Lipase-catalyzed asymmetric acylation in the chemoenzymatic synthesis of furan-based alcohols

Article abstract of DOI:10.1016/j.tetasy.2012.11.016

Eight racemic 1-(furan-2-yl)ethanols were prepared from the corresponding carbonyl compounds for enantioselective acylation studies, and seven of them were used in preparative-scale kinetic resolutions with Candida antarctica lipase B (Novozym 435) and vinyl acetate in dried diisopropyl ether. Mechanism-based competition between the (R)-acetate (enzymatic acylation product), vinyl acetate (added acylating reagent), and acetic acid (enzymatic hydrolysis product) toward CAL-B, together with the residual water of the lipase were shown to be potential reasons for side reactions, which affected the course of the kinetic resolution of 1-[5-(2-chlorophenyl) and (4-bromophenyl)furan-2-yl]ethanols. Clear effects were not observed with the other alcoholic substrates. Alcoholysis of the enantiomerically enriched (R)-acetates with methanol and CAL-B in diisopropyl ether was shown to be a potential method for the deprotection of the (R)-acetates and the formation of (R)-alcohols.

Full text of DOI:10.1016/j.tetasy.2012.11.016

Tetrahedron: Asymmetry 24 (2013) 142–150
Contents lists available at SciVerse ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Lipase-catalyzed asymmetric acylation in the chemoenzymatic synthesis
of furan-based alcohols
Piia Hara a, Mihaela-Claudia Turcu a, Riku Sundell a, Monica Toßsa b, Csaba Paizs b, Florin-Dan Irimie b,
Liisa T. Kanerva a,
⇑
a Institute of Biomedicine, Department of Pharmacology, Drug Development and Therapeutics/Laboratory of Synthetic Drug Chemistry and Department of Chemistry University of
Turku, FIN-20014 Turku, Finland
b Department of Biochemistry and Biochemical Engineering, Babeßs-Bolyai University Cluj-Napoca, Ro-400028 Cluj-Napoca, Arany János 11, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 November 2012
Accepted 21 November 2012
Eight racemic 1-(furan-2-yl)ethanols were prepared from the corresponding carbonyl compounds for
enantioselective acylation studies, and seven of them were used in preparative-scale kinetic resolutions
with Candida antarctica lipase B (Novozym 435) and vinyl acetate in dried diisopropyl ether. Mechanism-
based competition between the (R)-acetate (enzymatic acylation product), vinyl acetate (added acylating
reagent), and acetic acid (enzymatic hydrolysis product) toward CAL-B, together with the residual water
of the lipase were shown to be potential reasons for side reactions, which affected the course of the
kinetic resolution of 1-[5-(2-chlorophenyl) and (4-bromophenyl)furan-2-yl]ethanols. Clear effects were
not observed with the other alcoholic substrates. Alcoholysis of the enantiomerically enriched (R)-ace-
tates with methanol and CAL-B in diisopropyl ether was shown to be a potential method for the depro-
tection of the (R)-acetates and the formation of (R)-alcohols.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
commercial availability of lipases in free and variously immobi-
lized forms. Secondary alcohol functionalities have been subjected
The reactivity of the aromatic furan ring in Diels–Alder cycload-
ditions, in reduction and oxidation reactions and in various ring
opening reactions has made furans versatile building blocks for
the synthesis of biologically and pharmacologically active com-
pounds.1 For instance, 5-aryl-2-vinyl furans have been transformed
into furyl carbinols, which have been further applied to the synthe-
sis of papulacandins;2 glycolipid-antifungal agents which as 1,3-b-
to lipase-catalyzed acylations, affording enantiomerically enriched
resolution counterparts (one enantiomer as an unreacted alcohol
and the other as an acylated product). The lipase-catalyzed acyla-
tion of benzofuranylethanols, 3-hydroxy-3-(furanyl)-propanoates
as well as phenylfuran-based cyanohydrins and 3-hydroxypro-
panenitriles, are good examples related to the present work.6 In
these reactions, lipases such as Candida antarctica lipase B (CAL-
B) as a Novozym 435 preparation (CAL-B adsorbed on a divinylben-
zene-crosslinked hydrophobic methacrylate polymer), generally
favor reaction of the (R)-enantiomer for alcohols with the large
group at the asymmetric center having CIP priority over the med-
ium size group, as is the case with (R)-1-phenylethanol.7
In the present chemoenzymatic work, racemic furan-based alco-
hols rac-2a–h were first prepared from the corresponding carbonyl
compounds and then subjected to lipase-catalyzed asymmetric
acylations in organic solvents, leading to the enantiomerically en-
riched resolution products (R)-3a–h and (S)-2a–h (Scheme 1). 1-
(Furan-2-yl)ethanol rac-2a and 1-(5-bromofuran-2-yl)ethanol rac-
2b were studied since they lack the phenyl substitution at the furan
ring that is common to the other alcohols. Alcohols rac-2b (optimal
behavior in kinetic resolutions) and rac-2d (anomalous behavior in
kinetic resolutions) were used as model substrates. Specific rota-
tions for (R)- and (S)-2a are known, and this together with the gen-
erally observed enantiopreferences of the lipases were used to
confirm the absolute configurations of the resolution products
D-glucan synthase inhibitors affect the cell wall construction of
fungal cells; and for the synthesis of 3-deoxy-D-manno-2-octulo-
sonic acid,3 vital in the growth and proliferation of Gram negative
bacteria.4 When a furyl-based structure is part of a chiral molecule,
the preparation of a single enantiomer is often required, especially
in the synthesis of pharmaceuticals and natural products.
The enzymatic kinetic resolution of a racemate with a suitable
hydrolytic enzyme is one of the most commonly used biocatalytic
methods for the preparation of enantiomers. Lipases (E.C. 3.1.1.3.)
are especially attractive hydrolytic enzymes for the enantioselec-
tive acylation of racemic alcohols and for the enantioselective
deacylation of the corresponding esters in organic solvents.5 This
is due to the high stability and enantioselectivity as well as good
⇑
Corresponding author. Tel.: +358 2 333 6773.
E-mail address: lkanerva@utu.fi (L.T. Kanerva).
0957-4166/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2012.11.016

P. Hara et al. / Tetrahedron: Asymmetry 24 (2013) 142–150
143
O
O
R1
1f-h
ii
O
R2
O
OH
OH
i
iii
O
+
O
O
O
O
R1
R1
R1
R1
1b-e
rac-2a-h
(R)-3a-h
(S)-2a-h
R1:
2-Cl-Ph
4-Br-Ph
H
a
Br
b
iv
c
d
OH
4-Cl-Ph
4-NO2-Ph
e
f
O
4-CO2Et-Ph
2-Me, 4-NO2-Ph
g
R1
(R)-2a and c
h
Scheme 1. Chemoenzymatic preparation of 1-furylethanols: (i) Grignard reaction with MeMgBr in Et2O; (ii) reduction with NaBH4 in ethanol; (iii) lipase-catalyzed kinetic
resolution with R2CO2R3; (iv) lipase-catalyzed alcoholysis with MeOH.
shown in Scheme 1.7,8 We have also considered side-reactions
which become possible when lipase-catalyzed acylations are per-
formed in organic solvents. This kind of consideration is general
especially when taking into account that the acyl donor and the
acylated product might both be activated esters and as such might
be subjected to hydrolysis by the so-called residual water (the
water in the seemingly dry catalyst) of the lipase catalyst.
ficity pocket capable of accommodating groups smaller than pro-
pyl at the active site.10 The screening results showed signs which
were interpreted as hydrolytic side reactions, and accordingly the
solvents were carefully dried before use in further studies. Alcohol
rac-2d (in addition to other alcohols studied later) was not chem-
ically acylated in the absence of the lipase under the otherwise
identical reaction conditions.
Next rac-2d (0.05 M) was subjected to acylation conditions with
vinyl butanoate (0.1 M) in dried DIPE at different CAL-B contents
(from 2 to 20 mg mLꢀ1; Fig. 1). The acylation first proceeded rap-
idly at rates almost independent of the enzyme content until the
sudden cessation in the formation of (R)-3d took place at 45–50%
conversion. The acylation then continued slowly with time,
although without the enantiomeric purification of the less reactive
(S)-2d, as was expected. Evidently two separate processes are in-
2. Results and discussion
2.1. Chemical synthesis
While rac-2a was a commercial product, alcohols rac-2b–h
were prepared from the corresponding carbonyl compounds,
usually with good chemical yields, using traditional methods [the
Grignard reaction with MeMgBr (rac-2b–e) and reduction with
NaBH4 (rac-2f–h), Scheme 1]. Details of these syntheses can be
found in the Section 4.
