Acidic ionic liquid-catalyzed homologation of the polyene chain in α,β-enals (polyenals)

Article abstract of DOI:10.1016/j.tet.2010.11.005

An efficient green-chemistry synthesis of conjugated polyenals from α,β-enals, orthoesters, and alkyl vinyl ethers promoted by acidic ionic liquid [emim][HSO₄] is described. The ionic liquid can be reused at least three times without any decrease in product yields.

Full text of DOI:10.1016/j.tet.2010.11.005

Tetrahedron 67 (2011) 173e178
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Acidic ionic liquid-catalyzed homologation of the polyene chain in
(polyenals)
a,b-enals
,
Galina V. Kryshtal a, Galina M. Zhdankina a, Nikolai V. Ignat’ev b, Michael Schulte b, Sergei G. Zlotin a
*
a N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky prosp., Moscow 119991, Russia
b Merck KGaA, PC-RL, Frankfurter Strasse 250, D-64293 Darmstadt, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 July 2010
Received in revised form 10 October 2010
Accepted 1 November 2010
Available online 5 November 2010
An efficient green-chemistry synthesis of conjugated polyenals from
vinyl ethers promoted by acidic ionic liquid [emim][HSO4] is described. The ionic liquid can be reused at
least three times without any decrease in product yields.
a,b-enals, orthoesters, and alkyl
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Enals
Polyenals
Acetals
One-pot reactions
Ionic liquids
Recycle
1. Introduction
Conjugated polyene units are incorporated into a variety of
natural1 and biologically active compounds, in particular leukotri-
enes,2 pheromones,3 antibiotics,4 and carotinoids.5 Commonly
these complex molecules are synthesized from simpler (poly)enes
that contain a conjugated aldehyde group,5d via the aldol con-
densation,5h,6,7 Peterson olefination,8 organometallic synthesis,9
Wittig type,2c,3c,10e12 and some other reactions. However, these
reactions often have low selectivity due to high reactivity of the
enals and require preliminary introduction of protecting groups
(PG).7e9,11 In particular, a useful algorithm for the homologation of
Scheme 1. Synthesis of polyenals.
the polyene chain in
actions including the acetalization of
a
,
b
-enals is a sequence of acid-catalyzed re-
-enals 1, condensation of
a
,b
FeCl3),5a,13e15 whereas the acetalization of enals 1 to acetals 2 re-
quires Brùnsted acid catalysis (H3PO4, TsOH, NH4Cl, NH4NO3).5a,13d
The use of different catalysts necessitates the isolation and purifi-
cation of intermediates 2 and 3, which makes the experimental
procedure more difficult.
We assumed that compounds 3 could be prepared from enals 1
in one-pot in the presence of an ionic liquid (IL) catalyst. Some ILs
possess both Lewis17a,b and Brùnsted acidity17c,d,18 and find appli-
cations in various catalytic reactions.19 For instance, they catalyze
the formation of acetals from alkyl (aryl) aldehydes and alcohols17
or triethyl orthoformate.18 However there is no information on the
acetals 2 with vinyl or 1,3-dienyl ethers, and deprotection of acetals
3 to polyenals 4 (Scheme 1).5aed,13e15
However, implementation of this scheme is complicated by
concurrent addition of enol (dienol) ethers to acetals 3 affording
oligomeric by-products under reaction conditions.5d,13d,16 This side
process can be suppressed by carrying out the vinylogation reaction
(2/3) in the presence of a mild Lewis acid (BF3$Et2O, ZnCl2,
synthesis of acetals from
a,b-enals and on the condensation of ac-
* Corresponding author. E-mail address: zlotin@ioc.ac.ru (S.G. Zlotin).
etals with vinyl ethers in the presence of ILs.
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.11.005

174
G.V. Kryshtal et al. / Tetrahedron 67 (2011) 173e178
2. Results and discussion
isolation of compounds 3a,b, though their purification by column
chromatography led to a significant mass loss, possibly due to the
hydrolysis of the acetal group. Therefore, we further hydrolyzed
crude alkoxyacetals 3 to more stable dienals 4 without purification
by a routine procedure (treatment with aqueous acetate buffer) and
fully characterized the latter. Yields of thereby synthesized com-
pounds 4a,b (calculated for acetals 2) were higher than those
reported in literature.20 The IL, poorly-soluble in the reaction mix-
ture, could be easily separated and reused without a catalytic activity
decrease. We attempted to carry out the homologation and hydro-
lysis/elimination reactions as a one-pot process by adding water
directly to 3a (or 3b)/[emim][HSO4]/MeOH mixtures, which formed
at the CeC bond-forming stage. However, unlike reactions in the
acetate buffer, these reactions led to complex mixtures of products.
On the other hand, similar conditions for the synthesis of acetals
2a,b and alkoxyacetals 3a,b allowed carrying out both reactions
successively in the same reaction vessel. Upon completion of the
first stage, ethyl vinyl ether was added directly to the reaction mass
affording compounds 3a,b, which were separated from the catalyst
via the decantation and converted to dienals 4a,b under the action
of AcOH/AcONa system.5a,13a,b Yields of dienals 4a,b in this reaction
sequence (Table 2, entries 1 and 2) were comparable with those
obtained in the reactions where isolated acetals 2a,b had been used
as starting compounds (Scheme 3).
