Acyl transfer reactions of carbohydrates, alcohols, phenols, thiols and thiophenols under green reaction conditions

Article abstract of DOI:10.1039/c4ra16916f

Acyl transfer reactions of various carbohydrates, alcohols, phenols, thiols and thiophenols were achieved at room temperature in high yields and catalytic efficiency in the presence of methane sulfonic acid, a green organic acid, under solvent-free conditions over short time periods. The method is mild enough to allow acid labile substituents such as isopropylidene acetals and trityl ethers on the reacting substrates to be left completely unaffected. Esterification of free mono- and dicarboxylic acids such as acetic acid, cinnamic acid, sialic acid and tartaric acid with alcohols such as menthol, ethanol, methanol or propylene glycol has also been achieved efficiently at room temperature. A comparative study of the method with the silica-sulfuric acid is also reported.

Full text of DOI:10.1039/c4ra16916f

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Acyl transfer reactions of carbohydrates, alcohols, phenols, thiols and
thiophenols under green reaction conditions
Santosh Kumar Giri and K. P. Ravindranathan Kartha*a
5
Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Acyl transfer reactions of various carbohydrates, alcohols,
more environmentꢀfriendly (than the free acid) and more
phenols thiols and thiophenols were achieved at room 60 convenient to handle. The solidꢀsupported reagents are also less
temperature in high yields and catalytic efficiency in the presence
corrosive and of lower toxicity with the added advantage that
they can be easily separated from the reaction mixtures by
filtration offering possibilities for reuse, particularly important in
industrial applications.16 It was rather surprising that a green acid
10 of methane sulfonic acid, a green organic acid, under solventꢀfree
conditions in short time periods. The method is mild enough to
allow acid labile substituents such as isopropylidene acetals and
trityl ethers on the reacting substrates to be left completely 65 such as MeSA has not so far been investigated in detail for
unaffected. Esterification of free monoꢀ and dicarboxylic acids
15 such as acetic acid, cinnamic acid, sialic acid and tartaric acid
with alcohols such as menthol, ethanol, methanol or propylene
glycol has also been achieved efficiently at room temperature. A
acylation reactions, a class of reactions requiring to be carried out
so routinely during organic synthesis, particularly in
carbohydrateꢀ and medicinal chemistry and natural product
synthesis. This was true of CSA as well. For these reasons the
comparative study of the method with the notꢀsoꢀgreen 70 potential of both MeSA as well as SiSA was evaluated for acyl
chlorosulfonic acid supported on silica is also reported.
transfer reactions of carbohydrates, alcohols and phenols, thiols
and thiophenols and the results are presented herein.
20 Introduction
Results and discussion
New methods for the protection and deprotection of organic
compounds required during synthetic processes addressing
environmental concerns are of increasing topical relevance. Acyl
transfer reactions of carbohydrates, alcohols, phenols, amines and
25 thiols form perhaps one of the commonest reactions required to
be carried out routinely during synthetic transformations of these
classes of organic compounds. Both acids as well as bases serve
equally well as catalysts for this purpose. Among the wide variety
of Lewis acids that have been in use in this context, a majority
30 such as BF3/BF3.Et2O,1 FeCl3,2 SnCl4,3 TiCl4,4 ZnCl2,5 TMSOTf,6
etc. are extremely sensitive to moisture and are difficult to
handle. Among the more recently introduced metalꢀbased Lewis
acid substances are the different triflates such as In(OTf)3,7,8
Cu(OTf)2,9 Sc(OTf)3,10 La(OTf)3,11 Bi(OTf)3,12,13 etc. Among
35 others, iodine14 and sulfamic acid15 also have been found good
for various acyl transfer reactions, although in the case of the
latter the reaction is required to be carried out at 60 °C to be
effective. Immobilized catalysts such as solidꢀsupported acids16
have also been used for acidꢀcatalyzed acetylations. Apart from
40 the acids, basic reagents have also been used extensively as acyl
transfer agents. Pyridine,17 by far the most widely used reagent,
DMAP18 and DABCO19 have all been found to be well suited for
this purpose although they are used in stoichiometric quantities
(or in excess of it, for example, as solvent in the case of pyridine).
45 Pyridine and their derivatives are toxic/hazardous compounds.
New catalysts of high catalytic efficiency and low hazard have
therefore been always in utmost demand.
75
MeSA-Catalyzed acetylation (Method A): Considering the
polyhydroxylic nature of carbohydrates, acetylation of this class
of compounds, considered relatively more demanding than simple
alcohols or phenols, was chosen as the first set of substrates to be
80 investigated. MeSA is freely miscible with acetic anhydride and
hence was taken as the reagent of choice for the perꢀOꢀacetylation
to be tried. Moreover, the byꢀproduct (acetic acid) formed in the
reaction serves as an excellent solvent for the product. Also,
AcOH being miscible (soluble) in water, the possibility for its
85 removal from the reaction mixture by washing either as such or
after neutralization with an alkali is also attractive. Reactions
would be carried out using the acyl donor reagent as required by
stoichiometry (or in slight excess of it) and MeSA in only
catalytic amounts. As a representative example, when 10 ꢀL
90 MeSA was added to a suspension of methyl αꢀDꢀglucoside 1 (1 g,
entry 1, Table 1) in Ac2O (2.3 mL, 1.1 mol equiv/OH group) and
stirred, the sugar went into solution within 6 min and TLC
analysis of the reaction mixture at this stage revealed the
formation of a product identical with authentic methyl tetraꢀOꢀ
95 acetylꢀαꢀ
entry 1, Table 1). The reaction was considerably exothermic and
no unreacted methyl αꢀ ꢀglucoside 1 or any partially acetylated
Dꢀ glucoside 2 in Rf value (TLC, EtOAc:nꢀHeptane, 2:3;
D
product could be detected (TLC) in the reaction mixture.
