Evaluation of carbohydrates and lignocellulosic biomass from different wood species as raw material for the synthesis of 5-bromomethyfurfural

Article abstract of DOI:10.1016/j.carres.2013.04.002

The influence of different parameters on the conversion of carbohydrates and biomass into the potential biofuel intermediate 5-bromomethylfurfural (BMF) has been studied. Our optimized conditions avoid the use of lithium salt additives, making this method cheaper and environmentally more benign compared to previously reported methods. Different wood species and their potential as a raw material in BMF and furfural production have also been evaluated. In addition, we report a very simple and efficient procedure for conversion of 5-hydroxymethylfurfural (HMF) into BMF or 5-chloromethylfurfural (CMF).

Full text of DOI:10.1016/j.carres.2013.04.002

Carbohydrate Research 375 (2013) 63–67
Contents lists available at SciVerse ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Evaluation of carbohydrates and lignocellulosic biomass from
different wood species as raw material for the synthesis of
5-bromomethyfurfural
a
b
a,
⇑
Aleksei Bredihhin , Uno Mäeorg , Lauri Vares
a
Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia
Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The influence of different parameters on the conversion of carbohydrates and biomass into the potential
biofuel intermediate 5-bromomethylfurfural (BMF) has been studied. Our optimized conditions avoid the
use of lithium salt additives, making this method cheaper and environmentally more benign compared to
previously reported methods. Different wood species and their potential as a raw material in BMF and
furfural production have also been evaluated. In addition, we report a very simple and efficient procedure
for conversion of 5-hydroxymethylfurfural (HMF) into BMF or 5-chloromethylfurfural (CMF).
Ó 2013 Elsevier Ltd. All rights reserved.
Received 28 November 2012
Received in revised form 4 March 2013
Accepted 3 April 2013
Available online 10 April 2013
Keywords:
Biomass
Biofuels
Cellulose
5
5
-Ethoxymethylfurfural
-Bromomethylfurfural
1
. Introduction
suitable derivatives. For example 5-ethoxymethylfurfural (EMF,
4
) and ethyl levulinate (EL, 6) are considered to be promising bio-
1
5,16
Depletion of oil reserves, increasing CO
2
emissions, and rising
diesel candidates.
prices of gasoline and diesel make the development of new biofu-
els an extremely important task. Most of currently produced biofu-
els are based on edible raw materials and large-scale production,
for example, ethanol production from corn, is competing with
Our goal is to contribute to the development of a new biofuel
and the current study concerns several aspects of biofuel produc-
tion: evaluation of different biomass samples and carbohydrates
as starting materials, and development of improved procedures
for the preparation of BMF and EMF.
1
the food industry and has already led to an increase of food prices.
Current focus in this area is, however, directed toward using non-
edible lignocellulosic raw materials like wood and straw. Also var-
ious types of cellulose-containing wastes including old paper, cot-
ton clothes, and sawdust can be converted to fuel.
2
. Results and discussion
Inspired by recent progress in this field we started off by opti-
Currently, bioethanol is the most widely used biofuel. However,
during the production of bioethanol one third of the carbon con-
tent of the starting material is expelled in the fermentation pro-
cess, making this a rather inefficient process. It has also been
noted that the heat of combustion of ethanol is considerably lower
than that of gasoline and diesel fuel.2 Therefore intense research
has concentrated on furanic compounds like 5-hydroxymehtylfurf-
mizing the conditions for the conversion of glucose/cellulose into
BMF (2) (Scheme 1). The results from this study are summarized
in Table 1.
Hydrobromic acid was chosen as an acidic reagent because it
has several advantages compared to hydrochloric acid: it is a sta-
ble, stronger acid, can be easily distilled, and is non-volatile.
The choice of the solvent is an important factor when producing
biofuel and several parameters must be evaluated. We have found
that the most suitable solvent is 1,2-dichloroethane (DCE) (entries
3
,4
5–9
ural (HMF, 1), 5-chloromethylfurfural
(CMF, 3), 5-bromom-
1
0–13
14
ethylfurfural
(BMF, 2), and levulinic acid (LA, 5, Fig. 1).
