Organocatalysis for the acid-free O-arylidenation of carbohydrates

Article abstract of DOI:10.1002/ejoc.201301116

Methyl glycopyranosides of glucose, galactose, and mannose, their 2,3-di-O-benzyl-protected derivatives, as well as the unprotected sugars react with p-methoxybenzaldehyde dimethyl acetal (3) and with benzaldehyde dimethyl acetal (7) as reagents in the presence of thiourea 1 or squaramide 2 as the organocatalyst to afford regioselectively 4,6-O-arylidenated derivatives 5 and 8. With an excess amount of 3 or 7, diarylidenated derivatives are also obtained. In situ formation of acetals of type 3 and 7 from corresponding aldehydes 10 and 13 in the presence of an orthoester and organocatalyst 1 or 2 can be used to generate 5 and 8 directly from the aldehydes. Some substrates also lead to mixed orthoesters with this procedure. The reaction courses are discussed. A squaramide catalyzes the 4,6-O-arylidenation of glucose, mannose, and galactose with the use of arenecarbaldehyde dimethyl acetals or arenecarbaldehydes and orthoesters as the reagents. Glycoside derivatives also undergo this reaction. Copyright

Full text of DOI:10.1002/ejoc.201301116

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301116
Organocatalysis for the Acid-Free O-Arylidenation of Carbohydrates
Yiqun Geng,[a] Hassan M. Faidallah,[b] Hassan A. Albar,[b] Ibrahim A. Mhkalid,[b] and
Richard R. Schmidt*[a,b]
Keywords: Acetals / Carbohydrates / Organocatalysis / Protecting groups / Synthetic methods
Methyl glycopyranosides of glucose, galactose, and mann-
ose, their 2,3-di-O-benzyl-protected derivatives, as well as
the unprotected sugars react with p-methoxybenzaldehyde
dimethyl acetal (3) and with benzaldehyde dimethyl acetal
(7) as reagents in the presence of thiourea 1 or squaramide
2 as the organocatalyst to afford regioselectively 4,6-O-aryl-
idenated derivatives 5 and 8. With an excess amount of 3 or
7, diarylidenated derivatives are also obtained. In situ forma-
tion of acetals of type 3 and 7 from corresponding aldehydes
10 and 13 in the presence of an orthoester and organocatalyst
1 or 2 can be used to generate 5 and 8 directly from the
aldehydes. Some substrates also lead to mixed orthoesters
with this procedure. The reaction courses are discussed.
As these arylidenation reactions are generally catalyzed
by quite strong acids [e.g., p-toluenesulfonic acid (pTsOH),
etc.], the search for mild methods is important for compati-
bility with acid-sensitive substrates. Recently, the organoca-
talytic, “acid-free” acetalization of aldehydes and ketones
was reported by the Schreiner group (Scheme 1).[8] They
found that in the presence of an orthoester as a condensing
agent in alcohol as solvents thiourea 1[9] delivered excellent
yields of the corresponding acetal. Surprisingly, this method
was less efficient or even not successful for electron-rich ar-
enecarbaldehydes.
Introduction
Arylidene protection plays an important role in carbo-
hydrate chemistry, as it allows the regioselective protection
of two hydroxy groups in one reaction; in addition, subse-
quent regioselective reductive opening of these cyclic acetals
permits selective liberation of one of the hydroxy groups.[1,2]
For thermodynamic reasons, the formation of 1,3-dioxane
rings in the chair conformation with the aryl group in the
equatorial position is favored under equilibrating acid-ca-
talysis conditions, and the most important carbohydrates,
glucose, mannose, and galactose, react from the pyranose
form to generate 4,6-O-arylidene-protected derivatives as
preferred products.[3–5] This result is independent of the
presence or absence of groups at the anomeric oxygen atom
or at the oxygen atoms at the C-2 and/or C-3 position(s).
Generally, Lewis or Brønsted acids are employed as cata-
lysts for this reaction with arenecarbaldehydes (mainly
benzaldehyde and p-methoxybenzaldehyde); in addition,
the removal of water either by a condensing agent or
eventually by azeotropic distillation is required.[6] Alterna-
tively, arenecarbaldehyde acetals in the presence of Lewis
or Brønsted acid catalysts can be used,[7] as the formation
of cyclic six-membered acetals under release of simple
alcohols is thermodynamically also favored.
Scheme 1. Organocatalytic acetalization by the Schreiner group
(ref.[8]).
