Stereoselective β-mannosylations and β-rhamnosylations from glycosyl hemiacetals mediated by lithium iodide

Article abstract of DOI:10.1039/d1sc01300a

Stereoselective β-mannosylation is one of the most challenging problems in the synthesis of oligosaccharides. Herein, a highly selective synthesis of β-mannosides and β-rhamnosides from glycosyl hemi-acetals is reported, following a one-pot chlorination,

Full text of DOI:10.1039/d1sc01300a

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Stereoselective b-mannosylations and b-
rhamnosylations from glycosyl hemiacetals
Cite this: Chem. Sci., 2021, 12, 10070
mediated by lithium iodide†
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Imlirenla Pongener,
Dionissia A. Pepe, Joseph J. Ruddy
*
and Eoghan M. McGarrigle
Stereoselective b-mannosylation is one of the most challenging problems in the synthesis of
oligosaccharides. Herein, a highly selective synthesis of b-mannosides and b-rhamnosides from glycosyl
hemi-acetals is reported, following a one-pot chlorination, iodination, glycosylation sequence employing
cheap oxalyl chloride, phosphine oxide and LiI. The present protocol works excellently with a wide range
of glycosyl acceptors and with armed glycosyl donors. The method doesn't require conformationally
restricted donors or directing groups; it is proposed that the high b-selectivities observed are achieved
via an SN2-type reaction of a-glycosyl iodide promoted by lithium iodide.
Received 4th March 2021
Accepted 16th June 2021
DOI: 10.1039/d1sc01300a
rsc.li/chemical-science
necessary to use protecting groups that enforce conformational
control, or to use a directing group/intramolecular approach.
Introduction
Herein, we demonstrate proof-of-principle that b-man-
nosylations can be achieved without relying on specic protect-
ing group patterns to enforce conformational control or directing
group effects. A simple, practical, method for b-mannosylation
and b-rhamnosylation with broad scope is reported.
Carbohydrates are ubiquitous in nature; the heterogeneity and
complexity of biological glycosylation means that chemical
synthesis is required to access pure oligosaccharides for study. A
key challenge in the synthesis of oligosaccharides is the creation
of 1,2-cis-linkages, and of these, b-mannosides and b-rhamno-
sides are particularly important but difficult to make.1,2 Biologi-
cally important examples include high mannose-type N-linked
glycans,3 and pathogenic bacteria.4–8 Mannosylations and rham-
nosylations tend to show a high preference for formation of a-
glycosides unless special measures are taken, as both the
anomeric effect and neighbouring group participation favour a-
linkages, and approach to the b-face induces steric clashes with
the 2-O-substituent.9,10 Established methodologies for achieving
stereoselective b-mannosylations11,12 rely on: (a) conformational
control13–15 (b) intramolecular aglycone delivery16–23 (c) use of
directing groups22 and (d) anomeric O-alkylations (Scheme
1A).24,25 A general drawback of most of these approaches is the
need for donors bearing specic protecting group patterns (oen
requiring long synthetic sequences), which can be restrictive in
designing multi-step syntheses of complex molecules. Koenigs–
Knorr glycosylations employing mannosyl halides using insol-
uble silver salts is not in widespread use, possibly because of the
outcome being quite dependent on the donor/acceptor identities
(Scheme 1B).1,26 The prevailing paradigm seems to be that in
order to achieve high b-selectivity in mannosylation it is
Results and discussion
Previously, we reported the transformation of glycosyl hemi-
acetals to chlorides using catalytic Appel conditions.27–29 Our
Centre for Synthesis & Chemical Biology, UCD School of Chemistry, University College
Dublin, Beleld, Dublin 4, Ireland. E-mail: eoghan.mcgarrigle@ucd.ie
Scheme 1 (A) Key intermediates used in established methods for
accessing b-mannosides; (B) mannosylations mediated by insoluble
silver salts; (C) b-mannosylations and rhamnosylations in this work.
