Orthogonal protection of saccharide polyols through solvent-free one-pot sequences based on regioselective silylations

Article abstract of DOI:10.3762/bjoc.12.271

tert-Butyldimethylsilyl (TBDMS) and tert-butyldiphenylsilyl (TBDPS) are alcohol protecting groups widely employed in organic synthesis in view of their compatibility with a wide range of conditions. Their regioselective installation on polyols generally r

Full text of DOI:10.3762/bjoc.12.271

Orthogonal protection of saccharide polyols through solvent-
free one-pot sequences based on regioselective silylations
Serena Traboni, Emiliano Bedini and Alfonso Iadonisi*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 2748–2756.
Department of Chemical Sciences, University of Naples Federico II,
Via Cinthia 4, 80126, Naples, Italy
doi:10.3762/bjoc.12.271
Received: 30 August 2016
Accepted: 30 November 2016
Published: 14 December 2016
Email:
Alfonso Iadonisi* - iadonisi@unina.it
* Corresponding author
Associate Editor: S. Flitsch
Keywords:
© 2016 Traboni et al.; licensee Beilstein-Institut.
License and terms: see end of document.
carbohydrates; one-pot synthesis; regioselective protection; silyl
protecting group; solvent-free reaction
Abstract
tert-Butyldimethylsilyl (TBDMS) and tert-butyldiphenylsilyl (TBDPS) are alcohol protecting groups widely employed in organic
synthesis in view of their compatibility with a wide range of conditions. Their regioselective installation on polyols generally
requires lengthy reactions and the use of high boiling solvents. In the first part of this paper we demonstrate that regioselective sily-
lation of sugar polyols can be conducted in short times with the requisite silyl chloride and a very limited excess of pyridine
(2–3 equivalents). Under these conditions, that can be regarded as solvent-free conditions in view of the insolubility of the polyol
substrates, the reactions are faster than in most examples reported in the literature, and can even be further accelerated with a cata-
lytic amount of tetrabutylammonium bromide (TBAB). The strategy proved also useful for either the selective TBDMS protection
of secondary alcohols or the fast per-O-trimethylsilylation of saccharide polyols. In the second part of the paper the scope of the
silylation approach was significantly extended with the development of unprecedented “one-pot” and “solvent-free” sequences
allowing the regioselective silylation/alkylation (or the reverse sequence) of saccharide polyols in short times. The developed meth-
odologies represent a very useful and experimentally simple tool for the straightforward access to saccharide building-blocks use-
ful in organic synthesis.
Introduction
The application of an orthogonal set of protecting groups repre- with many other used alcohol protecting groups [1-3]. For this
sents a typical issue in organic synthesis in the elaboration of reason, silyl protecting groups are often serving as temporary
highly functionalized molecules such as carbohydrates, and protecting groups with polyol and saccharide substrates. The
lengthy multistep procedures are often needed to this aim [1,2]. most robust and adopted silyl protecting groups are featuring
Silyl groups are widely applied in organic chemistry in orthogo- the presence of hindered substituents at silicon such as in tert-
nal protection strategies owing to their stability to a broad range butyldimethylsilyl (TBDMS) and tert-butyldiphenylsilyl
of conditions and feasible removal under conditions compatible (TBDPS) groups. Their bulkiness allows in many cases regiose-
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Beilstein J. Org. Chem. 2016, 12, 2748–2756.
lective silyl protection of primary alcohols. Commonly, O-sily- effort led to a simple protocol for the selective benzylation of
lation is performed by exposing the alcohol to a suitably substi- primary saccharide alcohols [34], the first catalytic tin-medi-
tuted silyl chloride in the presence of a base, a catalyst (often ated procedure for regioselective benzylation/allylation of
coinciding with the base) and an aprotic solvent [2-9]. The use hydroxy groups incorporated into vicinal diols [35], and three
of more expensive silyl triflates is also reported, especially alternative acetalation protocols [36]. Besides the avoided use
when poorly reactive alcohols have to be protected [2,10-13]. of solvents, these approaches appear advantageous owing to
their experimental ease, the reactions being performed under air
A regioselective silylation of polar saccharide polyols is typical- by simply mixing the requisite reagents.
