Ehrlich's reaction of furanoeremophilanes

Article abstract of DOI:10.1246/bcsj.77.1737

6-Ethoxyfuranoeremophilane and its synthetic analogue, 4-ethoxy-4,5,6,7- tetrahydrobenzofuran, reacted with p-dimethylaminobenzaldehyde (Ehrlich's reagent) under acidic conditions in a 2:1 ratio to afford condensation products in 34% and 78% yields, respe

Full text of DOI:10.1246/bcsj.77.1737

Ó 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 1737–1740 (2004) 1737
Ehrlich’s Reaction of Furanoeremophilanes
ꢀ
Chiaki Kuroda, Tadashi Ueshino, and Hajime Nagano1
Department of Chemistry, Rikkyo University, Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501
1Department of Chemistry, Ochanomizu University, Otsuka, Bunkyo-ku, Tokyo 112-8610
Received April 7, 2004; E-mail: kuroda@rikkyo.ac.jp
6-Ethoxyfuranoeremophilane and its synthetic analogue, 4-ethoxy-4,5,6,7-tetrahydrobenzofuran, reacted with p-di-
methylaminobenzaldehyde (Ehrlich’s reagent) under acidic conditions in a 2:1 ratio to afford condensation products in
34% and 78% yields, respectively. 6-Hydroxyfuranoeremophilane and 4-hydroxy-4,5,6,7-tetrahydrobenzofuran afforded
the same products as in the case of ethoxy derivatives, respectively. The intermediate 1:1 adduct was not obtained from
6-ethoxyfuranoeremophilane, but obtained from the synthetic analogue in low yield. It was deduced that the colored
compound in Ehrlich’s test is the cationic species of the intermediate 1:1 adduct, and that the concentration of the col-
ored intermediate in the reaction mixture is very low.
Ehrlich’s reagent, p-dimethylaminobenzaldehyde (1) in hy-
drochloric acid, has a long history,1 and is known as the color-
ing reagent of pyrrole.2,3 As shown in Scheme 1, a Friedel–
Crafts type reaction occurs at the 2- or 3-position of pyrrole
to give a colored cation such as 2. Although this reagent does
not color a solution of non-substituted furan, many natural fur-
anosesquiterpenes such as furanoeremophilanes are colored.
The pinkish coloring of furanoeremophilanes is probably due
to the presence of the electron-rich trisubstituted furan ring.
The coloring of furanosesquiterpenes on TLC using this reac-
tion is called Ehrlich’s test, which is a very useful method for
searching for natural furanosesquiterpenes since it is easy to
detect the presence or absence of a furano-compound without
isolation. Using the method, various natural furanoeremophi-
lanes were isolated from Ligularia (Compositae)4 and related
genera of the family.5 However, to our surprise, no detailed
study on the reaction of p-dimethylaminobenzaldehyde (1)
with natural furanosesquiterpenes has been reported in spite
of its usefulness in natural product chemistry for over 40 years.
For example, researchers simply stated ‘‘compound I was posi-
tive to the Ehrlich test’’ without quoting any literature.4b,c,5a
Here we report a study on Ehrlich’s reaction of natural fura-
noeremophilanes as well as their synthetic analogues.
positae),8 which was collected in Nagano Prefecture, Japan.
Compound 3 is an artifact derived from 4 during the extraction
process with ethanol since this compound was not obtained
from the benzene extract.
When an ethanol solution of 3 was treated with p-dimethyl-
aminobenzaldehyde (1) and HCl at room temperature, the so-
lution immediately changed to pinkish purple and then slowly
changed to dark blue. After 1 day of stirring, the reaction was
quenched by the addition of aqueous alkali, and the product
was separated by silica-gel column chromatography. Although
many products were detected on TLC, only the major product
could be isolated, and was identified as the 2:1 adduct 5 (34%
yield) from its spectral data. Related syntheses of 2:1 adducts
from 2-methylfuran and various aldehydes have been report-
ed,9 however, a 2:1 adduct is not obtained from pyrrole.3 When
4 was treated under the same reaction conditions, compound 5
was afforded in 21% yield instead of 6 (Scheme 2). This must
be the result of substitution of the 6-OH group with solvent
ethanol, as in the case of the ethanol extraction process.