To verify this hypothesis we compared the catalytic activities
of 1-ethyl-3-methyl- (emim) and 1-butyl-3-methylimidazolium
(bmim) salts with HSO4ꢀ, BF4ꢀ, PFꢀ6 or OTfꢀ anions (5 mol %) in the
reaction between citral (1a) and trimethyl orthoformate (Scheme 2).
Scheme 2. Acetalization of a,b-enals 1a,b.
Aldehyde 1a was slowly added to IL-containing ꢀ(5 mol %)
orthoformate at 5 ꢁC. It turned out that ILs with BF4ꢀ, PF6 , and OTfꢀ
anions did not catalyze an acetalization reaction under the studied
conditions (Table 1, entries 1e3). However acetal 2a was formed in
92% yield in the presence of [emim][HSO4] (Table 1, entry 4). The
reaction proceeded faster when methanol was added to the re-
action mixture (Table 1, entries 5 and 6), and a pronounced effect
was detected in the presence of 5 mol % of CH3OH. Furthermore, the
addition of methanol simplified the regeneration of IL, which after
the product decantation retained catalytic activity over at least two
reaction cycles (Table 1, entry 6). The catalytic system [bmim]
[HSO4]/MeOH (5 mol % each) exhibited similar activity (Table 1,
entry 7). Acetal 2b was obtained from myrtenal (1b) in 93% yield
under the proposed conditions (Table 1, entry 8).
The proposed method is applicable to the synthesis of dienals 4
having various aliphatic, alicyclic or aromatic groups at the double
bond. The acetalization procedure is merely different for aldehydes
1a,b and 1cef. To suppress side reactions, acid-labile compounds
1a,b should be slowly (drop-wise) added to orthoester at 5 ꢁC
(method A), whereas the acetalization of more stable enals 1cef
proceeded efficiently at rt irrespective of the mode of mixing of
components (method B). According to 1H NMR data (dCH]CH
3
5.95e6.30 ppm, JCH]CH 15.0e16.0 Hz)8a,11g the double bond
Table 1
formed in compounds 4aef has E-configuration. No other available
unsaturated or functional groups (e.g., in 1d and 1f) affect the
process selectivity or efficacy.
Comparison of ILs and IL/MeOH systems in the synthesis of acetals 2a
Entry
Enal
IL
MeOH
(equiv)
Time
(h)
Product
Yieldb (%)
[run]
The proposed IL-based catalytic system allows further homol-
ogation of polyene chain in dienals 4. Trienals 6a,c,e with
E,E-configurations of double bonds were synthesized from corre-
sponding dienals 4b,c,e and ethyl vinyl ether in the presence of
this system (Table 3, entries 1, 3, and 5). Ethyl 1-propenyl ether can
also serve as an alkenyl group source to afford compounds
6b,d,f (Table 3, entries 2, 4, and 6) bearing a trisubstituted (E)-
double bond adjacent to the aldehyde group in high yields. The
E-configuration was assigned to the newly formed double bond by
1
2
3
4
5
6
1a
1a
1a
1a
1a
1a
[bmim][BF4]
[bmim][PF6]
[bmim][OTf]
[emim][HSO4]
[emim][HSO4]
[emim][HSO4]
d
15
15
15
5.5
3.5
1.5
d
d
d
2a
2a
2a
0
0
0
92
d
d
d
10
0.05
88
99[1], 97[2],
98[3]
98
7
8
1a
1b
[bmim][HSO4]
[emim][HSO4]
0.05
0.05
1.5
1.5
2a
2b
93
a
Reaction mixture: 1 (10 mmol), CH(OMe)3 (10 mmol), IL (0.5 mmol), and (if it is
analogy with known hydrolytic reactions of b-alkoxyacetals, which
used) MeOH.
b
The yield is given for the isolated and purified product.
afforded thermodynamically more stable E-isomers of corre-
sponding enals13,14 and based on the comparison of chemical shifts
of H-atoms incorporated into the aldehyde group in compounds
6b,d,f (dHC]O 9.40e9.53 ppm) with those reported for (E)-CH]C
Then we evaluated the catalytic properties of the system [emim]
[HSO4]/MeOH (5 mol % of each) in the condensation of acetals 2a,b
with ethyl vinyl etherand it appeared that desired alkoxyacetals 3a,b
had been formed as major products (Scheme 3). We succeeded in
(Me)eCHO (
d d
9.40e9.50 ppm)21 and (Z)-CH]CH(Me)eCHO (
10.20e10.27 ppm)21c,d fragments.
Scheme 3. Condensation of acetals 2a,b with ethyl vinyl ether.