None of the mentioned compounds can be used directly as biofuel
and must be converted to corresponding alkyl ethers or other
5
–14). We also looked for environmentally friendlier solvent which
does not contain halogens and tested toluene (entries 1 and 2) and
cycloalkanes (entries 3 and 4). However, toluene was found to
react partially under these reaction conditions yielding benzyl
bromide. Cycloalkanes are not suitable because of low solubility
⇑
Corresponding author. Tel.: +372 737 4808; fax: +372 737 4900.
E-mail address: lauri.vares@ut.ee (L. Vares).
0
008-6215/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.carres.2013.04.002

6
4
A. Bredihhin et al. / Carbohydrate Research 375 (2013) 63–67
The best reaction conditions (entry 5) were successfully tested
on 10% substrate loading (entry 6). Continuous extraction gave
no improvement compared to manual extraction (entries 7 vs 5).
The use of LiBr as an additive in this reaction is problematic due
to the cost of lithium compounds and difficulties associated with
its regeneration. Therefore, it would be highly desirable to reduce
the amount of LiBr used, or preferentially, to eliminate it com-
pletely. Therefore we studied the influence of the concentration
of LiBr using our optimized reaction conditions (entries 12–14).
We were pleased to discover that the decrease in the yield of
BMF (2) in the absence of LiBr is only 10%, thus making the proce-
dure much more suitable for industrial applications.
O
O
BMF (2)
O
O
CMF (3)
O
OH
O
O
Br
O
O
Cl
O
O
HMF (1)
O
OH
OEt
OEt
EMF (4)
LA (5)
EL (6)
Figure 1. Chemicals readily available from lignocellulosic biomass.
To the best of our knowledge we have completed the first eval-
uation of different wood species as a source of raw material for the
production of BMF (2) (Table 2). Wood species chosen for this
study (Quaking aspen, Silver birch, Black alder, Scots Pine, and Nor-
way Spruce) are common in Northern Europe. It is assumed that
dry wood in average contain 40% of cellulose, 20% of hemicellulose,
and 40% of lignin, however cellulose content significantly varies in
different species. Also not only cellulose is converted to BMF, but
all C carbohydrates (hexoses), while all C carbohydrates are con-
verted to furfural. Due to the variation of hexose and pentose con-
tent in different species we provide both mass and percent yields.
Percent yields are calculated based on hexose and pentose content
in given wood species. According to our observations the best
wood was aspen because of high mass and percent yields of both
BMF (2) and furfural. It was also the best from technological point
of view, because of the easiness of phase separation during extrac-
tion. Pine and spruce gave also good mass yields of BMF, however
these species are known to have higher content of cellulose, thus
making percent yields from these species less than that of birch.
Birch and alder were creating considerable difficulties during
extraction, producing hardly separable emulsions.
Conversion of aspen to BMF and furfural was also performed on
10 g scale with 5% substrate loading. In this experiment products
were separated by distillation to give 2.159 g (35% yield) of BMF
and 0.366 g (28% yield) of furfural. It should be noted that the in-
crease of substrate loading from 1% to 5% led to formation of emul-
sion during extraction. This was probably the cause for lower BMF
yield.
Yields of BMF from glucose and cellulose under the same condi-
tions are higher than yields from wood (compare Table 1, entries 5
and 8 with Table 2), however the difference between pure cellulose
and wood is only 3–10%, which is a very good result.
Glucose
HBr, additive
or
solvent
O
Cellulose
Br
O
2
Scheme 1. Synthesis of BMF (2).
of BMF (2) in them. Ether type solvents (THF, Me-THF, 1,4-dioxane,
,2-dimethoxyethane, MTBE, Et O) were rapidly hydrolyzed or
hydrobrominated in the presence of concentrated hydrobromic
acid.
Optimal temperature was found to be 65 °C, below this temper-
ature the reaction is slow or does not happen at all, and higher
temperatures lead to slightly lower yields of BMF (entries 5 vs 9
and entries 8 vs 10). At 80 °C the difference in yield for reactions
conducted for 3 or 24 h was only 4% (entries 9 and 11), indicating
that most of the product is formed during the first 3 h.
