Lipases in the regioselective preparation of glyceric acid esters of methyl glycosides

Article abstract of DOI:10.1002/ejoc.201300450

The lipase-catalyzed regioselective acylation of methyl α-D-galacto-, -gluco- and -mannopyranosides with isopropylidene-protected (R)- and (S)-glyceric acid methyl esters in organic solvents is presented. Factors affecting the formation of the 6-O-mono- and 2,6-O-diacylated products are examined and preparative-scale reactions are detailed. In addition, studies on partially protected methyl α-D-galactopyranosides are presented, with the 3,4-O-isopropylidene protected compound leading exclusively to the formation of the 6-O-monoacylated product even at high substrate concentrations. Application of acidic resin in methanol allows removal of the isopropylidene protection groups without disturbing the ester bond at C-6. Copyright

Full text of DOI:10.1002/ejoc.201300450

FULL PAPER
DOI: 10.1002/ejoc.201300450
Lipases in the Regioselective Preparation of Glyceric Acid Esters of Methyl
Glycosides
Riku Sundell[a] and Liisa T. Kanerva*[a]
Keywords: Enzyme catalysis / Biocatalysis / Acylation / Glycosides / Regioselectivity / Protecting groups
The lipase-catalyzed regioselective acylation of methyl α-
D
-
anosides are presented, with the 3,4-O-isopropylidene pro-
tected compound leading exclusively to the formation of the
6-O-monoacylated product even at high substrate concentra-
tions. Application of acidic resin in methanol allows removal
of the isopropylidene protection groups without disturbing
the ester bond at C-6.
galacto-, -gluco- and -mannopyranosides with isopropylid-
ene-protected (R)- and (S)-glyceric acid methyl esters in or-
ganic solvents is presented. Factors affecting the formation
of the 6-O-mono- and 2,6-O-diacylated products are exam-
ined and preparative-scale reactions are detailed. In ad-
dition, studies on partially protected methyl α-D-galactopyr-
with high stability and good commercial availability in both
free and immobilized forms, are especially attractive serine
hydrolases, and a number of reviews describing research on
the lipase-catalyzed acylation of carbohydrates, glycosides,
and protected derivatives are available.[7] We previously re-
ported the chemoenzymatic transformation of the aglycon
and C-6 positions of protected glycopyranosides into syn-
thetically potential cyanohydrin esters by using lipase catal-
ysis to control stereochemistry.[8] Herein, we focus on li-
pase-based regioselective modifications of unprotected
methyl glycopyranosides.
Our primary aim was the preparation of novel sugar es-
ters with a glyceric acid moiety (an α,β-dihydroxy acid) re-
gioselectively attached to methyl α-d-galacto-, gluco-, and
mannopyranosides 1a–c (Scheme 1). The glyceric acid moi-
ety was introduced through the reaction of 1a–c with
methyl (R)- and (S)-2,2-dimethyl-1,3-dioxolane-4-carboxyl-
ates (5, containing isopropylidene protection) in the pres-
ence of lipases. Such an extension of simple glycopyranos-
ides to obtain elongated structures mimicking naturally oc-
curring, biologically important sialic acids and similar
structures would further enhance the hydrophilic character
of carbohydrates. The new moiety was introduced as 5
(Step a) rather than as glyceric acid itself or as its ester,
to prevent dimerization/polymerization of the unprotected
compound by lipase catalysis. Finally, removal of the iso-
propylidene protection of 2 was examined (Step b) to ob-
tain the target products 4a–c. Galactopyranoside 1a was
used as a model compound for optimization. Moreover, the
lipase-catalyzed acylation of isopropylidene and acetyl pro-
tected galactosides (7a, 8a and 10a, see below) with (R)-5
was investigated. Products 2–4 have potential for further
synthetic development. The enantiomers of glyceric acid
and the protected form 5 are both also valuable building
blocks of pharmaceutically active compounds.[9]
Introduction
Carbohydrates and their conjugates form a majority of
natural biomolecules, acting in a number of diverse roles
from structural to cell-surface functions and to energy sup-
plies for living organisms.[1] The wide occurrence of sugars
in nature has prompted their use as bioactive, biocompat-
ible, and biodegradable compounds in cosmetic, pharma-
ceutical, food, and agricultural applications.[2] For example,
acidic sophorolipids and the corresponding galactopyr-
anose fatty acid esters can be used as nontoxic biosurfac-
tants with interesting applications in nanotechnology,
whereas glucose derivatives grafted on α-monohydroxy ac-
ids (glycolic, lactic and malic acids) have been prepared by
the cosmetic industry because of their high hydrating poten-
tial.[2a,3,4]
Protective group chemistry is an integral part of modern
organic and carbohydrate chemistry.[5] On the other hand,
the use of protective groups can be seen as a part of synthe-
sis that should be downplayed in terms of sustainable syn-
thesis. Sugars, as polyols, represent a good landscape and
challenge for regioselective enzymatic acylation with mini-
mal use of protective groups. Extensive development of en-
zyme-based approaches for the modification of carbo-
hydrates and their derivatives has taken place since
Klibanov et al. published their work on the use of serine
hydrolases as regioselective acylation catalysts in organic
solvents in the 1980s.[6] In this respect, lipases (E.C. 3.1.1.3),
[a] Institute of Biomedicine/Department of Pharmacology, Drug
Development and Therapeutics/Laboratory of Synthetic Drug
Chemistry, University of Turku,
20014 Turku, Finland
E-mail: lkanerva@utu.fi
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300450.
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Results and Discussion
Optimization of Reaction Conditions
Polar protic solvents such as water dissolve glycopyrano-
sides, whereas their solubility in many organic solvents is
known to be restricted. Accordingly, solubility was first ex-
amined by adding 1a (0.05 m) into ten organic solvents at
47 °C and comparing the solubility to that in water (100%;
see results in the Supporting Information). An HPLC/CAD
method was used in analysis, revealing good solubility in
organic solvents other than tert-butyl methyl ether (TBME,
4% dissolved) and diisopropyl ether (DIPE, Ͻ1% dis-
solved). Solubility differences in organic solvents were dra-
matic when 1a was added to give 0.51 m solutions with full
solubility. Finally, 2-methyl-2-butanol (tert-amyl alcohol,
tAmOH) was chosen as a solvent for further studies be-
cause many lipases may be inactivated in pyridine and N,N-
dimethylformamide (DMF), which were the best dissolving
solvents. For instance, Lipozyme TL IM was practically in-
active for the acylation of 1a with (R)-5 in these solvents.