G.V. Kryshtal et al. / Tetrahedron 67 (2011) 173e178
175
Table 2
One-pot synthesis of 2,4-dienals 4aef
1. HC(OAlk)3
OAlk OEt
OAlk
3a-f
2. CH2=CHOEt
AcOH/AcONa
H2O
R
R
O
O
R
[emim][HSO4]/AlkOH
(5 mol % each)
4a-f
1a-f
Entry
1
1
a
R
Alk
Me
Methoda
Yield of 4 (%),b [cycle]
Mp ꢁC or n2D0
Me
Me
A
76 [1], 76 [2], 77 [3] (lit.20 50)
1.5380 (lit.20 1.5375)
Me
Me
Me
2
3
4
bc
c
Me
Et
A
B
B
88 [1], 87 [2], 87 [3]
71 (lit.13a 66)
1.5620
MeeCH]CHe
Me Me
1.5346 (lit.13a 1.5354)
78e79 (lit.5a 77e78)
Me
d
Me
76 (lit.5a 64)
Me
5
e
f
PheCH]CHe
Me
Me
B
B
71 [1], 73 [2], 72[3] (lit.13b 66)
38e39 (lit.13b 37e38)
6
76
110e111
Me
MeO2C
a
Method A: compound 1 (10 mmol) was added to the mixture of orthoformate (10 mmol), [emim][HSO4] (0.5 mmol) and AlkOH (0.5 mmol) at 5 ꢁC; method B: the mixture
of 1 (10 mmol) and orthoformate (10 mmol) was added to emim][HSO4] (0.5 mmol) and AlkOH (0.5 mmol) at rt (method B).
b
Yield of isolated compounds 4 (purified by crystallization or via TLC) calculated for enals 1.
c
Compound 4b was prepared earlier from myrtenal and N-methoxy-N-methyl-diethylphosphono-acetamide in 75% yield.11e
Table 3
One-pot synthesis of 2,4,6-trienals 6aef
1. HC(OAlk)3
OAlk OEt
OAlk
2. R1CH=CHOEt
AcOH/AcONa
H2O
R
R
O
R
O
[emim][HSO4]/AlkOH
(5 mol % each)
R1
R1
6a-f
5a-f
4b-f
Entry
1
4
b
R
Alk
Me
R1
H
Method
6
a
Yield (%),a [cycle]
81
Mp ꢁC or n2D0
Me
Me
A
1.6250
Me
Me
2
3
b
c
Me
Et
Me
H
A
B
b
c
78
1.6370
MeeCH]CHe
Me Me
75 (lit.13a 54)
57e58 (lit.13a 57e58)
Me
4
d
Me
Me
B
d
73 (lit.5a 62)
67e68 (lit.5a 66e68)]
Me
5
6
e
f
PheCH]CHe
Me
Me
H
B
B
e
f
81 [1], 82 [2], 80 [3] (lit.13b 20)
71.5
114e115 (lit.13b 116)
120e121
Me
Me
MeO2C
a
Yield of isolated compounds 6 (purified by crystallization or via TLC) calculated for dienals 4.
The iterative methodology could be pushed further. Trienal 6e
was readily converted to the (E,E,E,E)-tetraenal 7 in 71% overall
yield under proposed conditions (Scheme 4).
The IL [emim][HSO4] could be multiply used in the studied re-
actions. Upon the reaction completion, compounds 3 or 5 were
separated and fresh portions of corresponding aldehydes 1 or 4

176
G.V. Kryshtal et al. / Tetrahedron 67 (2011) 173e178
Scheme 4. Synthesis of (E,E,E,E)-tetraenal 7.
were added to the remaining IL. Yields of products 4 or 6 did not
reduce over at least three reaction cycles (Table 2, entries 1, 2, and
5; Table 3, entry 5).
Fresh portions of the reagents were added to the remaining IL and
the synthesis was repeated as described above (Table 1, entry 6).
Some of the prepared compounds are analogs or intermediates
for the synthesis of natural and/or biologically active compounds
containing conjugated polyene units. Among them, 2,4,6-trienals
6a and 6b (Table 3, entries 1 and 2) are close vitamin A analogs and
polyenal 6d (Table 3, entry 4) serves as an intermediate in the in-
dustrial production of vitamin A and carotinoids.5aed Polyenal 6f
(Table 3, entry 6) is a precursor to 4-(nor-polyprenyl)benzoic acid
possessing anticancer activity.22 The IL-based procedure enhances
product yields and makes the synthesis of polyenals more envi-
ronmentally friendly.23
4.2.1. 1,1-Dimethoxy-3,7-dimethylocta-2,6-diene (mixture of E- and
Z-isomers 65/35, 2a). Compound 2a was prepared from citral (1a)
in 92% yield. Colorless oil, n2D0¼1.4550 (lit.26 n2D0¼1.4548). 1H NMR
(300 MHz, CDCl3)
d
1.58e1.74 (m, 9H, 3ꢂCH3C]), 2.02e2.14 (m, 4H,
2ꢂCH2), 3.28 (s, 6H, 2ꢂOCH3), 4.97e5.02 [m, CH(OMe)2], 5.05e5.12,
5.20e5.26 (each m, each 1H, CH]).
4.2.2. 2,2-Dimethoxymethyl-6,6-dimethyl-bicyclo[3.1.1]hept-2-ene
(2b). Compound 2b was prepared from myrtenal (1b) in 93% yield.