From these results, it can be deduced that the pinkish
colored compound is the cationic species 7 (three mesomeric
structures 7a, 7b, and 7c, are shown in Scheme 3), i.e., the
product obtained by the Friedel–Crafts reaction of 3 with p-di-
methylaminobenzaldehyde (1) followed by elimination of the
resulting hydroxy group. The 2:1 adduct 5 is considered to
be formed from 7 via the attack of a second molecule of 3.
It is known that the pinkish color produced by Ehrlich’s test
of natural furanoeremophilanes on TLC fades slowly within
one hour. This phenomenon can be explained by the formation
Results and Discussion
Two furanoeremophilanes, 6-ethoxy- (3)6 and 6-hydroxy-
furanoeremophilanes (= petasalbin, 4),4d,7 were used in this
study. These compounds were isolated from the rhizome of
the cultivated plant Petasites japonicus var. giganteus (Com-
CHO
H
N
H
N
H
HCl
N
NMe2
NMe2
+
NMe2
2
1
Scheme 1.
Published on the web September 10, 2004; DOI 10.1246/bcsj.77.1737

1738 Bull. Chem. Soc. Jpn., 77, No. 9 (2004)
Ehrlich’s Reaction of Furanoeremophilanes
of the colorless compound 5 or other decomposed compounds
from 7. The isolation of the 1:1 adduct was tried by quenching
the reaction with aqueous alkali in a short reaction time, how-
ever, only the 2:1 adduct 5 was detected together with the sub-
strate 3. This indicates that the concentration of the colored
compound 7 in the flask (thus on TLC in the Ehrlich’s test)
is low, and to which a second molecule of furanoeremophilane
reacted quickly. Namely, the attack of the electron-rich furan
ring in 3 to the cationic species 7b (e.g., the reaction from 7
to 5) is considered to be faster than that to the protonated p-di-
methylaminobenzaldehyde (1) (e.g., from 3 to 7).
H
O
1
4
9
6
8
7
2
3
10
12
11
5
+
1
OR
3 R = Et
4 R = H
i
NMe2
The Ehrlich’s reaction of synthetic analogues 8 and 9 (rac-
emic) was also studied (Scheme 4). The substrate 9 was pre-
pared by the Feist–Benary method from 1,3-cyclohexanedione
and chloroacetaldehyde10 followed by reduction with LiAlH4.
Compound 8 was obtained by etherification of 9. Namely,
when 9 was treated with acidic ethanol, 8 was afforded, as
in the case of the above Ehrlich reaction.
H
H
O
O
CH
OR
RO
5 R = Et
6 R = H
The results of Ehrlich’s reaction of 8 was almost parallel
with that of the natural furanoeremophilane 3, except that
the 2:1 adduct 11 was afforded in good yield (78%) after a
Scheme 2. Reagents and conditions: i) HCl aq, EtOH, r.t.,
1 d, then NaHCO3 aq.
NMe2
NMe2
NMe2
H
H
H
O
O
O
3
OEt
7a
OEt
7b
OEt
7c
Scheme 3.
NMe2
NMe2
O
i
+
1
O
O
OR
OEt
OEt
8 R = Et
9 R = H
10
iii
ii
NMe2
NMe2
O
O
O
CH
OEt
OEt
OEt
EtO
12
11
Scheme 4. Reagents and conditions: i) HCl aq, EtOH, r.t. ii) NaHCO3 aq. after 2 d. iii) NaHCO3 aq. after 1 h.

C. Kuroda et al.
Bull. Chem. Soc. Jpn., 77, No. 9 (2004) 1739
room temperature for 1 day, aqueous NaHCO3 was added, and
the mixture was extracted with Et2O. The ethereal layer was
washed successively with H2O and brine, and then dried over an-
hydrous Na2SO4. Evaporation of the solvent afforded an oily res-
idue, which was chromatographed on silica gel (3 g) using hex-
ane/Et2O (98:2) as the eluent. The chromatography was repeated
using hexane/Et2O (99:1 to 95:5) until 5 (22.3 mg, 34%) was ob-
tained.