Important steps in these reactions are the activation of
the carbonyl group by catalyst 1, which leads to adduct A
[Scheme 2, Equation (1)], and the heterolysis of orthoester
by 1, which leads to ion-pair B [Scheme 2, Equation (2)].[8]
Hence, the question is whether the heterolysis by 1 also
takes place in arenecarbaldehyde acetals, which would thus
lead to reactive ion-pair intermediates C [Scheme 2, Equa-
tion (3)] that initiate the formation of cyclic arylidene acet-
als.
[a] Fachbereich Chemie, Universität Konstanz
Fach 725, 78457 Konstanz, Germany
E-mail: richard.schmidt@uni-konstanz.de
http://www.chemie.uni-konstanz.de/~agschm/homepage.htm
[b] Chemistry Department, Faculty of Science, King Abdulaziz
University
Jeddah 21589, Saudi Arabia
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301116.
Eur. J. Org. Chem. 2013, 7035–7040
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Y. Geng, H. M. Faidallah, H. A. Albar, I. A. Mhkalid, R. R. Schmidt
SHORT COMMUNICATION
organocatalyst 1 (5 mol-%) in dichloromethane as the sol-
vent was studied (Table 1). After 28 h at room temperature,
almost complete formation of desired 4,6-O-p-methoxy-
benzylidene-protected mannopyranoside 5a[11] was ob-
served (Table 1, Entry 1). The same result was obtained
with squaramide 2[12] as the organocatalyst (Table 1, En-
try 2), even with a shorter reaction time. Hence, for the fol-
lowing investigations, catalyst 2 was mainly employed. THF
as the solvent inhibited the reaction (Table 1, Entry 3), pre-
sumably as a result of the interaction of 2 with the ring
oxygen atom of THF. Acetonitrile as the solvent led to 5a
(PMP = p-methoxyphenyl; Table 1, Entry 4), although a
longer reaction time was required. The same result was ob-
tained for corresponding methyl 2,3-O-benzylglucopyran-
oside 4b[13] (Table 1, Entries 5 and 6): 4,6-O-arylidenated
product 5b[14] was formed more quickly in dichloromethane
as the solvent than in acetonitrile as the solvent. However,
owing to better solubility of most substrates in aceto-
nitrile,[6a] in particular less-protected carbohydrates, this
solvent was used for most of the following reactions. Thus,
for 2,3,4,6-O-unprotected methyl glucopyranoside 4c[13] and
3 as the reagent under the same reaction conditions with
both catalysts (i.e., 1 and 2), the desired 4,6-O-protected
product 5c[15,6a] was selectively obtained in a rather short
reaction time (Table 1, Entries 7 and 8). Very good results
were also obtained for methyl galactoside 4d[13] and methyl
mannoside 4e[13] (generating 5d[14,6a] and 5e;[11] Table 1, En-
tries 9 and 10). As expected, treatment of, for example, 4e
with an excess amount of 3 (3 equiv.) led to 2,3:4,6-bis(O-
p-methoxybenzylidene)-protected 6e[11] as a ca. 1:1 mixture
of two diastereomers (Table 1, Entry 11). These results en-
Scheme 2. Activation of carbonyl compounds, orthoesters, or ar-
enecarbaldehyde acetals by organocatalyst 1 or 2.
Results and Discussion
As the cleavage of an alkoxide group from the orthoester
is a decisive step in the acetalization reaction [Scheme 2,
Equation (2)], we selected p-methoxybenzaldehyde dimethyl
acetal (3) for our studies, though 3 is not accessible by the
Schreiner method. However, the electron-donating effect of
the p-methoxyphenyl group should strongly support the
formation of ion-pair intermediate C. In addition, owing to
acid-catalyzed cleavage, oxidative cleavage, hydrogenolysis,
and regioselective reductive ring opening, the p-methoxy-
benzylidene protection has become popular in carbohydrate
chemistry.[1,2] Hence, the reaction of 3 (1.5 equiv.) with 4,6-
O-unprotected mannopyranoside 4a[10] in the presence of
Table 1. Introduction of the p-methoxybenzylidene group.