Aqueous workup gave a colorless crystalline product identical in
100 all respects (physical constants and spectroscopic data) with
authentic 2 in 98% yield. When the reaction under identical
conditions was carried out using Dꢀglucose (3, entry 2, Table 1)
Methanesulfonic acid (MeSO3H, MeSA) is a high boiling liquid,
50 nonꢀcorrosive, nonꢀvolatile, stable, inexpensive and readily
available green chemical reagent.20 These unique features of
MeSO3H attracted us to exploring its potential as an acid catalyst
for various organic reactions. It was also felt that another closely
related acid, namely, chlorosulfonic acid (ClSO3H, CSA) will be
55 also evaluated for comparative purposes. The latter however is
corrosive, volatile/fuming and is a very strong acid and hence
would be used as supported on silica (SiO2ꢀOSO3H, SiSA, in the
form of freeꢀflowing powder)21 thus making it comparatively
as the starting material, the corresponding pentaacetate 4 was
obtained in 99% yield. A generalized view of this acidꢀcatalysed
105 acetylation has been depicted in Scheme 1.
Scheme 1 MeSO3Hꢀcatalysed acetylation of carbohydrates using
Ac2O as the acyl donor agent
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It becomes evident that MeSA serves to extremely efficiently 65 After having demonstrated successfully the suitability of the
activate the acyl centre of the acid anhydride for a facile reaction
of alcoholic nucleophiles that are known to be only poorly
nucleophilic in nature giving rise to the desired esters. Moreover,
the AcOH generated as a byꢀproduct of the reaction being an
MeSA ꢀAc2O system for the efficient acetylation of carbohydrate
substrates, its applicability in the acetylation of nonꢀcarbohydrate
natural products such as cholesterol (39), menthol (41) and
linalool (43) was also evaluated (entries 20ꢀ22, Table 1).
5
excellent solvent serves to bring the desired acetylated sugars to 70 Satisfyingly, all of the reactions led to the formation of the
solution, thus perhaps also aiding further in the overall process. It
may be recalled that the vast majority of unprotected sugars and
their simple glycosides are insoluble in acetic anhydride.
respective desired acetates 40, 42 and 44 efficiently and in
excellent yields without affecting the optical purity in the case of
the optically active substrates. Likewise, the reaction of poorly
nucleophilic phenols such as pꢀnitroꢀ (45) and mꢀnitroꢀ (47)
10 Therefore a reaction mixture that is heterogeneous at start
culminates as a clear colourless solution at finish. In view of the 75 phenols (entries 23 and 24, Table 1) with Ac2O in the presence of
excellent results obtained in the acetylation reactions described
above, and with a view to demonstrating the generality of its
application to other carbohydrates, the method was extended to
15 some of the other commonly required carbohydrate substrates as
catalytic amounts (2 mol%) of MeSA yielded the desired
products 46 and 48, respectively, in near quantitative yields in 25
min at rt. These results are well comparable to those obtained by
using Mg(ClO4)2 (a highly efficient and powerful Lewis acid
well. Thus, excellent results were also obtained in the perꢀOꢀ 80 used at 1 mol% catalyst concentration) for the same examples
acetylation of monosaccharides [hexoses:
galactose (7)], disaccharides [ ꢀlactose (9),
maltose (13)] and trisaccharide [
20 desired products 6, 8, 10, 12, 14 and 16 respectively in near
D
ꢀmannose (5),
D
ꢀ
with the added advantage that doing the reaction at ambient
temperature was adequate under the current protocol unlike the
Mg(ClO4)2ꢀcatalyzed reaction that required heating at 80 °C for
the reaction to be effective.16b In the same manner, when mꢀ
D
Dꢀcellobiose (11),
ꢀraffinose (15)] and yielded the
Dꢀ
D
quantitative yields (entries 3ꢀ8, Table 1).8a, 15 Importantly, while 85 hydroxy benzoic acid (49, entry 25, Table 1) was subjected to
the results are well comparable to those obtained using iodine as
the Lewis acid catalyst for the reaction,14 it is much superior to
the more recent sulfamic acidꢀassisted acetylation.15 Thus, not
25 only that the current method is faster but also require no heating,
acetylation under similar conditions, it also led to the formation
of the desired neat monoꢀOꢀacetate 50 in very short reaction time
without any difficulty. From among the classes of thiols and
thiophenols also
a set of representative compounds were
unlike the method using sulfamic acid in which it is done at 60 °C 90 acetylated in order to evaluate the applicability of the method to
to be effective.