† Electronic supplementary information (ESI) available: Detailed experimental
procedures, characterisation, scope limitations and copies of NMR spectra. See
DOI: 10.1039/d1sc01300a
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efforts to intercept the postulated alkoxyphosphonium inter- (MeBn) proved to be excellent substrates. The MeBn group has
mediate30–37 with an acceptor in direct glycosylations were been reported to be orthogonal to Bn, Bz, and PMB groups44 and
unsuccessful (a one-pot protocol employing a urea catalyst38–41 thus donor 1c could be an efficient building block in 1,4-b-
was successful). We hypothesised that if glycosyl iodides could mannan synthesis. Donors 1g,h gave only moderate yields,
be generated in situ from hemiacetals using a modication of although high to excellent selectivity was still obtained. In the
the catalytic Appel conditions, then a one-pot glycosylation case of benzoyl-protected donor 1h, the yield was lowered due to
could be developed. During these investigations we made formation of elimination product, competing trans-
a serendipitous discovery of conditions that led to highly esterication under glycosylation conditions and subsequent
selective b-mannosylation and b-rhamnosylation using simple intramolecular reaction to give an anhydro sugar. Disarmed
donors (Scheme 1C).
donors e.g., peracetylated mannose showed poor reactivity and
Based on procedures by Denton28 and Konzer,42 we chose LiI gave complex mixtures under our conditions, elimination to
in CHCl3 for halide metathesis.43 We had previously explored give glycals and migration of acetyl groups to acceptor 3a were
various solvents for the synthesis of glycosyl chlorides from noted (see limitations section in ESI†). However, pivaloyl-
glycosyl hemiacetals and found CHCl3 and CH2Cl2 to be the best protected donor 1i gave excellent yield and no trans-
but CHCl3 was found to be superior for the following reac- esterication was observed. We demonstrated the method on
tions.27 We found that a one-pot, two-stage process generated 1 mmol scale for the synthesis of 2a, which was successful at
ꢀ
ꢀ
mannoside 2a in high yield and b-selectivity (Table 1). Thus, 30 C (on this scale at 45 C the a : b ratio was 10 : 90).
glycosyl chloride was generated using Ph3PO/(COCl)2 (Scheme A range of acceptors were also tested to establish the scope of
1C), and following removal of (COCl)2, LiI, base and alcohol the method (Table 2). For secondary alcohols, a larger excess of
were added. Following some exploration, we settled on the iPr2NEt was found to give better conversion to the desired
reaction conditions shown in Table 1 and tested the generality glycoside (more unreacted acceptor and elimination product
of the procedure for mannosyl hemi-acetal donors. Excellent resulted with lower amounts of base). High b-selectivity was
selectivity and yields were obtained with armed mannose obtained with a wide range of glycosyl acceptors. Mono-
donors 1a,d–f bearing orthogonal ether and silyl ether protect- saccharide acceptors bearing primary and secondary alcohols at
ing groups. Donors 1b and 1c, bearing the 4-methylbenzyl group positions 3 and 4 were tolerated. In the case of benzylidene-
protected acceptor 3d, some cleavage of the acetal was also
observed which lowered the yield of 4d. The excellent selectivity
with glucosamine derivative 3h enables the synthesis of a b-D-
Man(1/4)-GlcN linkage. The lower yield in this case reects
Table 1 Donor scope for b-mannosylations (isolated yield and a/
b ratio)
the poor nucleophilicity of this acceptor and thus the elimina-
tion reaction on the donor competes. In addition, phenolic
Table 2 Acceptor scope for b-mannosylations (isolated yield and a/
b ratio)
General procedure: a solution of the hemiacetal (1 eq.) and Ph3PO (0.5
eq.) in anhydrous CHCl3 (0.5 M) was treated with (COCl)2 (1.2 eq.) and
le to stir at rt for 1 h. Solvent and excess (COCl)2 were removed.
General procedure: as for Table 1 except: Ph3PO (1 eq.) was used and
Powdered LiI (4 eq.), iPr2NEt (2.5 eq.) and ROH (0.7 eq.) were added,
step 1 stirred for 30 min. In step 2 iPr2NEt (4 eq.) was used; aPh3PO
the mixture was re-dissolved in anhydrous CHCl3 (0.4 M w.r.t. donor)
(0.5 eq.) was used and step 1 le for 1 h. bReaction was carried out at
and stirred at 45 ꢀC. a/b ratios were determined by 1H NMR
60 ꢀC. ciPr2NEt (2.5 eq.) was used and reaction was carried out at 25
spectroscopy of the crude reaction mixture. aPh3PO (1 eq.) was used
ꢀC. dROH (0.5 eq.) was used.
and step 1 le for 30 min. bStep 2 was carried out at 30 ꢀC.