Entry
Substrate
Catalyst
Equiv. of 3
Solvent
t [h]
Product
Yield[a] [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
4a
4a
4a
4a
4b
4b
4c
4c
4d
4e
4e
4f
1
2
2
2
2
2
1
2
2
2
2
2
2
2
2
2
2
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.0
3.0
1.0
3.0
1.0
1.0
1.5
1.5
CH2Cl2
CH2Cl2
THF
28
20
48
36
0.42
10
1
0.17
14
1
36
16
48
5
24
48
168
5a
5a
81
83
–
80
89
81
67
90
82
88
85
83
81
[b]
–
CH3CN
CH2Cl2
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
5a
5b
5b
5c
5c
5d
5e
6e
5f
4f
6f
4g
4h
4i
5g, 6g
5h, 6h
5i
62, 11[c]
74, 9
76
4j
5j
76[d]
[a] Yield of isolated product. [b] No reaction. With less basic Et2O as the solvent, the reaction was slow: after 7 d, 5a (61%) was obtained.
[c] The reaction was performed at reflux. At r.t., the reaction was very slow. [d] The reaction was performed at 40 °C; after 48 h, 5j (47%)
was formed. At r.t., the reaction was very slow.
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Organocatalysis for the Acid-Free O-Arylidenation of Carbohydrates
couraged us to study the important regioselective mono-
As O-benzylidene-protected carbohydrates are com-
arylidenation of totally unprotected carbohydrates. Thus, monly employed as building blocks for various purposes,
under the same reaction conditions, d-glucose (4f)[13] deliv- benzaldehyde dimethyl acetal (7) was also studied in our
ered 4,6-O-p-methoxybenzylidene-protected 5f[16] in very reaction protocol. As the support for the generation of ion-
good yield (Table 1, Entry 12), and excess 3 (3 equiv.) fur- pair intermediate C [Scheme 2, Equation (3)] was dimin-
nished 1,2:4,6-bis(O-p-methoxybenzylidene)-protected 6f ished, it was no surprise that the room-temperature reaction
again as a 2:1 mixture of diastereomers (Table 1, Entry 13). of glucopyranoside 4c with thiourea 1 as the organocatalyst
Reaction of galactose (4g)[13] with 3 in the presence of 2 in acetonitrile gave almost no reaction (Table 2, Entry 1).
required heating to reflux and led, in addition to expected However, organocatalyst 2 led cleanly to desired 4,6-O-
product 5g (62%), to a minor amount of diarylidenated benzylidenated product 8c[19] (Table 2, Entry 2). The same
product 6g (11%; Table 1, Entry 14). Reaction of mannose results were obtained for galactoside 4d and mannoside 4e,
(4h)[13] at room temperature led to expected product 5h which upon reaction with 7 furnished 4,6-O-benzylidenated
(74%) and a minor amount of diarylidenated product 6h product 8d[20] and 8e,[21] respectively, in very good yield
(9%; Table 1, Entry 15). Owing to the very mild reaction (Table 2, Entries 3 and 4). Upon the addition of 7 (3 equiv.)
conditions, it was expected that acid-sensitive glycals, for to the reaction mixture containing 4e at 40 °C, dibenzylid-
instance galactal (4i),[13] would not be degraded but rather enated mannoside 9e[22] was furnished in very good yield
arylidenated under these conditions, and indeed, after a (Table 2, Entry 5). At a slightly elevated temperature, even
rather long reaction time under concomitant Ferrier re- the reaction of unprotected glucose (4f) with reagent 7 led
arrangement at room temperature desired 4,6-O-aryliden- to 4,6-O-benzylidenated derivative 8f[23] in acceptable yield
ated product 5i[17] was obtained in good yield (Table 1, En- (Table 2, Entry 6). However, under reflux conditions, un-
try 16). According to the recent work of Galan et al.,[18] for- protected galactose (4g) did not afford the desired product
mation of methyl 2-deoxy-4,6-O-(4-methoxybenzylidene)-α- (Table 2, Entry 7). Mannose (4h) gave 4,6-O-benzylidenated
d-galactopyranoside was expected; however, this compound derivative 8h[22] only in modest yield (Table 2, Entry 8).
was not found. The reaction with acetal 3 and catalyst 2
The Schreiner group found that under their experimental
was also extended to substrates having acid-sensitive silyl conditions (Scheme 1) arenecarbaldehyde acetals with elec-
protecting groups, as shown for thexyldimethylsilyl (TDS) tron-donating substituents were less readily accessible.[8]
galactopyranoside (4j), which underwent a slow reaction at This result was also confirmed in our studies; as shown in
40 °C to afford 4,6-O-protected 5j (Table 1, Entry 17). The Scheme 3, from p-methoxybenzaldehyde (10) only a trace
large difference in the reaction rates found for differently amount of diethyl acetal 11 was formed with 2 as the cata-
protected substrates is presumably the result of different lyst. This is presumably the result of the high preference of
solubilities and/or eventually aggregation phenomena.