48 h reaction. In contrast to the reaction of the natural product,
of course, 11 was obtained as a mixture of diastereomers.
Compound 9 also afforded 11 in 64% yield by the same treat-
ment. The coloring reaction, 8 (or 9) to 10, was slower than in
the case of natural furanoeremophilanes, since compounds 8
and 9 have only a disubstituted furan ring. The presence of a
more reactive trisubstituted furan ring in natural furanoeremo-
philanes is probably the cause of the complex reaction for 3.
This reactivity difference based on the substitution also
explains the difference in yields of 5 and 11.
Compound 5. An oil; IR (neat) 1614 (C=C), 1520, 1444,
1
1082, and 737 cmꢁ1; H NMR ꢀ 0.63–1.72 (34H, m), 2.05 (3H,
s, Me on furan), 2.07 (3H, s, Me on furan), 2.46 (6H, s, NMe2),
2.37–2.55 (4H, m CH2–furan ꢂ 2), 3.29–3.40 (2H, m, OCHHMe
ꢂ 2), 3.58–3.68 (2H, m, OCHHMe ꢂ 2), 4.17 (2H, br s, CHOEt ꢂ
2), 5.87 (1H, s, (furan)2–CH–Ar), 6.58 (2H, d, J ¼ 8:3 Hz, Ar),
and 7.24 (2H, d, J ¼ 8:3 Hz, Ar); 13C NMR ꢀ 9.2, 15.8, 15.9,
17.4, 20.4, 25.7, 26.4, 29.8, 31.1, 34.9, 40.3, 41.4, 41.7, 65.4,
76.1, 113.2, 115.6, 119.0, 119.1, 129.1, 148.6, 148.9, and 149.8;
MS (FAB) m=z 655 (Mþ, 100%), 610 (23), and 394 (57); HRMS
(FAB) Found: m=z 655.4559 (Mþ). Calcd for C43H61NO4: M,
655.4603.
Compound 11. An oil; IR (neat) 1614 (C=C), 1520, 1265,
1084, and 737 cmꢁ1; 1H NMR ꢀ 1.09 (6H, two sets of triplet, each
J ¼ 6:9 Hz, OCH2CH3 ꢂ 2), 1.33–1.54 (4H, m), 1.74–1.98 (4H,
m), 2.19–2.47 (4H, m), 2.47, 2.49, 2.50 (6H, three singlets, ratio
1:2:1, NMe2), 3.25–3.50 (4H, m, OCH2CH3 ꢂ 2), 4.14–4.20
(2H, m, CHOEt ꢂ 2), 5.54, 5.56 (1H, two singlets, ratio 1:3,
ArCH(furan)2), 6.25, 6.27, 6.28, 6.30 (2H, four singlets, ratio
1:1:1:1, furan), 6.50–6.59 (2H, m, Ar), and 7.31–7.39 (2H, m,
Ar); 13C NMR for the major isomer ꢀ 15.9, 19.3, 23.4, 29.7,
40.3, 45.3, 63.7, 71.1, 108.4, 113.1, 119.5, 129.6, 150.1, 152.1,
and 154.1 (one carbon signal could not be specified because of
overlapping with the solvent signal); MS m=z 463 (Mþ, 10%),
429 (10), 341 (19), 281 (49), and 207 (100); HRMS Found: m=z
463.2677 (Mþ). Calcd for C29H37NO4: M, 463.2723.
When the reaction of 9 was quenched with aqueous alkali in
a shorter reaction time (1 h), the presence of the 1:1 adduct 12
was detected on TLC, together with the etherified compound 8.
Compound 12 was obtained in only 1% yield after column
chromatography, however, it was difficult to obtain 12 in pure
form, and the structure was deduced to be the ethoxy deriva-
tive from the spectral data.