ly performed with the appropriate silyl chloride in the presence
of a high boiling solvent such as DMF or pyridine, often in the In this paper, we wish to report the extension of the scope of the
presence of a nucleophilic catalyst (more frequently imidazole solvent-free strategies to the silylation reaction, and the feasible
and DMAP) [14-21]. The protection generally takes several incorporation of this step into unprecedented one-pot, fully sol-
hours and the work-up is burdened by necessary removal of the vent-free sequences yielding orthogonally protected saccharide
high boiling solvent. A good regioselective control was also re- building-blocks under simple experimental conditions and in
ported in an alternative silylation approach based on a dehydro- short times.
genative mechanism in which expensive trialkyl silanes were
used as silylating agents [22]. Very recently, Vogel and Results and Discussion
co-workers reported an original strategy based on unusual sily- In preliminary experiments, methyl mannopyranoside (1) was
lating agents such as silyl methallylsulfinates; this approach selected as the model substrate and exposed to TBDMSCl in
proved very high-yielding under neutral conditions, but the pre- the presence of a slight or moderate excess of several bases
liminary synthesis of the requisite reagent relied on a non-trivial (Table 1).
two-step procedure starting from the corresponding silyl chlo-
ride [23].
In all cases 6-O-silylated derivative 2 was obtained as the main
product, but yields and rates were strongly dependent on the
The regioselective silylation of secondary saccharide alcohols adopted base: pyridine (Table 1, entries 6–9) gave much better
can also be achieved either taking advantage of the inherent results than tertiary amines (Table 1, entries 1–5) that, on the
difference of reactivity among the hydroxy functions [24-32] or other hand, had previously performed better than pyridine in the
exploiting the activation effect of boron complexes [33].
tin-catalyzed solvent-free regioselective benzylation or allyla-
tion of sugars [35]. The silylation rate was not appreciably
Over the last years, we have addressed our interest towards the influenced by tin catalysis (compare entries 2 and 3 in Table 1),
development of solvent-free protocols aimed at regioselective although a previous report described the stoichiometric use of
protection of highly functionalized saccharide substrates. This stannylene acetals in the regioselective silylation of saccharide
Table 1: Regioselective silylation of 1 under solvent free conditionsa.
Entry
Base (equiv)
Additive (equiv)
Temperature, time
Isolated yield
1
2
3
4
5
6
7
8
9
DIPEA (5)
DIPEA (5)
DIPEA (5)
DIPEA (5)
TEA (5)
–
50 °C, 5 h
50 °C, 5.5 h
50 °C, 5.5 h
50 °C, 5.5 h
50 °C, 5.5 h
50 °C, 1 h
rt, 1 h
<15
49
52
48
53
89
80
84
70
TBAB (0.3)
TBAB (0.3), Bu2SnO (0.1)
TBAI (0.3), Bu2SnO (0.1)
TBAB (0.3)
pyridine (5)
pyridine (2.5)
pyridine (2.2)
pyridine (2.2)
TBAB (0.3)
TBAB (0.3)
TBAB (0.1)
rt, 1.5 h
–
rt, 1.5 h
aGeneral conditions: substrate, base, additive, TBDMSCl (1.2 equiv for entries 1–6, 1.1 equiv for entries 7–9).