Colorless oil, nD20¼1.4695. 1H NMR (300 MHz, CDCl3)
d 0.83 (s, 3H,
CH3), 1.15 (d, J¼8.5 Hz, 1H, CH), 1.33 (s, 3H, CH3), 2.07e2.14,
2.28e2.45 (2m, 5H, CH, 2ꢂCH2), 3.27 (s, 3H, OCH3), 3.32 (s, 3H,
OCH3), 4.51 (s, 1H, CH(OMe)2), 5.64 (br s, 1H, CH]) (lit.27 NMR
spectroscopic data for myrtenyl dioxolane).
3. Conclusion
In summary, we have developed an efficient procedure for the
homologation of the polyene chain in
a,b-enals (-polyenals) by
4.3. General procedure for the condensation of acetals 2a or
2b with ethyl vinyl ether
means of an acidic IL-catalyzed sequence of acetalization and CeC
bond-forming reactions. The method meets the requirements of
environmentally friendly processes due to a one-pot methodology,
neat conditions, and multiple use of the catalyst. It is of importance
as an alternative to the existing procedures for the preparation of
valuable polyene compounds.
Ethyl vinyl ether (0.72 g, 10 mmol) was added drop-wise for
10e15 min to the stirred mixture of acetal 2 (10 mmol), [emim]
[HSO4] (0.10 g, 0.5 mmol) and MeOH (15 mg, 0.5 mmol) at the
temperature ꢃ40e45 ꢁC. The reaction mixture was left stirring for
6e8 h at rt, the product was separated from IL by decantation and
dried over L2CO3. Volatile impurities were removed at 40 ꢁC
(40 Torr) for 30 min and the residue was passed through an Al2O3
pad (eluent: hexane) to afford alkoxyacetals 3a,b.
4. Experimental
4.1. General
The NMR spectra were recorded with
a Bruker AM-300
4.3.1. 10-Ethoxy-8,10-dimethoxy-2,6-dimethyldeca-2,6-diene (mix-
ture of E- and Z-isomers 65/35, 3a). Compound 3a was prepared
from 1,1-dimethoxy-3,7-dimethylocta-2,6-diene (2a). Colorless oil,
n2D0¼1.4570 (lit.20 n2D0¼1.4590, data for corresponding triethyl
[300.13 MHz (1H), 75.47 MHz (13C)] spectrometer in CDCl3 solution
unless otherwise noted. 1H and 13C chemical shifts are given rela-
tive to Me4Si and acetone-d6 as reference substances, respectively.
The IR spectra were recorded in KBr pellets on a Specord M-82.
Microanalysis was carried out on a PerkineElmer 2400. The re-
actions were monitored by TLC [Silicagel or aluminium oxide plates
from Merck KGaA; eluent: ethyl acetate/benzene (9/19) or benzene,
visualization by I2 or UV]. Ionic liquids [bmim][BF4], [bmim][PF6],
[bmim][ OTf], [emim][HSO4], and [bmim][HSO4] were supplied by
Merck KGaA, Darmstadt, Germany. Citral, myrtenal, crotonalde-
hyde, cinnamaldehyde, trimethyl orthoformate, triethyl ortho-
formate, ethyl vinyl ether, and ethyl 1-propenyl ether were
purchased from Acros and used without further purification.
Compounds 1d24 and 1f25 were synthesized according to the
reported methods.
alkoxyacetal). 1H NMR (300 MHz, CDCl3)
d 1.13e1.24 (m, 3H, CH3),
1.61, 1.68, 1.75 (3s, 9H, 3ꢂCH3C]), 1.88e2.16 (m, 6H, 3ꢂCH2), 3.21,
3.32 (2s, 6H, 2ꢂOCH3), 3.40e3.67 (m, 2H, OCH2), 3.96e4.03 (m, 1H,
CHeOCH3), 4.48e4.55 (m, 1H, OeCHeO), 4.98 (d, J¼11.0 Hz, 1H,
CH]), 5.02e5.08 (m, 1H, CH]). Anal. Calcd for C16H30O3: C 71.07, H
11.18. Found C 69.92, H 11.08.
4.3.2. 2-(3-Ethoxy-1,3-dimethoxypropyl)-6,6-dimethyl-bicyclo[3.1.1]
hept-2-ene (3b). Compound 3b was prepared from 2,2-dimethox-
ymethyl-6,6-dimethyl-bicyclo[3.1.1]hept-2-ene (2b). Colorless oil,
n2D0¼1.4710. 1H NMR (300 MHz, CDCl3)
d 0.83 (s, 3H, CH3), 1.11
(d, J¼8.5 Hz, 1H, CH), 1.15e1.25 (m, 3H, CH3), 1.26 (s, 3H, CH3),
1.55e1.82, 2.08e2.40 (2m, 7H, CH, 3ꢂCH2), 3.22, 3.31 (each s, each
3H, OCH3), 3.41e3.70 (m, 3H, OCH2, CHeOCH3), 4.50e4.65 (m, 1H,
4.2. General procedure for the acetalization of
and 1b
a,b-enals 1a
OeCHeO), 5.43 (br s, 1H, CH]). 13C NMR (75 MHz, CDCl3)
d 15.4,
21.5, 26.3, 31.3, 32.0, 37.7, 41.0, 41.5, 52.6, 53.1, 56.4, 61.8, 63.7, 101.4,
123.5, 147.5. Anal. Calcd for C16H28O3: C 71.60, H 10.52. Found C
71.53, H 10.48.