70
60
2.2. Enzymatic kinetic resolution
50
Conv. = 50%
For the kinetic resolution, lipase screening with common com-
mercial lipase preparations (10 mg mLꢀ1) was first performed
using the acylation of rac-2d (0.05 M) with vinyl butanoate
(0.1 M) as a model reaction in diisopropyl ether (DIPE, directly
from a bottle, water content 800 ppm). Screening at room temper-
ature (23 °C) gave CAL-B (Novozym 435) with E = 27 as the most
enantioselective catalyst, while other lipases from Burkholderia
cepacia, Candida antarctica A, Pseudomonas fluorescens, and Ther-
momyces lanuginosus gave E values ranging between 3 and 23.
CAL-B was the most attractive of the enzymes; this was not unex-
pected since the lipase is known to catalyze the acylation of vari-
ous 1-phenylethanols with excellent enantioselectivity.9 The high
enantioselectivity of CAL-B with such alcohol substrates was
previously explained by the presence of the so-called stereospeci-
40
30
20
10
0
0
4
8
12
Time (h)
16
20
24
Figure 1. Formation of (R)-3d from rac-2d (0.05 M) with vinyl butanoate (0.1 M) in
dry DIPE with CAL-B [2 mg mLꢀ1 (.), 5 mg mLꢀ1
20 mg mLꢀ1 (d)].
(
N
), 10 mg mLꢀ1 (j) and

144
P. Hara et al. / Tetrahedron: Asymmetry 24 (2013) 142–150
volved. First, the slope of the slow stage clearly increased when
increasing the lipase content. This stage, as observed for the acyla-
tion of all of the alcohol substrates, was later confirmed as the slow
CAL-B-catalyzed acylation of the less reactive (S)-alcohols by sub-
jecting (S)-2a (98% ee) to the CAL-B-catalyzed acylation conditions
in dried DIPE. Second, the fast stage of the reaction of rac-2d fin-
ished too early. Moreover, the enantiomer ratio E (measure of
enantioselectivity) for the acylation varied with conversion rather
than being constant, going through a maximum value at around
45–50% conversion. This result meant that the Sih and Chen equa-
tion (see Section 4), commonly used to calculate E (the measure of
enzymatic enantioselectivity), was not valid for the acylation of
rac-2d (E is replaced as a suggestive ‘E’ value at the given conver-
sion in Table 1).11 The very slow acylation of (S)-2d, although
unlikely to be able to disturb the formation of (R)-3d at the fast
stage of the reaction, cannot be completely ruled out as an expla-
nation for the observed variation of E with conversion. Further-
more, it cannot alone explain the fact that the less reactive
(S)-2d was not purified with conversion or that both ee(S)-2d and
ee(R)-3d started to decrease after the fast stage of the acylation
was over. Thus, a side-reaction connected to the amount of the
residual water was suggested, leading to the need to keep the
amount of the enzyme preparation low. Finally, 5 mg mLꢀ1 of
CAL-B (see the curve N) was chosen for further studies.
40 ppm), enantioselectivities according to the ee(R)-3d values at
the given conversions were high while the reaction with 2,2,2-tri-
fluoroethyl butanoate in DIPE (TFEB, entry 5) and with ethyl but-
anoate as an acyl donor and a solvent (entry 6) gave low
selectivities. On the basis of both the ee(R)-3d and ‘E’ values, vinyl
acetate gave a slightly more enantioselective acylation than vinyl
butanoate, with DIPE being the best solvent (entry 7). Thus, inves-
tigations with vinyl acetate in DIPE were continued, since vinyl
acetate is also economically the most favorable choice. Vinyl esters
were also preferred to make the acylation irreversible as
CH2@CHOH liberated in step A1 of Scheme 2 immediately tauto-
merizes to acetaldehyde.
For substrate scope studies, alcohols rac-2a–h were subjected to
acylation with vinyl acetate in the presence of CAL-B (1 or
5 mg mLꢀ1) in dried DIPE (Table 2). Since the solubility of the alco-
hols varied from case to case, 0.1 M rac-2a–c and 0.05 M rac-2d–h
were used. Furanyl alcohols rac-2a and b (entries 1 and 2) reacted
rapidly and with excellent enantioselectivity (E >200). The acyla-
tion of phenylfuran-based alcohols mostly proceeded with good
(E 90 to <200) to excellent (E >200) enantioselectivity, reaching
50% conversion without difficulty. However, the acylation of rac-
2c and d (entries 3 and 4), although it proceeded with high
ee(R)-3 values and with good enantioselectivity, tended to stop too
early, with E tending to vary with conversion as already mentioned
with rac-2d. It is worth noting that ee(S)-2 and ee(R)-3 values both
stayed unchanged for at least 24 h when CAL-B was filtered off
from the resolved mixture. Accordingly, the anomalous behaviors
with rac-2c and d must be connected to the lipase catalysis.
Finally, the preparative-scale kinetic resolution of rac-2a–d and
f–h (0.05 M or 0.1 M) with vinyl acetate (2 equiv) in the presence
of CAL-B (1 or 5 mg mLꢀ1) was performed successfully in dried
DIPE. The reactions proceeded as expected, and enantiomerically
enriched unreacted (S)-2a–d and f–h together with the corre-
sponding acetate products (R)-3a–d and f–h were separated
mostly with good chemical yields by column chromatography
(Table 3). However, in accordance with the previous kinetic resolu-
tion of rac-2a,8 a decrease in the ee(R)-3 values was evident during
the purification although it was possible to minimize the drops by
adding triethylamine (0.5–1%) into the dichloromethane eluent
(Table 3, compare columns 4 and 5). This decrease was exception-
ally large (from 91% to 75%) with (R)-3d (entry 10). When silica
(100 mg mLꢀ1) and (R)-3d (0.05 M, 56% ee) were mixed in dichloro-
methane, 0% ee was detected after one hour indicating that silica
gel or dichloromethane was the reason for this decrease in ee. Since
other solvents (e.g., ethyl acetate/hexane or petroleum ether/
Table 1
Acylation of rac-2d (0.05 M) with an acyl donor (2 equiv) and CAL-B (5 mg mLꢀ1) in
dried organic solvents
Entry Solvent
Acyl donor
Time (h) Conv.a (%) ee(R)ꢀ3d (%) [E’]b
1
2
3
4
5
6
7
8
9
10
DIPE
Vinyl butanoate
2.5 49
3.5 48
3.5 49
0.5 48c
3.5 47
3.5 40
1.5 50
1.5 44
1.5 47
1.5 51
90 [50]
86 [35]
85 [35]
87 [25]
85 [26]
80 [15]
93 [90]
93 [60]
92 [60]
92 [80]
Toluene Vinyl butanoate
TBME
Hexane
DIPE
Ethyl butanoate
DIPE
Toluene Vinyl acetate
TBME
Hexane
Vinyl butanoate
Vinyl butanoate
TFEBc
Vinyl acetate
Vinyl acetate
Vinyl acetate
a
b
c
Calculated from the ee-values.
Ee(R)ꢀ3d at the given conversion; E value is inaccurate and marked as E0.
2,2,2-Trifluoroethyl butanoate.
When the acylation of rac-2d (0.05 M) carried out with vinyl
butanoate (Table 1, entries 1–4), or with vinyl acetate (entries
7–10), was studied in four dried solvents (water contents 20–
H2O
CH3CHO
R2CO2CH=CH2
+
HO-protein
A)
R2CO2-protein
acyl-enzyme
intermediate
R2CO2H
A1)
B)
H2O
(R)-2
(R)-2
A2)
HO protein
OCOR2
O
R1
(R)-3
Scheme 2. Competitive mechanistic pathways for the CAL-B-catalyzed acylation of rac-2.

