Highly Selective β-Mannosylations and β-Rhamnosylations Catalyzed by Bis-thiourea

Article abstract of DOI:10.1021/jacs.0c04255

We report highly β-selective bis-thioureas-catalyzed 1,2-cis-O-pyranosylations employing easily accessible acetonide-protected donors. A wide variety of alcohol nucleophiles, including complex natural products, glycosides, and amino acids were β-mannosylated and β-rhamnosylated successfully using an operationally simple protocol under mild and neutral conditions. Less nucleophilic acceptors such as phenols were also glycosylated efficiently in excellent yields and with high β-selectivities.

Full text of DOI:10.1021/jacs.0c04255

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Article
Highly Selective #-Mannosylations and #-
Rhamnosylations Catalyzed by a Bis-thiourea
Qiuhan Li, Samuel Levi, and Eric N. Jacobsen
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c04255 • Publication Date (Web): 11 Jun 2020
Downloaded from pubs.acs.org on June 13, 2020
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Journal of the American Chemical Society
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Highly Selective -Mannosylations and -Rhamnosylations Catalyzed
by a Bis-thiourea
Qiuhan Li, Samuel M. Levi†, and Eric N. Jacobsen*
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Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States
ABSTRACT: We report highly -selective bis-thiourea-catalyzed 1,2-cis‐O‐pyranosylations employing easily accessible ace-
tonide-protected donors. A wide variety of alcohol nucleophiles, including complex natural products, glycosides, and amino
acids were -mannosylated and rhamnosylated successfully using an operationally simple protocol under mild and neutral
conditions. Less nucleophilic acceptors such as phenols were also glycosylated efficiently in excellent yields and with high -
selectivities.
INTRODUCTION
glycosylation of less reactive nucleophiles such as neutral
phenols have been identified to date. In particular, electron-
rich O-aryl glycosides are difficult to access as they are
prone to Fries-type rearrangement pathways to afford C-
glycosides under Lewis acidic conditions.16
Pyranosides bearing -1,2-cis-O-glycosidic linkages are
found in many biologically important oligosaccharides that
play crucial roles ranging from structural support to cellular
recognition.1 For example, -mannosides are key constitu-
ents of plant primary cell wall2 and are found at the branch-
points of asparagine-linked glycopeptides, the structure of
which is known to define unique antibody-effector function
for a given isotype.3 -Rhamnosides, although largely xeno-
biotic to humans, are the antigenic components of many
pathogenic bacterial polysaccharides,4 and improved un-
derstanding of their immunological effects may be of value
in the development of therapeutics.4a The construction of
well-defined -1,2-cis-linkages is therefore an important
objective in synthetic chemistry, but one that presents fun-
damental challenges. -1,2-cis-O-Glycosides are intrinsi-
cally less stable than their -diastereomers as a result of di-
minished anomeric stabilization5 and increased steric con-
gestion. These effects are also manifested in glycosylation
pathways, resulting in a strong kinetic preference for -
products in typical mannosylation and rhamnosylation re-
actions (Figure 1A).
