCl3): 11.5; 17.0; 22.4; 22.5; 25.7; 27.0; 39.9; 40.9; 59.4; 168.9. GC/MS: 154
‡
‡
([M À 1] ), 140, 127, 113, 98, 84, 71, 56, 43, 39, 29. HR-MS: 155.1671 (M , C10H21N; calc. 155.1674).
Compound (R)-2: yield: 58%. [a]2D0 ˆ À17.5 (c ˆ 0.18, MeOH). HR-MS: 155.1709 (M , C10H21N; calc.
‡
155.1674).
(2S,2'S)-2-Methyl-N-(2'-methylbutylidene)butanamine ((2S,2'S)-3): yield: 51%. B.p. 598/2.0 kPa. [a]D20
ˆ
À4.61 (c ˆ 0.13, MeOH). IR (film): 1379, 1460, 1671, 2821, 2875, 2929, 2962. 1H-NMR (300 MHz, CDCl3):
0.85 (dd, J ˆ 1.7, 4.6, 3 H); 0.90 (d, J ˆ 5.1, 3 H); 0.90 (dd, J ˆ 5.1, 11.0, 3 H); 1.05 (d, J ˆ 6.8, 3 H); 1.08 1.73
(m, 5 H); 2.18 2.28 (m, 1 H); 3.10 (dd, J ˆ 7.3, 11.0, 1 H); 3.34 (dd, J ˆ 5.8, 11.0, 1 H); 7.41 (d, J ˆ 5.9, 1 H).
13C-NMR (75 MHz, CDCl3): 11.3; 11.6; 17.1; 17.4; 27.1; 27.3; 35.5; 40.9; 67.6; 169.4. GC/MS: 154 ([M À 1] ), 140,
‡
127, 112, 98, 84, 70, 56, 43, 39, 29. HR-MS: calc. for C10H21N, 155.1674, found 155.1776.
Compound (2R,2'R)-3: yield: 50%. [a]D20 ˆ 5.17 (c ˆ 0.12, MeOH). HR-MS: 155.1683 (M , C10H21N; calc.
‡
155.1674).
Compound (2R,2'S)-3: yield: 55%. B.p. 588/2.0 kPa. [a]2D0 ˆ 3.63 (c ˆ 0.11, MeOH). IR (film): 1379, 1460,
1671, 2821, 2875, 2929, 2962. 1H-NMR (300 MHz, CDCl3): 0.85 (dd, J ˆ 1.5, 6.6, 3 H); 0.90 (d, J ˆ 4.9, 3 H); 0.90
(dd, J ˆ 5.1, 11.0, 3 H); 1.05 (d, J ˆ 6.8, 3 H); 1.08 1.73 (m, 5 H); 2.18 2.28 (m, 1 H); 3.10 (dd, J ˆ 7.3, 11.0,
1 H); 3.34 (dd, J ˆ 5.8, 11.0, 1 H); 7.41 (d, J ˆ 5.9, 1 H). 13C-NMR (75 MHz, CDCl3): 11.3; 11.6; 17.1; 17.4; 27.1;
‡
27.3; 35.5; 40.9; 67.6; 169.4. GC/MS: 154 ([M À 1] ), 140, 127, 112, 98, 84, 70, 56, 43, 39, 29. HR-MS: 155.1617
‡
(M ; C10H21N; calc. 155.1674).
‡
Compound (2S,2'R)-3: yield: 53%. [a]D20 ˆ À3.84 (c ˆ 0.10, MeOH). HR-MS: 155.1614 (M , C10H21N; calc.
155.1674).
3-Methyl-N-(3'-methylbutylidene)butanamine (4): yield: 58%. B.p. 668/2.0 kPa. IR (film): 1366, 1467, 1670,
1
2869, 2955. H-NMR (300 MHz, CDCl3): 0.89 (d, J ˆ 6.3, 6 H); 0.94 (d, J ˆ 6.6, 6 H); 1.31 1.89 (m, 4 H); 2.11
(t, J ˆ 6.2, 2 H); 3.37 (t, J ˆ 7.2, 2 H); 7.64 (t, J ˆ 5.3, 1 H). 13C-NMR (75 MHz, CDCl3): 22.5; 26.2; 26.4; 39.9;
‡
‡
44.7; 59.5; 164.2. GC/MS: 154 ([M À 1] ), 140, 126, 113, 98, 84, 70, 57, 43, 39, 27. HR-MS: 155.1679 (M ,
C10H21N; calc. 155.1674).
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Received July 18, 2002