Synthesis of 2,5-substituted tetrahydrofurans by Pd(II)-catalyzed cyclization

Article abstract of DOI:10.2174/157017812802850258

The Pd(II)-catalyzed cyclization of hydroxyl-substituted allylic alcohols quantitatively afforded a cis/trans mixture of 2,5-substituted tetrahydrofurans.

Full text of DOI:10.2174/157017812802850258

Letters in Organic Chemistry, 2012, 9, 563-567
563
Synthesis of 2,5-Substituted Tetrahydrofurans by Pd(II)-Catalyzed
Cyclization
Beata Olszewska, Bogusꢀaw Kryczka, Anna Zawisza*
Department of Organic and Applied Chemistry, University of Lodz, Tamka 12, 91-403 Lodz, Poland
Received April 24, 2012: Revised June 29, 2011: Accepted July 01, 2012
Abstract: The Pd(II)-catalyzed cyclization of hydroxyl-substituted allylic alcohols quantitatively afforded a cis/trans
mixture of 2,5-substituted tetrahydrofurans.
Keywords: Cyclization, diastereoselectivity, homogeneous catalysis, oxygen heterocycles, palladium.
INTRODUCTION
RESULTS AND DISCUSSION
Substituted tetrahydrofuran moieties constitute one of the
most important structural units in natural products and bio-
logically active compounds [1-10].
The starting unsaturated alcohols 4a-e were prepared ac-
cording to Scheme 1. Reaction of (Z)-but-2-en-1,4-diol (1)
with thionyl chloride in pyridine afforded (Z)-4-chlorobut-2-
en-1-ol (2) in 85% yield [18]. The condensation of 2 with
ethyl acetoacetate in the presence of sodium in ethyl alcohol
gave the corresponding unsaturated carbonyl compound 3 in
Among them, 2-alkenyl and 2,5-dialkenyl tetrahydro-
furans are widely recognized as the most useful because the
alkenyl moiety can be transformed into a wide range of func-
tionalities, thereby facilitating further elongation. The litera-
ture describes several methods of synthesis of such connec-
tions. Transition-metal-catalyzed heterocyclization reactions
4
8% yield [19]. Diols 4a-e were obtained in 38-98% yield,
after reaction of 3 with the corresponding Grignard reagent
followed by hydrolysis with NH Cl aqueous solution.
4
O
HO
OH SOCl
2
, Py
HO
Cl ethyl acetoacetate
Na/C H OH
1
. RMgX
OH
2
5
2. NH Cl
4
(aq)
1
2
3
X = I, Br
OH
R
R
PdCl (CH CN)
2
OH
2
3
O
5a-e
a: R = CH3
4
a-e
b: R = CH CH
3
2
c: R = CH =CH
2
d: R = CH =C(CH )
2
3
e: R = CH =CHCH
2
2
Scheme 1.
of unsaturated substrates are now a well used methodology
in organic synthesis which allows for highly valuable oxy-
gen-heterocyclic derivatives [11, 12]. Particularly, an in-
tramolecular nucleophilic attack of oxygen nucleophiles on
palladium ꢁ-olefin complexes [13, 14] or palladium ꢂ -allyl
complexes [15-17] afforded the corresponding oxygen het-
erocycles in a generally stereo- and regioselective manner.
The palladium(II)-catalyzed heterocyclization was first
studied with alcohol 4a as the substrate (Table 1). Ring clo-
sure of 4a occurred in THF at room temperature in the pres-
2 3 2
ence of a catalytic amount of PdCl (CH CN) providing tet-
3
rahydrofuran derivative 5a in 32% yield (Table 1, Entry 1).
Reactions were also carried out in other solvents (Table 1,
Entries 2-3) to yield 5a in a quantitative yield.
II
Here we present our results concerning the palladium(II)-
catalyzed synthesis of 2,5-substituted tetrahydrofurans from
readily available hydroxyl-substituted allylic alcohols.