P. Hara et al. / Tetrahedron: Asymmetry 24 (2013) 142–150
145
Table 2
Accordingly, the step with vinyl esters is irreversible. In the second
mechanistic step, any nucleophile [here preferably (R)-2 and
water] present at the active site may compete to react with the
intermediate in the formation of the corresponding enantiomeri-
cally enriched (R)-3 and R2CO2H. At high enough concentrations,
both new products are potential acyl donors for the serine hydro-
xyl, that is, enzymatic hydrolysis of the vinyl ester through route
A1/A; enzymatic hydrolysis of (R)-3 through route A2/A; and enzy-
matic esterification of the carboxylic acid through route B can take
place in addition to the desired acylation of (R)-2. Hydrolytic routes
consume, while route B forms, water. The residual water plays a
crucial role in initiating the hydrolytic process, while esterification
serves as a continuous source of new water. We propose that for
reactions in relatively hydrophobic organic solvents, the water
produced by enzymatic esterification does not leave the active site.
Instead, it effectively competes with (R)-2 from the intermediate.
This proposal is in accordance with the previous explanation for
the high interesterification (also mechanistically an alcoholysis
reaction) reactivities detected compared to the corresponding
reactivities for alcoholysis in organic solvents.5,12
Lipase-catalyzed hydrolysis of activated acyl donors, such as the
hydrolysis of vinyl esters and enantiomerically enriched acylated
products by the residual water, is well recognized for reactions
in organic solvents (Scheme 2).13 It is clear that when the CAL-B-
catalyzed hydrolysis of enantiomerically enriched (R)-3 takes place
through route A2/A in the resolution mixture, both ee(S)-2 and ee(R)-3
decrease. The extent of the ester hydrolysis and esterification
cannot be separately determined during the enzymatic acylation
of rac-2 whereas the total amount of the carboxylic acid in unde-
rivatized samples can be easily obtained by GC. When the CAL-B-
catalyzed acylation of rac-2 proceeded in DIPE, acetic (filled signs)
or butanoic acid (open signs) was present in the reaction mixture
depending on the acyl donor used. The amount of acetic acid was
higher than that of the corresponding butanoic acid under other-
wise identical conditions (Fig. 2). Acid concentrations increased
with conversion and time, a burst occurring when the more reac-
tive enantiomer (R)-2 was nearly acylated at close to 50% conver-
sion. The presence of the acid can affect the enantiomeric
outcome of the lipase-catalyzed kinetic resolution, at least through
esterification (route B) producing enantiomerically enriched (R)-3
with its own enantioselectivity and by changing the pH of the
microenvironment and accordingly the active conformation of
the enzyme.
Acylation of rac-2a–i (0.05 M) with vinyl acetate (2 equiv) and CAL-B (5 mg mLꢀ1) in
dried DIPE
Entry Substrate Time (h) Conv. (%) ee(S)-2 (%) ee(R)-3 (%)
E
1
2
3
4
5
6
7
8
rac-2aa
rac-2ba
rac-2ca
rac-2d
rac-2e
rac-2f
2
2
2
1.5
1.5
1.5
1.5
1.5
50
50
39
49
48
44
50
50
98
98
60
91
78
76
95
94
99
99
96
93
83
97
95
94
>200
>200
—
—
25
177 21
150 10
90 13
5
rac-2g
rac-2h
a
rac-2 (0.1 M), vinyl acetate 0.2 M and CAL-B (1 mg mLꢀ1).
Table 3
Isolated yields after preparative-scale kinetic resolution of rac-2 with vinyl acetate
and CAL-B in DIPE
eeb (%)
eec (%)
½
a 2D5
ꢁ
(c 1, CHCl3)
ꢀ20.8
+164
+20.7
Entry
Product
Yield (%)
1
2
3
4
5
6
7
8
(S)-2a
(R)-3a
(R)-2a
(S)-2b
(R)-3b
(S)-2c
(R)-3c
(R)-2c
(S)-2d
(R)-3d
(S)-2f
(R)-3f
(S)-2g
(R)-3g
(S)-2h
(R)-3h
42a
43a
82
99
98
99
96
95
99
98
99
92
91
94
96
98
94
93
94
98
92
99
95
94
98
90
99
91
75
89
90
95
88
93
86
45a
45a
48a
49a
76
ꢀ20.1
+159
ꢀ1.6
+146
+1.6
9
21a
55a
28a
27a
46a
46a
48a
43a
+5
+54
+2.3
10
11
12
13
14
15
16
+225
ꢀ2.3
+198
+5.8
+178.8
a
b
c
Yield calculated from the racemic mixture.
Ee in the resolution mixture.
Ee after purification by column chromatography.
acetone) for the elution gave poor separations of (S)-2 from (R)-3,
dichloromethane was mostly used with rapid chromatography.
Alcohol rac-2e was not used in the preparative scale studies owing
to the poor enantioselectivity (E = 25; Table 2).
Alcoholysis studies in the presence of CAL-B (5 mg mLꢀ1) re-
vealed that the reaction of rac-3a (0.1 M, E >>200) and c (0.1 M,
E = 96) with methanol (0.5 M) proceeded with high enantioselec-
tivity in dried DIPE. This method was used for the deprotection
of the purified ester products (R)-3a (92% ee) and c (90% ee) to give
(R)-2a and c with 99% ee at 83 and 76% isolated yields, respectively.
The routine process, not repeated with the other (R)-esters, was ex-
pected to be usable in general.
Conv. = 50%
40
30
20
10
0
2.3. Mechanism-based side-reactions
As discussed above, conversion tended to cease too early and E
varied with conversion for the CAL-B-catalyzed acylation of rac-2c
and d. With the other substrates, this behavior cannot be excluded
although it was not detectable within experimental accuracy. This
led us to consider reaction mechanism-based possibilities for side-
reactions and the involvement of the residual water (or water
otherwise in the system) for the acylation of rac-2 in DIPE
(Scheme 2). In the first step of the two-step reaction mechanism
10
20
30
40
50
60
70
of serine hydrolases,5 an acyl donor (here
a vinyl ester
Conv. (%)
R2CO2CH@CH2) acylates the serine hydroxyl (Ser105 in CAL-B) at
the active site in the formation of an ester intermediate (R2CO2-
protein) called an acyl-enzyme intermediate and vinyl alcohol,
the latter tautomerizing resulting in the formation of acetaldehyde.
Figure 2. Acid formation in the acylation of rac-2a-c (0.1 M, rac-2a (j), rac-2b (d),
rac-2c, (N)) and rac-2d (0.05 M, .) with 2 equiv. of vinyl acetate (filled) or vinyl
butanoate (open) in dried DIPE with CAL-B (1 mg mLꢀ1 for rac-2a-c, 5 mg mLꢀ1 for
rac-2d).

146
P. Hara et al. / Tetrahedron: Asymmetry 24 (2013) 142–150
Table 4
Esterification of rac-2b (0.1 M) and rac-2d (0.05 M) with acetic and butanoic acid in dried DIPE in the presence of CAL-B (1 mg mLꢀ1
)
Entry
Alcohol
RCO2H
vo
(
l
mol minꢀ1 mgꢀ1
)
Yield [(R)-3]a (%)
ee(R)ꢀ3b (%)
1
2
3
4
5
6
7
8
9
10
rac-2b
rac-2b
rac-2b
rac-2b
rac-2d
rac-2b
rac-2b
rac-2b
rac-2b
rac-2dc
AcOH (0.05 M)
AcOH (0.02 M)
AcOH (0.01 M)
AcOH (0.005 M)
AcOH (0.05 M)
PrCO2H (0.05 M)
PrCO2H (0.02 M)
PrCO2H (0.01 M)
PrCO2H (0.005 M)
PrCO2H (0.05 M)
13.6 0.7
6.9 0.1
5.0 0.4
2.5 0.3
—
13.5 0.3
4.4 0.5
1.7 0.3
0.7 0.1
—
10
7
4
98
97
95
94
79
98
95
98
99
85
3
26
11
6
3
1
22
a
b
c
Yield of enantiomerically enriched (R)-3; reaction time 2 h.
ee after the reaction of 2 h.
CAL-B content 10 mg mLꢀ1; reaction time 1.5 h.