On the other hand, the reaction in tetrahydrofuran (THF)
and 1,4-dioxane was slow compared with that in tAmOH.
It has been previously shown that the solubility of sucrose
and α-d-glucose in tAmOH noticeably increased when the
corresponding 6-O-laurate ester was present in the sys-
tem.[12] Accordingly, the results of our solubility studies evi-
dently provide only a limited view on the real situation dur-
ing the lipase-catalyzed acylation conditions. It is worth em-
phasizing that products 2 and 3 should be more soluble
than the corresponding starting material 1 in the organic
solvents examined and that the chosen low concentration
of 1 (0.05 m) ensures good solubility for all compounds
studied.
Lipase screening for the acylation of 1a–c (0.05 m) with
(R)- and (S)-5 (5 equiv.) was first performed in the presence
of commercial lipase preparations (15 mgmL–1) in tAmOH.
Due to slow initial solubility of the glycopyranosides at
room temperature, the acylations were performed at 47 °C.
Samples taken after 24 h from the reaction mixtures re-
vealed that the lipases all regioselectively produced mono-
substituted 2a–c followed by the slow formation of diacyl-
ated 3a and 3b (Table 1). Mannopyranoside 1c, with the
axial HO at C-2, exclusively gave the monoacylated product
2c, in accordance with earlier observations that lipases pref-
erentially acylate equatorial hydroxyl groups of cyclohex-
anols.[13] Other acylation products were not detected in the
HPLC chromatograms over the course of 2–3 days. Conver-
sions after 24 h (reactivity) and product distributions
strongly depended on the lipase preparation and on the
structure of the glycopyranoside 1. The lipases all poorly
discriminated between (R)- and (S)-5, the R-enantiomer be-
ing slightly more favorable, except with lipase RM IM for
which the S-isomer reacted faster (entries 9 and 10). The
poor enantioselectivity observed is in accordance with that
previously reported for the hydrolysis of racemic 2,2-di-
methyl-1,3-dioxolane-4-carboxylate with Candida cylindra-
cea and porcine pancreatic lipases.[9c] Lipozyme TL IM (en-
Scheme 1. Synthetic strategy: (a) lipase-catalyzed regioselective
acylation of 1 with (R)- or (S)-5 and (b) deprotection of the glyceric
acid moiety.
The importance of water activity for lipase catalysis in
organic solvents has been pointed out, and the subject has
been thoroughly discussed and accepted.[10] Another side of
the story, the involvement of so-called residual water (the
water adsorbed in the seemingly dry lipase preparation),
even when there is no added water in the system, is often
forgotten. The residual water may be enough to cause
mechanism-based hydrolytic side reactions for ester com-
pounds, as previously discussed.[11] Side reaction possibil-
ities for the monoacylation of 1 with (R)-5 as an acyl donor
are best considered in terms of the two-step reaction
mechanism of serine hydrolases with the involvement of an
acyl-enzyme intermediate (acyl-enz; Scheme 2). In principle,
the intermediate can react with all possible nucleophiles
bound at the active site. Thus, in addition to the desired
acylation of 1 (Steps a and aЈ), the product 2 (Steps b and
bЈ) and the acyl donor (R)-5 (Steps a and bЈ) may both
become hydrolyzed by the residual water, and the acyl-OH
can be esterified with 1a–c (Steps c and aЈ). Moreover, the
acylation of 1 with (R)-5 is a typical reaction leading up to
an equilibrium, the position of which depends on the molar
ratio [acyl donor]/[substrate] according to the mass transfer
rule. To minimize such side reactions, all lipase-catalyzed
acylations in this work were performed in the presence of
molecular sieves (50 mgmL–1, 3 Å) using the acyl donor in
excess over 1.
Scheme 2. Competitive mechanistic pathways for the formation of
2a–c in the acylation of 1a–c with (R)-5 and a lipase enzyme (enz-
OH).
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Glyceric Acid Esters of Methyl Glycosides
tries 11 and 14) and CAL-B (entries 1 and 2) were shown mation of 3a (the corresponding open signs). However, the
to be applicable, whereas CAL-A (entries 3 and 4) was un- formation of 3a cannot alone explain the extent of the de-
practical with all three glycopyranosides. Lipase prepara- crease observed in the formation of 2a at longer reaction
tions PS-D and PS-C II (entries 5–8) and lipase RM IM times and the fact that the maximal yields were obtained at
(entries 9 and 10) gave lower conversions than Lipozyme somewhat lower conversions with the highest enzyme con-
TL IM and CAL-B catalysts. However, reactivity compari- tent. Moreover, the amount of 3a tended to decrease with
sons should be considered with care because the absolute time. These observations support the idea of the involve-
lipase amount of each commercial preparation was un- ment of hydrolytic side reactions. To estimate the potential
known, and the formation of the diacylated product 3 evi- for side reactions, 2a (0.05 m) was subjected to the enzy-
dently depends on the formation of 2 and mechanism-based matic reaction conditions in the absence of acyl donor (R)-
side reactions might affect the final outcomes. Lipozyme 5, keeping in mind that the extent of hydrolyses may be
TL IM [61, 80 and 70% conversions with (R)-5 and 61, 68 overestimated due to the lack of competition between acyl-
and 62% conversions with (S)-5 for 1a–c, respectively, en- ation and hydrolysis. The results indicated that 68% (Lipo-
tries 11 and 14] was applicable to all glycopyranosides and zyme TL IM, 30 mgmL–1) and 49% (lipase PS-D,
was therefore chosen for further optimization. Because no 30 mgmL–1) of 2a had hydrolyzed by the residual water in
product formation could be observed when 1a (0.05 m) was 24 h (Scheme 2, Steps b and bЈ). At the same time, the for-
subjected to the acylation conditions without enzyme, it mation of diacylated 3a (1% with Lipozyme TL IM and
was clear that the presence of molecular sieves did not cause 5% with lipase PS-D) was detected, indicating that (R)-2,2-
any background reaction.
dimethyl-1,3-dioxolane-4-carboxylic acid (acyl-OH), the hy-
drolysis product of 2a, had enzymatically esterified the 2-
position of 2a. Chemical C-6 to C-2 acyl migration (nonen-
zymatic)[14] in the formation of 3a can be ruled out because
2a (0.05 m) remained intact when shaken under the same
conditions without lipase. On the other hand, when diacyl-
ated 3a (0.05 m) was subjected to the reaction conditions in
the absence of (R)-5, Lipozyme TL IM released 23% of 2a
(not a monoacylated product with the acyl group at C-2) in
24 h, the formation of 1a remaining negligible (ca. 1%).