A) Challenges:
RO
O
OR'
RO
O
Poor overlap between
Steric congestion imposed
by C2 substituent
RO
n
O and 
O
C–O
RO
R'
LG
B) Prior work:
RO
RO
O
O
R'OH
RO
RO
LG
Lewis Acid/ Oxidant
OR'

(LG= leaving group)
selectivity via:
O
Ph
O O
RO
O
O
O
OR
LG
OR
O
RO
RO
OTf
Protecting group control
Intramolecular aglycone delivery
1,2-Anhydropyranose donors
C) This work:
selectivity via catalyst control
Me
Me
Me
O
O
O
O
Me
Cat
O
Me
O
Me
Significant efforts have been devoted to overcoming the in-
herent -selectivity and developing -1,2-cis glycosylation
methods.6 Successful approaches include intramolecular
aglycone delivery,7 hydrogen-bond-mediated aglycone de-
livery,8 the use of 1,2-anhydropyranose donors with borinic
and boronic acid catalysts,9 protecting-group control such
as with 4,6-benzylidene acetals10 and 2,6-lactones,11 and
anomeric O-alkylation12 (Figure 1B). Each of these strate-
gies employs donors with specific protecting group
schemes requiring multi-step syntheses. Although the
strongly Lewis acidic or oxidizing reaction conditions re-
quired for these couplings are generally compatible with
carbohydrate substrates and can therefore be applied to ol-
igosaccharide synthesis,13 they display poor tolerance to-
ward a wide variety of common functional groups and
therefore have limited utility for the glycosylation of com-
plex acceptors.14,15 Finally, no general methods for 1,2-cis
O
Me
O
O
R
O
P
O
Me
OPh
O
PhO
O
O
OR
OR
H
RO
2 steps from D-mannose
O
O
LG
OPh
NR2
P
H
N
H
N
O
Me
BnO
O
OPh
Ar
Me
O
O
-Cat
ent
O
BnO
S
O
O
Me
Me
Dual activation mode
Me
Me
5 steps from L-rhamnose
-1,2-cis linkages
Figure 1. (A) Challenges associated with -1,2-cis-pyranosyl
glycosidic bond construction. (B) Traditional approaches to
achieving selectivity in glycosylation. (C) Catalyst-controlled
approach to -1,2-cis linkages with a bis-hydrogen-bond donor.
The use of hydrogen-bond-donor and chiral phosphoric
acid organocatalysts in glycosylation reactions has been re-
ported by several groups.14,17 With the goal of developing a
mild, functional-group-tolerant, and broadly applicable
method for -1,2-cis glycosylations employing easily
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alcohol 3a. (B) Protecting-group effect on mannosylation reac-
accessible donors, we hypothesized that our recently
reported stereospecific approach catalyzed by precisely
designed bis-thiourea derivatives could be applied in this
most challenging context (Figure 1C).18 While construction
of -1,2-trans linkages18a,b and -1,2-cis-O-furanosyl18c link-
ages has been demonstrated using the appropriate -glyco-
sylphosphate donors with linked bis-thiourea catalysts
such as 1, very limited success has been achieved as yet in
the application of this strategy to -1,2-cis-O-pyranosyl link-
ages. Herein, we describe the discovery of remarkable pro-
tecting group effects in catalytic glycosylations of mannosyl
and rhamnosyl phosphate derivatives, leading to the devel-
opment of highly selective, operationally simple, and gen-
eral protocols for -1,2-cis glycosylations of amino acids,
carbohydrates, complex natural products, and pharmaceu-
ticals.
1
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tions with secondary alcohol nucleophile 3b catalyzed by 1. (C)
Stoichiometry optimization. Conversions were determined by
1H NMR analysis of crude product mixtures with mesitylene as
an internal standard. Selectivities were determined by 1H NMR
analysis of crude product mixtures. aConversions after 3 h. Se-
lectivies remained constant over time. bReaction run at –40 °C.
RESULTS AND DISCUSSION
Bis-thiourea 1, the enantiomer of which had been identified
previously as an effective catalyst for stereospecific -1,2-
trans-pyranosylation reactions,18b was evaluated in the
mannosylation of the model acceptor 3a (Figure 2A).19 Var-
iation of the protecting groups on the mannosyl diphe-
nylphospate donor resulted in pronounced effects on ano-
meric selectivity.20 Simple alkyl protecting schemes such as
perbenzyl (2a) or permethyl (2b) resulted in largely unse-
lective coupling reactions (1:1 to 1:2 ). Incorporation of
the 4,6-benzylidene acetal protecting group (as in 2c) pio-
neered by Crich for -selective mannosylations under
strong Lewis acid conditions again led to an unselective re-
action in the presence of catalyst 1. However, a dramatic in-
crease in -selectivity was observed under catalytic condi-
tions when the C2 and C3 substituents were bridged by an
acetonide protecting group. Thus, coupling reactions with
donors 2d-f all afforded high -selectivity (1:16–32 )
with the relatively unhindered acceptor 3a.21 Mannosyla-
tion of the more sterically congested secondary nucleophile
3b was found to be substantially more -selective with 2f
than with 2d (1:24 vs 1:10 Figure 2B). The bis-ace-
tonide-protected mannose derivative 2f also offers im-
portant practical advantages, being prepared in a simple
two-step sequence from D-mannose, and affording glycosyl-
ation products that may be selectively or fully deprotected
under mild conditions (vide infra).22 As observed previously
in related bis-thiourea-catalyzed glycosylations of glycosyl
phosphates, the inclusion of 4 Å molecular sieves is crucial
for minimizing undesired hydrolysis of donors and seques-
tering the phosphoric acid by-product, which has been
shown to promote glycosyations unselectively.18b,c Effects of
varying the stoichiometry of donor 2f and acceptor 3c were
also investigated (Figure 2C). While slightly higher conver-
sions were obtained when the stoichiometry of either 2f or
3c was increased, those improvements were offset by ob-
servable decreases in -selectivity. Accordingly, subsequent
scope studies (Figures 4 and 5) were carried out with
equimolar ratios of donor and acceptor.