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Beilstein J. Org. Chem. 2016, 12, 2748–2756.
primary alcohols [37]. At this stage, it should be noted that pyri- yield (Table 2, entry 5), whereas the corresponding α-anomer
dine is frequently employed as the solvent for silylations, but was almost quantitatively recovered (Table 2, entry 4). As will
the conditions herein described can be referred to as “solvent- be shown below, partial protection of allyl α-galactoside in
free” because of the very limited amount of pyridine used, that itinere can render this substrate much more reactive towards
is by far not sufficient for dissolution of the polar polyol sub- these silylation conditions. High-yielding silylation of allyl
strates. Interestingly, reactions with pyridine were found to give β-galactoside (6) required the employment of a higher stoichio-
slightly improved yields (within comparable times) on using a metric excess of both pyridine and TBDMSCl (Table 2, entry
catalytic amount of TBAB (compare in Table 1 entries 6–8 with 5), exhibiting an extent of conversion apparently ruled by an
entry 9), which may be accounted for by a possible role of the equilibrium-like control.
bromide ion in the activation of the silylating agent as also sug-
gested by literature [38]. Taking into account several parame- Among the screened polyols in Table 2, glycals 7 and 8 were
ters such as the used amount of pyridine and TBAB, the reac- the only substrates to exhibit a lower regioselectivity, the silyla-
tion yield and its length, we elected conditions of Table 1, entry tion rate being comparable at the primary (O-6) and the allylic
8 as the optimized conditions. Notably, the conversion of 1 to 2 (O-3) position, to provide a mixture of products (Table 2,
under these conditions took a much shorter time than previ- entries 6 and 7). A similar competitive reactivity was very
ously reported by using the same silylating agent [14,15,18]. recently described also with the above mentioned method based
With optimized conditions in hand, the TBDMS regioselective on silyl methallylsulfinates [23]. On the other hand, the method
installation was tested on a range of saccharide building-blocks, herein proposed was found compatible with a reducing sugar
and good yields were achieved in short times with several such as D-mannose, which was converted in a good yield into
polyols (Table 2, entries 1–3, 5 and 8).
the corresponding 6-O-silylated product 13, isolated after in situ
peracetylation (Table 2, entry 8).
Interestingly, galacto-configured substrates exhibited a pecu-
liar reactivity with the reaction outcome depending on their The scope of the TBAB-catalyzed silyl protection under sol-
anomeric configuration. In accordance with yields obtained by vent-free conditions was next examined for the regioselective
Lee and Taylor with similar substrates under standard condi- attachment of TBDPS (Table 2, entries 9–13), a commonly used
tions [14], only allyl β-galactoside (6) was silylated in a good silyl protecting group bulkier than TBDMS and more resistant
Table 2: Regioselective silylation of saccharide primary alcohols under solvent-free conditionsa.
Entry
Substrate
Pyridine/silylating agent/TBAB
(equivalents)b
Time (h)
Product, isolated yield
pyridine/TBDMSCl/TBAB
(2.2:1.1:0.1)
1
1.5
2, 84%
1
3
pyridine/TBDMSCl/TBAB
(2.2:1.1:0.1)
2
3
1.5
1.5
10, 83%
pyridine/TBDMSCl/TBAB
(2.2:1.1:0.1)
4
5
11, 86%
pyridine/TBDMSCl/TBAB
(2.2:1.1:0.1)
4
1.5
low conversion
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Table 2: Regioselective silylation of saccharide primary alcohols under solvent-free conditionsa. (continued)
pyridine/TBDMSCl/TBAB
5
2.5
(3.0:2.0:0.1)
6
7
8
12, 82%
pyridine/TBDMSCl/TBAB
(2.2:1.1:0.1)
6
7
1.5
1.5
2
complex mixture
pyridine/TBDMSCl/TBAB
(2.2:1.1:0.1)
complex mixture
pyridine/TBDMSCl/TBAB
(2.2:1.1:0.1)
8c
9
1
1
13, 73% (α/β 1:2)
pyridine/TBDPSCl/TBAB
(2.2:1.1:0.2)
9
3
3
14, 68%
14, 92%
pyridine/TBDPSCl/TBAB
(3.0:1.1:0.2)
10
11
pyridine/TBDPSCl/TBAB
(3.0:1.1:0.2)
3
3
15, 84%
pyridine/TBDPSCl/TBAB
(3.0:1.1:0.2)
12
13
4
7
2
5
16, 98%
pyridine/TBDPSCl/TBAB
(3.0:1.1:0.2)
very sluggish
aGeneral conditions: polyol substrate, pyridine, silylating agent (TBDMSCl or TBDPSCl), and TBAB at rt. See pertinent entries for stoichiometric
ratios. bWith respect to the polyol substrate. cThe crude silylation mixture was acetylated in situ (direct addition of pyridine and acetic anhydride), prior
to purification of the product.