When the esterification of rac-2b was studied with the carbox-
as an enzymatic hydrolysis product, was present in the reaction
mixture and that CAL-B-catalyzed the enantioselective esterifica-
tion between the acid and rac-2 and proceeded even at very low
acid concentrations (0.005 M) with possible effects on kinetic
resolution.
ylic acids and CAL-B (1 mg mLꢀ1) in dried DIPE, esterification took
place even at 0.005 M acid concentrations (Table 4, entries 4 and
9). Reactivity (initial rate vo) and ester yields of (R)-3b after 2 h,
increased when the acid concentration increased (entries 1–4
and 5–9). Esterifications typically attained their equilibrium posi-
tions in the 2 h samples. The high ee(R)-3b values (94-99%) indicated
excellent enantioselectivity for the esterification. Thus, visible
effects on the outcome of the highly enantioselective (E >200) acyl-
ation of rac-2b with vinyl esters were not detected (Table 2, entry
2). For the acylation of rac-2d (0.05 M) with the acids (0.05 M), the
esterification yields of (R)-3d were doubled when compared to
those of (R)-3b at the same time (for instance 26%, entry 5 vs
10%, entry 1); the values of ee(R)-3d = 79% and 85% indicate moder-
ate enantioselectivity for the esterifications (entries 5 and 10).
These results suggest that depending on the structure of rac-2, es-
ter hydrolysis/esterification side-reactions for lipase-catalyzed
acylations in organic solvents may well stop the CAL-B catalyzed
acylation too early and affect the enantiomeric outcome of the
acylation with conversion.
4. Experimental part
4.1. Materials and methods
Candida antarctica lipase B (CAL-B) as a Novozym 435 prepara-
tion was from Novo Nordisk. Methanol and other solvents used
were of the highest analytical grade from J.T. Baker, Lab-Scan Ltd,
Aldrich and Mallinckrodt. Vinyl butanoate and 1-(furan-2-yl)etha-
nol rac-2a were purchased from Fluka, vinyl acetate and the alde-
hydes from Aldrich. MeMgBr (3 M in diethyl ether) and aldehydes
1b–e were products of Aldrich. For analytical purposes and for
alcoholysis experiments, the esters (acetates 3, propanoates 4,
and butanoates 5) of rac-2a–h were prepared from the correspond-
ing acid anhydride and alcohol using traditional methods.
Lipase promiscuity, also based on the catalytic properties of the
active site, is well known to lead to C–C bond forming side-reac-
tions for reactions in organic solvents and is thus noteworthy.14
However, the formation of new compounds other than the ester
products (R)-3 were not observed after chromatography for the
CAL-B catalyzed acylation of rac-2.
The progress of the enzymatic acylation of rac-2 was followed
by taking samples (50 or 100 lL) at intervals and analyzing them
with an Agilent 6850 GC/FID equipped with a Chrompack CP-
Chirasil-DEX CB column (25 m ꢂ 0.25 mm) (Table 5). The unre-
acted (S)-2 in the samples (50 or 100 lL) was derivatized as ester
3–5 with the corresponding anhydride (5–10 lL) in the presence
of 4-dimethylaminopyridine (DMAP, 1% in pyridine) prior to injec-
tion to achieve good baseline separation. Underivatized samples
were separately injected in order to determine the acetic and buta-
noic acid contents in the samples against an internal standard.
The 1H and 13C NMR spectra were recorded on a Bruker Avance
500 Spectrometer at 25 °C. Tetramethylsilane (TMS) was used as
an internal standard in chloroform (CDCl3). Mass spectra (MS)
were measured on a VG ZabSpec mass spectrometer (EI+). DIPE
was dried by refluxing with CaH2 followed by distillation before
use or by molecular sieves (4 Å) to attain the water content of
20-40 ppm. Water contents were measured using a Metrohm 831
KF Coulometer. The determination of ‘E’ was based on the equation
E = ln[(1 ꢀ c)(1 ꢀ eeS)]/ln[(1 ꢀ c)(1 + eeS)] (eeS is the enantiomeric
excess of the unreacted substrate at conversion c) and E values
on the use of linear regression (E as the slope of the line
ln[(1 ꢀ c)(1 ꢀ eeS)] versus ln[(1 ꢀ c)(1 + eeS)] and).11 Analytical
thin layer chromatography (TLC) was carried out on Merck Kiesel-
gel 60F254 sheets. Spots were visualized by treatment with 5% eth-
3. Conclusions
The CAL-B-catalyzed kinetic resolution of racemic 1-(furan-2-
yl)ethanols has been studied in dried diisopropyl ether. Seven
enantiomerically enriched counterparts, (S)-2 as the unreacted
alcohol enantiomers and (R)-3 as the produced acetate enantio-
mers were prepared.
Acylations of rac-2 with vinyl esters in DIPE revealed two note-
worthy aspects. First, the progression curves showed a rapid stage
when mainly the (R)-alcohols reacted, which was followed by a
slow stage when the less reactive (S)-enantiomers reacted. Second,
progress for the kinetic resolution of 1-[5-(2-chlorophenyl)furan-
2-yl]ethanol rac-2c and 1-[5-(4-bromophenyl)furan-2-yl]ethanol
rac-2d ceased at 45–50% conversion, while E varied with conver-
sion and ee(S)-2c/d was not further purified. This behavior was
anomalous to the above. With the other alcohols studied, the reac-
tions did not undergo this behavior. We concluded that mecha-
nism-based side-reactions, initiated by the residual water of CAL-
B and leading to complicated enzymatic ester hydrolysis/esterifica-
tion equilibria (Scheme 2) might be a potential reason for the
anomalous behavior. We have shown that acetic (or butanoic) acid
anolic
Chromatographic separations were performed using column chro-
matography on Kieselgel 60 (0.063–0.200 m). Optical rotations
were determined with a Perkin Elmer 341 Polarimeter (c 1, CHCl3,
phosphomolybdic
acid
solution
and
heating.
l

P. Hara et al. / Tetrahedron: Asymmetry 24 (2013) 142–150
147
Table 5
GC methods and retention times for compound analysis.
R1
a
R2 in 3-4a–h /acid
tR (S/R) [min]
tR (acid) [min]
tR (Std) [min]
3a
5.06/5.90
7.28/7.66
11.25/11.65
4a
5a
AcOHb
3.04
5.93
10.22 (dodecane)
8.47 (acetophenone)
Butanoic acida
b
c
3c
10.17/12.40
15.20/16.64
24.44/26.20
4c
5c
AcOHd
2.90
4.07
6.40 (decane)
6.91 (dodecane)
Butanoic acidc
3e
28.85/30.28
39.96/41.13
60.96/62.30
4e
5e
AcOHf
7.47
4.31
19.54 (dodecane)
6.06 (acetophenone)
Butanoic acidg
d
e
3h
4h
AcOHi
3j
72.44/77.46
103.18/108.03
2.04
12.66 (dihexyl ether)
35.58/37.77
51.12/52.37
70.81/72.48
112.02/117.79
155.47/159.52
119.06/126.39
167.44/171.15
119.46/125.42
167.69/171.88
4j
5j
f
3k
5k
g
h
3l
4l
3m
5m
a
b
c
0.1
l
l
l
l
l
l
l
l
l
l
l
l
l
L inj, vol, inlet temp 300 °C, 12.21 psi, 25:1 split, 28.2 mL minꢀ1 total flow, oven temp 110 °C (12.5 min), then +50 °C minꢀ1 up to 165 °C (4 min).
L inj, vol, inlet temp 300 °C, 12.21 psi, 25:1 split, 28.2 mLminꢀ1 total flow, oven temp60 °C (6 min), +40 °C minꢀ1, 100 °C (5 min), +50 °C minꢀ1, 165 °C (3 min).
L inj, vol, inlet temp 300 °C, 11.42 psi, 70:1 split, 66.6 mL minꢀ1 total flow, oven temp 114 °C (21 min).
0.1
1.0
1.0
0.1
0.1
0.1
0.2
0.2
0.5
0.5
0.5
0.5
d
e
f
L inj, vol, inlet temp 300 °C, 11.42 psi, 70:1 split, 66.6 mL minꢀ1 total flow, oven temp 80 °C (5 min), +20 °C minꢀ1, 120 °C (15 min).
L inj, vol, inlet temp 300 °C, 12.21 psi, 25:1 split, 28.2 mL minꢀ1 total flow, oven temp 160 °C (45 min).
L inj, vol, inlet temp 300 °C, 12.21 psi, 25:1 split, 28.2 mLminꢀ1 total flow, oven temp (70 °C (10 min), +40 °C minꢀ1, 110 °C (21 min).