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Figure 2. (A) Protecting-group effects on bis-thiourea-cata-
lyzed and TMSOTf-promoted mannosylations of primary
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protection cannot be ascribed to destabilization of an -se-
1
2
lective SN1-type pathway.
Recently, high -selectivity has been demonstrated in man-
nosylations utilizing 4,6-benzylidene acetal-protected do-
nors in systems in which the formation of transient -man-
nosyl triflates is not possible.26 In those cases, the stereose-
lectivity has been ascribed to preferential axial attack of nu-
cleophiles on the energetically accessible B2,5 conformation
of the oxocarbenium ion intermediates,26b,27 a similar con-
formation to that calculated for 5d and 5e. However, if this
mode of stereoselectivity were operative in reactions cata-
lyzed by bis-thiourea 1, comparable levels of -selectivity
might be expected with 2c and 2f because of the similarity
in the conformations of 5b_B2,5 and 5e. In fact, dramatically
higher -selectivity is observed with donor 2f (=1:32 vs
1:1 for 2cFigure 2A). Furthermore, the divergent selectiv-
ities observed with 2d–f in 1-catalyzed and trimethylsilyl
triflate (TMSOTf)-promoted reactions (1:16–32 vs 3–8:1
Figure 2A) appear to be inconsistent with stereoselec-
tive SN1 addition to B2,5 oxocarbenium ion intermediates in
both pathways. We propose instead that the enhancement
in -selectivity is achieved by acceleration of a stereospe-
cific SN2-type pathway promoted by the bis-thiourea cata-
lyst 1, consistent with the observation that increased reac-
tivity is observed with 2,3-acetonide protection 2d relative
to unbridged analogs 2a–b (39% vs 11%, 15% conversion
respectively). However, the basis for the acceleration effect
resulting from the interplay between protecting group and
catalyst remains to be determined, and the possibility that
reactions proceed through a catalyst-promoted -selective
SN1 pathway cannot be strictly ruled out. Further examina-
tion of the mechanism of substitutions catalyzed by 1 and
related bis-thioureas is the focus of continuing investiga-
tion.28
3
4
5
6
7
8
9
10
11
12
13
Figure 3. Computed energies of oxocarbenium ion intermedi-
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ates relative to the corresponding -mannosyl diphenyl phos-
phates. Calculations were performed at B3LYP/6-
311+G(d,p)//B3LYP/6-31G(d) with D3BJ dispersion correc-
tions and PCM (H2O). Free energy corrections were determined
at the B3LYP/6-31G(d) level of theory at 298.15 K using
Grimme’s quasi-harmonic free energy correction. Similar
trends were observed in ether solvation model and other levels
of theory (see SI for full details).
