Volatile constituents of Semnostachya menglaensis Tsui

Article abstract of DOI:10.1021/jf051632d

Semnostachya menglaensis Tsui (Acanthaceae) is a rare plant indigenous to Mengla in the tropical rainforest of the Xishuangbanna prefecture in the south of Yunnan province, People's Republic of China. When the leaves are crushed, a characteristic smell of basmati rice or pandan leaves develops. Their hexane extract, prepared from a specimen growing in a greenhouse of the botanical garden of the Kunming Institute of Botany, contains 1-(3,4,5,6-tetrahydro-2- pyridyl)-1-propanone (41.2%) and 1-(1,4,5,6-tetrahydro-2-pyridyl)-1-propanone (37.5%) which constitute the main part of the volatile compounds. Minor components are 1-(3,4,5,6-tetrahydro-2-pyridyl)-1-ethanone (4.9%), 1-(1,4,5,6-tetrahydro-2-pyridyl)-1-ethanone (4.8%), 1-(2-piperidyl)-1-propanone (5.2%), 1-octen-3-ol (3.2%), 1-octen-3-one (1.9%), and 3-octanol and 1-(2-pyridyl)-1-propanone in trace amounts.

Full text of DOI:10.1021/jf051632d

J. Agric. Food Chem. 2005, 53, 9161−9164 9161
Volatile Constituents of Semnostachya menglaensis Tsui
†
†
†
REGULA NAEF, ALAIN VELLUZ, FABIENNE MAYENZET,
CHRISTIAN STARKENMANN,* AND HAN-DONG SUN‡
,
†
Firmenich SA, Corporate R&D Division, P.O. Box 239, CH-1211 Geneva 8, Switzerland, and Key
State Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany,
Chinese Academy of Sciences, Kunming, 650204 Yunnan, People’s Republic of China.
Semnostachya menglaensis Tsui (Acanthaceae) is a rare plant indigenous to Mengla in the tropical
rainforest of the Xishuangbanna prefecture in the south of Yunnan province, People’s Republic of
China. When the leaves are crushed, a characteristic smell of basmati rice or pandan leaves develops.
Their hexane extract, prepared from a specimen growing in a greenhouse of the botanical garden of
the Kunming Institute of Botany, contains 1-(3,4,5,6-tetrahydro-2-pyridyl)-1-propanone (41.2%) and
1-(1,4,5,6-tetrahydro-2-pyridyl)-1-propanone (37.5%) which constitute the main part of the volatile
compounds. Minor components are 1-(3,4,5,6-tetrahydro-2-pyridyl)-1-ethanone (4.9%), 1-(1,4,5,6-
tetrahydro-2-pyridyl)-1-ethanone (4.8%), 1-(2-piperidyl)-1-propanone (5.2%), 1-octen-3-ol (3.2%),
1-octen-3-one (1.9%), and 3-octanol and 1-(2-pyridyl)-1-propanone in trace amounts.
KEYWORDS: Semnostachya menglaensis; Acanthaceae; tetrahydropyridines; synthesis
INTRODUCTION
Model Reaction (2). L-Proline (10 mmol, 1.15 g) and glucose (10
mmol, 1.80 g) were added to a phosphate buffer 0.1 M, pH 7.00 (200
Semnostachya menglaensis Tsui (Acanthaceae) is a rare plant
indigenous to Mengla situated in the tropical rainforest of the
Xishuangbanna prefecture in the south of Yunnan province,
People’s Republic of China. When the long, lanceolate leaves
are crushed, a strong odor reminiscent of basmati rice or pandan
leaves is liberated.
To the best of our knowledge, only nonvolatile phenolic
glycosides have been previously identified from this plant (1).
The characteristic odor principle has not yet been investigated.
mL, 2.4 g of NaHPO
apparatus and heated for 6 h, the volatiles formed being simultaneously
4 2
in 200 mL of H O) in a Likens-Nickerson
extracted with pentane (150 mL). The solvent was dried over MgSO