these classes of compounds. Thus, the dithiothreitol 51 (entry 26,
Table 1) gave the tetraꢀOꢀ/Sꢀacetylated 52 almost instantaneously
in the presence of catalytic MeSA. The acetylation was so quick
that no chemoꢀdiscrimination in the –OH and the –SH groups of
Further, to evaluate the applicability of MeSA for the acylation of
30 compounds bearing acidꢀsensitive groups, in particular those that
are easily amenable to acetolysis by the iodineꢀAc2O system, 95 51 towards the reaction was observed. Likewise, the substituted
various anomeric Oꢀ/Sꢀacetals and cyclic Oꢀacetonides were
subjected to acetylation conditions as optimized in the foregoing
studies. Acetylation of the βꢀOꢀglycoside 17 (entry 9, Table 1) to
35 yield the desired tetraꢀOꢀacetate 18 occurred, as in the case of 1,
without any difficulty. More importantly, the phenylthioꢀ and
methylthioglucosides (19, 21, entries 10 and 11, Table 1) when
subjected to the acetylation conditions as above, both could be
globally acetylated without affecting either the thioacetal
40 protection at the anomeric centre or its configuration. The
acetolytically unstable acetonide 23 (entry 12, Table 1) also on
subjecting to the acetylation reaction yielded the desired acetate
24 as neat product with the NMR spectra of even the crude
product (see Supplementary Information for the spectra) showing
45 it to be virtually pure and suitable for any subsequent further
reaction. The acid sensitive trityl ethers 25 and 27 (entries 13 and
14, Table 1) on the other hand when subjected to the above
conditions of acetylation, although gave the desired triacetates 26
and 28 in excellent yields of 93 and 92% respectively, did also
50 yield small amounts of some acetolytic byꢀproducts, methyl
thiophenols, pꢀtoluenethiol (53, bearing electron donating paraꢀ
substituent) and pꢀnitrothiophenol (55, bearing an electron
withdrawing paraꢀsubstituent) could also be acetylated neatly and
in very short time by acetic anhydride in the presence of MeSA to
100 yield 54 and 56 respectively(entries 27 and 28, Table 1). Notably
thus, acetylation of sugars, alcohols, phenols, thiols and
thiophenols took place extremely efficiently yielding products
without having to resort to any chromatographic purification for
the isolation of the product in practically pure form. In the
105 majority of cases studied the products were obtained following a
simple aqueous workꢀup involving precipitation and separation of
solids by filtration. The exceptions to this were those at entries 3,
4, 14, 15, 21, 22, 27 and 30 (Table 1) wherein the respective
products were obtained as syrup following an extractive workꢀup
110 in a separatory funnel, the organic solvent used for the extraction
being EtOAc.
Based on the fact that perꢀOꢀacetylated sugars usually serve as
bulk starting materials in multiꢀstep synthetic carbohydrate
115 reactions, a preparative scale model acetylation using Dꢀglucose
(3) as the substrate (at 50 gꢀscale) was also carried out. As can be
seen from entry 29 (Table 1), the desired pentaacetate 4 could be
isolated in excellent yields making this method one of the best
hitherto reported procedures for acidꢀcatalyzed acetylation
120 reactions. In large scale reactions, in order to avoid the reaction
mixture getting overheated, adding the sugar portionꢀwise to a
solution of the catalyst in the required measure of acetic
anhydride was found, however, beneficial. It was observed that
usually the reaction was complete with the completion of the
125 addition and the dissolution of the sugar. Further, as a part of the
attempt to evaluate the scope of the reaction in other acylations,
2,3,4,6ꢀtetraꢀOꢀacetylꢀαꢀDꢀglucopyranoside (2) and phenyl
2,3,4,6ꢀtetraꢀOꢀacetylꢀ1ꢀthioꢀβꢀ
respectively, in 6ꢀ7% yields.
D
ꢀglucopyranoside
(20),
55 In the presence of MeSA, alditols [
D
ꢀsorbitol (31) and
D
ꢀmannitol
(33)] could also be successfully acetylated to yield the desired
products [32 and 34 respectively] in excellent yields (entries 16
and 17, Table 1). Further, αꢀcyclodextrin (35, entry 18, Table 1)
when subjected to the MeSA ꢀcatalyzed acetylation, also gave the
60 desired perꢀOꢀacetylated αꢀcyclodextrin 36 in excellent yields.
The perꢀOꢀacetylation of the 2ꢀacetamidoꢀ2ꢀdeoxy sugar, N-
acetylꢀDꢀglucosamine (37) was comparatively slow, but also
yielded the desired acetylated derivative 38 in good yields (entry
19, Table 1).
methyl βꢀDꢀgalactoside 17 (1 g, entry 30, Table 1) was
propionylated using propionic anhydride under the same
conditions as optimized for acetylation. Indeed, the desired tetraꢀ
2
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Oꢀpropionate 57 was also obtained in excellent yields as for the
corresponding acetylation reaction.
CSA-Catalyzed acetylation (Method B): CSA is a strong organic
acid but unlike MeSA, suffers from two major deficiencies that it
is a fuming liquid and is corrosive. It may have been for the latter
5
Table 1 MeSAꢀ/SiSAꢀCatalyzed perꢀacetylation of sugars, alcohols, phenols, thiols and thiophenols
Method Aa
Method Ba
(Using MeSA) (Using SiSA)
Entry
Substrate
Product
Ref.