We sought to elucidate the origin of the remarkably benefi-
cial effect of the 2,3-acetonide protecting group on -selec-
tive mannosylations with catalyst 1 through density func-
tional theory computations (Figure 3).23 B3LYP/6-
311+G(d,p)//B3LYP/6-31G(d) were selected because they
have been previously applied to compute the transition
states of glycosylation reactions and geometries and ener-
gies of oxocarbenium ion intermediates.24 Oxocarbenium
ion intermediates were computed in isolation due to the ob-
servation of undesired ion-pair collapse into glycosyl phos-
phates when computations were performed in the presence
of diphenyl phosphate anions. Crich has proposed that 4,6-
benzylidene acetal protection of mannose serves to desta-
bilize the corresponding oxocarbenium ion species relative
to acyclic protection schemes (e.g. 5b vs 5a), thereby favor-
ing stereospecific glycosylation pathways via transient -
mannosyl triflate intermediates.10d–e,25 Computational mod-
eling of the oxocarbenium ions 5a and 5b fully supports that
hypothesis, with the 4,6-benzylidene-protected derivative
5b calculated to be destabilized by 4.4 and 3.2 kcal/mol (4H3
and B2,5 conformation respectively relative to the unbridged
analog 5a. The oxocarbenium ion 5c derived from 4.6-ace-
tonide-protected donor is conformationally similar to 5b,
and displays a similar destabilizing effect. However, a simi-
lar analysis of 2,3-acetonide-protected oxocarbenium ion
5d revealed that it was instead stabilized by 1.3 kcal/mol
relative to the unbridged analog 5a. Thus, the dramatic in-
creases in -selectivity resulting from 2,3-acetonide
The scope of catalytic, -selective mannosylation reactions
with the bis-acetonide donor 2f was explored, with particu-
lar attention to cases where the mild and neutral conditions
could provide a unique advantage over traditional Lewis
acid-promoted glycosylation protocols (Figures 4 and S1).
Sugar derivatives were found to be excellent substrates for
the methodology, with both primary (6-OH-galactose 3c
and 6-OH-glucose 3d) and secondary glycosyl acceptors (2-
OH-galactose 3e and 4-OH-rhamnose 3f) undergoing man-
nosylation with high -selectivity.29 The -mannoside of C4-
linked N-acetylglucosamine derivative 3g is of particular in-
terest as it is a conserved branch-point on all N-glycans.3a
A
wide variety of other alcohols containing Lewis basic func-
tional groups including esters, carbamates, and tertiary
amines (3a, 3h–i, and S3f) were also found to undergo -
mannosylations selectively and cleanly, with no decomposi-
tion of nucleophiles observed.
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Me
Me
Me
O
Me
O
Me
Me
1
2
3
4
Lewis Acid Promoted:
Catalytic Approach:
1 (
Me
Me
Me
O
O
O
O
10 mol%)
TMSOTf (1.5 equiv.)
O
O
O
O
Me
Me
Me
O
+
O
O
ROH
(1.0 equiv.)
3
O
O
CH2Cl2, –78 °C, 40 min.
4 Å MS, iPr2O (0.1 M)
OR
50 °C, 24 h
O
P
O
OR
2f
-4
-4


OPh
PhO
5
6
7
8
Me
Me
O
OH
O
OMe
OH
OBn
O
Me
Me
O
O
OTBDPS
O
Me
HO
O
O
O
BnO
BnO
HO
BnO
HO
OMe
O
OMe
O
O(p-MeO)Ph
O
O
O
BnO
NHBoc
OH
N3
OMe
Me
3f
Me
Me
(4-Rha)
Me
9
3a
3c
3d
3e
3g
(4-GlcN3)
(L-Ser)
(6-Gal)
(6-Glu)
(2-Gal)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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43
44
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49
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60
Catalyst 1
82% (1:19 :)a
41% (8:1 :)
94% (1:19 :)a,b
90% (4:1 :)
92% (1:50 :)a
85% (9:1 :)
73% (1:38 :)c
88% (6:1 :)
71% (1:17 :)a,c
80% (3:1 :)
43% (1:14 :)d
78% (16:1 :)
TMSOTf
Me
Me
O
O
Me
OH
Me
HO
O
N
HO
Me
O
Me
Cl
Me Me
O
HO
O
Me
Me
Me
N
OMe
H
N
HO
O
NBOM
N
O
N
OMe
O
O
Me
Me
O
Cl
HO
Me
N3
S
O
OH
O
Me
HO
OMe
3h
3i
3j
3k
(FK-506)
(Quetiapine)
(BOM-zidovudine)
(Pleuromutilin)
3l
(Clofoctol)
63% (1:19 :)c
61% (1:18 :)c
30% (1:15 :<1:50 r.r.) c
67% (<1:50 :<1:50 r.r.)