L inj, vol, inlet temp 300 °C, 12.21 psi, 25:1 split, 28.2 mL minꢀ1 total flow, oven temp 120 °C (7 min).
g
h
i
L inj, vol, inlet temp 150 °C, 12.42 psi, 20:1 split, 21.7 mL minꢀ1 total flow, oven temp 150 °C (115 min).
L inj, vol, inlet temp 150 °C, 12.42 psi, 20:1 split, 21.7 mL minꢀ1 total flow, oven temp 80 °C (5 min), + 20 °C minꢀ1, 120 °C (10 min).
L inj, vol, inlet temp 300 °C, 12.21 psi, 10:1 split, 22.4 mL minꢀ1 total flow, oven temp 165 °C (77 min).
j
k
l
L inj. vol, inlet temp 300 °C, 13.42 psi, 8:1 split, 44.3 mL minꢀ1 total flow, oven temp 165 °C (170 min).
L inj, vol, inlet temp 300 °C, 13.42 psi, 20:1 split, 21.7 mL minꢀ1 total flow, oven temp 165 °C (175 min).
m
L inj, vol, inlet temp 300 °C, 12.42 psi, 20:1 split, 21.7 mL minꢀ1 total flow, oven temp 165 °C (175 min).
25 °C), and ½a 2D5
ꢁ
values are given in units of 10ꢀ1 deg cm2 gꢀ1. All
enzymatic reactions were performed at room temperature (23 °C).
J = 7.9 Hz, 1H, 50-H), 7.42 (dd, J = 1.3 Hz, J = 8.0 Hz, 1H, 30-H), 7.84
(dd, J = 1.7 Hz, J = 7.9 Hz, 1H, 60-H) ppm. 13C NMR (126 MHz,
25 °C, CDCl3): d = 21.38 (–CH(OH)CH3), 63.75 (–CH(OH)CH3),
107.32 (3-C), 111.56 (4-C), 126.82 (50-C), 127.81 (60-C), 128.01
(30-C), 129.07 (20-C), 130.06 (10-C), 130.69 (40-C), 149.48 (2-C),
157.20 (5-C) ppm. MS(EI+): m/z (rel. int.) = 222 (53) [M+], 207
(100), 179 (5), 149 (14), 139 (9), 125 (2), 115 (20); HRMS: M+ found
(M+ calculated for C12H11O2Cl): 222.04450 (222.04476).
4.2. Preparation of racemic alcohols
4.2.1. Synthesis of racemic alcohols rac-2b–e
Alcohols rac-2b–e were prepared by a Grignard reaction from
the corresponding aldehydes (Scheme 1) as exemplified for the
preparation of 1-(5-(2-chlorophenyl)furan-2-yl)ethanol rac-2c.
Aldehyde 1c (2.5 g, 12.10 mmol) in diethyl ether (60 mL) was
added to 10 mL of MeMgBr (3 M solution in diethyl ether,
30.25 mmol) in an ice-water bath and the mixture was stirred for
1 h at room temperature. The reaction mixture was cooled to
0 °C and satd NH4Cl aq (100 mL) with 30 mL of ice was added.
The organic layer was separated and the aqueous layer washed
with diethyl ether (3 ꢂ 75 mL). The combined organic layers were
washed with sat. NaHCO3 aq (1 ꢂ 50 mL) and water (3 ꢂ 50 mL),
dried over anhydrous MgSO4, filtered, and evaporated. When
impurities were observed by TLC, the crude product was purified
by filtering the product through silica (dichloromethane) to yield
1-(5-(2-chlorophenyl)furan-2-yl)ethanol rac-2c as a light yellow
semisolid: (2.47 g, 11.10 mmol, 92%). 1H NMR (500 MHz, 25 °C,
CDCl3): d = 1.60 (d, J = 6.6 Hz, 3H, –CH(OH)CH3), 2.10 (br, 1H, OH),
4.95 (q, J = 6.6 Hz, –CH(OH)CH3), 6.36 (dd, J = 0.6 Hz, J = 3.4 Hz,
1H, 3-H), 7.05 (d, J = 3.4 Hz, 1H, 4-H), 7.18 (ddd, J = 1.7 Hz,
J = 7.5 Hz, J = 7.9 Hz, 1H, 40-H), 7.30 (ddd, J = 1.3 Hz, J = 7.5 Hz,
4.2.1.1. 1-(5-Bromofuran-2-yl)ethanol, rac-2b.
From alde-
hyde 1b (1.0 g, 5.71 mmol) light yellow semisolid rac-2b
a
(1.02 g, 5.31 mmol, 93%) was obtained as presented for rac-2c
above. 1H NMR (500 MHz, 25 °C, CDCl3): d = 1.53 (d, J = 6.6 Hz,
3H, –CH(OH)CH3), 2.04 (br, 1H, –OH), 4.83 (q, J = 6.6 Hz, 1H, –
CH(OH)CH3), 6.21 (dd, J = 0.6 Hz, 3.2 Hz, 1H, 4-H), 6.24 (d,
J = 3.3 Hz, 1H, 3-H) ppm. 13C NMR (126 MHz, 25 °C, CDCl3):
d = 20.03 (–CH(OH)CH3), 62.48 (–CH(OH)CH3), 106.91 (3-C),
110.76 (4-C), 120.15 (2-C), 158.54 (5-C) ppm. MS(EI+): m/z (rel.
int.) = 190 (13) [M+], 175 (100), 160 (2), 145 (9), 95 (99), 65 (47);
HRMS: M+ found (M+ calculated for C6H7O2Br): 189.96240
(189.96294).
4.2.1.2. 1-[5-(4-Bromophenyl)furan-2-yl]ethanol, rac-2d. From
aldehyde 1d (1.0 g, 3.98 mmol) a light golden yellow semisolid
rac-2d (1.00 g, 3.75 mmol, 94%) was obtained as presented for rac-
2c above. 1H NMR (500 MHz, 25 °C, CDCl3): d = 1.59 (d, J = 6.6 Hz,

148
P. Hara et al. / Tetrahedron: Asymmetry 24 (2013) 142–150
3H, –CH(OH)CH3), 2.01 (br, 1H, –OH), 4.93 (q, J = 6.6 Hz, 1H, –
CH(OH)CH3), 6.31 (d, J = 3.2 Hz, 1H, 4-H), 6.58 (d, J = 3.4 Hz, 1H,
3-H), 7.50 (m, 4H, 20-H, 60-H, 50-H, 30-H) ppm. 13C NMR (126 MHz,
25 °C, CDCl3): d = 21.36 (–CH(OH)CH3), 63.77 (–CH(OH)CH3),
106.13 (4-C), 107.47 (3-C), 121.09 (2-C), 125.21 (20-C, 60-C), 129.65
(40-C), 131.79 (30-C, 50-C), 152.29 (10-C), 157.57 (5-C) ppm. MS: m/z
(rel. int.) = 267 (52) [81Br-M+], 266 (58) [79Br-M+], 251 (100), 185
(7), 172 (9), 144 (18), 122 (4), 115 (23), 76 (9); HRMS: M+ found
(M+ calculated for C12H11BrO2): 265.99320 (265.99424).
4.2.2.2. 1-[5-(2-Methyl-4-nitrophenyl)furan-2-yl]ethanol, rac-
2g. From ketone 1g (0.77 g, 3.14 mmol) a golden brown semi-
solid rac-2g (0.74 g, 3.00 mmol, 96%) was obtained as presented for
rac-2h above. 1H NMR (500 MHz, 25 °C, CDCl3): d = 1.62 (d,
J = 6.6 Hz, 3H, –CH(OH)CH3), 2.27 (br, 1H, –OH), 2.59 (s, 3H, –
CH3), 4.98 (q, J = 6.6 Hz, 1H, –CH(OH)CH3), 6.42 (d, J = 3.4 Hz, 1H,
3-H), 6.71 (d, J = 3.4 Hz, 1H, 4-H), 7.85 (d, J = 8.7 Hz, 1H, 60-H),
8.06 (dd, J = 2.3 Hz, J = 8.7 Hz, 1H, 50-H), 8.10 (d, J = 2.0 Hz, 1H, 30-
H) ppm. 13C NMR (126 MHz, 25 °C, CDCl3): d = 21.50 (–CH3),
22.36 (–CH(OH)CH3), 63.75 (–CH(OH)CH3), 107.79 (3-C), 112.76
(4-C), 121.27 (50-C), 126.30 (30-C), 126.87 (60-C), 135.18 (20-C),
135.71 (10-C), 145.96 (40-C), 150.56 (5-C), 158.78 (2-C)
ppm. MS(EI+): m/z (rel. int.) = 247 (49) [M+], 232 (100), 204 (3),
186 (22), 164 (5), 141 (3), 128 (20) 115 (16), 102 (6), 77 (10);
HRMS: M+ found (M+ calculated for C13H13NO4): 247.08500
(247.08446).