2 or 1 to form a complex with the basic carbonyl oxygen
Eur. J. Org. Chem. 2013, 7035–7040
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Y. Geng, H. M. Faidallah, H. A. Albar, I. A. Mhkalid, R. R. Schmidt
SHORT COMMUNICATION
Table 2. Introduction of the benzylidene group.
acetal 12 in good yield. Hence, it seemed to be possible
to combine arenecarbaldehyde acetal formation – even for
arenecarbaldehydes having electron-donating substituents –
with the thermodynamically favored arylidenation of carbo-
hydrates. Thus, from arenecarbaldehydes, orthoesters, and
carbohydrates as substrates in the presence of 1 or 2 as the
organocatalyst, arylidenated products should become di-
rectly accessible. The results in Table 3 confirm this hypoth-
esis. From aldehyde 10, glucoside 4c, ethyl orthoester, and
1 or 2 as the catalyst, desired product 5c was readily ob-
tained in very good yields (Table 3, Entries 1 and 2).
Clearly, the presence of the catalyst and the orthoester as
the condensing agent were required for product formation
(Table 3, Entries 3 and 4). Catalysis of this reaction with a
strong acid such as pTsOH was also possible; however, an
increase only in the rate but not in the yield was observed
(Table 3, Entry 5). Reaction of glycosides with axial hy-
droxy groups, as in galactoside 4d and mannoside 4e, led
not only to 4,6-O-arylidenated products 5d and 5e, respec-
tively, but also to orthoester derivatives 14d and 14e. Their
structures were confirmed by NMR spectroscopy and MS
data. Clearly, with 4d as the substrate, ion-pair intermediate
B [Scheme 2, Equation (2)] did not only support the forma-
tion of 4,6-O-p-methoxybenzaldehyde dimethyl acetal, but
En-
try
Sub-
strate
Cata-
lyst
Equiv. of t [h] Prod-
Yield[a]
[%]
7
uct
[b]
1
2
3
4
5
6
7
8
4c
4c
4d
4e
4e
4f
1
2
2
2
2
2
2
2
1.5
1.5
1.5
1.0
3.0
1.0
1.0
1.0
48
1
–
–
8c
8d
8e
9e
91
48
2.5
24
48
48
48
78[c]
86
81[c]
67[c]
–
8f
[b,d]
4g
4h
–
8h
39[c]
[a] Yield of isolated product. [b] No reaction. [c] The reaction was
performed at 40 °C. At r.t., the reaction was very slow. [d] The reac-
tion was performed at reflux.
atom of 10 [Ǟ A, Scheme 2, Equation (1)] over an ion pair
with the orthoester [Ǟ B, Scheme 2, Equation (2)], and this
pushes the reaction to acetal formation. This assumption
was confirmed by the reaction with 1,3-propanediol, which
Scheme 3. Acetal formation with p-methoxybenzaldehyde and 2 as
led to thermodynamically favored six-membered cyclic the catalyst.
Table 3. Combination of arenecarbaldehyde acetal formation with the arylidenation of carbohydrates.
Entry
Substrate
Catalyst
Aldehyde
Equiv. of aldehyde
Equiv. of orthoester
t [h]
Product
Yield[a] [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
4c
4c
4c
4c
4c
4d
4e
4e
4f
4c
4c
4d
4e
1
10
10
10
10
10
10
10
10
10
13
13
13
13
2.0
2.0
2.0
2.0
2.0
2.0
1.0
2.0
1.1
2.0
2.0
2.0
1.0
2.0
2.0
2.0
–
2.0
2.0
1.0
2.0
1.1
2.0
2.0
2.0
1.0
0.5
0.17
48
48
0.083
24
2
12
18
6
5c
5c
83
87
–
–
88
30, 18
65, 12
71
67
80
2
[b]
–
2
–
[b]
–
pTsOH
5c
5d, 14d
5e, 15e
15e
2
2
2
2
1
2
2
2
5f
8c
5
48
48
8c
81
59
38,15
14d
14e, 16e
[a] Yield of isolated product. [b] No reaction.
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Organocatalysis for the Acid-Free O-Arylidenation of Carbohydrates
ture was then quenched with a drop of Et3N and concentrated
in vacuo. The crude product was purified by flash column
chromatography [petroleum ether/ethyl acetate, petroleum ether/
acetone, or ethyl acetate/methanol containing 1% (v/v) Et3N] to
afford desired 4,6-O-arylidene-protected 5 or 8. Diarylidene-pro-
tected 6 or 9 could be obtained in some cases.