Since it is reasonable to assume that the ester group is hy-
drolyzed at C-6 rather than at C-2, this might indicate rela-
tively fast hydrolysis of the 2- and 6-positions of 3a followed
by esterification of 1a (Steps c and aЈ) or fast C-2 to C-6
acyl migration.
Table 1. Lipase screening for the acylation of 1a–c (0.05 m) with
(R)- or (S)-5 (5 equiv.) and a lipase preparation (15 mgmL–1) in
tAmOH (reaction time 24 h).
Entry
Lipase
(R)/(S)-5
2 or 3 [%][a]
2a
3a
2b
3b
2c
1
2
3
4
5
6
7
8
CAL-B
CAL-B
CAL-A
CAL-A
PS-D
R
S
R
S
R
S
R
S
R
S
R
R
R
S
51
24
3
6
1
Ͻ1
–
4
Ͻ1
4
Ͻ1
Ͻ1
Ͻ1
7
5
1
71
55
Ͻ1
Ͻ1
53
23
42
21
10
24
75
53
–
12
1
–
–
17
1
9
1
–
Ͻ1
5
71
41
Ͻ1
–
43
17
36
11
13
35
70
47
–
Ͻ1
48
38
45
33
26
38
54
51
40
52
40
PS-D
PS-C II
PS-C II
RM IM
RM IM
TL IM
TL IM[b]
TL IM[c]
TL IM
TL IM[b]
9
10
11
12
13
14
15
3
–
3
–
9
3
65
–
62
–
S
[a] ([2] or [3])/[1]0. [b] 5 (3 equiv.) and Lipozyme TL IM
(30 mgmL–1). [c] Methyl β-d-galactopyranoside (1d; 0.05 m) in
place of 1a, 5 (3 equiv.), and Lipozyme TL IM (30 mgmL–1).
Figure 1. Effect of lipase TL IM content on the formation of 2a
(filled signs) and 3a (open signs). (a) Reagents and conditions: 1a
(0.05 m), (R)-5 (5 equiv.), Lipozyme TL IM (5–50 mgmL–1),
tAmOH.
The acylation of 1a (0.05 m) with (R)-5 (5 equiv.) in the
presence of various amounts (5–50 mgmL–1) of Lipozyme
TL IM was next investigated in tAmOH. The reaction pro-
The acylation of 1a (0.05 m) with varying amounts of
gression curves (Figure 1) indicate maximal yields (50– (R)-5 (1.2–7.0 equiv.) and Lipozyme TL IM (30 mgmL–1)
55%) for the formation of 2a (filled signs) within 2–24 h, in tAmOH gave progression curves going through the maxi-
with the maximum shifting to longer reaction times with mal yields for 2a in 2 h and for 3a in 24 h, with the yields
lowering catalyst contents. It is reasonable to assume that increasing steadily with increasing acyl donor concentration
the amount of 2a decreased as it was consumed in the for- (Figure 2). Clearly, it would be optimal to choose the high-
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est possible acyl donor content with a short reaction time (40–57%) and 3a and 3b (8–11%) at isolated yields corre-
when the target is the regioselective preparation of 2a. sponding relatively well to those observed in the acylation
However, for reasons of economy [(R)- and (S)-5 are rela- mixture at the point at which the enzyme was filtered off to
tive expensive], we continued the work using only 3 equiv. stop the reaction.
of the acyl donor.
Utilizing Partially Protected Galactopyranosides
One of the major issues in the above acylation reactions
is the low solubility of 1a–c in many anhydrous organic sol-
vents and, accordingly, the need to use low sugar concentra-
tions. Because protected sugars have improved solubility
they can be seen as preferred substrates (although this in-
cludes extra protection/deprotection steps) for regioselective
acylations at a sugar skeleton. Thus, 1a was transformed
into 7a (2,3,4-tri-O-acetyl protection), 8a (3,4-O-isoprop-
ylidene protection), and 10a (2-O-acetyl-3,4-O-isopropylid-
ene protection) by well-known methods,[8b,15] and their
acylation (0.05 m) was studied with (R)-5 (3 equiv.) and a
Figure 2. Effect of acyl donor (R)-5 concentration on the formation
lipase preparation (30 mgmL–1) in the formation of the cor-
of 2a (filled signs) and 3a (open signs). Reagents and conditions:
responding esters 11–13a (Scheme 3). The results after 24 h
for the acylation of 10a remained poor (Table 3, entries 11
and 12), and because the protection scheme is quite labori-
ous, the approach was not viable. With Lipozyme TL IM
in tAmOH (entries 1 and 5), the acylations of 7a (product
yield 41%) and 8a (product yield 63%) gave comparable
results to those observed for unprotected 1a under the same
conditions (Table 2, entry 1), indicating that the protective
step is not necessary under the reaction conditions. How-
ever, the reaction conditions and substrate structure are
pivotal to the outcome of enzymatic reactions. Accordingly,
a range of lipases and solvents were tested, indicating that
lipase PS-D with 7a in DIPE (Table 3, entry 4), and CAL-
B with 8a in THF (entries 8–10) allowed improvements in
the corresponding product yields 11a (53%, entry 4) and
12a (close to 80%, entries 8–10) compared with those in
tAmOH. Even better, the reaction of 8a with (R)-5 and
CAL-B in THF allowed the substrate concentration to be
increased from 0.05 to 0.5 m and a reduction in the acyl
donor content to just 2 equiv. without an effect on the out-
come of the acylation (entry 8 vs. 10). These two facts make
the acylation of 8a beneficial over the acylation of unpro-
tected 1a with Lipozyme TL IM in tAmOH. An additional
benefit comes from the fact that isopropylidene protection
of the sugar skeleton can be removed at the same time as
deprotection of the glyceric acid moiety. Accordingly, 8a
was subjected to a preparative-scale acylation with (R)-5
(2 equiv.) in the presence of CAL-B (30 mgmL–1) in THF.