4.2.1.3. 1-[5-(4-Chlorophenyl)furan-2-yl]ethanol, rac-2e. From
aldehyde 1e (1.5 g, 7.26 mmol) a light yellow semisolid rac-2e
(1.45 g, 6.53 mmol, 90%) was obtained as presented for rac-2c
above. 1H NMR (500 MHz, 25 °C, CDCl3): d = 1.58 (d, J = 6.6 Hz, 3H,
–CH(OH)CH3), 2.14 (br, 1H, –OH), 4.92 (q, J = 6.6 Hz, –CH(OH)CH3),
6.31 (d, J = 3.2 Hz, 1H, 4-H), 6.56 (d, J = 3.2 Hz, 1H, 3-H), 7.33 (d,
J = 8.4 Hz, 2H, 20-H, 60-H), 7.57 (d, J = 8.4 Hz, 2H, 30-H, 500-H) ppm.
13C NMR (126 MHz, 25 °C, CDCl3): d = 21.35 (–CH(OH)CH3), 63.73
(–CH(OH)CH3), 106.00 (3-C), 107.44 (4-C), 124.93 (30-C, 50-C),
128.85 (20-C, 60-C), 129.22 (10-C), 132.96 (40-C), 152.25 (2-C),
157.50 (5-C) ppm.
4.3. Analytical-scale enzymatic reactions
In a typical small-scale experiment, CAL-B as the commercial
Novozym 435 preparation (1–20 mg mLꢀ1) was added to a solution
of one of the substrates rac-2a–h (0.05 or 0.1 M) and an acyl donor
(2 equiv) in an organic solvent. The mixture was then shaken at
room temperature. The progress and ee values of the products
were followed by taking samples at intervals and analyzing the
samples after derivatization by GC.
4.2.2. Synthesis of racemic alcohols rac-2f–h
The reaction between 1-(furan-2-yl)ethanone and the corre-
sponding diazonium salts by a variation of the Meerwein method15
produced ketones 1f–h, which were transformed into alcohols rac-
2f–h by NaBH4 reduction (Scheme 1, ii), as exemplified for the syn-
thesis of 1-[5-(-carboxyethylphenyl)furan-2-yl]ethanol, rac-2h.
Ketone 1h (0.66 g, 2.56 mmol) was dissolved in ethanol (25 mL)
after which NaBH4 (0.15 g, 3.84 mmol) in 15 mL of ethanol was
added slowly while stirring the mixture at room temperature.
The reaction mixture was poured into a glass with ice, water,
and HCl (1 M, 1 mL). The aqueous solution was extracted three
times with diethyl ether. The organic layer was washed with brine,
dried over MgSO4, and evaporated. The residue was purified on a
short column (dichloromethane) to afford a yellow semisolid 1-
4.4. Preparative scale kinetic resolutions
4.4.1. (R)-3a, (S)-2a and (R)-2a
CAL-B (1 mg/mL) was added to the solution of rac-2a (1.0 g,
8.93 mmol) and vinyl acetate (1.64 mL, 18.0 mmol) in dried DIPE
(89 mL). The reaction was stopped after 5 h at 50% conversion by
filtering off the enzyme. The product mixture was evaporated
and purified by filtering through silica (dichloromethane), afford-
ing light yellow oils (S)-2a [0.42 g, 3.75 mmol, 42%, 98% ee,
[5-(4-carboxyethylphenyl)furan-2-yl]ethanol
rac-2h:
(0.61 g,
½
a 2D5
ꢁ
¼ ꢀ20:8 (c 1, CHCl3); literature data8
½
a 2D5
ꢁ
¼ ꢀ24:4 (neat),
2.36 mmol, 92%). 1H NMR (500 MHz, 25 °C, CDCl3): d = 1.40 (t,
J = 7.1 Hz, 3H, –CO2CH2CH3), 1.60 (d, J = 6.6 Hz, 3H, –CH(OH)CH3),
2.25 (br, 1H, –OH), 4.37 (q, J = 7.1 Hz, 2H, –CO2CH2CH3), 4.95 (q,
J = 6.6 Hz, 1H, –CH(OH)CH3), 6.35 (d, J = 3.4, 1H, 3-H), 6.71 (d,
J = 3.6 Hz, 1H, 4-H), 7.69 (d, J = 8.5 Hz, 2H, 30-H, 50-H); 8.03 (d,
J = 8.5 Hz, 2H, 20-H, 60-H) ppm. 13C NMR (126 MHz, 25 °C, CDCl3):
d = 14.35 (–CO2CH2CH3), 21.42 (–CH(OH)CH3), 61.01 (–
CO2CH2CH3), 63.76 (–CH(OH)CH3), 107.64 (3-C), 107.82 (4-C),
123.26 (20-C, 60-C), 128.83 (10-C), 130.04 (30-C, 50-C), 134.57 (40-C),
152.24 (5-C), 158.40 (2-C), 166.40 (–CO2CH2CH3) ppm. MS(EI+):
m/z (rel. int.) = 260 (66), [M]+, 245 (100), 217 (25), 197 (7), 172
(11), 145 (7), 115 (21), 100 (5), 77 (5); HRMS: M+ found (M+ calcu-
lated for C15H16O4): 260.10470 (260.10486).
98–99% ee] and (R)-3a [0.58 g, 3.78 mmol, 43%, 92% ee,
ꢁ
½
a 2D5
¼ þ164 (c 1, CHCl3)]. Compound (R)-3a was further purified
by adding CAL-B (5 mg mLꢀ1) to a solution of (R)-3a (0.50 g,
3.25 mmol, 92% ee) and methanol (0.66 mL, 16 mmol) in dried DIPE
(32.5 mL). The reaction was stopped after 6 h at 95% conversion by
filtering off the enzyme. The filtrate was poured through silica
(dichloromethane) to afford (R)-2a as a light yellow oil [0.30 g,
2.66 mmol, 82%, 99% ee, ½a D25
ꢁ
¼ þ20:7 (c 1, CHCl3); literature data8
½
a 2D5
ꢁ
¼ þ24:3 (neat), 96% ee]. For (R)-3a: 1H NMR (500 MHz,
CDCl3): d = 1.58 (d, J = 6.7 Hz, 3H, –CH(OCOCH3)CH3), 2.06 (s, 3H,
–CH(OCOCH3)CH3), 5.96 (q, J = 6.7 Hz, 1H, –CH(OCOCH3)CH3),
6.32 (d, J = 3.3 Hz, 1H, 3-H), 6.34 (dd, J = 1.8 Hz, J = 3.2 Hz, 1H,
4-H), 7.39 (d, J = 1.7 Hz, 1H, 5-H) ppm. 13C NMR (126 MHz, CDCl3):
d = 18.23 (–CH(OCOCH3)CH3), 21.22 (–CH(OCOCH3)CH3), 65.07
(–CH(OCOCH3)CH3), 107.75 (3-C), 110.20 (4-C), 142.49 (5-C),
153.47 (2-C), 170.30 (–CH(OCOCH3)CH3) ppm. MS(EI+): m/z (rel.
int.) = 154 (51) [M+], 127 (1), 115 (1), 110 (6), 95 (13), 43 (100);
HRMS: M+ found (M+ calculated for C8H10O3): 154.06310
(154.06299). For (S)-2a: 1H NMR (500 MHz, CDCl3): d = 1.54 (d,
J = 6.5 Hz, 3H, –CH(OH)CH3), 2.05 (br, 1H, –OH), 4.88 (q,
J = 6.5 Hz, 1H, –CH(OH)CH3), 6.22 (d, J = 3.2 Hz, 1H, 3-H), 6.32
(dd, J = 1.3 Hz, J = 3.2 Hz, 4-H), 7.37 (d, J = 1.0 Hz, 1H, 5-H) ppm.