15
Time Yield, % Time Yield, %
(min) (α:β) (min) (α:β)
OH
O
OAc
O
HO
HO
AcO
AcO
1
5
5
2
96
4
3
2
95
HO
AcO
OMe
OMe
1
2
OAc
O
OH
O
AcO
AcO
HO
HO
99
(1:1)
99
(2:1)
2
3
15
15
OAc
OH
OAc
HO
4
3
OH
OH
O
OAc
OAc
O
HO
HO
95
(1:1.2)
98
(1:2)
AcO
AcO
OH
OH
6
OAc
OAc
5
HO
HO
AcO
AcO
OH
O
OAc
O
4
3
98b
2
98b
15
15
22
AcO
HO
8
7
AcO
AcO
HO
HO
OAc
O
OH
O
OAc
O
OH
O
95
(3:4)
96
(1:1)
5c
15
25
10
15
O
O
HO
AcO
HO
AcO
HO
OH
AcO
OAc
9
10
OAc
O
OH
O
OAc
O
OH
O
AcO
AcO
HO
HO
97
(1:3)
95
(1:3)
6c
O
O
AcO
HO
OAc
HO
OH
OAc
OAc
HO
OH
11
12
OAc
O
O
HO
HO
AcO
AcO
OAc
OH
98
(1:1)
97
(1:1)
7c
20
10
15
HO
AcO
O
O
O
HO
O
AcO
13
14
OH
AcO
OH
OAc
AcO
HO
HO
AcO
OH
O
O
AcO
AcO
HO
O
O
O
O
AcO
AcO
HO
HO
8c
HO
AcO
AcO
O
O
O
20
94
98
10
92
96
16c
15
HO
O
HO
AcO
15
16
OH
OAc
OH
AcO
HO
HO
OH
O
AcO
AcO
OAc
O
OMe
OMe
9
6
5
HO
AcO
17
18
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OH
OAc
O
O
HO
HO
AcO
SPh
SPh
10
11
10
10
92
98
8
93
97
19a
23
AcO
OH
AcO
20
19
OAc
OH
O
O
HO
HO
AcO
AcO
SMe
15
SMe
OH
21
OAc
22
OH
OAc
O
O
O
O
O
O
12
13
15
15
99
93
10
10
99
95
19a
19b
O
O
O
O
23
24
OTr
OTr
O
O
HO
HO
AcO
AcO
HO
AcO
OMe
SPh
OMe
SPh
25
26
OTr
O
OTr
O
HO
HO
AcO
AcO
14
15
16
17
10
60
5
92
95
96
99
8
45
4
93
93
95
98
ꢀꢀ
OH
OAc
27
28
O
O
O
O
HO
HO
29
AcO
AcO
30
24
25
25
OH
OAc
OH
OAc
OH OH
OAc OAc
OAc
OH
31
AcO
AcO
HO
HO
OAc OAc
OH OH
OH OH
32
OAc OAc
OAc
OH
33
60
25
OAc OAc
OH OH
34
CH2OH
O
CH2OAc
O
O
O
18
10
98
8
96
19a
15
AcO
OAc
HO
OH
6
6
36
35
OH
O
OAc
O
HO
HO
AcO
AcO
80
(2:1)
85
(2:1)
19c
120
60
AcNH OH
AcNH OAc
37
38
20
21
30
10
95
20
5
92
19a
16a
H
H
H
H
H
H
AcO
40
HO
39
OH
OAc
100
100
42
41
4
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HO
R1
AcO
22
20
90
10
92
16a
44
43
R1
R2
R3
R2
R3
R4
R4
45 R1 = OH, R2 = R3 =H, R4 = NO2
47 R1 = OH, R2 = R4 =H, R3 = NO2
46 R1 = OAc, R2 = R3 =H, R4 = NO2
48 R1 = OAc, R2 = R4 =H, R3 = NO2
23
24
25
25
25
5
98
97
95
15
15
5
96
95
95
16a
16b
26
49 R1 = COOH, R2 = R4 =H, R3 = OH 50 R1 = COOH, R2 = R4 =H, R3 = OAc
OAc
OAc
SAc
51
SAc
52
26
5
98
5
98
27
AcS
AcS
OAc
OAc
53 R1 = SH, R2 = R3 =H, R4 = Me
55 R1 = SH, R2 = R3 =H, R4 = NO2
54 R1 = SAc, R2 = R3 =H, R4 = Me
56 R1 = SAc, R2 = R3 =H, R4 = NO2
27
28
15
10
95
99
10
5
94
97
16b
28
29
30
99
ꢀꢀ
ꢀꢀ
15
3 (at 50 g scale)
4
30d
10
96
10
98
29
17
57
a The substrate was treated with Ac2O (1.1 mol equiv per OH/SH group) in the presence of either MeSA, 1.5ꢀ3 mol% (Method A) or SiSA, 1 mol %
(Method B) and the yields reported are those obtained after isolation of the pure product; b Was obtained as a mixture of furanose:pyranose, 1:1; c 2 mol%
SiSA was used under Method B; d For the propionylation 1.05 mol equiv. of propionic anhydride per OH group was used at room temperature.