In conclusion, the primary result of this study is that the
Ehrlich’s coloring reaction of natural furanoeremophilanes
gives a complex product mixture. This is consistent with the
known fact that some natural furanoeremophilanes are not sta-
ble enough under weak acid conditions and decompose during
handling, even in CDCl3. This can be attributed to the presence
of the reactive trisubstituted furan ring. Among the above com-
plex mixture of products, the 2:1 adduct (two furanoeremophi-
lanes and one p-dimethylaminobenzaldehyde; the second step
is not addition but substitution) was obtained as the major
product. It could be deduced that the coloring compound is a
dehydrated cationic intermediate of the 1:1 adduct.
Experimental
General Procedure. IR spectra were recorded on a Jasco FT/
1
IR-230 spectrometer. Both H and 13C NMR spectra were meas-
Compound 12. An oil; IR (neat) 1606 (C=C), 1520, 1265,
1
ured on a Jeol GSX-400 (400 MHz for 1H; 100 MHz for 13C)
spectrometer in C6D6 as the solvent. Chemical shifts were record-
ed on the ꢀ scale (ppm) with tetramethylsilane as an internal
and 739 cmꢁ1; H NMR ꢀ 1.13 (3H, t, J ¼ 7:1 Hz, OCH2CH3),
1.16 (3H, t, J ¼ 7:0 Hz, OCH2CH3), 1.07–1.73 (4H, m), 1.96–
2.04 (1H, m), 2.37–2.43 (1H, m), 2.52 (6H, s, NMe2), 3.30 (1H,
dq, J ¼ 9:2, 7.1 Hz, OCHHCH3), 3.38 (2H, AB, each q, J ¼
7:0 Hz, OCH2CH3), 3.61 (1H, dq, J ¼ 9:2, 7.1 Hz, OCHHCH3),
3.90 (1H, ddd, J ¼ 1:6, 5.5, 11.0 Hz, CH2CHOEt), 5.47 (1H, s,
ArCHOEt), 6.25 (1H, d, J ¼ 1:6 Hz, furan), 6.65 (2H, d, J ¼
8:8 Hz, Ar), and 7.90 (2H, d, J ¼ 8:8 Hz, Ar); MS m=z 343
(Mþ, 78%), 314 (8), 298 (100, Mþ ꢁ OEt), 268 (14), 252 (25),
and 134 (17); HRMS Found: m=z 343.2101 (Mþ). Calcd for
C21H29NO3: M, 343.2147.
standard. For 13C NMR, the signal of the solvent (C6D6
=
128.0) was used as the reference. Both low-resolution mass spec-
tra (MS) and high-resolution mass spectra (HRMS) were obtained
on a Jeol SX-102A, CMATE II, JMS-700, or Shimadzu GCMS-
QP5050 mass spectrometer with the EI method unless otherwise
noted. Analytical TLC was done on precoated TLC plates (Kiesel-
gel 60 F254, layer thickness 0.2 mm). Wakogel C-200 was used
for column chromatography.
Natural Furanosesquiterpenes. Compounds 3 and 4 were
isolated from Petasites japonicus var. giganteus (Compositae)
collected in Nagano Prefecture, Japan. Roots of P. japonicus
var. giganteus (ca. 200 g) were collected and, without drying, ex-
tracted with ethanol at room temperature for several days. After
filtration, the aqueous ethanol solution of the extract was concen-
trated under reduced pressure to afford a mixture of oily residue
and aqueous phase. AcOEt was added, and the organic phase
was separated and concentrated. The resulting crude extract was
chromatographed on silica-gel (20 g) using hexane/AcOEt as an
eluent to afford 3 (117 mg) and 4 (253 mg).
Ehrlich’s Reaction. Compound 3 (52.3 mg, 0.200 mmol) was
dissolved in commercial EtOH (1 cm3), and to this was added p-
dimethylaminobenzaldehyde (45.0 mg, 0.302 mmol) and 2
mol dmꢁ3 HCl aq (2 drops). The reaction mixture became pinkish
purple, and slowly changed to dark blue. After being stirred at
Etherification of 9. Compound 9 (411 mg, 2.98 mmol) was
dissolved in EtOH (5 cm3), and conc. HCl (0.25 cm3) was added
to this solution at room temperature with stirring. After 4 h, an
aqueous solution of NaOH was added to pH 7–8, and the mixture
was extracted with Et2O. Drying over anhydrous MgSO4 followed
by evaporation of the solvent gave an oily residue, which was
chromatographed on silica gel (15 g) using hexane/Et2O (98:2)
as the eluent to afford 8 (342 mg, 69%).