The Pd -catalyzed cyclization was extended to other al-
cohols 4b-e bearing different substituents. In all cases the
corresponding tetrahydrofurans 5b-e were obtained in excel-
lent yields (Table 1, Entries 4-9) as a mixture of cis/trans
diastereoisomers in a ratio close to 50:50 (Fig. 1). Only un-
saturated alcohol 4d gave high diastereoselectivity 68:32
(cis:trans) in THF, 70:30 in Et O in favor of the cis di-
astereoisomer and what was found to be remarkable, 38:62
*
Address correspondence to this author at the Department of Organic and
2
Applied Chemistry, University of Lodz, Tamka 12, 91-403 Lodz, Poland;
Tel: +48 42 6355764; Fax: +48 42 6655162;
E-mail: azawisza@chemia.uni.lodz.pl
2 2
in favor of the trans diastereoisomer in CH Cl . The differ-
1
875-6255/12 $58.00+.00
© 2012 Bentham Science Publishers

5
64 Letters in Organic Chemistry, 2012, Vol. 9, No. 8
Olszewska et al.
a
Table 1. Palladium(II)-Catalyzed Cyclization of Compounds 4a-e.
c
Ratio of isomer
b
Entry
Alcohol
Solvent
Conversion [%]
d
cis : trans
1
2
3
4
5
6
7
8
9
4a
4a
4a
4b
4b
4b
4c
4c
4c
4d
4d
4d
4e
4e
4e
THF
32
98
40
40
98
86
67
98
98
98
98
98
93
93
95
-
CH
2
2
Cl
O
2
2
2
2
2
-
Et
-
THF
55:45
50:50
53:47
51:49
47:53
57:43
68:32
38:62
70:30
50:50
50:50
53:47
CH
2
2
Cl
O
Et
THF
CH
2
2
Cl
O
Et
1
1
1
1
1
1
0
1
2
3
4
5
THF
CH
2
2
Cl
O
Et
THF
CH
2
2
Cl
O
Et
a
b
c
d
2 3 2
Conditions: PdCl (CH CN) /[4] = 1:20
1
Conversion was determined by H-NMR analysis.
1
Ratios of the isomers were determined by H-NMR analysis.
Relative configuration of the methl group (C-2) to the C-5 vinyl group.
ence could be due to the presence of a bulkier group as com-
pared to the other substituents.
Therefore, the cis/trans configuration of 5b-e can be as-
signed analogously by the chemical shift difference of the
methyl group on the C-2 position as summarized in Table 2.
R
H
R
The mechanism for the formation of compounds 5a-e is
quite similar to the one previously described [24] (Scheme
O
H C
O
3
H C
H
3
2
).
The first step is the coordination of PdCl
bond to give the ꢀ complex, which is in equilibrium with the
complex II obtained by a simple hydroxyl group exchange
at Pd.
2
to the double
cis
trans
ꢀ
Fig. (1).
The cis/trans configuration of each diastereoisomer of
b-e was determined by H-NMR analysis. In 2,5-substituted
The next step is an intramolecular syn attack of this hy-
1
5
droxyl group from the same side of the Pd complex [25, 26]
occurring in a 5-exo-trig fashion to give the ꢁ-Pd complex
III. A syn elimination of PdCl(OH) from intermediate III
generates the oxygen heterocycles 5a-e.
tetrahydrofurans, the chemical shifts of the protons of the
substituents at C-2, such as Me and t-Bu, being in configura-
tionally different positions are magnetically affected by the
C-5 substituents (vice versa) and the signals of the cis-
isomer appear at the lower fields [20-23].
In conclusion, the palladium(II)-catalyzed cyclization of
unsaturated alcohols afforded the mixture of cis- and trans-
Table 2. The Methyl Signals (ꢀ) of the C-2 Substituents (Relative Configuration to the C-5 Vinyl Group).
Compounds
Cis
Trans
5
b
1.13
1.28
1.29
1.16
1.12
1.26
1.17
1.15
5
c
5
d
5
e

Synthesis of 2,5-Substituted Tetrahydrofurans by Pd(II)-Catalyzed
Letters in Organic Chemistry, 2012, Vol. 9, No. 8
565
OH
R
O
R
OH
4
a-e
5a-e
PdCl2
-PdCl(OH)
H
H
O
O
HO
Cl
Cl
Cl Cl
Pd
H
Pd
Cl Pd
HO
R
R
O
O
-HCl
R
I
III
II
Scheme 2.