The reactions with 0.05 m 8a went to completion in 72 h
and the product was isolated at a theoretical yield. How-
ever, the reaction with 0.5 m 8a only slowly increased from
77% conversion in 24 h to 90% conversion at 72 h. Whereas
the protective scheme for 1a in the formation of 8a is a
simple one-step process, more ingenuity and synthetic steps
are necessary to successfully apply the protection to 1b and
1c. For instance, the formation of a 2,3-O-isopropylidene
protected 1c required two steps in a recent example.[16]
1a (0.05 m), (R)-5 (1.2–7 equiv.), Lipozyme TL IM (30 mgmL–1),
tAmOH.
Finally, the reactions were carried out on preparative-
scale by introducing one of 1a–c (0.05 m), (R)-5 (3 equiv.),
Lipozyme TL IM (30 mgmL–1), and molecular sieves
(50 mgmL–1, 3 Å) in dried tAmOH at 47 °C (Table 2). To
also prepare and characterize the diacylated products 3a
and 3b, reaction times were extended over that optimal for
the preparation of 2a and 2b. The reactions were stopped
at 65 (1a), 70 (1b), and 60% (1c) conversions yielding 2a–c
Table 2. Preparative-scale acylation of
1 (0.05 m) with (R)-5
(3 equiv.) and Lipozyme TL IM (30 mgmL–1) in tAmOH and sub-
sequent deprotection with acidic resin DR2030.
Entry
Substrate
Time [h]
Yield [%]
2[a]
3[a]
4[b]
1
2
3
4
1a
48
48
24
72
42
57
40
Ͼ99
8
11
–
93
81
83
76
1b
1c
8a[c]
–
[a] Isolated yield from 1. [b] Isolated yield from 2. [c] Reaction con-
ditions: 8a (0.05 m), (R)-5 (2 equiv.), CAL-B (30 mgmL–1), THF.
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Glyceric Acid Esters of Methyl Glycosides
Scheme 3. Protection of methyl α-d-galactopyranoside (1a) and acylation of 7, 8 and 10a with (R)-5.
Table 3. Lipase-catalyzed acylation of protected methyl galactopyr- FeCl3·6H2O in dichloromethane/acetone (4:1),[23] and 2,3-
anosides 7–10a with (R)-5 (reaction time 24 h).
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in aceto-
nitrile/water (9:1)[24] were tested with 2a without complete
success either due to concomitant splitting of the ester
bond, solubility issues, or difficulties in product isolation
and purification. Finally, a simple method relying on the
use of an acidic resin [Dowex DR2030 (H+-form)][25] turned
out to be successful. Thus, simply mixing one of the 6-O-
acylated products 2a–c in methanol together with the resin
and stirring the mixture overnight at room temperature, fol-
lowed by filtration and solvent evaporation, allowed the
preparation of the corresponding 4a–c at 81–93% isolated
yields (Table 2, entries 1–3). When the same deprotection
procedure was applied to 12a, both of the isopropylidene
protecting groups were removed in a single step, delivering
4a in 76% isolated yield (Table 2, entry 4).
Entry Substrate
Lipase
(R)-5
[equiv.]
Solvent
Yield[a]
[%]
[m]
[mgmL–1]
1
2
3
4
5
6
7
8
7a [0.05]
7a [0.05]
7a [0.05]
7a [0.05]
8a [0.05]
8a [0.05]
8a [0.05]
8a [0.05]
8a [0.1]
TL IM [30]
TL IM [30]
CAL-B [30]
PS-D [30]
TL IM [30]
TL IM [15]
CAL-B [15]
CAL-B [30]
CAL-B [30]
CAL-B [30]
TL IM [30]
TL IM [30]
3
3
3
3
3
5
5
3
2
2
3
3
tAmOH
DIPE
DIPE
DIPE
tAmOH
THF
THF
THF
THF
THF
41
44
16
53
63
2
23
79
79
77
29
14
Conclusions
9
10
11
12
8a [0.5]
10a [0.05]
10a [0.05]
A method that can be used to generate novel sugar esters
with a glyceric acid moiety regioselectively attached to
methyl α-d-galacto-, gluco- and mannopyranosides 1a–c is
described. As the main result, Lipozyme TL IM in tAmOH
for unprotected 1a–c, and CAL-B in THF for 3,4-O-iso-
propylidene-protected methyl α-d-galactopyranoside 8a,
are shown to lead to regioselective acylation at the primary
DIPE
tAmOH
[a] Relative yield according to the HPLC peak areas in the reaction
mixture.
Deprotection of 2a and 12a
Isopropylidene groups, which are frequently encountered alcohol functionality at C-6 with (R)-5. Acylation of 1a–c
to protect 1,2- and 1,3-diols, are typically deprotected under with both (R)- and (S)-5 is shown to proceed in very similar
acidic conditions. In the course of the work, a number of manner with negligible enantioselection. Although the ga-
methods such as 80% AcOH aq.,[17] CF3CO2H in dichloro- lacto-, gluco and mannopyranosides 1a–c mainly provide 6-
methane,[18] and para-toluenesulfonic acid in methanol,[19] O-acylated products (isolated yields 42, 57 and 40% for 2a,
catalytic I2 in methanol,[20] pyridinium para-toluenesulf- 2b and 2c, respectively), small amounts (5–10%) of 2,6-di-
onate in methanol,[21] SnCl2 in dichloromethane,[22] O-acylated 3a and 3b are also obtained. We show that a
Eur. J. Org. Chem. 2013, 4971–4978
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3a (53 mg, 0.12 mmol, 8%) as a clear transparent syrup. Rf (eth-
anol/ethyl acetate, 1:20) = 0.24 (2a), 0.68 (5a).
Compound 2a: M.p. 130–131 °C; [α]2D2 = +120.4 (c = 0.75, MeOH).
1H NMR (500 MHz, [D6]DMSO): δ = 1.30 (s, 3 H, CH3), 1.37 (s,
3 H, CH3), 3.25 (s, 3 H, OCH3), 3.52 (ddd, J3,4 = 3.3, J3,OH = 5.6,
simple one-step protection of 1a with 2,2-dimethoxyprop-
ane might be reasonable in terms of efficiency because 8a
is more soluble in organic solvents than 1a, allowing con-
siderable enhancement in substrate concentration and the
isolation of the new ester product 12a practically at theoret-
ical yields from the acylation mixture in THF. Because the
isopropylidene protecting groups of both 2a and 12a can be
removed in a single step by simple treatment with an acidic
resin, the protection step of 1a in the formation of 8a is the
only extra step in the process. Unfortunately, protection of
1b and 1c is more complex. On the other hand, the benefit
of using free methyl glycosides 1 as substrates for enzymatic
acylation is that the extra protection step can be avoided.