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acceptors 3e–g gave excellent selectivities and yields of the O- acceptors, except for cholesterol (see ESI†). Thioglycosides, and
glycoside. 4-Nitrophenol b-mannopyranoside 4g, which is of acceptors bearing azido and NHBoc groups were tolerated,
interest for enzymatic studies was synthesised in excellent yield which could be helpful in complex molecule synthesis.
and selectivity.45 A limitation was that ester-substituted accep-
From a practical perspective, we believe the method is
tors either gave no reaction or gave complex mixtures; we attractive as it doesn't require protecting groups that impose
believe the latter result arises, at least in part, from trans- conformational restraints on the donor. All the reagents are
esterication and thus multiple acceptors being present in commercially available and the hemi-acetal donors are
solution. We conrmed that ester migration occurred on straightforward to synthesize and are bench stable.51 The reac-
a benzoylated acceptor with iPr2NEt in CHCl3 at 45 ꢀC in the tion is straight-forward to implement and cryogenic conditions
absence of other reagents (see limitations section in ESI†).
are not required. Semi-orthogonal protecting group patterns
and tolerance of thioglycosides should enable its use in oligo-
saccharide synthesis. Nonetheless, the method is not without
limitations, e.g., the poorer results with acyl and benzoyl ester
protecting groups are a drawback (see ESI†).
Rhamnosylations
Delighted with the excellent results for b-mannosylations, we
explored the reaction conditions for rhamnosylations (Table 3).
Approaches to stereoselective b-rhamnosylations include use of
an electron-withdrawing group on the 2-O-position,46 use of 2,3-
O- and 3,4-O-tethered rhamnosyl donors.47–49
Recently, Bols and Pedersen showed that bulky silyl-
protected rhamnosyl donors gave b-selectivity as they adopt
an axially-rich conformation.50 Rhamnose donors 1j,k gave
excellent yields and selectivities. In the case of benzoyl-
protected donor 1l, excellent selectivity was achieved however
the yield was only moderate due to transesterication. Excellent
b-selectivity was obtained with a wide range of glycosyl
Preliminary mechanistic investigations
The high selectivities observed in these glycosylations are
notable and some preliminary mechanistic investigations are
described here (Table 4). Previously, it has been suggested that
a/b-mixtures of glycosyl bromides and iodides are equilibrated
by phosphine oxides via alkoxyphosphonium intermediates and
that a-selective glycosylations proceed through a b-alkox-
yphosphonium salt (Scheme 2).36–43
However, in contrast to our initial hypothesis, and to litera-
ture reports invoking these alkoxyphosphonium salts as key
intermediates, we did not nd that the phosphine oxide is
benecial in the glycosylation step in our system. When glycosyl
chloride 6a was synthesised as normal and then the Ph3PO
removed prior to glycosylation, excellent b-selectivity was still
obtained (entry 1).
Table 3 Donor and acceptor scope for b-rhamnosylations (isolated
yield and a/b ratio)
Our evidence suggests that glycosyl iodides 7 are key inter-
mediates and that the presence of LiI promotes the stereo-
selective glycosylation step. In the absence of LiI no glycosylation
products were observed (entry 3). With NaI in place of LiI no
stereoselectivity was obtained (entry 4) and some unreacted
glycosyl chloride was present. In the absence of added alcohol,
the a-glycosyl iodide 7 was formed with complete consumption of
the chloride 6a.52,53 Thus glycosyl iodides can be generated
directly from glycosyl hemiacetals using the combination of
Appel conditions and LiI in CHCl3 – this approach may be
advantageous compared to other methods in many circum-
stances, especially in terms of practicality.54–56
Mannosyl iodides have been reported to react with cyclic
ethers and thioethers to give b-selectivity57–60 but under in situ
anomerisation conditions a-mannosides are usually ob-
tained.61,62 We note that the b-mannosylations using cyclic
ethers were conducted in the presence of MgO and that lithium
and magnesium ions have similar ionic radii.57–60 The salts LiI,
General procedure: as for Table 1 except: Ph3PO (1 eq.) was used and
step 1 stirred for 30 min. In step 2 iPr2NEt (4 eq.) was used; aPh3PO
(0.5 eq.) was used and step 1 le for 1 h. biPr2NEt (2.5 eq.) was used
and reaction was carried out at 25 ꢀC. cROH (0.5 eq.) was used.