,5-substituted tetrahydrofurans in quite a good yield. At
2
2 4
ganic phase was dried (Na SO ) and evaporated. The residue
diastereoselectivity the reaction is influenced by the solvent
and the size of the substituent in the starting alcohol.
was purified by flash chromatography (hexanes:ethyl acetate
2:3) to give ketone 3 (10.9 g, 46%).
Work is currently in progress in order to find the condi-
tions allowing enantioselective access to the tetrahydrofuran
structure.
General procedures for the preparation of alcohols 4a-e
The appropriate Grignard reagent (25.2 mmol) was added
2
dropwise to a solution of the ketone 3 (12 mmol) in Et O (50
mL) at 0 °C. The reaction mixture was heated to room tem-
perature and stirred until the transformation of the alcohol
was complete (2-3 h), as indicated by thin layer chromatog-
raphy. The reaction mixture was then quenched with satu-
EXPERIMENTAL
All commercially available chemical reagents were used
without further purification. All reactions were monitored by
TLC analysis (TLC plates Merck silica gel 60 F254). Column
chromatography was performed on silica gel 60 (230-400
mesh, Merck). NMR spectra were recorded with a Bruker
rated NH
0 mL), the combined organic phases were dried over
Na SO . Evaporation of the solvents under reduced pressure
4
Cl solution. After extraction with ethyl acetate (3 x
3
1
2
4
Avance III spectrometer and referenced as the following: H
gave oil that was purified by flash chromatography on silica
gel to give the appropriate alcohol 4a-e.
1
3
(
4
600 MHz), internal SiMe at ꢀ = 0.00 ppm, C (150 MHz),
internal standard at ꢀ = 77.23 ppm. Mass spectra were ob-
(
E)-6-methylhept-2-ene-1,6-diol (4a) [19]: Colorless
oil, 1.3 g, 77% yield. R = 0.20 (hexanes/ethyl acetate, 1:2).
H NMR (600 MHz, CDCl ): ꢀ 1.23 (s, 6H, 2xCH ), 1.54 (t,
tained with a Varian 500-MS spectrometer.
f
(
Z)-4-Chlorobut-2-en-1-ol (2) was prepared according
1
3
3
to the literature method [18]. Thionyl chloride (8.8 mL, 120
mmol) was added dropwise over a period of 2.5 h to a cooled
solution of 2-butene-1,4-diol (1) (10.0 g, 110 mmol) in
2
7
H, J = 2.4, H-5), 1.72 (s, 2H, 2xOH), 2.18 (dt, 2H, J = 9.0,
.2, H-5), 4.19 (d, 1H, J = 6.6, H-1), 5.56-5.66 (m, 2H, H-2,
13
H-3). C-NMR (150 MHz, CDCl
5
6
3
): ꢀ 22.51, 29.35, 43.33,
: C,
Et
stirred overnight at room temperature and then poured into
ice water (9 mL) and extracted with Et O (2 x 20 mL). The
combined organic layers were washed successively with
saturated NaHCO solution, brine, and dried (Na SO ). The
concentration of the dried extract afforded the pure product
19.9 g, 85%).
2
O/pyridine (2.5/1, 35 mL). The reaction mixture was
8.33, 70.99, 128.41, 133.26. Anal. Calcd for C
8 16 2
H O
6.68; H, 11.18. Found: C, 66.41; H, 11.49.