J2,3 = 10.1 Hz, 1 H, 3-H), 3.58 (ddd, J1,2 = 3.6, J2,OH = 6.5, J2,3
=
10.1 Hz, 1 H, 2-H), 3.69 (ddd, J4,5 = 1.2, J3,4 = 3.3, J4,OH = 4.5 Hz,
1 H, 4-H), 3.78 (dd, J = 5.3, J = 7.3 Hz, 1 H, 5-H), 4.01 (dd, J8,9a
= 4.5, J9a,9b = 8.6 Hz, 1 H, 9a-H), 4.17 (m, 2 H, 6a-H, 6b-H), 4.19
(dd, J8,9b = 7.3, J9a,9b = 8.6 Hz, 1 H, 9b-H), 4.56 (d, J1,2 = 3.6 Hz,
1 H, 1-H), 4.59 (d, J = 6.5 Hz, 1 H, 2-OH), 4.64 (d, J = 5.4 Hz, 1
H, 3-OH), 4.66 (dd, J8,9a = 4.3, J8,9b = 7.3 Hz, 1 H, 8-H), 4.69 (d,
J = 4.5 Hz, 1 H, 4-OH) ppm. 13C NMR (126 MHz, [D6]DMSO):
δ = 25.22 (CH3), 25.72 (CH3), 54.38 (OCH3), 64.55 (6-C), 66.53 (9-
C), 67.99 (2-C and 5-C), 68.87 (4-C), 69.04 (3-C), 73.29 (8-C),
100.00 (1-C), 110.19 [C(CH3)2], 170.76 (C=O) ppm. HRMS: m/z
calcd. for C13H22O9Na+ [M + Na+] 345.11560; found 345.11654.
Compound 3a: [α]2D2 = +73.9 (c = 0.75, MeOH). 1H NMR
(500 MHz, [D6]DMSO): δ = 1.30 (s, 3 H, CH3), 1.32 (s, 3 H, CH3),
1.38 (s, 3 H, CH3), 1.40 (s, 3 H, CH3), 3.26 (s, 3 H, OCH3), 3.78
(m, 2 H, 3-H, 4-H), 3.84 (t, J5,6 = 6.0 Hz, 1 H, 5-H), 3.97 (dd,
J8Ј,9aЈ = 4.6, J9aЈ,9bЈ = 8.5 Hz, 1 H, 9aЈ-H), 4.02 (dd, J8,9a = 4.5,
J9a,9b = 8.7 Hz, 1 H, 9a-H), 4.19 (dd, J8,9b = 7.4, J9a,9b = 8.6 Hz, 1
H, 9b-H), 4.20 (d, J5,6 = 5.9 Hz, 2 H, 6a-H, 6b-H), 4.22 (dd, J8Ј,9bЈ
Experimental Section
Materials and Methods: Reagents and solvents were purchased
from commercial sources, and solvents were dried with molecular
sieves (3 Å) prior to use. Burkholderia cepacia lipase preparations,
lipase PS-D (lipase adsorbed on Celite), and lipase PS-C II (immo-
bilized on ceramic beads), were products of Amano, Europe. Li-
pases from Candida antarctica (CAL-B; as the Novozym 435 prepa-
ration), Thermomyces lanuginosus (Lipozyme TL IM), and Rhizo-
mucor miehei (Lipozyme RM IM) were from Novozym. Candida
antarctica lipase A (CAL-A; as the NZL-101-IMB preparation)
was obtained from Codexis. HPLC analyses were performed with
a Waters 2690 Separations Module equipped with an ESA Corona
CAD detector (HPLC/CAD), and with an Agilent Technologies
ZORBAX Eclipse XDB-C8 (4.6 mmϫ150 mmϫ5 μm) column.
Eluents were gradient mixtures of water (A) and MeOH (B) as
specified in the Supporting Information. Analytical thin-layer
chromatography (TLC) was carried out with Merck Kieselgel
60F254 sheets, visualizing the spots by treatment with 5% ethanolic
phosphomolybdic acid solution or 10% sulfuric acid in ethanol and
heating. Chromatographic separations were performed by using
column chromatography on Kieselgel 60 (0.063–0.200 μm). 1H and
13C NMR spectra were recorded with a Bruker Avance 500 spec-
trometer at 298 K in [D6]DMSO or CDCl3 against an internal stan-
dard (TMS). Mass spectra were recorded with a Bruker Daltonics
micrOTOF-Q ESI-TOF in positive mode. Specific rotations were
measured with a Perkin–Elmer 341 Polarimeter (c = 0.75, MeOH,
22 °C) and values are presented as [α]2D2 = 10–1 degcm–2 g–1. Melting
points were measured with a Gallenkamp device and are uncor-
rected.
= 7.5, J9aЈ,9bЈ = 8.6 Hz, 1 H, 9bЈ-H), 4.66 (dd, J8,9a = 4.5, J8,9b
7.4 Hz, 1 H, 8-H), 4.67 (dd, J8Ј,9aЈ = 4.5, J8Ј,9bЈ = 7.4 Hz, 1 H, 8Ј-
H), 4.77 (d, J1,2 = 3.7 Hz, 1 H, 1-H), 4.89 (dd, J1,2 = 3.7, J2,3
=
=
9.8 Hz, 1 H, 2-H), 5.04 (d, J3,OH = 6.1 Hz, 1 H, 3-OH), 5.08 (dd,
J4,OH = 4.2 Hz, 1 H, 4-OH) ppm. 13C NMR (126 MHz, [D6]-
DMSO): δ = 25.21 (CH3), 25.28 (CH3), 25.64 (CH3), 25.72 (CH3),
54.52 (OCH3), 64.27 (6-C), 66.35 (3-C or 4-C), 66.52 (9-C or 9Ј-
C), 66.62 (9-C or 9Ј-C), 68.09 (5-C), 68.84 (3-C or 4-C), 71.33 (2-
C), 73.11 (8Ј-C), 73.27 (8-C), 96.39 (1-C), 110.21 [C(CH3)2], 110.34
[C(CH3)2], 170.73 (C=O), 170.77 (C=O) ppm. HRMS: m/z calcd.
for C19H30O12Na+ [M + Na+] 473.16295; found 473.16178.