and evaporated using a Vigreux column.
4
Syntheses. Preparation of 1-(3,4,5,6-Tetrahydro-2-pyridyl)-1-pro-
panone (3) and 1-(1,4,5,6-Tetrahydro-2-pyridyl)-1-propanone (4) (3).
1-(2-Pyridyl)-1-propanone (6, 8.4 g, 62.2 mmol, MP Biomedical cat.
no. 220569) in ethanol (75 mL) was hydrogenated at room temperature
using rhodium on alumina (5%, 1.5 g) as catalyst. An amount of 1.2 L
of hydrogen was consumed after 6 h and 5.6 L after 2 days. The crude
mixture was filtered on Celite, concentrated, and distilled using a
Vigreux column. Bp (0.11 mbar): 53 °C. The solid was recrystallized
from hexane to give 5.27 g of pure 1-(2-piperidyl)-1-propanol (7), mp
EXPERIMENTAL PROCEDURES
Preparation of the Natural Extract. Crushed leaves (10 g), picked
from a specimen of Semnostachya menglaensis grown in a greenhouse
of the botanical garden of the Kunming Institute of Botany were
immersed in hexane (100 mL). The plant material was then filtered
off, and the extract was concentrated at atmospheric pressure using a
Vigreux column (yield 20 mg).
GC/MS Analysis. The concentrated extract was analyzed by GC/
MS on a GC 6890 coupled to an MSD 5973 inert (Agilent, Palo Alto,
CA) equipped with an apolar column (SPB-1, 30 m × 0.25 mm fused
silica capillary column, film thickness 1 µm). After 5 min at 60 °C,
the oven temperature was raised by 5°/min to 250 °C, at an ion source
temperature of 156 °C and a helium flow of 1 mL/min. The mass spectra
in the electron impact mode (EI) were measured at 70 eV in a scan
range from 35 to 300.
)
89-90 °C, yield 60%.
1-(2-Piperidyl)-1-propanol (7, 5 g, 35.2 mmol) in benzene (250 mL)
in the presence of silver carbonate on Celite (57 g) (4) was heated
under reflux for 11 h. The mixture was filtered, concentrated, and bulb-
to-bulb distilled under 1 mbar at 100-120 °C to give 2.1 g of a yellow
oil which was purified by flash-chromatography on SiO
Et O 3:2) and again bulb-to-bulb distilled to remove traces of solvent.
Compounds 3 (56% GC) and 4 (40% GC) were obtained (1.23 g, yield
5%).
Preparation of 1-(2-Piperidyl)-1-propanone (5). 1-(2-Pyridyl)-1-
2
(30 g, pentane/
2
2
propanone (6, 6 g, 44.4 mmol, MP Biomedical cat. no. 220569) in
toluene (60 mL), ethylene glycol (5.5 g, 88.7 mmol), and p-toluene-
sulfonic acid (9.3 g, 48.8 mmol) were heated at reflux under N
water was removed by using a Dean Stark apparatus during 2 h. The
crude mixture was cooled, water was added (50 mL), and then Na
CO was added portionwise (pH 12). The organic phase was separated,
and the water phase was extracted twice with Et O. The organic phase
was dried over MgSO and concentrated in vacuo to give crude 2-ethyl-
2
. The
3
NMR Analysis. NMR spectra were measured in CDCl with a
Bruker DPX 400 instrument with tetramethylsilane as the internal
standard, δ ) 0.00 ppm.
2
-
3
2
*
To whom correspondence should be addressed. Phone: +41 22 780
4
3
4 77. Fax: +41 22 780 33 34. E-mail christian.starkenmann@firmenich.com.
+
•
†
‡
2-(2-pyridyl)-1,3-dioxolane (8, 7.75 g, yield 97%). MS: 179 (M , 0),
Firmenich SA.
Chinese Academy of Sciences.
1
150 (73), 136 (8), 134 (8), 106 (25), 101 (100), 78 (53), 57 (27). H
1
0.1021/jf051632d CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/06/2005