2
(
E)-6-methyloct-2-ene-1,6-diol (4b): Colorless oil, 1.2
1
g, 60% yield. R
NMR (600 MHz, CDCl
(s, 3H, CH ), 1.44-1.49 (m, 4H, H-4, H-7), 1.78 (bs, 2H,
xOH), 2.11-2.15 (m, 1H, H-5), 4.16 (d, 1H, J = 6.4, H-1),
.51-5.59 (m, 2H, H-2, H-3). C-NMR (150 MHz, CDCl
8.22, 22.04, 26.20, 34.46, 40.85, 58.28, 72.95, 128.40,
33.26. Anal. Calcd for C : C, 68.31; H, 11.47. Found:
f
= 0.26 (hexanes/ethyl acetate, 1:2). H
3
2
4
3
): ꢀ 0.86 (t, 3H, J = 7.5, H-8), 1.27
(
3
2
5
ꢀ
1
(
E)-7-hydroxyhept-5-en-2-one (3) was prepared accord-
ing to the literature method [19]. Ethyl acetoacetate (24.1 g,
85 mmol) was added to a solution of EtONa prepared from
1
3
3
):
1
18 2
H O
sodium (4.26 g, 185 mmol) and EtOH (150 mL). The reac-
9
C, 68.07; H, 11.65.
tion mixture was cooled to 0 °C and then alcohol 2 (19.7 g,
1
85 mmol) was added dropwise. The mixture was refluxed
for 1 h. Then the solvent was evaporated, the residue was
treated with water (50 mL) and extracted with Et O (3 x 30
mL). The combined organic layers were dried over Na SO
and evaporated to give a yellow oil. The oil was treated with
a solution of Na CO (26.65 g, 250 mmol) in H O (270 mL)
(
E)-6-methylocta-2,7-diene-1,6-diol (4c): Colorless oil,
1
1
.3 g, 68% yield. R
f
= 0.43 (hexanes/ethyl acetate, 1:2). H
): ꢀ 1.21 (s, 3H, CH ), 1.50-1.57 (m,
H, H-4), 2.01-2.12 (m, 4H, H-5, 2xOH), 4.17 (ddd, 2H, J =
.8, 3.0, 0.7, H-1), 5.07 (dd, 1H, J = 10.7, 1.2, H-8), 5.71
2
NMR (600 MHz, CDCl
3
3
2
4
,
2
6
(
2
3
2
dd, 1H, J = 17.3, 1.2, H-8), 5.44-5.56 (m, 2H, H-2, H-3),
.91 (dd, 1H, J = 17.3, 10.8, H-7). C NMR (150 MHz,
and refluxed for 7 h. Next, the mixture was saturated with
NaCl and extracted with ethyl acetate (3 x 20 mL). The or-
13
5

5
66 Letters in Organic Chemistry, 2012, Vol. 9, No. 8
Olszewska et al.
CDCl
32.94, 144.84. Anal. Calcd for C
Found: C, 69.03; H, 10.72.
3
): ꢀ 22.19, 27.98, 41.74, 58.22, 73.32, 111.93, 128.53,
MHz, CDCl
3
): ꢀ 26.92, 27.38, 31.10, 32.38, 37.04, 38.30,
1
9
H
16
O
2
: C, 69.19; H, 10.32.
80.72, 79.98, 83.27, 83.43, 111.67, 111.83, 115.04, 115.59,
139.77, 140.15, 144.10, 144.82. ES-MS: 161 ([M+Na] ).
+
(
E)-6,7-dimethylocta-2,7-diene-1,6-diol (4d): Colorless
oil, 1.0 g, 51% yield. R = 0.39 (hexanes/ethyl acetate, 1:2).
H NMR (600 MHz, CDCl ): ꢀ 1.28 (s, 3H, CH ), 1.56-1.67
m, 2H, H-5), 1.72 (dd, 3H, J = 1.5, 0.7, CH -C=), 1.82 (bs,
H, 2xOH), 1.95-2.00 (m, 1H, H-4), 2.06-2.12 (m, 1H, H-4),
.13-4.18 (m, 2H, H-1), 4.82 (m, 1H, CH =C(CH )), 4.97
=C(CH )), 5.49-5.59 (m, 2H, H-3, H-2).
NMR (150 MHz, CDCl ): ꢀ 19.60, 22.37, 28.03, 40.05,
8.52, 75.73, 110.01, 128.76, 133.21, 150.35. Anal. Calcd
: C, 70.55; H, 10.66. Found: C, 70.53; H, 10.71.