5
reasons that a literature search on the applications of this acid
revealed that it has not yet been investigated in any detail for its
possible applications in some of the routine organic reactions
Thus, when SiSA (1 mol%) was added to a suspension of
glucose (1 g, 5.55 mmol) in Ac2O the sugar went into solution
Dꢀ
such as those described above. Nevertheless, besides being an 30 within 3 min and the subsequent TLC of the reaction mixture
10 excellent sulfonating agent, it indeed finds applications in
alkylation, halogenation, rearrangements, cyclization and
polymerization reactions.30 In view of these factors, alongside the
work on MeSA reported above, an investigation of the
revealed complete conversion of the sugar (3) to its pentaacetate
4 which was obtained in nearly quantitative yield (98%, entry 2,
Table 1). The same reaction was also performed using free CSA
in place of the bound form (SiSA) for comparison (results not
applications of CSA in the context of the acyl transfer reactions 35 presented in the Table). The yield of the pure product 4 obtained
15 described above was also undertaken. Further, as it was possible
to contain the corrosive and fuming nature of the acid by
immobilizing it on to the surface of silica, the silicaꢀbound acid
(SiSA) prepared in the lab was also investigated simultaneously
in the latter was also excellent, but up to 92% only. It was
observed that both reactions were exothermic in nature, as was
the case when it was carried out in the presence of MeSA, but
between the CSAꢀpromoted and the SiSAꢀpromoted acetylations,
(see Scheme 2) for its suitability in the acyl transfer reactions as 40 the latter was seen as undoubtedly more attractive. There were
20 enumerated above. Interestingly, the acid in both the free and the
bound forms were found to be exceedingly efficient for all the
acyl transfer reactions investigated as can be seen from the results
presented below.
two important differences observed in the two latter reactions.
Firstly, the reaction mixture in the case of the CSAꢀcatalyzed
acetylation invariably appeared slightly colored (light amber) in
contrast to the colorless reaction mixture that had always resulted
45 in the case of the SiSAꢀcatalyzed reactions. And secondly the
somewhat reduced yields obtained in the case of the reactions
employing the neat acid, CSA. On close observation it was found
that at the time of the addition of the neat acid to the sugar, the
solid particles (sugar) that happen to come into its immediate
50 contact gets partially burnt. However, as the dissolution of the
acid in Ac2O is also very fast, they become homogeneous with
each other very soon thereby leading to an effective dilution of
25
Scheme 2 SiO2ꢀOSO3Hꢀcatalysed acetylation of free sugars
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the acid. Thus the detrimental effect caused on the sugar substrate
(that affects the yield) gets fortuitously limited. It was found that
vigorous stirring at the time of the introduction of the acid could
effectively address this problem with nearꢀcomplete restoration of
compared, not surprisingly though because it is also employed as
the solvent in the reaction. Incidentally, it also must generate
significantly higher amounts of waste products.
In general, although environmental benefits accrued from
5
yields to those comparable to the SiSAꢀmediated reactions. 60 catalyst/byꢀproduct recovery in chemical reactions may outweigh
Addition of the acid prior to the addition of the sugar was also
useful in addressing this deficiency. In any case, it may be
recalled, the difficulty in handling the corrosive liquid in the case
of CSA when it is needed to be used neat indeed remains. In
the cost concern, considering the fact that the method using
MeSA is not only environmentally nonꢀhazardous but also is
cheap, a need for its recovery after the reaction was not felt very
rewarding (lest the cost of recovery overshoot the reagentꢀcost).
10 contrast, the SiSA reagent was very easy and convenient to 65 In the case of the SiSAꢀcatalyzed reactions also the catalyst is
handle. It can be seen from the results presented in Table 1 that
the SiSAꢀcatalyzed reactions are as efficient as those catalyzed by
MeSA although the latter may be rated as more green and hence
the catalyst of choice for the reactions described.
cheap but the possibility for an easy recovery of the silicaꢀbound
acid by simple filtration and washing does exist.
Table 3 Comparison of the reagentꢀcost involved in the perꢀOꢀ
15
70 acetylation of
Dꢀglucose on employing various catalysts for the
In the light of the above observations a comparison of the current
method of acetylation with some of the literature methods was
made using the acetonide 23, a somewhat difficult substrate
(sensitive to acidic conditions) as the example and the results are
reactiona
Entry Description Pack Priceb Required for
size
( ) 50 g Dꢀglucose
Amount Cost
( )
20 shown in Table 2. It was seen that the reaction took place very
successfully to afford the desired acetylated product in both the
MeSA ꢀ as well as the SiSAꢀcatalyzed reactions; and the yields
were as good as or better than the methods compared (Table 2).
While the literature methods require either controlled reaction
25 conditions or use of a solvent, the current protocols allow the
reaction to be performed solventꢀfree at room temperature.
1
2
3
4
5
6
7
8
9
Pyridine17 100 mL 3,551 250 mL 8877
DMAP18
DABCO19
Iodine14
In(OTf)3
Cu(OTf)2
25 g
25 g
1,690
2,950
27 g 1825
31 g 3658
100 g 1,520 2.5 g
38
7,8
25 g
5 g
5 g
3,114 100 mg 53
6,812 50 mg 68
10,666 150 mg 320
4,703 500 mg 94
9
10
Sc(OTf)3
La(OTf)3
Table 2 Comparison of the current method with some of the
reported methods using the acetonide 23 as the substrate for
30 acetylationa
11
25 g
33
CoCl2
25 g 23,786 1.8 g 1712
10 NH2SO3H15 100 g 1,968 10.7 g 212
Substrate
Catalyst
Time Yield of Ref.
11
MeSO3H 100 mL 3,692 0.2 mL
7
(min) 24 (%)
12 SiO2ꢀOSO3H 100 mL 11,660 1.0 g
7c
Pyridineb
Imidazolec
DABCO
Al2O3
Cu(OTf)2
Iodine
45
2
95
98
17
31
19
32
9
14
ꢀꢀ
ꢀꢀ
a The price of the reagent was based on the amount needed for a practicalꢀ
b
scale (of 50 g) reaction and was based on the respective literature;
25
12
5
30
15
10
98
Aldrich India catalogue price. Wherever possible the listed price of a
75 packꢀsize of 100 g (or mL) of the reagent was used for the calculation of
the cost. Wherever such a pack is not available commercially, the listed
price of 25 g or 5 g pack as applicable was used for the calculation. As far
as possible the cost of the same grade of the reagent was chosen for the
comparison. 100 = £1, approximately; c Price of the acid only has been
80 used for the calculation.