4-Ethoxy-4,5,6,7-tetrahydrobenzofuran (8).
(neat) 1626 (C=C), 1508, 1441, 1086, and 725 cmꢁ1
An oil; IR
1H NMR
;
ꢀ 1.15 (3H, t, J ¼ 6:9 Hz, Me), 1.34–1.57 (2H, m), 1.75–1.99
(2H, m), 2.22–2.46 (2H, m), 3.37 (1H, dq, J ¼ 8:8, 7.0 Hz,
OCHHCH3), 3.44 (1H, dq, J ¼ 8:8, 7.0 Hz, OCHHCH3), 4.18
(1H, t, J ¼ 4 Hz, CHOEt), 6.28 (1H, d, J ¼ 1:8 Hz, furan), and
7.08 (1H, d, J ¼ 1:8 Hz, furan); 13C NMR ꢀ 15.9, 19.2, 23.3,
29.7, 63.6, 70.9, 110.7, 118.8, 140.6, and 152.9; MS m=z 166

1740 Bull. Chem. Soc. Jpn., 77, No. 9 (2004)
Ehrlich’s Reaction of Furanoeremophilanes
(Mþ, 34%), 138 (75), 121 (Mþ ꢁ OEt, 100), 109 (80), and 91
(52); HRMS Found: m=z 166.0990 (Mþ). Calcd for C10H14O2:
M, 166.0994; Anal. Found: C, 72.31; H, 72.26%. Calcd for
C10H14O2: C, 8.59; H, 8.49%.
T. Takahashi, Bull. Chem. Soc. Jpn., 48, 112 (1975). d) H. Ishii,
T. Tozyo, and H. Minato, Tetrahedron, 21, 2605 (1965).
5
a) H. Nagano, Y. Tanahashi, Y. Moyiyama, and T.
Takahashi, Bull. Chem. Soc. Jpn., 46, 2840 (1973). b) C. Kuroda,
T. Murae, M. Tada, H. Nagano, T. Tsuyuki, and T. Takahashi,
Bull. Chem. Soc. Jpn., 55, 1228 (1982). c) K. Naya, M. Nakagawa,
M. Hayashi, K. Tsuji, and M. Naito, Tetrahedron Lett., 1971,
2961.
References
1
F. Feigl, ‘‘Spot Tests, 2,’’ Elsevier (1954), p. 198; E. Stahl,
‘‘Dunnschicht-Chromatographie,’’ Springer-Verlag, Berlin (1962),
pp. 498, 503.
6 H. Nagano, C. Kuroda, Y. Moriyama, T. Tsuyuki, and T.
Takahashi, Bull. Chem. Soc. Jpn., 55, 1221 (1982).
2
L. F. Fieser and M. Fieser, ‘‘Reagents for Organic Synthe-
sis,’’ Wiley, New York (1967).
R. S. Alexander and A. R. Butler, J. Chem. Soc., Perkin
Trans. 2, 1976, 696.
7
(1979).
8
K. Yamakawa and T. Satoh, Chem. Pharm. Bull., 27, 1747
´
L. Novotny, K. Kotva, J. Toman, and V. Herout, Phyto-
3
chemistry, 11, 2795 (1972).
4
a) Y. Ishizaki, Y. Tanahashi, and T. Takahashi, J. Chem.
9 A. Riad, Z. Mouloungui, M. Delmas, and A. Gaset, Synth.
Commun., 19, 3169 (1989).
10 E. Bisagni, J.-P. Marquet, J.-D. Bourzat, J.-J. Pepin, and
Soc., Chem. Commun., 1969, 551. b) M. Tada, Y. Moriyama, Y.
Tanahashi, and T. Takahashi, Bull. Chem. Soc. Jpn., 47, 1999
(1974). c) T. Sato, Y. Moriyama, H. Nagano, Y. Tanahashi, and
´
J. Andre-Louisfert, Bull. Soc. Chim. Fr., 1971, 4041.