to acidic conditions. Optimization on mannoside 1 indicated The only disappointing result observed in attempted TPDPS
that in this case a moderate increase of the pyridine excess (3.0 protections was the poor yield recorded with glucal 7 (Table 2,
instead of 2.2 equivalents) is beneficial for the achievement of a entry 13), a surprising outcome which is somehow consistent
higher yield while maintaining the use of a minimal excess of with the very slow rate observed for the same reaction under
the silylating agent (Table 2, entries 9 and 10). As with TBDMS standard conditions [39,40]. As already observed for the synthe-
protection of 6 (Table 2, entry 5), in this case a sort of steady sis of 2, the herein described conditions for regioselective
state was observed when adopting less than three equivalents of TBDMS and TBDPS protections entail shorter reaction times
pyridine, with coexistence of the reagent and the product.
than most of the reported protocols in the literature on monosac-
charide polyols [14-18,20,41-53]; comparable silylation rates
were indeed be found in a few examples, often involving rela-
tively less polar thioaryl glycosides as the substrates and DMF
as the solvent [19,54-59]. Having established the scope of
The TBDPS selective protection of polyols 3 and 4 proceeded
in high yields (Table 2, entries 11 and 12), and expectedly the
reactions took slightly longer times than for TBDMS protection.
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TBAB-catalyzed mono silylations with a minimal excess of an anomeric mixture) was 3-O-silylated in a good yield, and the
pyridine, some effort was devoted to ascertain the feasible anomeric composition of the products revealed the higher reac-
exploitation of a similar strategy to either regioselective di-O- tivity of the α-anomer (Table 3, entry 5). The same regioselec-
silylations or the protection of secondary alcohols in absence of tivity was also found starting from the corresponding 4-O-
primary ones (Table 3).
benzoylated precursor 24 [61] (Table 3, entry 6) and a satis-
fying yield and an excellent conversion were observed in spite
As a matter of fact, upon doubling the stoichiometric amount of of the increased hindrance and deactivation of the O-3 hydroxy
TBAB, pyridine and TBDMSCl, the regioselective synthesis of group due to the adjacent electron-withdrawing benzoyl group.
di-O-TBDMS derivatives was achieved at 50 °C in satisfying As already described above in Table 1 for mono-silylations, tin
yields from glycosides 1 and 3 (Table 3, entries 1 and 2), and catalysis did not affect the double silylation processes as evi-
glycal 8 (entry 3).
denced by the reaction of Table 3, entry 1 that in the presence of
0.1 equiv of Bu2SnO gave 17 in the same yield within the same
Double silylation at primary positions of the disaccharide lacto- time. Not unexpectedly, secondary alcohols exhibited a recalci-
side 20 [60] also proved feasible at room temperature within trant reactivity towards TBDPSCl under these solvent-free
short times (Table 3, entry 4). Model substrates devoid of a pri- conditions, and the di-O-silylation process resulted in limited
mary alcohol were also examined; thioethyl rhamnoside 22 (as synthetic usefulness (data not shown).
Table 3: Solvent-free regioselective silylations committing secondary alcoholsa.
Entry
1
Substrate
Time (h)
Product, isolated yield
1
4
17, 72%
2
3
3
8
4.5
5
18, 60%
19, 56%
4
1
20
21, 56%
5
6
6
6
22 (α:β ca. 2.5)
24 (α:β ca. 2.5)
23 (α:β ca. 4), 75%
25 (α:β ca. 4), 50% (95%)b
aGeneral conditions (entries 1–3): pyridine (5 equiv), TBDMSCl (2.5 equiv), TBAB (0.3 equiv), 50 °C. For entry 4: pyridine (6 equiv), TBDMSCl
(3.5 equiv), TBAB (0.3 equiv), rt. For entries 5 and 6: pyridine (3 equiv), TBDMSCl (1.5 equiv), TBAB (0.15 equiv), 50 °C. bIn parenthesis is indicated
the conversion yield.