For deprotection, 2a (56 mg, 0.17 mmol) was dissolved in methanol
(15 mLmmol–1, 2 mL) and Dowex DR2030 (H+-form, 28 mg) was
added. The mixture was stirred at room temp. for 24 h before fil-
tration to yield the 6-O-acylated sugar 4a (46 mg, 0.16 mmol, 93%)
as a clear syrup. [α]2D2 = +116.0 (c = 0.75, MeOH). 1H NMR
(500 MHz, [D6]DMSO): δ = 3.25 (s, 3 H, OCH3), 3.50–3.60 (m, 4
H, 2-H, 3-H, 9a-H, 9b-H), 3.70 (m, 1 H, 4-H), 3.76 (m, 1 H, 5-H),
4.06 (dd, J = 5.3, 10.2 Hz, 1 H, 8-H), 4.15 (d, J5,6 = 5.8 Hz, 2 H,
6a-H, 6b-H), 4.56 (d, J1,2 = 3.5 Hz, 1 H, 1-H), 4.58 (d, J2,OH
6.6 Hz, 1 H, 2-OH), 4.64 (m, 2 H, 3-OH, 4-OH), 4.82 (t, J9,OH
=
=
5.8 Hz, 1 H, 9-OH), 5.38 (d, J8,OH = 6.2 Hz, 1 H, 8-OH) ppm. 13C
NMR (126 MHz, [D6]DMSO): δ = 54.32 (OCH3), 63.48 (9-C),
64.03 (6-C), 68.05 (2-C), 68.16 (5-C), 68.95 (4-C), 69.11 (3-C), 71.86
(8-C), 99.92 (1-C), 172.48 (C=O) ppm. HRMS: m/z calcd. for
C10H18O9Na+ [M + Na+] 305.08430; found 305.08433.
Small-Scale Enzymatic Acylation with Methyl Glycosides 1a–c: A
lipase preparation (5–50 mgmL–1), molecular sieves (3 Å,
50 mgmL–1), and (R)- or (S)-4 (0.06–0.38 m, 9–54 μL) were added
to a solution of one of the sugars 1a–c, 7a, 8a or 10a (0.05–0.5 m)
in an organic solvent (1 mL). The reaction mixture was shaken
(170 rpm) at 47 °C. Samples (50 μL) were taken at intervals, diluted
with acetonitrile (100 μL), filtered, and analyzed by HPLC/CAD.
Preparation of (2–4)b: As described above, 1b (291 mg, 1.50 mmol)
gave 6-O-acylated 2b (277 mg, 0.86 mmol, 57%) and 2,6-di-O-acyl-
ated 3b (73 mg, 0.16 mmol, 11%) as clear syrups after a 48 h reac-
tion and purification by column chromatography (ethanol/ethyl
acetate, 1:20). Rf (ethanol/ethyl acetate, 1:20) = 0.25 (2b), 0.56 (3b).
Compound 2b: [α]2D2 = +113.3 (c = 0.75, MeOH) 1H NMR
(500 MHz, [D6]DMSO): δ = 1.31 (s, 3 H, CH3), 1.38 (s, 3 H, CH3),
3.07 (ddd, J3,OH = 5.8, J3,4 = 8.7, J2,3 = 9.9 Hz, 1 H, 3-H), 3.20
(ddd, J1,2 = 3.7, J2,OH = 6.5, J2,3 = 9.9 Hz, 2-H), 3.26 (s, 3 H,
OCH3), 3.38 (ddd, J4,OH = 4.9, J3,4 = 8.9, J4,5 = 9.2 Hz, 1 H, 4-H),
3.54 (ddd, J5,6a = 1.9, J5,6b = 6.5, J4,5 = 9.8 Hz, 1 H, 5-H), 4.00
Preparation of 2–4a: Compound (R)-5 (3 equiv., 4.50 mmol,
652 μL) was added to a mixture of 1a (291 mg, 1.50 mmol) in
tAmOH (30 mL) together with molecular sieves (3 Å; 50 mgmL–1,
1.5 g) and lipase TL IM (30 mgmL–1, 900 mg). The reaction mix-
ture was shaken (170 rpm) at 47 °C. The reaction was stopped after
48 h by filtering off the lipase and molecular sieves and thereafter
evaporating the filtrate. Column chromatography (ethanol/ethyl
acetate, 1:20) gave 6-O-acylated product 2a (193 mg, 0.60 mmol,
40%) as an amorphous white solid and 2,6-di-O-acylated product
(dd, J8,9b = 4.5, J9a,9b = –8.6 Hz, 1 H, 9b-H), 4.11 (dd, J5,6b
=
4976
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 4971–4978

Glyceric Acid Esters of Methyl Glycosides
6.5 Hz, J6a,6b = –11.7 Hz, 1 H, 6b-H), 4.20 (dd, J8,9a = 7.4 Hz, J9a,9b
= –8.6 Hz, 1 H, 9a-H), 4.38 (dd, J5,6a = 2.2 Hz, J6a,6b = –11.6 Hz, 1
H, 6a-H), 4.53 (d, J1,2 = 3.6 Hz, 1 H, 1-H), 4.65 (dd, J8,9b = 4.5 Hz,
25.25 (CH3), 25.72 (CH3), 53.99 (OCH3), 64.21 (6-C), 66.62 (9-C),
66.64 (4-C), 69.94 (2-C), 70.59 (3-C), 70.63 (5-C), 73.30 (8-C),
100.98 (1-C), 110.19 [C(CH3)2], 170.77 (C=O) ppm. HRMS: m/z
J8,9a = 7.3 Hz, 1 H, 8-H), 4.80 (d, J2,OH = 6.4 Hz, 1 H, HO-2), 4.89 calcd. for C13H22O9Na+ [M + Na+] 345.11560; found 345.11504.