Procedure B: Monosaccharide substrate 4 (0.4 mmol, 1.0 equiv.)
was dissolved in anhydrous CH3CN (4.0 mL) at room temperature.
p-Methoxybenzaldehyde (10) or benzaldehyde (13; 0.8 mmol,
2.0 equiv.) and triethyl orthoformate (0.8 mmol, 2.0 equiv.) were
added under nitrogen, followed by the addition of organocatalyst
1 or 2 (0.01 mmol, 0.025 equiv.). The reaction mixture was stirred
at the same temperature until TLC indicated the complete con-
sumption of the starting material. The reaction mixture was then
concentrated in vacuo. The crude product was purified by flash col-
umn chromatography [petroleum ether/ethyl acetate, petroleum
ether/acetone, or ethyl acetate/methanol containing 1% (v/v) Et3N]
to afford desired 4,6-O-arylidene-protected 5 or 8. Orthoesters 14–
16 were formed in some cases.
it also attacked the substrate directly to give orthoester 14d,
which was not cleaved under the reaction conditions
(Table 3, Entry 6). In the case of mannoside 4e, if 1.0 equiv.
of each reagent was used, 4,6-O-arylidenated 5e was partly
transformed into 2,3-O-orthoester 15e, which reduced the
yield of the expected product (Table 3, Entry 7); an increase
in the amounts of both reagents led exclusively to 15e
(Table 3, Entry 8). The same result was obtained for unpro-
tected sugars. With glucose (4f), 4,6-O-arylidenated 5f was
obtained (Table 3, Entry 9). This arylidenation methodol-
ogy was also extended to benzaldehyde (13) and ethyl or-
thoformate as reagents, as shown for glucoside 4c; both cat-
alysts (i.e., 1 and 2) furnished 4,6-O-benzylideneglucopyr-
anoside 8c in very good yields (Table 3, Entries 10 and 11).
However, galactoside 4d afforded only orthoester 14d, and
mannoside 4e led to a mixture of orthoesters 14e and 16e
(Table 3, Entries 12 and 13). Hence, glucose and glucosides
have a much higher tendency to undergo 4,6-O-aryliden-
ation than mannose and galactose and their glycosides. In
particular, galactose and galactosides compete successfully
for intermediate B. Owing to the accumulation of orbitals
for the lone pairs of electrons of the oxygen atom on the β-
side (at C-3, C-4, and C-6), galactopyranose is more nucleo-
philic than mannopyranose and particularly more nucleo-
philic than glucopyranose.
Supporting Information (see footnote on the first page of this arti-
1
cle): Experimental details, H and 13C NMR spectra of new com-
1
pounds, and H NMR spectra of known compounds.
Acknowledgments
The authors acknowledge the generous support of this work by the
University of Konstanz. The work outlined in this paper was also
funded by the Deanship of Scientific Research (DSR), King Abd-
ulaziz University (KAU) under grant No. 26-3-1432/HiCi. The au-
thors therefore acknowledge technical and financial support of
KAU.
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Conclusions
Arenecarbaldehyde acetals 3 and 7 can be activated by
thiourea 1 or squaramide 2 as the organocatalyst to afford
cyclic acetals from carbohydrates. 4,6-O-Arylidenated com-
pounds can be readily obtained under mild conditions from
glucose, galactose, mannose, and their glycopyranosides.
Acid-sensitive galactals can also be arylidenated at the 4,6-
O-positions. The combination of this method with in situ
acetal formation from arenecarbaldehydes and orthoesters
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Experimental Section
General Procedure for the Arylidenation Reaction
Procedure A: p-Methoxybenzaldehyde dimethyl acetal (3) or benz-
aldehyde dimethyl acetal (7; 0.6 mmol, 1.5 equiv.) was added to a
solution of monosaccharide 4 (0.4 mmol, 1.0 equiv.) in anhydrous
CH3CN or CH2Cl2 (4.0 mL) under nitrogen. Organocatalyst 1 or
2 (0.02 mmol, 0.05 equiv.) was then added, and the reaction mix-
ture was stirred at the same temperature until TLC indicated the
complete consumption of the starting material. The reaction mix-
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Eur. J. Org. Chem. 2013, 7035–7040
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Y. Geng, H. M. Faidallah, H. A. Albar, I. A. Mhkalid, R. R. Schmidt
SHORT COMMUNICATION
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Received: July 26, 2013
Published Online: October 2, 2013
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Eur. J. Org. Chem. 2013, 7035–7040