Enal 1 (10 mmol) was added drop-wise for 8e10 min to the
stirred mixture of trimethyl orthoformate (1.06 g, 10 mmol), IL
(0.5 mmol) and, if specified, MeOH (see Table 1) at 5 ꢁC. The re-
action mixture was left stirring for 30 min at 5e7 ꢁC and then at rt
for the time given in Table 1. The products were separated from IL
by decantation and dried over L2CO3. Volatile impurities were re-
moved at 40 ꢁC (40 Torr) and the residue was passed through an
Al2O3 pad to afford compounds 2a,b (yields are specified in Table 1).
4.4. General procedure for the one-pot synthesis of
compounds 4a,b and 6a,b (method A)
Enal 1 or 4 (10 mmol) was added drop-wise for 8e10 min to the
stirred mixture of orthoformate (10 mmol), [emim][HSO4] (0.10 g,

G.V. Kryshtal et al. / Tetrahedron 67 (2011) 173e178
177
0.5 mmol), and AlkOH (0.5 mmol) at 5 ꢁC. The reaction mixture was
left stirring for 30 min at 5e7 ꢁC and then for 1.5 h at rt. Ethyl vinyl
or ethyl 1-propenyl ether (10 mmol) was added to the reaction
mixture at the temperature ꢃ40e45 ꢁC and it was left stirring for
4e8 h (TLC monitoring). Crude alkoxyacetal 3 or 5 was separated
from the reaction media (reused in the next run) and heated at
95 ꢁC for 3e6 h (TLC monitoring) with the Isler mixture (6 mL;
prepared from 25 g of sodium acetate, 100 mL of acetic acid and
15 mL of water). The mixture was poured onto ice and the product
was extracted with diethyl ether (2ꢂ10 mL). The combined extracts
were washed successively with water (10 mL), aqueous Na2CO3
(3ꢂ10 mL), and water (10 mL) and dried over MgSO4. The solvent
was evaporated and the residue was purified by column chroma-
tography (SiO2, eluent: hexane; hexane/benzene 1/1; benzene) to
afford compound 4 or 6.
The reaction mixture was left stirring for 2 h and the above de-
scribed working-up procedure was used next to afford compounds
4cef, 6cef, and 7. 1H NMR data for compounds 4c,e, 6c,e, and 7
were in accordance with those described in the literature.8a
4.5.1. (E,E)-Hexa-2,4-dienal (sorbic aldehyde) (4c). Compound 4c
was prepared from trans-crotonaldehyde (1c) in 71% yield. Color-
less oil, n2D0¼1.5346 (lit.13a nD20¼1.5375). 1H NMR (300 MHz, CDCl3)
d
1.88 (d, J¼4.6 Hz, 3H, CH3), 6.03 (dd, J¼15.0, 8.0 Hz, 1H, ]CHCHO),
6.30e6.35 (m, 2H, 2CH]), 7.10e7.14 (m, 1H, CH]), 9.51 (d,
J¼8.0 Hz, 1H, CH]O).
4.5.2. (E,E)-4-Methyl-6-(2,6,6-trimethylcyclohex-1-enyl)hexa-2,4-di-
enal (4d). Compound 4d was prepared from 4-(20,60,60-trime-
thylcyclohex-10-enyl)-2-methylbut-2-enal (1d) in 76% yield. Mp
78e79 ꢁC (lit.5a mp 78e79 ꢁC). 1H NMR (300 MHz, CDCl3)
d 0.94 (s,
4.4.1. 5,9-Dimethyldeca-2,4-8-trienal (mixture of 2E,4E- and 2E,4Z-
isomers 65/35, 4a). Compound 4a was prepared from citral (1a) in
77% yield. Colorless oil, n2D0¼1.5380 (lit.20 n2D0¼1.5375). 1H NMR
6H, 2ꢂCH3), 1.51, 1.83 (each s, each 3H, CH3CH]), 1.38e1.43,
1.48e1.60, 1.87e1.92 (each m, each 2H, CH2), 2.90 (d, J¼7.0 Hz, 2H,
CH2eCH]), 5.85 (t, J¼7.0 Hz, 1H, CH]), 6.6 (dd, J¼16.0, 8.0 Hz, 1H,
]CHCHO), 7.08 (d, J¼16.0 Hz,1H, CH]), 9.50 (d, J¼8.0 Hz,1H, CHO).
(300 MHz, CDCl3)
d
1.58, 1.62, 1.91 (each s, each 3H, 3ꢂCH3C]),
2.12e2.30 (m, 4H, 2ꢂCH2), 5.02e5.08 (m, 1H, CH]), 6.00 (dd,
J¼16.0, 8.0 Hz, 1H, ]CHCHO), 6.12 (d, J¼16.0 Hz, 1H, CH]),
7.28e7.42 (m, 1H, CH]), 9.52 (dd, J¼8.0, 1.5 Hz, 1H, CHO).