Scheme 2 Alkoxyphosphonium intermediates previously invoked in
rationalising outcomes of glycosylations.
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Table 4 Experiments to probe mechanism
Entry
Route
Deviation from standard procedure
b/aa
1
2
3
4
5
6
7
8
A
A
A
A
A
A
A
A
A
A
B
B
B
C
C
No Ph3POb
0.5 eq. Ph3PO
No LiI
NaI in lieu of LiI
Filtration of insoluble LiCl/LiI salts
Filtration of insoluble LiCl/LiI salts; no Ph3POb
2 eq. LiI in lieu of 4 eq. LiI
2 eq. Ph3PO
$20 : 1
$20 : 1
No glycosylation
1 : 1
1 : 1
$20 : 1
2 : 3
1 : 1
10 : 1
$20 : 1
2 : 3
3 : 1
9
2 eq. Ph3PO, 8 eq. LiI
b
˚
10
11
12
13
14
15
No Ph3PO 4 A MS
0.5 eq. Ph3PO
4 eq. LiI
4 eq. LiCl
—
2 : 3
2 : 3
$20 : 1
4 eq. LiI
a
Determined by 1H NMR spectroscopy of the reaction mixture. b Aer synthesis of 6a, the Ph3PO was removed by chromatography.
LiCl, NaI and NaCl are insoluble in CHCl3. Since glycosylations is important for more than one fundamental role in achieving
of mannosyl chlorides and bromides promoted by insoluble the desired reactivity and b-selectivity during glycosylation.
silver salts have been reported,1,2,63 a ltration of the reaction Investigation of the underlying mechanism and the role of LiI is
mixture was carried out prior to addition of alcohol to remove the subject of ongoing work in our laboratory.
insoluble LiCl and LiI – this did not lead to a change in selec-
We discount an SN1-like mechanism as this would be expected
tivity when carried out in the absence of Ph3PO (entry 5 vs. to give a-selectivity. Although we did not observe b-iodide 7, we
entry 6); it is possible that the excess LiI is important for pre- cannot rule out the possibility that the a/b-anomers are in rapid
venting the erosion of b-selectivity due to the presence of Ph3PO equilibrium under reaction conditions.65,66 The minor a-glycoside
(entry 7, 8 vs. 9), or that a lithium salt in solution may be acti- could arise from an SN2-like pathway from b-iodide 7 (which
vating glycosyl iodides, or that lithium alkoxides are the active would be expected to be more reactive), whereas the major b-
nucleophiles. When TMSI was used to generate glycosyl iodide glycoside could come from SN2-like reaction with the major a-
from glycosyl acetate 8a (route B), the glycosylation was slightly iodide 7 promoted by LiI (Scheme 3). Given the high b-selectivity
a-selective (entry 11), however, when LiI was added it was observed, it seems that either the position of equilibrium strongly
moderately b-selective (entry 12); the moderate selectivity might favours the a-iodide, or that equilibration is very slow under our
arise because TMSOAc interferes. We conrmed that the pres- conditions. An increase in the rate of equilibration at higher
˚
ence of 4 A MS doesn't impact on the selectivity (entry 10). temperatures may explain why some substrates exhibited better
Replacing LiI with LiCl also led to poor selectivity (entry 13). selectivity at 25 ꢀC than 45 ꢀC. Mechanistically our method may be
Route C was used to generate the glycosyl iodide through related to the earlier work with insoluble silver salts1,2 but has
reaction of hemiacetal 1a with an iodophosphonium salt greater applicability, especially with respect to scope. In part, this
generated in situ.55,64 As in route B, the glycosylation was slightly may come from the greater reactivity of glycosyl iodides, but it also
a-selective (entry 14) but exhibited high b-selectivity when LiI suggests that other b-glycosylation methods could be developed
was added (entry 15). These preliminary experiments suggest LiI that do not rely on restricting the conformation of the donor.
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Yannick Ortin and Dr Guanghua Jin for NMR support, and Dr
Jimmy Muldoon for Mass Spectrometry. We thank Laurentia
Berzan and Saranna Kavanagh for synthesis of an acceptor and
a donor.
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