2-Methyl-2-(prop-1-en-2-yl)-5-vinyltetrahydrofuran
1
(
5d): R
MHz, CDCl
1.67 (m, 2H, H-3), 1.69 (dd, 3H, J = 1.4, 0.7, CH
cis), 1.71 (dd, 3H, J = 1.3, 0.8, CH =CCH trans), 1.91-1.97
(m, 2H, H-4), 4.31 (dt, 1H, J = 6.7, 6.6, H-5 cis), 4.38 (dt,
f
= 0.89 (hexanes/ethyl acetate, 2:1). H NMR (600
f
1
3 3 3
): ꢀ 1.27 (s, CH trans), 1.29 (s, CH
cis), 1.56-
=CCH
3
3
(
2
4
3
2
3
2
3
2
3
1
3
(
m, 1H, CH
2
3
C
1H, J = 7.0, 6.0, H-5 trans), 4.67-4.70 (m, 1H, CH=CH
2
2
2
),
),
).
4
.92-4.95 (m, 1H, CH=CH
5.15-5.19 (m, 1H, CH=CH
2
), 4.97-5.02 (m, 1H, CH=CH
), 5.58-5.84 (m, 1H, CH=CH
3
5
2
1
3
for C10
H
18
O
2
3
C-NMR (150 MHz, CDCl ): ꢀ 19.75, 19.35, 26.66, 27.15,
3
1
1
2.33, 32.44, 36.27, 36.99, 80.01, 80.32, 85.60, 85.86,
08.90, 109.35, 115.03, 115.42, 139.73, 140.26, 149.84,
(
E)-6-methylnona-2,8-diene-1,6-diol (4e): Colorless oil,
1
2
.0 g, 98% yield. R
f
= 0.27 (hexanes/ethyl acetate, 1:2). H
): ꢀ 1.12 (s, 3H, CH ), 1.44-1.48 (m,
H, H-4), 1.70 (bs, 2H, 2xOH), 2.17 (dd, 2H, J = 7.4, 0.8, H-
+
50.30. CI-MS: 153 ([M+H] ), 93, 79.
NMR (600 MHz, CDCl
3
3
2
5
6
2-Allyl-2-methyl-5-vinyltetrahydrofuran (5e): R
f
=
1
), 2.30 (dd, 1H, J = 14.3, 6.8, H-7), 2.48 (dd, 1H, J = 6.8,
.8, H-7), 4.13 (d, 2H, J = 6.8, H-1), 5.03-5.09 (m, 2H, H-9,
CH
0.70 (hexanes/ethyl acetate, 2:1). H NMR (600 MHz,
CDCl ): ꢀ 1.15 (s, CH trans), 1.16 (s, CH cis), 1.57-1.70
(m, 2H, H-3, H-4), 1.76-1.84 (m, 1H, H-3), 1.98-2.03 (m,
1H, H-4), 2.22 (ddt, 2H, J = 7.1, 5.3, 1.0, CH -CH=CH ),
.29 (dt, 1H, J = 6.8, 6.6, H-5 cis), 4.33 (dt, 1H, J = 6.9, 6.7,
3
3
3
=
2
), 5.47-5.61 (m, 2H, H-2, H-3), 5,75-5.82 (m, 1H, CH -
3
13
CH=). C NMR (150 MHz, CDCl
3
): ꢀ 21.73, 22.24, 26.90,
1.53, 43.13, 46.77, 58.42, 58.57, 72.35, 118.99, 128.69,
29.87, 131.23, 133.32, 134.02. Anal. Calcd for C10
2
2
4
4
1
H
18
O
2
:
H-5 trans), 4.97-5.2 (m, 3H, CH=CH ), 5.14-5.19 (m, 1H,
2
13
C, 70.55; H, 10.66. Found: C, 69.83; H, 10.17.