6
5
1
2
Table 1
a
Optimization of carbohydrate conversion to BMF
Temp (°C) Additive Yieldb (%)
Entry Substrate Solvent
1
2
3
4
5
6
7
8
9
0
1
2
3
4
Glucose
Cellulose
Glucose
Glucose
Glucose
Glucose
Glucose
Cellulose
Glucose
Cellulose
Glucose
Glucose
Cellulose
Aspen
Toluene
Toluene
Cyclohexane
Methylcyclohexane 100
65
65
80
LiBr
LiBr
LiBr
LiBr
LiBr
LiBr
LiBr
LiBr
LiBr
LiBr
LiBr
—
47
44
20
7
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
65
65
65
65
80
80
80
65
65
65
64
d
53
e
60
59
59
1
1
1
1
1
51
c
55
Addressing significant community interest and recent advances
in conversion of sugars and biomass to HMF (1), we have also
developed a simple and very efficient method for conversion of
HMF (1) to CMF (3) and BMF (2) (Scheme 2). Current methods2
for making compounds 1 and 2 are complicated, requiring the use
of solutions of gaseous hydrogen halides in organic solvents or
54
48
41
—
—
0,21
a
b
c
Conditions: HBr, additive, solvent, 24 h; substrate loading is 1%.
Yield of isolated product.
Reaction time 3 h.
0% Substrate loading.
Continuous extraction.
d
e
1
other reagents like PCl
3 3 2
, PBr , SOCl etc. It is known that HMF (1)
is sensitive to acidic conditions, and undergoes decomposition
Table 2
a
Conversion of different wood species
Hexose contentb (%)
BMF yieldc
Pentose contentb (%)
Furfural yieldc
(
mg)
(%)
(mg)
(%)
Aspen,1 Populus tremula
8
53
43
48
56
58
237
197
197
232
230
55
56
50
51
49
18
28
24
9
34
42
30
9
35
29
25
20
33
Birch,1 Betula pendula
7
1
8
Alder, Alnus glutinosa
17
Pine, Pinus sylvestris
Spruce,17 Picea abies
9
15
a
b
c
Conditions: HBr, LiBr, DCE, 65 °C, 24 h; 700 mg of wood shavings.
Adopted from Refs. 17,18.
Yield of isolated product.

A. Bredihhin et al. / Carbohydrate Research 375 (2013) 63–67
65
3
. Conclusion
HX(aq)/DCE
O
O
Carbohydrates and wood of different species have been evalu-
ated as a raw material for making 5-bromomethylfurfural (BMF,
). Individual carbohydrates (glucose, cellulose) generally gave
OH
O
X
O
1
X= Br (2), Cl (3)
2
Scheme 2. Conversion of HMF (1) to halomethylfurfurals.
better yields compared to wood, although the difference was not
significant. The Quaking aspen gave highest yields of BMF and fur-
fural among the wood species. The impact of several reaction
parameters on the conversion of carbohydrates to BMF (2) has also
been studied. Optimized procedure allows direct conversion of car-
bohydrates and raw wood biomass to BMF in good yields, substrate
loadings as high as 10% are allowed. Also LiBr additive can be elim-
inated from the process, which is advantageous compared to the
current methods. A mild and efficient method for the conversion
of HMF (1) to halomethylfurfurals is reported. And finally, a proce-
dure for converting BMF (2) to a perspective biodiesel candidate
EMF (4) is reported.
Table 3
Conversion of HMF (1) to BMF (2) or CMF (3)
Conditions
Product
Yielda (%)
HBr (aq)/DCE, 24 h, rt
HBr (aq)/DCE, 1 h, 65 °C
HCl (aq)/DCE, 24 h, rt
HCl (aq)/DCE, 4 h, 65 °C
BMF
BMF
CMF
CMF
92
94
86
79
a
Yield of isolated product.
Table 4
4
4
. Experimental section
Optimization of EMF (4) synthesis
Entry
Solvent
Conditions
Yielda
of 4 (%)
Contentb
of 6 (%)
Yielda
of 7 (%)
.1. General methods
1
2
3
4
5
96% EtOH
96% EtOH
Abs EtOH
Abs EtOH
Abs EtOH
3 h, reflux
1 h, reflux, CaCO
3 h, reflux
1 d, rt, CaCO
5 h, 80 °C, CaCO
99
95
20
9
—
3
—
—
—
30
29
All reagents and solvents (except absolute ethanol) were ob-
tained from commercial sources and used without further purifica-
tion. Absolute ethanol was obtained by drying commercial 96%
3
dec
c
c
3
50
c
c
2
ethanol over CaH (boiling 2 h with reflux, then distillation) and
3
64
5
was stored under inert atmosphere. NMR spectroscopy was per-
formed on a Bruker AVANCE II 400 MHz spectrometer using resid-
ual solvent peak (CDCl , 7.27 ppm for H and 77.0 for C NMR
3
a
b
c
Yield of isolated product.