13C NMR (126 MHz, CDCl3): d = 21.27 (–CH(OH)CH3), 63.63
(–CH(OH)CH3), 105.11 (3-C), 110.13 (4-C), 141.92 (5-C), 157.61
(2-C) ppm. MS(EI+): m/z (rel. int.) = 112 (51) [M+], 97 (100), 94
(45), 84 (11), 81 (6), 65 (24), 55 (16), 44 (92); HRMS: M+ found
(M+ calculated for C6H8O2): 112.05240 (112.05243). NMR and MS
spectra of (R)-2a were found identical to (S)-2a.
4.2.2.1. 1-[5-(4-Nitrophenyl)furan-2-yl]ethanol, rac-2f.
From
ketone 1f (1.0 g, 4.33 mmol) a brown semisolid rac-2f (0.97 g,
4.14 mmol, 96%) was obtained as presented for rac-2h above. 1H
NMR (500 MHz, 25 °C, CDCl3): d = 1.62 (d, J = 6.6 Hz, 3H,
CH(OH)CH3), 2.12 (br, 1H, –OH), 4.97 (q, J = 6.6 Hz, 1H,
–
–
CH(OH)CH3), 6.41 (d, J = 3.4 Hz, 1H, 3-H), 6.82 (d, J = 3.4 Hz, 1H, 4-
H), 7.77 (d, J = 7.0 Hz, 2H, 20-H, 60-H), 8.23 (d, J = 8.9 Hz, 2H, 3’-H,
50-H) ppm. 13C NMR (126 MHz, 25 °C, CDCl3): d = 21.48 (–
CH(OH)CH3), 63.78 (–CH(OH)CH3), 108.08 (3-C), 109.70 (4-C),
123.82 (30-C, 50-C), 124.33 (20-C, 60-C), 136.35 (10-C), 146.35 (40-C),
151.02 (5-C), 159.56 (2-C) ppm. MS(EI+): m/z (rel. int.) = 233 (45)
[M+], 218 (100), 172 (36), 160 (6), 150 (6), 115 (22), 83 (10),
76(7); HRMS: M+ found (M+ calculated for C12H11NO4): 233.06880
(233.06881).

P. Hara et al. / Tetrahedron: Asymmetry 24 (2013) 142–150
149
4.4.2. (R)-3b and (S)-2b
4.4.5. (R)-3f and (S)-2f
As above, the kinetic resolution of rac-2b (0.50 g, 2.62 mmol)
As above, the kinetic resolution of rac-2f (500 mg, 2.14 mmol)
yielded golden brown semisolids (S)-2f [140 mg, 0.597 mmol,
with CAL-B (1 mg mLꢀ1) afforded a light brown semisolid (S)-2b
[0.23 g, 1.19 mmol, 45%, 95% ee, ½a D25
ꢁ
¼ ꢀ20:1 (c 1, CHCl3)] and a
28%, 89% ee), ½a 2D5
¼ þ2:3 (c 1, CHCl3)] and (R)-3f [161 mg,
ꢁ
yellow-brown oily acetate (R)-3b [0.27 g, 1.17 mmol, 45%, 94% ee,
0.585 mmol, 27%, 90% ee, ½a D25
¼ þ225 (c 1, CHCl3)] after purifica-
ꢁ
½
a 2D5
ꢁ
¼ þ159 (c 1, CHCl3)] after purification by column chromatog-
tion by column chromatography (22:3 petroleum ether/acetone).
For (R)-3f: 1H NMR (500 MHz, CDCl3): d = 1.66 (d, J = 6.7 Hz, 3H, –
CH(OCOCH3)CH3), 2.10 (s, 3H, –CH(OCOCH3)CH3), 6.02 (q,
J = 6.7 Hz, 1H, –CH(OCOCH3), 6.48 (d, J = 3.4 Hz, 1H, 3-H), 6.83 (d,
J = 3.4 Hz, 1H, 4-H), 7.79 (d, J = 8.9 Hz, 2H, 20-H, 60-H), 8.25 (d,
J = 8.9 Hz, 2H, 30-H, 50-H) ppm. 13C NMR (126 MHz, CDCl3):
d = 18.29 (–CH(OCOCH3)CH3), 21.23 (–CH(OCOCH3)CH3), 64.98 (–
CH(OCOCH3)CH3), 109.55 (3-C), 110.52 (4-C), 124.02 (30-C, 50-C),
124.32 (20-C, 60-C), 136.19 (10-C), 146.53 (40-C), 151.44 (5-C),
155.28 (2-C), 170.20 (–CH(OCOCH3)CH3) ppm. MS(EI+): m/z (rel.
int.) = 275 (42), [M]+, 233 (21), 216 (100), 185 (8), 170 (36), 141
(13), 115 (14); HRMS: M+ found (M+ calculated for C14H13O5N)
275.07890 (275.07937). Spectroscopic data for (S)-2f were identi-
cal to the data for rac-2f.
raphy (dichloromethane). For (R)-3b: 1H NMR (500 MHz, CDCl3):
d = 1.56 (d, J = 6.7 Hz, 3H, –CH(OCOCH3)CH3), 2.06 (s, 3H, –CH(O-
COCH3)CH3), 5.88 (q, J = 6.7 Hz, 1H, –CH(OCOCH3)CH3), 6.26 (d,
J = 3.2 Hz, 1H, 4-H), 6.30 (d, J = 3.1 Hz, 3-H) ppm. 13C NMR
(126 MHz, CDCl3): d = 18.03 (–CH(OCOCH3)CH3), 21.21 (–CH(O-
COCH3)CH3), 64.76 (–CH(OCOCH3)CH3), 110.64 (3-C), 111.94
(4-C), 121.97 (5-C), 155.35 (2-C), 170.16 (–CH(OCOCH3)CH3) ppm.
MS(EI+): m/z (rel. int.) = 234 (18) [M81Br+], 232 (18) [M79Br+], 192
(10), 190 (10), 175 (41), 173 (34), 145 (6), 119 (2), 111 (4), 93
(62), 82 (3), 65 (28); HRMS M+ found (M+ calculated for C8H9O3Br)
231.97360 (231.97351). Spectroscopic data for (S)-2b were identi-
cal to the data of rac-2b.
4.4.3. (R)-3c, (S)-2c and (R)-2c
As above, the kinetic resolution of rac-2c (1.0 g, 4.49 mmol)
4.4.6. (R)-3g and (S)-2g
with CAL-B (1 mg mLꢀ1
) yielded an amber semisolid (S)-2c
As above, the kinetic resolution of rac-2g (500 mg, 2.02 mmol)
[0.48 g, 2.17 mmol, 48%, 98% ee, ½a D25
¼ ꢀ1:6 (c 1, CHCl3)] and a yel-
ꢁ
with CAL-B (5 mg mLꢀ1
)
yielded
a
yellow semisolid (S)-2g
¼ ꢀ2:3 (c 1, CHCl3)] and a
low semisolid (R)-3c [0.58 g, 2.19 mmol, 49%, 90% ee, ½a D25
¼ þ146
ꢁ
[232 mg, 0.94 mmol, 46%, 93% ee, ½a D25
ꢁ
(c 1, CHCl3)] after purification by column chromatography (dichlo-
romethane). The enzymatic methanolysis of the enantiomerically
enriched (R)-3c (0.50 g, 1.89 mmol) under the conditions (stopped
at 24 h) used for (R)-3a above produced an amber semisolid (R)-2c
yellow semisolid (R)-3g [272 mg, 0.94 mmol, 46%, 88% ee,
½
a 2D5
ꢁ
¼ þ198 (c 1, CHCl3)] after purification by column chromatog-
raphy (dichloromethane). For (R)-3g: 1H NMR (500 MHz, CDCl3):
d = 1.66 (d, J = 6.7 Hz, 3H, –CH(OCOCH3)CH3), 2.10 (s, 3H, –CH(O-
COCH3)CH3), 2.61 (s, 3H, –CH3), 6.03 (q, J = 6.7 Hz, 1H, –CH(O-
COCH3)CH3), 6.50 (d, J = 3.3 Hz, 1H, 3-H), 6.73 (d, J = 3.3 Hz, 1H,
4-H), 7.87 (d, J = 8.6 Hz, 1H, 60-H), 8.09 (d, J = 8.7 Hz, 1H, 50-H),
8.12 (s, 1H, 30-H) ppm. 13C NMR (126 MHz, CDCl3): d = 18.30
(–CH(OCOCH3)CH3), 21.23 (–CH3), 22.32 (–CH(OCOCH3)CH3),
64.97 (–CH(OCOCH3)CH3), 110.16 (3-C), 112.50 (4-C), 121.31 (50-
C), 126.34 (30-C), 127.14 (60-C), 135.44 (20-C), 135.55 (10-C),
146.20 (40-C), 151.15 (5-C), 154.52 (2-C), 170.22 (–CH(O-
COCH3)CH3) ppm. MS(EI+): m/z (rel. int.) = 289 (30), [M]+, 277
(5), 247 (10), 230 (100), 214 (5), 201 (5), 184 (23), 155 (8), 141
(8), 128 (16), 115 (12), 102 (4); HRMS: M+ found (M+ calculated
for C15H15NO5): 289.09510 (289.09502). Spectroscopic data for
(S)-2g were identical to the data for rac-2g.