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Beilstein J. Org. Chem. 2016, 12, 2748–2756.
The scope of the solvent-free conditions was further examined product 29 was obtained in a satisfying 61% yield within a few
in the synthesis of per-O-trimethylsilylated derivatives hours (Table 4, entry 2). It should be outlined that this result is
(Scheme 1), widely used precursors in one-pot strategies for or- especially relevant taking into account that the solvent-free 3-O-
thogonal protection of carbohydrates [62-67].
benzylation alone (the first step of the sequence) occurs in a
comparable yield under tin catalysis [35]. The reverse protec-
Application of the TBAB-catalyzed protocol on glucoside 3 tion sequence (6-O-silylation/3-O-benzylation) was found to be
gave product 26 in high yield within a few minutes (Scheme 1, less effective in terms of yields, apparently because of the
reaction 1). The method proved also applicable to glucosamine partial loss of the TBDMS group in the second step where rela-
hydrochloride, although in this case a higher excess of pyridine tively forced thermal conditions are needed (Table 4, entry 3).
was needed for the conversion to occur (Scheme 1, reaction 2). This latter one-pot sequence was instead much more rewarding
Consistent with previous literature reports [68-72], silylation when the more robust TBDPS group was installed first
left unaltered the amino functionality which could be protected (Table 4, entries 5 and 6). Interestingly, this latter group was
in situ with a Troc group without isolation of 27 [69], as shown satisfyingly stable under the especially forced conditions (a
in the one-pot, two-step sequence in Scheme 1, reaction 3.
reaction temperature of 90 °C) required to carry out the tin-
mediated 2-O-benzylation of a gluco-substrate (Table 4, entry
Owing to the importance of the one-pot functional diversifica- 6). In addition, the TBDPS installation also proceeded in satis-
tion of carbohydrates in modern organic synthesis [62-67,73- fying yields after a preliminary allylation (Table 4, entry 4) or
76], the scope of the solvent-free silylation approaches herein benzylation step (Table 4, entry 8).
introduced was next extended to the development of fully sol-
vent-free one-pot sequences leading to the sequential alkylation/ In the case of glycal 8, the application of a benzylation/silyla-
silylation of saccharide polyols with high regiocontrol. For this tion sequence resulted in very good yields with both silylating
purpose, we tried to combine the previously reported tin-cata- agents (Table 4, entries 7 and 8), although the 6-O-silylation
lyzed procedure for benzylation/allylation of saccharide second- alone had been low yielding in the initial set of experiments
ary alcohols [35] with the present protocol for silylation of pri- (Table 2, entries 6, 7 and 13). Comparison of these data seems
mary alcohols. Some experiments were carried out on methyl to indicate that the silylation step is much more effective on less
mannoside (1) in order to establish which order of steps (alkyl- polar, partially protected polyols. As a further evidence of this
ation/silylation or the reverse sequence) might be higher trend, the 3-O-benzylation/6-O-silylation sequence was succes-
yielding. Initial experiments indicated that the silylation step fully applied to access α-galactoside 35 (Table 4, entry 9), al-
was not apparently effective when performed after the tin-cata- though, as shown earlier in Table 2, entry 4, direct silylation of
lyzed 3-O-benzylation of mannoside (1, Table 4, entry 1). A the same precursor was quite unfruitful.
competitive reaction of residual benzyl bromide from the first
step with pyridine may account for this result. Indeed, on Conclusion
repeating the experiment suitably increasing the amount of pyri- In the first part of this paper we have introduced a very simple
dine in the second step (from 2.2 to 5 equivalents), the desired approach to carry out the selective TBDMS or TBDPS protec-
Scheme 1: Multiple O-trimethylsilylations of saccharide compounds.
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Beilstein J. Org. Chem. 2016, 12, 2748–2756.