CH=CH
MHz, CDCl
2
), 5.73-5.81 (m, 2H, CH=CH
): ꢀ 26.66, 27.45, 32.79, 36.56, 36.59, 46.21,
6.83, 79.82, 80.49, 83.12, 115.12, 115.20, 117.54, 117.63,
2
). C NMR (150
II
3
Typical Procedure for the Pd -Catalyzed Reaction: A
solution of PdCl
4
1
2
(CH
3
CN)
2
(5.2 mg, 0.02 mmol) in an ap-
+
35.17, 135.23, 139.96, 140.31. CI-MS: 153 ([M+H] ), 111.
propriate anhydrous solvent (3 mL) was added to a solution
of alcohol 4a-e (0.4 mmol) in an appropriate anhydrous sol-
vent at room temperature under an argon atmosphere. The
mixture was stirred at room temperature for 24 h. The sol-
vent was evaporated and the mixture was analyzed by NMR
analysis.
CONFLICT OF INTEREST
The author(s) confirm that this article content has no con-
flicts of interest.
2
,2-Dimethyl-5-vinyltetrahydrofuran (5a): R
f
= 0.66
hexanes/ethyl acetate, 2:1). H NMR (600 MHz, CDCl ): ꢀ
.19 (s, 3H, CH ), 1.20 (s, 3H, CH ), 1.65-1.73 (m, 4H, H-3,
H-4), 4.33 (dt, 1H, J = 7.0, 7.0, H-5), 4.99 (d, 1H, J = 10.3,
CHCH ), 5.16 (d, 1H, J = 17.1, CHCH ), 5.78 (ddd, 1H, J =
). C NMR (150 MHz, CDCl ): ꢀ
8.45, 29.29, 32.90, 38.81, 79.91, 81.27, 115.04, 140.40. ES-
1
(
1
3
ACKNOWLEDGEMENTS
3
3
The project entitled “Sugars as raw materials in the syn-
thesis of products with high added value” No
POIG.01.01.02-14-102/09 is part-financed by the European
Union within the European Regional Development Fund.
2
2
1
3
1
2
7.1, 10.3, 6.7, CHCH
2
3
+
+
MS: 149 ([M+Na] ), 127 ([M+H] ).
REFERENCES
2
-Ethyl-2-methyl-5-vinyltetrahydrofuran (5b): R
f
=
1
0
.78 (hexanes/ethyl acetate, 2:1). H NMR (600 MHz,
CDCl ): ꢀ 0.83-0.88 (m, 3H, CH CH ), 1.12 (s, CH trans),
.13 (s, CH cis), 1.42-1.53 (m, 2H, CH CH ), 1.56-1.76 (m,
H, H-3, H-4), 4.24-4.34 (m, 1H, H-5), 4.96-5.00 (m, 1H,
), 5.15 (ddd, 1H, J = 17.0, 6.2, 2.9, CH=CH ), 5.75-
). C NMR (150 MHz, CDCl ): ꢀ
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CH=CH
2
2
1
3
5
9
3
1
.80 (m, 1H, CH=CH
2
3
.17, 9.25, 25.91, 26.61, 32.89, 32.92, 34.14, 34.73, 36.49,
6.68, 79.56, 80.34, 83.92, 83.97, 114.98, 115.10, 140.15,
40.52. CI-MS: 141 ([M+H] ), 93, 79.
+
2
-Methyl-2,5-divinyltetrahydrofuran (5c): R
f
= 0.80
hexanes/ethyl acetate, 2:1). H NMR (600 MHz, CDCl ): ꢀ
.26 (s, CH trans), 1.28 (s, CH cis),1.61-1.74 (m, 2H, H-3,
H-4), 1.80-1.90 (m, 1H, H-3), 1.96-2.40 (m, 1H, H-4), 4.35
dt, J = 7.1, 6.4, H-5 cis), 4.38 (dt, J = 8.0, 6.5, H-5 trans),
.92 (dd, J = 10.6, 1.5, CH=CH ), 4.93 (dd, J = 10.8, 1.7,
CH=CH ), 5.02 (ddd, J = 10.6, 4.5, 1.5, CH=CH ), 5.11
ddd, J = 10.6, 3.9, 1.5, CH=CH ), 5.21-5.30 (m, 2H,
[6]
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1
(
1
3
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