4.5.3. (E,E)-5-Phenylpenta-2,4-dienal (4e). Compound 4e was pre-
pared from trans-cinnamaldehyde (1e) in 73% yield. Mp 38e39 ꢁC
(lit.13b mp 37e38 ꢁC). 1H NMR (300 MHz, CDCl3)
d
6.27 (dd, J¼15.2,
4.4.2. (E)-3-(6,6-Dimethyl-bicyclo[3.1.1]hept-2-en-2-yl)prop-2-enal
(4b). Compound 4b was prepared from myrtenal (1b) in 88% yield.
8.0 Hz,1H, CH]), 7.00e7.05 (m, 2H, 2CH]), 7.27 (dd, J¼15.2, 8.0 Hz,
1H, CH]), 7.30e7.40 (m, 3H, Ph), 7.45e7.52 (m, 2H, Ph), 9.62 (d,
J¼8.0 Hz, 1H, CHO).
Colorless oil, nD20¼1.5620. 1H NMR (300 MHz, CDCl3)
d 0.75 (s, 3H,
CH3), 1.12 (d, J¼8.5 Hz, 1H, CH), 1.32 (s, 3H, CH3), 2.10e2.15,
2.40e2.55 (2m, 5H, CH, 2CH2), 6.02 (dd, J¼16.0, 8.1 Hz, 1H, ]
CHCHO), 6.15 (br s, 1H, CH]), 7.07 (d, J¼16.0 Hz, 1H, CH]), 9.52 (d,
J¼8.1 Hz, 1H, CHO) (1H NMR data for 4b were in accordance with
those described in the literature28).
4.5.4. Methyl 4-((E,E)-2-methyl-5-oxopenta-1,3-dienyl)benzoate (4f).
Compound 4f was prepared from methyl 4-(2-methyl-3-oxoprop-1-
enyl)benzoate (1f) in 76% yield. Mp 110ꢀ111 ꢁC. IR (KBr)
n
1712 (C]
O), 1668 (C]O), 1604 (C]C). 1H NMR (300 MHz, CDCl3)
d 2.10 (s, 3H,
CH3C]), 3.91 (s, 3H, OCH3), 6.30 (dd, J¼16.0, 8.0 Hz, 1H, ]CHCHO),
6.93 (s, 1H, CH]), 7.29 (d, J¼16.0 Hz, 1H, CH]), 7.40e7.44 (m, 2H,
Ar), 8.03e8.07 (m, 2H, Ar), 9.65 (d, J¼8.0 Hz, 1H, CHO). 13C NMR
4.4.3. (E,E)-5-(6,6-Dimethyl-bicyclo[3.1.1]hept-2-en-2-yl)penta-
2,4-dienal (6a). Compound 6a was prepared from 3-(6,6-dimethyl-
bicyclo[3.1.1]hept-2-en-2-yl)prop-2-enal (4b) in 81% yield.
(75 MHz, CDCl3)
d 14.0, 52.2, 129.1, 129.4, 129.5, 129.6, 136.2, 139.2,
Colorless oil, nD20¼1.6250. IR (thin film)
n
1680 (C]O), 1616 (C]C),
0.79 (s, 3H,
140.7, 156.9, 166.5, 193.7. Anal. Calcd for C14H14O3: C 73.02, H 6.13.
Found C 72.93, H 6.05.
1600 (C]C), 1576 (C]C). 1H NMR (300 MHz, CDCl3)
d
CH3), 1.16 (d, J¼8.5 Hz, 1H, CH), 1.36 (s, 3H, CH3); 2.10e2.20,
2.40e2.50 (2m, 4H, 2ꢂCH2), 2.63 (t, J¼5.5 Hz,1H, CH), 5.95 (br s,1H,
CH]), 6.15 (dd, J¼16.0, 8.1 Hz, 1H, ]CHCHO), 6.34 (dd, J¼16.0,
11.2 Hz, 1H, CH]), 6.68 (d, J¼16.0 Hz, 1H, CH]), 7.17 (dd, J¼16.0,
11.2 Hz, 1H, CH]), 9.53 (d, J¼8.1 Hz, 1H, CHO). 13C NMR (75 MHz,
4.5.5. (E,E,E)-Octa-2,4,6-trienal (6c). Compound 6c was prepared
from hexa-2,4-dienal (4c) in 75% yield. Mp 57e58 ꢁC (lit.13a mp
57e58 ꢁC). 1H NMR (300 MHz, CDCl3)
d
1.87 (d, J¼7.5 Hz, 3H, CH3,),
6.01e6.05 (m, 1H, CH]), 6.12 (dd, J¼15.2, 8.0 Hz, 1H, ]CHCHO),
6.24e6.35 (m, 2H, 2ꢂCH]), 6.62 (dd, J¼15.2, 11.4 Hz, 1H, CH]),
7.12 (dd, J¼15.2, 11.4 Hz, 1H, CH]), 9.52 (d, J¼8.0 Hz, 1H, CHO).