Determined by 1H NMR.
Purified by column chromatography.
1
13
spectra) as internal standard. Infrared spectra were measured on
a Shimadzu IRAffinity-1 FTIR spectrometer, using ATR module with
ZnSe crystal. Melting points were determined on a Gallenkamp
melting point apparatus. Reactions were monitored by thin-layer
with the formation of levulinic acid or polymerization depending
on the conditions used.19 This issue was resolved by using a bipha-
sic acid/DCE reaction mixture which affords the corresponding
halomethylfurfurals in excellent yields (Table 3).
chromatography (TLC) and visualized by UV light or with KMnO
4
As CMF (3) and BMF (2) cannot be used directly as fuels, we
have also studied their conversion to EMF (4) (Table 4). Using the
solution. Reaction products were purified by flash chromatography
using silica gel 60 (0.040–0.063 mm, 230–400 mesh ASTM). Wood
shavings were dried in drying oven at 105 °C until constant mass is
achieved (20 h).
1
0
literature procedure we managed to obtain crude oil in up to
9% yield (Table 4, entry 1). However, analysis showed the product
to be a mixture of EMF (4) and EL (6) (up to 20%) which was not
separable by column chromatography. An attempt to use CaCO
9
3
4.2. Typical procedure for conversion of wood to 5-
bromomethylfurfural (2)
as a base in absolute ethanol (Table 4, entries 4 and 5) resulted
in a mixture of EMF (4), EL (6), and EMF diethyl acetal (7)
(
Scheme 3). Similar difficulties during conversion of HMF (1) to
Dry aspen shavings (700 mg) and anhydrous LiBr (7 g) were
mixed with HBr (aq, 48%, 70 mL) and 1,2-dichloroethane (70 mL),
and then heated on the oil bath at 65 °C. After 1 h organic layer
was separated and acidic fraction was extracted with 1,2-dichloro-
ethane (2 Â 70 mL). Organic fractions were combined and dried
1
5,22
EMF (4) in ethanol were also recently reported by others.
version of BMF (2) with 96% ethanol in the presence of CaCO
Con-
3
worked better, holding the levels of EL (6) below 10% (Table 4, en-
try 2). We have found that additional wash of this mixture with
NaOH (aq) furnishes pure EMF (4), thus providing a method for
obtaining EMF (4) in a pure form.
We have demonstrated a procedure for transformation of cellu-
lose to EMF, which does not require chromatography and purifica-
tion of intermediate BMF, thus making the whole process more
suitable for industrial application. Cellulose is converted to BMF
using optimized conditions (Table 1, entry 8) and organic extracts
are evaporated to yield crude BMF as black oil, which was then
2 4
with Na SO . A fresh 1,2-dichloroethane (70 mL) was added to
the acidic fraction and flask was placed again on the oil bath. Pro-
cedure was repeated after 1 and 2 h. Then reaction mixture was left
to stir with heating overnight. On the next day (24 h), organic layer
was separated and acidic residue extracted with 1,2-dichloroeth-
ane (3 Â 70 mL). All organic fractions were combined and evapo-
rated. Residue was subjected to column chromatography (eluent
2 2
CH Cl ) resulting in 2 (237 mg, 55%) as yellowish crystals and fur-
mixed with CaCO
evaporation, extraction, and washing with NaOH gave pure EMF in
0% yield from cellulose.
3
and EtOH (96%) and refluxed for 1 h. Subsequent
fural (34 mg, 35%). 2: mp 58.3–60.0 °C. FTIR (ATR): 3113, 3032,
2970, 2924, 2854, 1667, 1520, 1392, 1277, 1219, 1196, 1115,
1
4
1018, 968, 810, 772, 698, 655, 563. H NMR (400 MHz, CDCl
3
): d
O
EtOH
OEt
+
+
OEt
O
O
O
Br
O
OEt
O
OEt
OEt
O
2
4
6
7
Scheme 3. Synthesis of EMF (4).