[0.32 g, 1.43 mmol, 76%, 99% ee, ½a D25
¼ þ1:6 (c 1, CHCl3)] after col-
ꢁ
umn chromatography (dichloromethane). For (R)-3c: 1H NMR
(500 MHz, CDCl3): d = 1.64 (d, J = 6.7 Hz, 3H, –CH(OCOCH3)CH3),
2.08 (s, 3H, –CH(OCOCH3)CH3), 6.02 (q, J = 6.7 Hz, 1H, –CH(O-
COCH3)CH3), 6.45 (d, J = 3.4 Hz, 1H, 3-H), 7.07 (d, J = 3.4 Hz, 1H,
4-H), 7.20 (ddd, J = 1.6 Hz, J = 7.6 Hz, J = 7.7 Hz, 1H, 40-H), 7.31
(ddd, J = 1.2 Hz, J = 6.6 Hz, J = 7.5 Hz, 1H, 50-H), 7.43 (dd,
J = 1.1 Hz, J = 8.0 Hz, 1H, 30-H), 7.85 (dd, J = 1.6 Hz, J = 7.9 Hz, 1H,
60-H) ppm. 13C NMR (126 MHz, CDCl3): d = 18.28 (–CH(O-
COCH3)CH3), 21.28 (–CH(OCOCH3)CH3), 65.11 (–CH(OCOCH3)CH3),
109.84 (3-C), 111.52 (4-C), 126.86 (50-C), 127.97 (60-C), 128.18
(300-C), 128.97 (20-C), 130.18 (100-C), 130.71 (40-C), 149.96 (2-C),
152.99 (5-C), 170.29 (–CH(OCOCH3)CH3) ppm. MS(EI+): m/z (rel.
int.) = 264 (32) [M+], 220 (33), 205 (100), 194 (9), 176 (4), 169
(9), 149 (31); HRMS: M+ found (M+ calculated for C14H13O3Cl):
264.05520 (264.05532). Spectroscopic data for (R)- and (S)-2c were
identical to the data for rac-2c.
4.4.7. (R)-3h and (S)-2h
As above, the kinetic resolution of rac-2h (500 mg, 1.92 mmol)
with CAL-B (5 mg mLꢀ1) yielded a slightly yellow oil (S)-2h
[239 mg, 0.92 mmol, 48%, 93% ee, ½a D25
¼ þ5:8 (c 1, CHCl3)] and a
ꢁ
4.4.4. (R)-3d and (S)-2d
white semisolid (R)-3h [250 mg, 0.83 mmol, 43%, 86% ee,
As above, the kinetic resolution of rac-2d (300 mg, 1.12 mmol)
½
a 2D5
ꢁ
¼ þ178 (c 1, CHCl3)] after purification by column chromatog-
with CAL-B (5 mg mLꢀ1) yielded a light golden brown semisolid
raphy (dichloromethane with 1% TEA). For (R)-3h: 1H NMR
(500 MHz, CDCl3): d = 1.41 (t, J = 7.1 Hz, 3H, –CO2CH2CH3), 1.65
(d, J = 6.7 Hz, 3H, –CH(OCOCH3)CH3), 2.09 (s, 3H, –CH(O-
COCH3)CH3), 4.39 (q, J = 7.1 Hz, 2H, –CO2CH2CH3), 6.01 (q,
J = 6.7 Hz, 1H, –CH(OCOCH3)CH3), 6.44 (d, J = 3.4 Hz, 1H, 3-H),
6.73 (d, J = 3.4 Hz, 1H, 4-H), 7.71 (d, J = 8.5 Hz, 2H, 30-H, 50-H),
8.05 (d, J = 8.5 Hz, 2H, 20-H, 60-H) ppm. 13C NMR (126 MHz, CDCl3):
d = 14.36 (–CO2CH2CH3), 18.28 (–CH(OCOCH3)CH3), 21.28 (–CH(O-
COCH3)CH3), 61.01 (–CO2CH2CH3), 65.14 (–CH(OCOCH3)CH3),
107.74 (3-C), 110.23 (4-C), 123.44 (20-C, 60-C), 129.09 (10-C),
130.05 (30-C, 50-C), 134.41 (40-C), 152.74 (5-C), 154.06 (2-C),
166.32 (–CO2CH2CH3), 170.28 (–CH(OCOCH3)CH3) ppm. MS(EI+):
m/z (rel. int.) = 302 (60), [M]+, 257 (13), 243 (100), 215 (20), 197
(9), 170 (13), 141 (6), 128 (3), 115 (8); HRMS: M+ found (M+ calcu-
lated for C17H18O5) 302.11550 (302.11542). Spectroscopic data for
(S)-2h were identical to the data for rac-2h.
(S)-2d [62 mg, 0.232 mmol, 21%, 91% ee, ½a D25
¼ þ5:0 (c 1, CHCl3)]
ꢁ
and a light golden brown oil (R)-3d [(192 mg, 0.621 mmol, 55%,
75% ee, ½a 2D5
¼ þ54 (c 1, CHCl3)] after purification by column chro-
ꢁ
matography (dichloromethane with 0.5% TEA). For (R)-3d: 1H NMR
(500 MHz, CDCl3): d = 1.62 (d, J = 6.7 Hz, 3H, –CH(OCOCH3)CH3),
2.07 (s, 3H, –CH(OCOCH3)CH3), 5.96 (q, J = 6.7 Hz, 1 H, –CH(O-
COCH3)CH3), 6.39 (d, J = 3.4 Hz, 1H, 4-H), 6.58 (d, J = 3.4, 1H, 3-H),
7.48 (m, 4H, 20-H, 30-H, 50-H, 60-H) ppm. 13C NMR (126 MHz, CDCl3):
d = 18.24 (–CH(OCOCH3)CH3), 21.28 (–CH(OCOCH3)CH3), 65.15 (–
CH(OCOCH3)CH3), 106.10 (3-C), 110.06 (4-C), 121.31 (40-C),
125.36 (20-C, 60-C), 129.52 (10-C), 131.80 (30-C, 50-C), 152.74 (2-C),
153.33 (5-C), 170.28 (–CH(OCOCH3)CH3) ppm. MS(EI+): m/z (rel.
int.) 310 (44) [MBr-81]+, 308 (47) [MBr-79]+, 266 (3), 250 (100),
170 (9), 141 (12), 115 (9), 76 (3), 43 (23); HRMS: M+ found (M+ cal-
culated for C14H13O3Br): 308.00460 (308.00481). Spectroscopic
data for (S)-2d were identical to the data for rac-2d.

150
P. Hara et al. / Tetrahedron: Asymmetry 24 (2013) 142–150
Asymmetry 2011, 22, 315–322; (e) Turcu, M. C.; Perkiö, P.; Kanerva, L. T.
Acknowledgements
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1991, 56, 2656–2665.
8. Kaminska, J.; Górnicka, I.; Sikora, M.; Góra, J. Tetrahedron: Asymmetry 1996, 7,
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A grant from the Romanian National Authority for Scientific
Research, CNDI-UEFISCDI, project PN-II-PT-PCCA-2011-3.1-1268
(M. Tosa) is acknowledged.
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