CDCl3)
d 20.9, 26.2, 31.2, 33.0, 37.9, 40.8, 41.5, 123.0, 124.5, 125.6,
132.6, 146.8, 153.1, 193.5. Anal. Calcd for C14H18O: C 83.12, H 8.97.
Found C 83.26, H 8.81.
4.5.6. (E,E,E)-2,6-Dimethyl-8-(2,6,6-trimethylcyclohex-1-enyl)octa-
2,4,6-trienal (6d). Compound 6d was prepared from 4-methyl-6-
(2,6,6-trimethylcyclohex-1-enyl)hexa-2,4-dienal (4d) in 73% yield.
4.4.4. (E,E)-5-(6,6-Dimethyl-bicyclo[3.1.1]hept-2-en-2-yl)-2-methyl-
penta-2,4-dienal (6b). Compound 6b was prepared from 3-(6,6-
dimethyl-bicyclo[3.1.1]hept-2-en-2-yl)prop-2-enal (4b) in 78%
Mp 67e68 ꢁC (lit.5a mp 66e68 ꢁC). 1H NMR (300 MHz, CDCl3)
d 0.98
yield. Colorless oil, n2D0¼1.6370. IR (thin film)
n
1672 (C]O), 1616
0.80 (s, 3H,
(s, 6H, 2ꢂCH3), 1.54, 1.88, 1.90 (3s, 9H, 3ꢂCH3CH]), 1.41e1.47,
1.56e1.62, 1.86e1.97 (3m, 6H, 3ꢂCH2), 2.92 (d, J¼7.0 Hz, 2H,
CH2eCH]), 5.65 (t, J¼7.0 Hz, 1H, CH]), 6.55 (dd, J¼16.0, 11.2 Hz,
1H, CH]), 6.68 (d, J¼16.0 Hz, 1H, ]CH), 6.88 (d, J¼11.2 Hz, 1H,
CH]), 9.53 (s, 1H, CHO).
(C]C), 1596 (C]C) cmꢀ1. 1H NMR (300 MHz, CDCl3)
d
CH3),1.15 (d, J¼8.5 Hz,1H, CH),1.35 (s, 3H, CH3),1.85 (s, 3H, CH3C]),
2.10e2.18, 2.41e2.51 (m, 4H, 2CH2), 2.65 (t, J¼5.5 Hz, 1H, CH), 5.90
(br s, 1H, CH]), 6.51 (dd, J¼16.0, 11.2 Hz, 1H, CH]), 6.63 (d,
J¼16.0 Hz,1H, ]CH), 6.90 (d, J¼11.2 Hz,1H, CH]), 9.41 (s,1H, CHO).
13C NMR (75 MHz, CDCl3)
d
9.7, 20.9, 26.3, 31.3, 33.0, 37.9, 40.8, 41.5,
4.5.7. (E,E,E)-7-Phenylhepta-2,4,6-trienal (6e). Compound 6e was
prepared from 5-phenylpenta-2,4-dienal (4e) in 82% yield. Mp
120.2, 123.6, 125.7, 136.7, 147.0, 149.8, 194.7. Anal. Calcd for C15H20O:
C 83.28, H 9.32. Found C 83.32, H 9.18.
114ꢀ115 ꢁC (lit.13b mp 116 ꢁC). 1H NMR (300 MHz, CDCl3)
d 6.20 (dd,
J¼16.0, 8.0 Hz, 1H, ]CHCHO), 6.57 (dd, J¼14.6, 11.2 Hz, 1H, CH]),
6.81e6.93 (m, 3H, 3ꢂCH]), 7.19 (dd, J¼16.0, 11.2 Hz, 1H, CH]),
7.27e7.38 (m, 3H, Ph), 7.44e7.50 (m, 2H, Ph), 9.59 (d, J¼8.0 Hz,
1H, CHO).
4.5. General procedure for the one-pot synthesis of
compounds 4cef, 6cef, and 7 (method B)
The mixture of enal 1, 4 or 6e (10 mmol) and orthoformate
(10 mmol) was added drop-wise for 8e10 min to the stirred mix-
ture of [emim][HSO4] (0.10 g, 0.5 mmol) and AlkOH (0.5 mmol) at rt.
4.5.8. Methyl 4-((E,E,E)-2,6-dimethyl-7-oxohepta-1,3,5-trienyl)ben-
zoate (6f). Compound 6f was prepared from methyl 4-(2-methyl-

178
G.V. Kryshtal et al. / Tetrahedron 67 (2011) 173e178
5-oxopenta-1,3-dienyl)benzoate (4f) in 76% yield. Mp 120e121 ꢁC.
soued, M.; Reboul, E.; Salemkour, M. Synth. Commun. 1995, 25, 3097e3108; (d)
Bellassoued, M.; Salemkour, M. Tetrahedron 1996, 52, 4607e4624.