30d
80
98e
99
99
MeSO3H
SiO2ꢀOSO3H
a Substrate (1 g) was treated with Ac2O (1.2 mol equiv) at 25 °C in the
presence of either MeSA (2 mol%) or SiSA (1 mol%). For the literature
Acetylation using AcOH as the acyl donor reagent
b
catalysts, the method as reported was used; Pyridine also served as the
solvent; c MeCN was used as the solvent; d The reaction was not complete;
35 e The reaction was held at iceꢀbath temp or below.
Although the reactions using Ac2O reported thus far have been
solventꢀfree and only a small excess over the stoichiometrically
85 required acyl donor reagent had been employed in the
acetylations, considering using the free acid as the donor reagent
in the reaction wherever possible would also be in context. For
this reason a model reaction using (ꢀ)ꢀmenthol as the alcohol
substrate and HOAc (glacial) as the acyl donor was carried out in
90 the presence of MeSA (2 mol%, Scheme 3). It was found that a
neat reaction occurred and the acetate 4216a, 34 could be obtained
in quantitative yields when a menthol:AcOH mole ratio of 1:2
was used for the reaction (Scheme 3). Washing with cold aqueous
bicarbonate was the only major step involved in the isolation of
95 the product. At mole ratios of 1:1.2 and 1:1.5 the conversion of
the alcohol to the desired acetate occurred only to the extent of 90
and 95% respectively as determined by chromatography. The
latter reactions also required longer reaction time with the added
observation that extending the reaction time beyond 4 h did not
100 result in any significant enhancement in the per cent conversion
(Scheme 3). Measurement of the optical rotation of the product
isolated confirmed beyond doubt that the reaction did not affect
the optical purity of the chiral compound. It is noteworthy that for
On the MeSA ꢀcatalyzed acylations turning out to be efficient and
successful [with respect to factors such as: catalytic efficiencyꢀ
only 1.5ꢀ3 mol% catalyst in the case of MeSA and 1 mol% in the
40 case of SiSA), reaction time (short reaction time of a few minutes
to 1 h in most cases), reaction conditions (ambient temperatures
with no requirement of heating or cooling), solventꢀfree, yield
(nearly quantitative in most cases) and purity of product
(practically pure and suitable for further reaction with no
45 requirement for chromatographic purification)] as seen above, a
comparison of the cost of the reagent needed for the reaction was
attempted (Table 3). Both basic as well as acidic catalysts from
among the traditional and more recent categories were chosen for
the comparison using the amounts as specified in the respective
50 literature as the basis for the calculation of the cost. It can be seen
from the Table that the method using MeSA as the catalyst was
proved the cheapest among the lot. In general, it was observed
that, using basic catalysts was more expensive than others.
Pyridine, the traditional and one of the most trusted/popular
55 catalysts, was seen as the most expensive among the lot
6
|Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C4RA16916F
the same substrate HClO4ꢀSilica gave comparable yields of the
acetate, when used at 1 mol% level, in a reaction for 6 h at 80 °C
Conclusions
45 MeSO3H and SiO2ꢀOSO3H have been found to be two highly
efficient catalysts for acyl transfer reactions of monoꢀ, diꢀ and
trisaccharides and their various partially protected derivatives, as
well as various alcohols, phenols, thiols and thiophenols. The
method is tolerant to acid sensitive functional groups and can
50 therefore be present on the substrate for acylation. The new
method of acetylation using the green organic acid, MeSO3H, is
economical, solventꢀfree, and benign and serves to fulfil most
other qualities desired of a green method. Acetylation in the case
of an optically active alcohol, menthol by reaction with acetic
55 acid as well as other examples of successful alcoholꢀcarboxylic
acid esterifications, without affecting the chiral integrity of the
compound, clearly indicate future possibilities of application of
this method.
Menthol:AcOH Time Conversion
(mole ratio)
(h)
(%)
1:1.2
1:1.5
1:2
4
4
1
90
95
100
5
Scheme 3 Acetylation of (ꢀ)ꢀmenthol using acetic acid as the acyl
donor reagent (% Conversions were determined by GC & HPLC)
in excess of AcOH (35 mol equiv).16a The Kegginꢀtype
60 Experimental
10 heteropolyacids on the other hand had been reported to be
ineffective even under microwave irradiation.34
similar
esterification protocol for (ꢀ)ꢀmenthol catalyzed by
A
General
a
All reagents (Aldrich India Ltd) used were of analytical grade and
were used as purchased without further purification. Reactions
65 were monitored by TLC, which was performed with 0.2 mm
Merck preꢀcoated silica gel 60 F254 aluminium sheets.