62% (1:24 :)a,c
92% (1:1 :)
1
Catalyst
TMSOTf
0%
0%
79% (9:1 :)
49% (3:1 :)
Me
OH
Me
Me
H
BocHN
Me
Me
HO
O
H
H
Me
Me
S
H
N
O
N
H
Me
Me
N
OMe
O
H
HO
O(CH2)7CH3
HO
Me
O
NHBoc
3p
(L-Tyr)
NHBoc
HO
HO
O
Me
3o
(Serotonin)
3q
(Estradiol)
3m
3n
(-Tocopherol)
(Amoxicillin)
57% (1:18 :)c
27% (1:1 :)
94% (<1:50 :1:15 r.r.) b
1
Catalyst
95% (<1:50 :)
95% (<1:50 :)
80% (1:50 :) (X-ray structure)
TMSOTf
 
: )
92% (2:1
16% (4:1 :)
61% (1:1 :)
23% (3:1 :)
Figure 4. Acceptor scope of -mannosylations. Yields of reactions catalyzed by 1 reflect isolated yields of pure -products. Yields of
TMSOTf reactions reflect combined yield of the - and -products and were determined by 1H NMR analysis of crude product mix-
tures with mesitylene as an internal standard. Selectivities were determined by H NMR analysis of crude product mixtures. 2.0
equiv. nucleophilic coupling partners were used. Reaction was run at 40 °C. 48 h reaction time. 63 h reaction time. Additional
substrates are provided in Figure S1.
1
a
b
c
d
Substrates bearing multiple hydroxyl groups such as deox-
ycholic acid octyl ester (S3g), pleuromutilin (3j) and FK-
506 (3k) were found to undergo glycosylation at the least
hindered position with high site-selectivity.30,31 Highly ste-
rically congested secondary alcohols and tertiary alcohols
underwent mannosyation with reduced level of selectivity
and reactivity (Figure S1).
with high -selectivity. In contrast, TMSOTf-promoted reac-
tions yielded -anomers as the major glycosylation prod-
ucts in almost all cases, consistent with the expected intrin-
sic preference for -product in mannosylations proceeding
via oxocarbenium ion intermediates. Functionally complex
acceptors such as FK-506 (3k) were unstable to the Lewis
acid activation conditions, while substrates possessing
Lewis basic functionality such as quetiapine (3h) afforded
none of the desired glycosylation products. Other sensitive
substrates such as amoxicillin (3n) and serotonin (3o) un-
derwent mannosylation in low yield and anomeric selectiv-
ity under TMSOTf conditions, with formation of multiple
side products.
Phenolic acceptors were found to display excellent reactiv-
ity toward 2f under catalysis by bis-thiourea 1 to afford -
mannosylation products (Figures 4 and S1). A wide assort-
ment of phenol derivatives, including sterically hindered
substrates such as clofoctol (3l) and -tocopherol (3m)
were mannosylated with high -selectivity. Excellent func-
tional group tolerance was again demonstrated, such as in
the glycosylation of the -lactam-containing amoxicillin de-
rivative 3n. Fries-type rearrangement of O-aryl glycosides
to C-glycosides, which is common with electron-rich phenol
derivatives (e.g. 3o and 3q) under Lewis acidic conditions,16
was not observed in any reactions promoted by 1.