IR (KBr)
CDCl3)
n
1724 (C]O),1680 (C]O), 1604 (C]C). 1H NMR (300 MHz,
1.92 (s, 3H, CH3C]), 2.13 (s, 3H, CH3C]), 3.91 (s, 3H,
9. (a) Duhamel, L.; Ple, G.; Ramondenc, Y. Tetrahedron Lett. 1989, 30,
7377e7380; (b) Contreras, B.; Duhamel, L.; Ple, G. Synth. Commun. 1990, 20,
2983e2990; (c) Duhamel, L.; Ancel, J.-E. Tetrahedron 1992, 48, 9237e9250;
(d) Maeta, H.; Suzuki, K. Tetrahedron Lett. 1993, 34, 341e344; (e) Soullez, D.;
Ple, G.; Duhamel, L. J. Chem. Soc., Perkin Trans. 1 1997, 1639e1645; (f) Si-Fodil,
M.; Ferreira, H.; Gralak, J.; Duhamel, L. Tetrahedron Lett. 1998, 39,
8975e8978; (g) Villiers, P.; Soullez, D.; Ramondenc, Y.; Ple, G. Tetrahedron
1999, 55, 14031e14042.
10. (a) Cresp, T. M.; Sargent, M. V.; Vogel, P. J. Chem. Soc., Perkin Trans. 11974, 37e41;
(b) Bestmann, H. J.; Vostrowsky, O.; Paulus, H.; Billmann, W.; Stransky, W.
Tetrahedron Lett. 1977, 121e124; (c) Rokach, J.; Young, R. N.; Kakushima, M.; Lau,
C.-K.; Seguin, R.; Franette, R.; Guindon, Y. Tetrahedron Lett. 1981, 22, 979e981;
(d) Bestmann, H. J.; Roth, K.; Ettlinger, M. Chem. Ber. 1982, 115, 161e171; (e)
Baker, S. R.; Boot, J. R.; Morgan, S. E.; Osborn, D. J.; Ross, W. J.; Shrubsall, P. R.
Tetrahedron Lett. 1983, 24, 4469e4472; (f) Ernest, I.; Main, A.; Menasse, R.
Tetrahedron Lett. 1982, 23, 167e170; (g) Spangler, C. W.; McCoy, R. K. Synth.
Commun. 1988, 18, 51e60; (h) Ivanova, D. I.; Eremin, S. V.; Shvets, V. I. Tetra-
hedron 1996, 52, 9581e9588.
d
OCH3), 6.72e6.79 (m, 2H, 2CH]), 6.81 (s, 1H, CH]), 6.97
(d, J¼16.0 Hz, 1H, CH]), 7.38e7.43 (m, 2H, Ar), 7.99e8.07 (m, 2H,
Ar), 9.49 (s, 1H, CHO). 13C NMR (75 MHz, CDCl3)
d 14.2, 18.6, 52.0,
125.7, 126.3, 129.0, 129.4, 135.0, 136.8, 138.5, 141.2, 146.9,
167.0, 194.5. Anal. Calcd for C17H18O3: C 75.53, H 6.71. Found C
75.67, H 6.84.
4.5.9. (E,E,E,E)-9-Phenylnona-2,4,6,8-tetraenal (7). Compound 7 was
prepared from 7-phenylhepta-2,4,6-trienal (6e) in 71% yield. Mp
140e141 ꢁC (lit.8a mp 140 ꢁC). 1H NMR (300 MHz, CDCl3)
d 6.18 (dd,
J¼15.4, 8.0 Hz, 1H, ]CHCHO), 6.48 (dd, J¼14.6, 11.0 Hz, 1H, CH]),
6.64e6.78 (m, 2H, 2CH]), 6.81 (d, J¼15.6 Hz, 1H, CH]), 6.90 (dd,
J¼14.6, 11.0 Hz, 1H, CH]), 7.17 (dd, J¼15.6, 11.0 Hz, 1H, CH]),
7.35e7.40 (m, 3H, Ph), 7.42e7.52 (m, 3H, Ph, CH]), 9.58 (d, J¼8.0 Hz,
1H, CHO).
11. (a) Takahashi, H.; Fujiwara, K.; Ohta, M. Bull. Chem. Soc. Jpn. 1962, 35,
1498e1500; (b) Nagata, W.; Hayase, Y. Tetrahedron Lett. 1968, 4359e4362; (c)
Boutagy, J.; Thomas, R. Chem. Rev. 1974, 74, 87e89; (d) Meyers, A. J.; Tomoika,
K.; Fleming, M. P. J. Org. Chem. 1978, 43, 3788e3789; (e) Nuzillard, J.-M.; Bou-
mendjel, A.; Massiot, G. Tetrahedron Lett. 1989, 30, 3779e3780; (f) Dumahel, L.;
Guillemont, J.; Le Gallic, Y.; Ple, G.; Poirier, J.-M.; Ramondenc, Y.; Chabardes, P.
Acknowledgements
ꢁ
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This work was financially supported by Merck KGaA, Darmstadt,
Germany.
Supplementary data
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.tet.2010.11.005.
14. (a) Makin, S. M. Pure Appl. Chem. 1976, 47, 173e181; (b) Reuttimann, A. Eur.
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