9162 J. Agric. Food Chem., Vol. 53, No. 23, 2005
Naef et al.
Figure 1. 1H NMR spectra of the total extract of Semnostachya menglaensis and of a reference sample of a mixture of compounds 1 and 2.
Scheme 1. Compounds Identified in Semnostachya menglaensis
NMR: 0.88 (t, 3H), 2.07 (q, 2H), 3.83-3.87 (m, 2H), 4.07-4.01 (m,
H), 7.18-7.23 (m, 1H), 7.52 (d, 1H), 7.67-7.80 (m, 1H), 8.63-866
not caused, as first supposed, by 2-acetyl-1-pyrroline, the odor
principle of cooked rice (6) and of pandan leaves (7). In fact,
this compound could not be identified in the extract.
The mass spectra (MS) of the two main peaks were very
similar to the fragmentation patterns of 1-(3,4,5,6-tetrahydro-
2
(
1
3
m, 1H). C NMR: 160.6 (s), 149.6 (d), 136.2 (d), 122.7 (d), 120.2
d), 110.5 (s), 65.1 (2t), 31.5 (t), 7.7 (q).
(
2
-Ethyl-2-(2-pyridyl)-1,3-dioxolane (8, 7.75 g, 43 mmol) in ethanol
(70 mL) was hydrogenated at room temperature using rhodium on
2
1
-pyridyl)-1-ethanone (1) and 1-(1,4,5,6-tetrahydro-2-pyridyl)-
-ethanone (2) (3, 8), with a molecular weight (MW) of 139
alumina (5%, 1.6 g) as catalyst. Hydrogen (2.7 L) was consumed after
0 h. The organic phase was dried over MgSO and concentrated to
give 2-ethyl-2-(2-piperidyl)-1,3-dioxolane (9, 7.8 g, yield 98%). MS:
9
4
instead of 125 (see Scheme 1).
+
•
1
1
85 (M , 2), 156 (5), 110 (5), 101 (30), 84 (100), 57 (10), 56 (10). H
The NMR-spectrum (see Figure 1) of the total extract of the
leaves showed an ethyl group for each isomer (2.6, 2.8 ppm
for -CH2- and 1.1 ppm for -CH3) replacing the methyl groups
NMR: 0.89 (t, 3H), 1.19-1.47 (m, 3H), 1.52-1.59 (m, 1H), 1.62-
1
4
2
.87 (m, 5H), 2.55-2.62 (m, 2H), 3.08-3.14 (m, 1H), 3.95-4.03 (m,
13
H). C NMR: 112.4 (s), 65.8 (t), 65.5 (t), 61.9 (d), 47.4 (t), 27.0 (t),
6.5 (t), 26.4 (t), 24.9 (t), 7.4 (q).
(2.29 and 2.36 ppm) of 1 and 2, but the signals at 3.17, 3.82,
and 5.65 ppm, respectively, were identical with the signals for
the imine 1 and the enamine 2.
2
-Ethyl-2-(2-piperidyl)-1,3-dioxolane (9, 4.4 g, 23.8 mmol) was
heated at 70 °C during 15 h in HCl 10% (50 mL) and acetone (15
mL). The reaction mixture was cooled, and NaOH 15% and NaCl were
added. The crude product was extracted with EtOAc and bulb-to-bulb
distilled (0.9 mbar, 90 °C) to give 1-(2-piperidyl)-1-propanone (5, 3.2
These spectral data were in agreement with the data published
for 1-(3,4,5,6-tetrahydro-2-pyridyl)-1-propanone (3) and 1-(1,4,5,6-
tetrahydro-2-pyridyl)-1-propanone (4) by De Kimpe and Kep-
pens, who synthesized and fully characterized the compounds
(9) and by Hofmann and Schieberle who prepared them in
Maillard-type model reactions and established the organoleptic
descriptors (roasted, popcorn-like) and their thresholds (0.2 ng/L
in air) (2) by AEDA analysis of a mixture. It is worth mentioning
that the first to identify these types of compounds in model
reactions involving proline and sugars was Tressl et al. in 1981
+•
g, yield 95%). MS: 141 (M , 1), 84 (100), 56 (20), 55 (8), in agreement
1
with the data published by Tressl et al. (5). H NMR: 1.07 (t, 3H),
1
1
3
.22-1.62 (m, 5H), 1.59 (m, 1H), 1.83-1.91 (m, 1H), 1.92-2.00 (m,
H), 2.43-2.52 (m, 2H), 2.59-2.65 (m, 1H), 3.08-3.12 (m, 1H), 3.30-
1
3
.32 (m, 1H). C NMR: 211.6 (s), 65.3 (d); 45.8 (t), 32.20 (t), 29.6
(t), 26.2 (t), 24.8 (t), 7.7 (q).
RESULTS AND DISCUSSION
(10).
The extract of this rare plant with its extraordinary smell of
basmati rice, bread, and popcorn was preferentially analyzed
by injection onto an apolar column (SPB-1), where perfect
separation was observed, the polar phase (Supelcowax) produc-
ing broad and badly resolved peaks. The characteristic odor was
Compounds 3 (41.2%) and 4 (37.5%) constitute the main part
of the volatile compounds; minor components are 1 (4.9%), 2
(4.8%), 1-(2-piperidyl)-1-propanone (5, 5.2%, MS (see ref 5)),
1-octen-3-ol (3.2%), 1-octen-3-one (1.9%), and 3-octanol and
1-(2-pyridyl)-1-propanone (6) in trace amounts (see Figure 2).