We sought to explore whether the 2,3-acetonide protection
strategy applied successfully in -1,2-cis mannosylations
catalyzed by 1 might be extended to other interesting gly-
cosyl donors. Rhamnose is the C6-deoxy analogue of man-
nose, and it presents another long-standing challenge as a
coupling partner in -1,2-cis glycosylation reactions. Be-
cause the C6 hydroxyl of mannose is lacking, the 4,6-benzyl-
idene acetal protecting group strategy pioneered by Crich
for -mannosylations is not applicable. Alternative
Mannosylations with donor 2f using catalyst 1 were com-
pared to reactions promoted by the Lewis acid TMSOTf.32 In
all the examples illustrated in Figure 4, the reactions cata-
lyzed by 1 afforded the mannosylation products cleanly and
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O
OPh
OR
P
Lewis Acid Promoted:
Catalytic Approach:
O
1
2
3
Me
BnO
OPh
Me
BnO
OR
O
O
TMSOTf (1.5 equiv.)
1 (
ent- 10 mol%)
Me
BnO
O
+
O
O
ROH
O
O
CH2Cl2, –78 °C, 40 min
4 Å MS, iPr2O (0.1 M)
O
(1.0 equiv.)
O
Me
Me
30 °C, 24 h
Me
Me
Me
Me
4
-7
6
3
-7
5
6
7
8
9
Me
O
Me
Me
Me
Me
Me
O
OH
O
HO
Me
Me
O
N
Me
HO
O
HO
O
Me
N
N
O
OMe
OMe
N
O
NBOM
O
O
O
N
O
O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
Me
Me
N3
Me
S
HO
OH
O
Me
HO
OMe
3c
3h
3i
3k
3r
(Cholecalciferol)
(6-Gal)
(Quetiapine)
(BOM-zidovudine)
(FK-506)
96% (<1:50 :)
80% (2:1 :)
76% (1:18 :)a
82% (1:10 :)b
55% (<1:50 : <1:50 r.r.) c
94% (1:47 :)
61% (3:1 :)
ent–1
0%
0%
85% (7:1 :)
TMSOTf
OH
Me
O
O
Me
HO
O(CH2)7CH3
Me
Me
Me
Me
Me
HO
O
NMe2
H
N
O
MeO
O
HO
O
OH
Me
H
O
O
Me
Me
Me
OMe
NHBoc
HO
H
H
O
Me
O
O
NHBoc
(Serotonin)
83% (1:20 :)
 
O
Me
HO
Me
HO
Me
H
3s
O
O
Me
3o
3p
(L-Tyr)
84% (1:26 :)c
 
3m

3t
(
-Tocopherol)
 
(Deoxycholicoctyl ester)
(Midecamycin)
91% (1:25 :<1:50 r.r.) c
64% (3:1 :)
51% (1:16 : <1:50 r.r.) b,c
41% (6:1 :)
96% (<1:50
: )
ent–1
31% (2:1
:
)
67% (2:1 : )
61% (6:1 :)
TMSOTf
Figure 5. Acceptor scope of -rhamnosylations. Yields of reactions catalyzed by ent-1 reflect isolated yields of pure -prod-
ucts. Yields of TMSOTf reactions reflect combined yield of the - and -products and were determined by 1H NMR analysis of
crude product mixtures with mesitylene as an internal standard. Selectivities were determined by 1H NMR analysis of crude
product mixtures. aReaction was run at 50 °C for 48 h. b48 h reaction time. cReaction was run at 40 °C. Additional substrates
are provided in Figure S2.
strategies for -rhamnoside synthesis have been identified,
but the reported successful examples have been limited to
(A) Deprotection protocol
Me
Me
OH
HO
HO
HO
Me
simple alcohols as nucleophilic coupling partners.7e,8b,9c,23f,33
The 2,3-acetonide-protected L-rhamnose phosphate donor
6 was evaluated with 3a as a model acceptor (Figure S14).