6
6
A. Bredihhin et al. / Carbohydrate Research 375 (2013) 63–67
9
2
1
1
1
1
.61 (s, 1H), 7.19 (d, J = 3.6 Hz, 1H), 6.58 (d, J = 3.6 Hz, 1H), 4.48 (s,
2 2
(eluent CH Cl ) yielding after drying under vacuum 3 (0.249 g,
86%) as yellowish crystals. mp 37.8–38.6 °C. FTIR (ATR): 3117,
1
3
H) ppm. C NMR (100 MHz, CDCl
3
): d 177.6, 156.1, 152.7, 121.7,
12.0, 21.5 ppm. Furfural: H NMR (400 MHz, CDCl ): d 9.64 (s,
H), 7.68 (m, 1H), 7.24 (dd, J = 3.6, 0.7 Hz, 1H), 6.59 (dd, J = 3.6,
1
3
3024, 2970, 2854, 1667, 1524, 1396, 1265, 1184, 1142, 1022,
1
980, 814, 772, 714, 687, 606. H NMR (400 MHz, CDCl
3
): d 9.62
1
3
3
.8 Hz, 1H) ppm. C NMR (100 MHz, CDCl ): d = 177.8, 152.9,
48.0, 120.9, 112.5 ppm.
(s, 1H), 7.19 (d, J = 3.6 Hz, 1H), 6.58 (d, J = 3.6 Hz, 1H), 4.60 (s,
1
3
2H), ppm. C NMR (100 MHz, CDCl
3
): d 177.6, 156.0, 152.8,
1
21.6, 111.9, 36.4 ppm.
4
.2.1. Conversion of wood to 5-bromomethylfurfural (2), 10 g
scale
According to typical procedure described above, dry aspen
shavings (10 g) and anhydrous LiBr (20 g) were mixed with HBr
aq, 48%, 200 mL) and 1,2-dichloroethane (200 mL), and then
4.4.2. Method b
Compound
1
2 2
(252 mg, 2 mmol) was dissolved in CH Cl
(10 mL), then HCl (aq, 37%, 5 mL) was added. Biphasic reaction
mixture was heated for 4 h at 65 °C, then organic layer was sepa-
(
heated on the oil bath at 65 °C. Solvent replacement and extraction
with 1,2-dichloroethane (2 Â 200 mL) were performed after 1, 2, 3,
and 24 h. All organic fractions were combined and evaporated.
rated, and acidic fraction was extracted with CH
Organic fractions were combined, dried with Na
rated. Residue was purified by filtering through short silica gel col-
umn (eluent CH Cl ) yielding after drying under vacuum 3
2
Cl
2
(3 Â 20 mL).
2
SO4, and evapo-
Crude oil was filtered through the plug of silica (eluent CH
2
Cl
2
)
2
2
and evaporated. Residue (2.566 g) was distilled with Kugelrohr
(229 mg, 79%) as yellowish crystals.
apparatus to give 2 (2.159 g, 35%) as yellowish crystals and furfural
(
0.366 g, 28%) as colorless oil.
4.5. 5-Ethoxymethylfurfural (4)
4
.2.2. Conversion of glucose into 5-bromomethylfurfural (2)
Compound
2
(500 mg, 2.65 mmol) and CaCO
3
(265 mg,
According to typical procedure described above, glucose
721 mg) gave crude product that after purification with column
chromatography and drying under vacuum resulted in 2 (482 mg,
2.65 mmol) were stirred in ethanol (10 mL) until dissolution of 2.
Then reaction mixture was placed on the oil bath and refluxed
for 1 h. After this reaction mixture was cooled down, evaporated
(
6
4%) as yellowish crystals.
to the small volume and partitioned between Et
water (30 mL). Water fraction was additionally extracted with
Et
O (3 Â 30 mL). Organic fractions were combined and dried with
Na SO , then volatiles were removed to give crude 4 (387 mg, 95%)
as yellowish oil. Analysis showed that crude product contained
also 9% of 6. To obtain pure 4, the crude oil was dissolved in Et
2
O (30 mL) and
4
.2.3. Conversion of cellulose into 5-bromomethylfurfural (2)
According to typical procedure described above, cellulose
2
2
4
(
700 mg, 5% water by mass) gave crude product that after purifica-
tion with column chromatography and drying under vacuum re-
2
O
sulted in 2 (452 mg, 59%) as yellowish crystals.