(d, J4,OH = 4.9 Hz, 1 H, HO-4), 5.16 (d, J3,OH = 5.8 Hz, 1 H, HO-
As described above for 2a, the deprotection of 2c (60 mg,
3) ppm. 13C NMR (126 MHz, [D6]DMSO): δ = 25.25 (CH3), 25.70
0.19 mmol), gave 4c (44 mg, 0.16 mmol, 83%) as a transparent
(CH3), 54.35 (OCH3), 64.15 (C-6), 66.60 (C-9), 69.39 (C-5), 70.20
1
syrup. [α]2D2 = +60.8 (c = 0.75, MeOH). H NMR (500 MHz, [D6]-
(C-3), 71.65 (C-2), 72.98 (C-4), 73.34 (C-8), 99.66 (C-1), 110.23
DMSO): δ = 3.24 (s, 3 H, OCH3), 3.35–3.46 (m, 2 H, 3-H, 4-H),
[C(CH3)2], 170.78 (C=O) ppm. HRMS: m/z calcd. for
3.47 (ddd, J5,6b = 1.7, J5,6a = 7.2, J4,5 = 8.9 Hz, 1 H, 5-H), 3.51–
C13H22O9Na+ [M + Na+] 345.11560; found 345.11497.
3.62 (m, 3 H, 2-H, 9a-H, 9b-H), 4.04 (dd, J5,6a = 7.2 Hz, J6a,6b
=
Compound 3b: [α]2D2 = +56.2 (c = 0.75, MeOH). 1H NMR
(500 MHz, [D6]DMSO): δ = 1.31 (s, 3 H, CH3), 1.32 (s, 3 H, CH3),
1.39 (s, 3 H, CH3), 1.40 (s, 3 H, CH3), 3.22 (ddd, J3,OH = 5.9, J3,4
= 8.5, J2,3 = 9.8 Hz, 1 H, 3-H), 3.27 (s, 3 H, OCH3), 3.57–3.65 (m,
2 H, 4-H, 5-H), 3.98 (dd, J8,9a = 4.5 Hz, J9a,9b = –8.7 Hz, 1 H, 9aЈ-
H), 4.01 (dd, J8,9a = 4.4 Hz, J9a,9b = –8.6 Hz, 1 H, 9a-H), 4.16 (dd,
–11.4 Hz, 1 H, 6a-H), 4.06 (m, 1 H, 8-H), 4.43 (dd, J5,6b = 1.7 Hz,
J6a,6b = –11.5 Hz, 1 H, 6b-H), 4.49 (d, J1,2 = 1.4 Hz, 1 H, 1-H),
4.72 (br., 1 H, 3-OH), 4.83 (br., 2 H, 2-OH and 9-OH), 5.01 (br.,
1 H, 4-OH), 5.38 (d, J8,OH = 5.4 Hz, 1 H, 8-OH) ppm. 13C NMR
(126 MHz, [D6]DMSO): δ = 53.93 (OCH3), 63.55 (9-C), 63.90 (6-
C), 66.74 (4-C), 70.63 (2-C), 70.75 (3-C), 70.90 (5-C), 71.86 (8-C),
100.87 (1-C), 172.48 (C=O) ppm. HRMS: m/z calcd. for
C10H18O9Na+ [M + Na+] 305.08430; found 305.08598.
J5,6a = 6.3 Hz, J6,6b = –11.7 Hz, 1 H, 6a-H), 4.20 (dd, J8,9b
7.3 Hz, J9a,9b = –8.5 Hz, 1 H, 9b-H), 4.22 (dd, J8Ј,9bЈ = 7.5 Hz,
J9aЈ9bЈ = –8.5 Hz, 1 H, 9b-H), 4.41 (dd, J5,6b = 2.0 Hz, J6a,6b
=
=
Preparation of 12 and 2a: Compound (R)-5 (2 equiv., 1.50 mmol,
217 μL) was added to a mixture of methyl 3,4-O-isopropylidene-α-
d-galactopyranoside (8a; 176 mg, 0.75 mmol), CAL-B (Novozym
435, 30 mgmL–1, 450 mg) and molecular sieves (3 Å; 50 mgmL–1,
750 mg) in anhydrous THF (15 mL). The mixture was shaken
(170 rpm) at 47 °C for 72 h before being stopped by filtering and
evaporating the mixture. Finally, 6-O-acylated product 12a
(273 mg, 0.75 mmol, Ͼ99%) was obtained as a clear syrup after
purification by column chromatography (ethyl acetate/2-propanol/
water, 30:20:1) and re-evaporation from toluene (2ϫ 100 mL). Rf
(ethanol/ethyl acetate, 1:20) = 0.75. [α]2D2 = +124.8 (c = 0.75,
MeOH). 1H NMR (500 MHz, [D6]DMSO, 298 K): δ = 1.26 (s, 3
H, CH3), 1.31 (s, 3 H, CH3), 1.38 (s, 3 H, CH3), 1.40 (s, 3 H, CH3),
3.28 (s, 3 H, OCH3), 3.48 (ddd, J1,2 = 3.6, J2,OH = 6.5, J2,3 = 7.6 Hz,
1 H, 2-H), 4.02 (dd, J8,9a = 4.4 Hz, J9a,9b = –8.8 Hz, 1 H, 9a-H),
4.04 (dd, J3,4 = 5.8, J2,3 = 7.7 Hz, 1 H, 3-H), 4.09 (ddd, J4,5 = 2.5,
–11.7 Hz, 1 H, 6b-H), 4.53 (dd, J1,2 = 3.7 Hz, J2,3 = 10.0 Hz, 1 H,
2-H), 4.67 (dd, J8,9a = 4.4, J8,9b = 7.2 Hz, 1 H, 8-H), 4.68 (dd, J8Ј,9aЈ
= 4.4, J8Ј,9bЈ = 7.4 Hz, 1 H, 8Ј-H), 4.75 (d, J1,2 = 3.7 Hz, 1 H, 1-
H), 5.30 (d, J4,OH = 5.5 Hz, 1 H, 4-OH), 5.45 (d, J3,OH = 6.1 Hz,
1 H, 3-OH) ppm. 13C NMR (126 MHz, [D6]DMSO): δ = 25.36
(CH3), 25.39 (CH3), 25.76 (CH3), 25.82 (CH3), 54.65 (OCH3), 63.84
(6-C), 66.72 (9-C or 9Ј-C), 66.77 (9-C or 9Ј-C), 69.54 (5-C), 70.18
(3-C), 70.24 (2-C), 73.22 (4-C), 73.45 (8-C or 8Ј-C), 73.52 (8-C or
8Ј-C), 96.22 (1-C), 110.37 [C(CH3)2], 110.51 [C(CH3)2], 170.72
(C=O), 170.88 (C=O) ppm. HRMS: m/z calcd. for C19H30O12Na+
[M + Na+] 473.16295; found 473.16669.