Volatile Constituents of Semnostachya menglaensis Tsui
J. Agric. Food Chem., Vol. 53, No. 23, 2005 9163
Figure 2. GC/MS profile of Semnostachya menglaensis.
Figure 3. GC/MS profile of a model reaction (glucose
Scheme 2 Synthesis of Compounds 3 and 4
+ proline).
Scheme 3. Synthesis of Compound 5
1
-(2-Piperidyl)-1-propanone (5), known as conhydrinone, was
The compounds were specifically synthesized starting from
the commercial 1-(2-pyridyl)-1-propanone (6), as outlined in
Scheme 2 following the method of B u¨ chi and W u¨ est (3). The
overall yield for 3 and 4 (3:2 ratio) was 15% for a purity
of 96%. The intermediate 1-(2-piperidyl)-1-propanol (7, â-
conhydrine) was first isolated from the flowers of hemlock
(Conium maculatum) in 1856 (12, 13) and is widely docu-
mented.
Compound 5 was prepared by conversion of ketone 6 to the
dioxolane 8 followed by reduction of the aromatic ring to
compound 9 and deprotection (Scheme 3). This compound
decomposed within 3 days when stocked as a pure compound
at 4 °C.
described first as a hemlock alkaloid (11) and later on in model
reactions of proline with sugars by Tressl et al. (5). It exhibits
a strong odor reminiscent of rice, cereals, and Chinese
noodles.
We identified compounds 3 and 4 as minor constituents in a
Maillard-type model reaction of proline with glucose (following
ref 2, slightly modified), where the acetyl-derivatives 1 and 2
were the main compounds, to check the MS and retention times
(
see Figure 3).
Compounds 1 and 2 as well as 3 and 4 are tautomeric pairs,
the ratios of the imino forms 1 and 3 to the enamino forms 2
and 4 approaching 1:1 in the natural extract.

9164 J. Agric. Food Chem., Vol. 53, No. 23, 2005
Naef et al.
All the major pyridine derivatives have a skeleton of eight
carbons. Interestingly, the two most important compounds
containing no nitrogen are unsaturated C8-compounds: 1-octen-
-ol and 1-octen-3-one. It is most likely that these structures
are related.
(7) Buttery, R. G.; Juliano, B. O.; Ling, L. C. Identification of rice
aroma compound 2-acetyl-1-pyrroline in pandan leaves. Chem.
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(
8) Hofmann, T.; Schieberle, P. New and convenient syntheses of
the important roasty, popcorn-like smelling food aroma com-
pounds 2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine from
their corresponding cyclic amino acids. J. Agric. Food Chem.
3
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JF051632D