(15 mL) mixed with NaOH (1 M, 10 mL) in separation funnel. Then
organic layer was separated and aqueous layer washed with ethyl
ether (1 Â 10 mL). Organic layers were combined, dried with
4
.3. Conversion of 5-hydroxymethylfurfural (1) into 5-
bromomethylfurfural (2)
2 4
Na SO . Then volatiles were removed to furnish 4 (301 mg, 74%)
as pale yellow oil. FTIR (ATR): 3121, 2978, 2870, 1674, 1520,
1
4
.3.1. Method a
1346, 1277, 1192, 1092, 1018, 968, 945, 806, 756. H NMR
Compound 1 (252 mg, 2 mmol) was dissolved in 1,2-dichloro-
3
(400 MHz, CDCl ): d = 9.59 (s, 1H), 7.19 (d, J = 3.5 Hz, 1H), 6.50 (d,
ethane (7 mL), then HBr (aq, 48%, 7 mL) was added. Biphasic reac-
tion mixture was stirred at rt for 24 h, then organic layer was
J = 3.5 Hz, 1H), 4.50 (s, 2H), 3.56 (q, J = 7.1 Hz), 1.21 (t, J = 7.1 Hz)
1
3
3
ppm. C NMR (100 MHz, CDCl ): d = 177.6, 158.7, 152.5, 121.8,
separated, and acidic fraction was extracted with CH
3 Â 20 mL). Organic fractions were combined, dried with Na
and evaporated. Residue was purified by filtering through short sil-
ica gel column (eluent CH Cl ) yielding after drying under vacuum
(348 mg, 92%) as yellowish crystals.
2 2
Cl
2
SO4,
110.9, 66.5, 64.7, 15.0 ppm.
(
4.5.1. Conversion of cellulose into 5-ethoxymethylfurfural (4)
According to typical procedure Section 4.2, cellulose (700 mg,
contains 5% water by mass) gave crude BMF product (574 mg,
black oil, contains 7% 1,2-dichloroethane) which was mixed with
2
2
2
4
.3.2. Method b
3
CaCO (300 mg, 3.0 mmol) and ethanol (10 mL). Then reaction mix-
Compound 1 (252 mg, 2 mmol) was dissolved in 1,2-dichloro-
ethane (7 mL), then HBr (aq, 48%, 7 mL) was added. Biphasic reac-
ture was placed on the oil bath and refluxed for 1 h. After this reac-
tion mixture was cooled down, evaporated to the small volume
tion mixture was heated for 1 h at 65 °C, then organic layer was
and partitioned between Et
fraction was additionally extracted with Et
fractions were combined and dried with Na
moved to give crude 4 (363 mg, contains 6% ethyl levulinate) as
brownish oil. To obtain pure 4, the crude oil was dissolved in
2
O (15 mL) and water (15 mL). Water
O (3 Â 15 mL). Organic
SO , volatiles were re-
separated and acidic fraction was extracted with CH
3 Â 20 mL). Organic fractions were combined, dried with Na
and evaporated, yielding after drying under vacuum 2 (355 mg,
2
Cl
2
2
(
2
SO4,
2
4
9
4%) as black crystals.
2
Et O (15 mL) and mixed with NaOH (1 M, 10 mL) in separation fun-
4
.4. Conversion of 5-hydroxymethylfurfural (1) into 5-
nel. Organic layer was separated and aqueous layer washed with
chloromethylfurfural (3)
ethyl ether (1 Â 15 mL). Organic layers were combined and dried
2 4
with Na SO . The removal of volatiles afforded pure 4 (251 mg,
4
.4.1. Method a
Compound
10 mL), then HCl (aq, 37%, 5 mL) was added. Biphasic reaction
mixture was stirred at rt for 24 h, then organic layer was separated,
and acidic fraction was extracted with CH Cl
(3 Â 20 mL). Organic
fractions were combined, dried with Na SO4, and evaporated.
Residue was purified by filtering through short silica gel column
40% from cellulose) as yellowish oil.
1
2 2
(252 mg, 2 mmol) was dissolved in CH Cl
(
Acknowledgements
2
2
The authors would like to thank Archimedes Foundation (pro-
ject no 3.2.0501.10-0004) and Estonian Ministry of Education (pro-
ject no. SF0180073s08) for financial support.
2

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