As described above for 2a, the deprotection of 2b (81 mg,
0.25 mmol) gave 4b (58 mg, 0.20 mmol, 81%) as a transparent
syrup. [α]2D2 = +109.2 (c = 0.50, MeOH). 1H NMR (500 MHz, [D6]-
DMSO): δ = 3.05 (ddd, J3,OH = 5.7, J3,4 = 8.7, J2,3 = 9.7 Hz, 1 H,
3-H), 3.20 (ddd, J1,2 = 3.7, J2,OH = 6.3, J2,3 = 9.8 Hz, 1 H, 2-H),
3.26 (s, 3 H, OCH3), 3.38 (m, 1 H, 4-H), 3.50–3.61 (m, 3 H, 5-H,
9a-H, 9b-H), 4.03 (dd, J5,6a = 7.1 Hz, J6a,6b = –11.7 Hz, 1 H, 6a-
H), 4.05 (m, 1 H, 8-H), 4.38 (dd, J5,6b = 1.6 Hz, J6a,6b = –11.6 Hz,
J5,6b = 4.2, J5,6a = 8.0 Hz, 1 H, 5-H), 4.21 (dd, J3,4 = 5.5, J4,5
=
2.4 Hz, 1 H, 4-H), 4.21 (dd, J8,9b = 7.3 Hz, J9a,9b = –8.7 Hz, 1 H,
9b-H), 4.22 (dd, J5,6a = 8.1 Hz, J6a,6b = –11.9 Hz, 1 H, 6a-H), 4.27
(dd, J5,6b = 4.2 Hz, J6a,6b = –11.3 Hz, 1 H, 6b-H), 4.56 (d, J1,2
=
3.6 Hz, 1 H, 1-H), 4.68 (dd, J8,9a = 4.4, J8,9b = 7.3 Hz, 1 H, 8-H),
5.13 (d, J2,OH = 6.5 Hz, 1 H, 2-OH) ppm. 13C NMR (126 MHz,
[D6]DMSO, 298 K): δ = 25.23 (CH3), 25.71 (CH3), 26.13 (CH3),
27.93 (CH3), 54.67 (OCH3), 63.73 (6-C), 64.96 (5-C), 66.61 (9-C),
68.97 (2-C), 72.56 (4-C), 73.29 (8-C), 75.81 (3-C), 99.33 (1-C),
108.31 [C(CH3)2], 110.26 [C(CH3)2], 170.76 (C=O) ppm. HRMS:
1 H, 6b-H), 4.53 (d, J1,2 = 3.6 Hz, 1 H, 1-H), 4.80 (d, J2,OH
=
6.3 Hz, 1 H, 2-OH), 4.82 (t, J9,OH = 5.7 Hz, 1 H, 9-OH), 4.90 (d,
J4,OH = 4.5 Hz, 1 H, 4-OH), 5.16 (d, J3,OH = 5.8 Hz, 1 H, 3-OH),
5.38 (d, J8,OH = 6.1 Hz, 1 H, 8-OH) ppm. 13C NMR (126 MHz,
[D6]DMSO): δ = 54.23 (OCH3), 63.52 (9-C), 63.77 (6-C), 69.53 (5-
C), 70.30 (3-C), 71.66 (2-C), 71.86 (8-C), 73.04 (4-C), 99.53 (1-C),
172.43 (C=O) ppm. HRMS: m/z calcd. for C10H18O9Na+ [M +
Na+] 305.08430; found 305.08510.
m/z calcd. for C16H26O9Na+ [M
+
Na+] 385.14690; found
385.14590.
From 12a (76 mg, 0.21 mmol), deprotected sugar 4a (45 mg,
0.16 mmol, 76%) was isolated after filtration. NMR spectra and
HRMS correspond to those presented above. [α]2D2 = +116.5 (c =
0.75, MeOH).
Preparation of
2 and 4c: As described above, 1c (291 mg,
1.50 mmol) gave 6-O-acylated product 2c (191 mg, 0.59 mmol,
40%) as a transparent syrup after a 24 h reaction and purification
by column chromatography (ethanol/ethyl acetate, 1:20). Rf (eth-
anol/ethyl acetate, 1:20) = 0.27 (2c). [α]2D2 = +68.1 (c = 0.75,
MeOH). 1H NMR (500.13 MHz, [D6]DMSO): δ = 1.31 (s, 3 H,
CH3), 1.38 (s, 3 H, CH3), 3.24 (s, 3 H, OCH3), 3.41 (m, 1 H, 4-H),
3.43 (m, 1 H, 3-H), 3.48 (ddd, J4,5 = 8.8, J5,6a = 6.8, J5,6b = 1.8 Hz,
1 H, 5-H), 3.60 (ddd, J1,2 = 1.7, J2,3 = 3.0, J2,OH = 4.4 Hz, 1 H, 2-
H), 4.04 (dd, J8,9a = 4.6 Hz, J9a,9b = –8.7 Hz, 9a-H), 4.09 (dd, J5,6a
= 6.8 Hz, J6a,6b = –11.6 Hz, 1 H, 6a-H), 4.19 (dd, J8,9b = 7.4 Hz,
Supporting Information (see footnote on the first page of this arti-
cle): 1H and 13C NMR spectra of the prepared compounds, synthe-
sis of the partially protected galactopyranosides and HPLC meth-
ods and conditions.
Acknowledgments
J9a,9b = –8.6 Hz, 1 H, 9b-H), 4.46 (dd, J5,6b = 1.9 Hz, J6a,6b
=
–11.6 Hz, 1 H, 6b-H), 4.49 (d, J1,2 = 1.4 Hz, 1 H, 1-H), 4.65 (dd,
J8,9a = 4.5 Hz, J8,9b = 7.4 Hz, 1 H, 8-H), 4.69 (d, J3,OH = 5.7 Hz,
Financial support from the Finnish Funding Agency for Technol-
ogy and Innovation (Technology Programme SYMBIO Project
#40168/07: Developing New Chemoenzymatic Methods and Bio-
1 H, 3-OH), 4.82 (d, J2,OH = 4.4 Hz, 1 H, 2-OH), 4.99 (d, J4,OH
=
5.4 Hz, 1 H, 4-OH) ppm. 13C NMR (126 MHz, [D6]DMSO): δ = catalysts) is gratefully acknowledged.
Eur. J. Org. Chem. 2013, 4971–4978
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4977

R. Sundell, L. T. Kanerva
FULL PAPER
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Received: March 26, 2013
Published Online: June 17, 2013
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