Table 4: One-pot, regioselective protection of sugar polyols with silyl and alkyl groupsa.
Entry
Substrate
First step conditions (equiv)
Second step conditions (equiv)
Product, isolated yield
low silylation yields
1
2
1
1
DIPEA (2.5), Bu2SnO (0.1), TBAB pyridine (2.2), TBDMSCl (2.0),
(0.3), BnBr (4), 70 °C, 3.5 h
rt, 3 h
DIPEA (2.5), Bu2SnO (0.1), TBAB pyridine (5.0), TBDMSCl (2.0),
(0.3), BnBr (4), 70 °C, 3.5 h
rt, 3 h
29, 61%
29, 39%
3
4
1
1
pyridine (2.2), TBDMSCl (1.1),
TBAB (0.3), rt, 1.5 h
DIPEA (3.5), Bu2SnO (0.1),
BnBr (5.5), 70 °C, 4.5 h
DIPEA (4), Bu2SnO (0.1), TBAB
(0.3), AllBr (8), 90 °C, 3.5 h
pyridine (5.0), TBDPSCl (1.5),
rt, 3.5 h
30, 59%
31, 62%
5
6
1
3
pyridine (3.0), TBDPSCl (1.1),
TBAB (0.3), rt, 3 h
DIPEA (2.5), Bu2SnO (0.1),
BnBr (6), 80 °C, 4 h
pyridine (3.0), TBDPSCl (1.1),
TBAB (0.3), rt, 3 h
DIPEA (2.5), Bu2SnO (0.2),
BnBr (7), 90 °C, 4 h
32, 42%
33, 69%
34, 75%
7
8
9
8
8
5
DIPEA (2.5), Bu2SnO (0.1), BnBr
(2), TBAB (0.3) 70 °C, 2.5 h
pyridine (3.5), TBDMSCl (2.0),
TBAB (0.3), rt, 1 h
DIPEA (2.5), Bu2SnO (0.1), BnBr
(2), TBAB (0.3), 70 °C, 2.5 h
pyridine (3.5), TBDPSCl (1.5),
rt, 2.5 h
DIPEA (2.5), Bu2SnO (0.1), BnBr
(4), TBAB (0.3), 70 °C, 2 h
pyridine (6), TBDMSCl (2), rt,
1.5 h
35, 57%
aGeneral conditions: upon completion of the first step (see times in pertinent entries), the temperature was modified according to conditions of the
second step, and the requisite reagents added.
tion of carbohydrate polyols taking advantage of reactions per- proposed method is endowed with further practical advantages
formed in the presence of a very limited amount of pyridine such as the experimental ease (all reaction herein reported were
(3 equivalents or less for the hydroxy group to be protected). conducted in air by simply mixing the reagents), and high reac-
The method can indeed be regarded as a solvent-free approach tion rates, often comparing favourably with literature examples,
because of the limited stoichiometric amount of the base that is in spite of the poor solubility of the starting substrates in the
not sufficient to dissolve the highly polar saccharide substrates. reaction medium. The scope of the silylation approach was also
Under these conditions, a catalytic role played by TBAB was extended to secondary alcohols and to the per-O-trimethylsily-
also evidenced. Besides the limited amount of the used base, the lation of saccharides.
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12.Boschelli, D.; Takemasa, T.; Nishitani, Y.; Masamune, S.
In the second part of the paper, we have explored the feasible
combination of the silylation methodology here introduced with
a previously developed tin-catalyzed methodology for regiose-
lective alkyl protection of carbohydrates. The merging of both
methods led to unprecedented one-pot and fully solvent-free
synthetic sequences, providing a straightforward and experi-
mentally simple access to saccharide building-blocks orthogo-
nally protected with a silyl at the primary position and a benzyl
(or an allyl) group at well defined and predicatable secondary
positions.
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Supporting Information
Supporting Information File 1
Experimental and analytical data.
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Acknowledgements
This paper is dedicated to Prof. Michelangelo Parrilli on the
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