Compounds were detected by dipping the TLC plates in an
ethanolic solution of sulphuric acid (5%, v/v) and heating or
using a reagent prepared by dissolving a mixture of 4.2 g
70 ammonium molybdate, 0.2 g cerium sulphate tetrahydrate and 6.2
mL sulphuric acid in 94 mL water. Column chromatography was
functionalized carbon material that appeared while this
manuscript was under preparation35 required heating in heptane
15 for a period of 8ꢀ10 h at 70 °C to be complete. Likewise, another
esterification reaction of the same kind, but using a functionalized
ionic liquidꢀbased mesoporous organosilica as the catalyst,
described during the preparation of this manuscript, also required
heating at 60ꢀ75 °C for 12ꢀ24 h for the reaction to be effective.36
20 As in the case of menthol, treatment of other alcohols such as nꢀ
octanol (58) and propylene glycol (60) with acetic acid also led to
the expected monoꢀ and diꢀOꢀacetates (59 and 61, respectively) in
near quantitative yields (entries 2 and 3, Table 4). Similarly,
treatment of other carboxylic acids such as cinnamic acid (62)
25 and tartaric acid (64) with EtOH also afforded with ease the
expected cinnamate 63 and tartrate 65 (monoꢀ and diesters)
respectively (entries 3 and 4, Table 4). It may be recalled that a
recently reported Nꢀmethylꢀboronopyridinium iodide40a required
heating at reflux for 18 h for the conversion of the dicarboxylic
30 acid 64 to its diester 65. Again, when sialic acid (66) was stirred
in anhydrous MeOH in the presence of catalytic MeSA, a
complete conversion of the acid to its methyl ester 67 also
occurred with ease. The latter reactions were conveniently held in
the respective alcoholic solutions, and on completion of the
35 reaction, were recovered at ease by distillation under reduced
pressure; and the products in these cases were crystalline and
sufficiently pure for subsequent reactions.
1
performed using silica gel (100ꢀ200 mesh ASTM). H/13C NMR
spectra were recorded in CDCl3 at 400/100 MHz on a Bruker
AvanceIII spectrometer. The chemical shifts have been reported in
75 ppm using TMS as the internal standard. The multiplicity of the
signals have been shown as, s = singlet, d = doublet, t = triplet, q
= quartet, m = multiplet) with the coupling constant (J value)
expressed in Hz. The anomeric ratio of products reported in Table
1 were determined from their NMR spectra. Mass spectra were
80 obtained on an Ultraflex TOF/TOF MALDI or Maxis HRMS
equipment. Specific rotations were determined on an AUTOPOL
IV polarimeter at 20 °C. IR spectra were recorded on a Nicolet
FTꢀIR Impact 410 instrument either as neat or KBr pellets.
Melting points were determined on a Buchi melting point
85 apparatus. All compounds reported here (except compound 28)
have been reported previously (2,15 4,15 6,15 8,15 10,15 12,21 14,15
16,16c 18,15 20,19a 22,22 24,19a 26,19b 30,23 32,24 34,24 36,19a 38,15
40,19a 42,16a 44,16a 46,16a 48,16b 50,25 52,26 54,16b 56,27 57,28 59,37
61,38 63,39 6540 and 6741), many are commercially available and
90 therefore only the spectral data for the new compound (28) are
reported.
Table 4 Esterification of various monoꢀ and dicarboxylic acids
40 under neat conditions in the presence of catalytic MeSA a
Entry
Acid
(R1ꢀCOOH)
Acetic
Alcohol
(R2ꢀOH)
(ꢀ)ꢀMenthol
(41)
Time
(h)
2
Product
(% Yield)
(ꢀ)ꢀMenthyl acetate
(42) (100)
Preparation of SiO2-sulfuric acid (SiSA) catalyst
CSA (1.45 g, 12.5 mmol) was added to a suspension of SiO2
95 (23.7 g, 230ꢀ400 mesh) in Et2O (70.0 mL). Release of HCl gas
that followed immediately after the addition of CSA was found to
subside in a short while. The mixture after stirring for a few
minutes was concentrated to dryness followed by heating at 100
οC for 48 h in a vacuum oven to furnish SiSA as a free flowing
100 powder. 10 mg of the powder contains approximately 0.005
mmol of the acid enough for the acetylation of a gram of the
sugar.
1
2
3
4
5
6
Acetic
nꢀOctanol (58) 10 Octyl acetate (59)
(98)
1,2ꢀDiacetoxy
propane (61) (97)
Acetic 1,2ꢀPropanediol
6
(60)
Ethanol
Cinnamic
(62)
15 Ethyl cinnamate
(63) (95)
(+)ꢀDiethylꢀ
tartrate (65) (99)
Lꢀ(+)ꢀ
Ethanol
12
Lꢀ
Tartaric (64)
Sialic (66)
Methanol
10 Sialic acid methyl
Typical Procedure for acetylation using Ac2O as the acyl
105 donor reagent: Acetylation of D-Glucose
ester (67) (98)
a The reactions were held at rt in the presence of 2 mol% MeSA.
This journal is © The Royal Society of Chemistry [year]
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Method A, using MeSO3H:
To a suspension of ꢀglucose (entry 2, Table 1, 1 g, 5.55 mmol)
Phenyl
2,3,4-
tri-O-acetyl-6-O-trityl-1-thio-β-D-
20
D
glucopyranosi-de (28): Colourless liquid; [
α]
= +1.2 (c = 1,
D
1
ο
in acetic anhydride (2.88 mL, 30.53 mmol) taken in a round
bottom flask was added, while being stirred at rt (approximately
30ꢀ35 ˚C), MeSA (5ꢀ7 ꢁL, 1.5ꢀ1.8 mol%) and the stirring was
continued until complete dissolution of the sugar occurred.
Warming of the reaction mixture occurred immediately following
the addition of the catalyst and was indicative of the onset of
acetylation. It was soon led to the dissolution of the sugar
CH2Cl2); H NMR (400 MHz, CDCl3, 25 C, TMS): δ (ppm) =
7.64ꢀ7.61 (m, 2H, ArH), 7.45ꢀ7.42 (m, 6H, ArH), 7.30ꢀ7.20 (m,
70 12H, ArH), (5.17 (t, J = 9.2 Hz, 1H, Hꢀ2), 5.09 (dd, Ja = 9.8 Hz,
Jb = 6.8 Hz, 1H, Hꢀ3), 5.04 (dd, Ja = 7.2 Hz, Jb = 9.1 Hz, 1H, Hꢀ
4), 4.73 (d, J = 10.0 Hz, 1H, Hꢀ1), 3.58 (m, 1H, Hꢀ5), 3.27 (dd, Ja
= 2.1 Hz, Jb = 10.1 Hz, 1H, Hꢀ6a), 3.15 (dd, Ja = 5.2 Hz, Jb = 10.5
Hz, 1H, Hꢀ6b), 2.09 (s, 3H, COCH3), 1.96 (s, 3H, COCH3), 1.69
75 (s, 3H, COCH3). 13C NMR: (100 MHz, CDCl3, 25 οC): δ (ppm) =
170.3 (CO of COCH3), 169.3 (CO of COCH3), 168.9 (CO of
COCH3), 143.5, 133.2, 131.8, 129.0, 128.7, 128.3, 127.8, 127.0,
86.7 (Cꢀ1), 85.6, 74.3, 70.1, 68.3, 62.0, 20.8 (CH3 of COCH3),
20.6 (CH3 of COCH3), 20.3 (CH3 of COCH3); IR (cmꢀ1): 2924,
80 2853, 2104, 1756, 1634, 1490, 1448, 1373, 1242, 1218, 1155,
1047, 912, 765, 748, 703, 632. HRMSꢀESI (m/Z) calculated for
C37H36NaO8S, [M + Na] + 663.2029, found 663.2021.
5
10 indicating completion of the reaction which was confirmed by
TLC (EtOAc: nꢀHeptane, 2:3, v/v) by comparison with authentic
pentaꢀOꢀacetyl
ꢀD
ꢀglucopyranose.
A
colorless homogeneous
solution was obtained within a few minutes. The reaction mixture
was poured into a beaker containing crushed ice and aqueous
15 NaHCO3 during stirring which resulted in the separation of the
product as a colorless solid. It was left for a few hours in a
refrigerator for solidification of the product to complete. The
solids were separated by filtration at the pump and, after washing
with cold deionized water to neutrality, was dried (first in the air
20 and then in a vacuum oven/Kugelrohr) when the title product was
obtained (2.14 g) as a colorless solid (99% yield).
In large scale preparations (5ꢀ50 g or more), the sugar was added
portionꢀwise to acetic anhydride containing the catalyst (180 ꢁL
MeSA in 145 mL Ac2O for a 50 gramꢀscale reaction) and upon
25 completion of the reaction, the product was isolated as described
above and was obtained (107 g pentaacetate from 50 g of
glucose) as white powder in near quantitative yield.
Acknowledgements
85 S.K.G. thanks NIPER, S.A.S. Nagar and the Ministry of
Chemicals and Fertilizers, Govt. of India for permission to pursue
PhD under its staff development scheme.
Notes and references
aDepartment of Medicinal Chemistry, National Institute of
90 Pharmaceutical Education and Research, SAS Nagar, Punjab 160062,
India, *E-mail: rkartha@niper.ac.in
Method B, using SiO2-OSO3H (SiSA):
30 To a suspension of Dꢀglucose (entry 2, Table 1, 1 g, 5.55 mmol)
†Electronic Supplementary Information (ESI) available: [Archive entries
of 1H and 13C spectra of parent compounds synthesized.]. See
DOI: 10.1039/b000000x/
in acetic anhydride (2.88 mL, 30.53 mmol) taken in a round
bottom flask was added, while being stirred at rt (approximately
30ꢀ35 ˚C), SiSA (10 mg, 1 mol%) and the stirring was continued
until complete reaction occurred. The product was isolated as
35 described under Method A above. The title compound was
obtained in near quantitative yield as colourless solid (2.14 g,
99%).
95
1
2
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105
110
115
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Acylation using free carboxylic acid as the acyl donor
40 reagent: Acetylation of (-)-menthol (41)
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temperature and the stirring was continued for an hour. TLC at
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45 reaction mixture was poured into crushed ice in a beaker
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separatory funnel. Yield, 1.26 g, quantitative, colorless liquid.
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50 literature data.16a
4
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7
8
Esterification of tartaric acid: Preparation of (+)-diethyl-L-
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55 acid (64, 1 g, 6.6 mmol) in absolute alcohol (50 mL) and the
solution was stirred for 12 h at room temperature when TLC
(EtOAc:nꢀHept) showed complete disappearance of the tartaric
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60 EtOAc (25 mL) and was washed successively with cold aqueous
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ꢀtartrate (65).40
65
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DOI: 10.1039/C4RA16916F
Graphical Abstract
Acyl transfer reactions of carbohydrates, alcohols, phenols, thiols and thiophenols under green
reaction conditions
5
Santosh Kumar Giri and K. P. Ravindranathan Kartha