L-Rhamnose is pseudoenantiomeric with D-mannose, and
we observed that a large improvement in -selectivity was
achieved by using the enantiomer of catalyst 1 (1:7  with
O
O
AcOH:H2O (4:1)
23 °C, 18 h
O
O
O
Me
OR
O
OR
4
8
Me
O
O
Me
HO
OH
HO
HO
O
Me
O
HO
OMe
OMe
O
O
N
BnO
BnO
catalyst 1 vs 1:32  with ent-catalyst 1, Figure S14). Such
stereochemical matching points to precise catalyst-sub-
strate interactions in the catalytic glycosylation event and is
consistent with observations we have made previously in
less challenging glycosylation reactions.18b,c The nucleophile
scope in -rhamnosylations catalyzed by ent-1 was exam-
ined and proved to be similarly broad to that documented
above in -mannosylations (Figures 5 and S2). Thus, excel-
lent functional group compatibility was demonstrated with
substrates bearing Lewis basic groups such as tertiary
amines (3h), imide (3i), and carbamates (3o–p), as well as
with complex natural products (e.g. 3k and 3t).34 Excellent
-selectivity and reactivity were observed in the rhamno-
sylation of phenols as well. Reactions promoted by TMSOTf
displayed limited functional group tolerance and generally
favored the -rhamnosylation products in cases where gly-
cosylation product was obtained.
O
O
BnO
Me
OMe
OH
HO
OH
O
Me
O
HO
O
OMe
HO
8b
8a (Man--(1,6)-Glu)
81% (no anomerization)
Man-FK506
86% (no anomerization)
(B) One-pot mannosylation and deprotection
Me
Me
1) 1 (1 mol%)
Me
O
O
MeOH (20 equiv.)
4 Å MS, iPr2O (0.1 M)
23 °C, 48 h
OH
HO
O
O
Me
O
O
HO
HO
OMe
O
P
O
2) AcOH:H2O (4:1)
60 °C, 2 h
2f
2.5 mmol
8c
Quant.
1:99
OPh
PhO

Figure 6. (A) Protocol for bis-acetonide deprotection under
mildly acidic hydrolytic conditions. (B) Multi-millimole-scale
synthesis of methyl -D-mannopyranoside. Yields reflect iso-
lated yields of pure -products. Selectivities were determined
by 1H NMR analysis of crude product mixtures.
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ACKNOWLEDGMENT
The acetonide-protected glycosylation products are readily
unmasked under mild conditions.22c Thus, both acetonides
on disaccharide 4d were hydrolyzed in a 4:1 acetic acid:wa-
ter mixture at room temperature, with the -mannoside
product isolated in 81% yield with no erosion of anomeric
purity (Figure 6A). Similarly, mannosylated FK-506 (4k)
was also deprotected in 86% yield with no detectable anom-
erization, demonstrating how even highly sensitive func-
tional groups are tolerant to the mildly acidic hydrolytic
conditions. The potential practicality of the mannosylation
procedure was demonstrated in a multi-millimole-scale
synthesis of methyl -D-mannopyranoside 8c. The shortest
prior reported stereoselective synthesis of 8c required 7
steps from D-mannose.35 Using the approach outline herein
and in Figure 6B, 8c was prepared in 2 steps from 2f and
isolated in >95% purity without column chromatography (4
steps from D-mannose). The glycosylation step was per-
formed on 2.5 mmol scale using 1 mol% catalyst 1 to pro-
vide the acetonide protected-mannoside quantitatively,36
and that material was subjected to the deprotection proto-
col directly to afford the fully deprotected -mannoside 8c
in 1:99  selectivity.
1
2
3
4
5
6
7
8
This work was supported by the NIH through GM132571 and
the Common Fund Glycoscience Program (U01 GM116249),
and by an NSF pre-doctoral fellowship to S.M.L. We thank Dr.
Shao-Liang Zheng (Harvard University) for X-ray data collec-
tion and structure determination.
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CONCLUSION
We have demonstrated that -mannosides and -rhamno-
sides can be mildly and selectively accessed under catalyst
control using readily accessible 2,3-acetonide-protected
glycosyl phosphate donors. The catalytic protocol is com-
patible with extensive functional group complexity on the
nucleophilic coupling partner, so we anticipate this method
may enable exploration of the effect of specific O-glycosyla-
tion on a wide range chemical matter, although the applica-
tion of the current methodology to highly sterically con-
gested nucleophiles is still limited. Work is currently under-
way to elucidate the origins of selectivity imparted by the
2,3-acetonide protecting group in connection with the bis-
thiourea catalyst, and to further illuminate the scope of po-
tential coupling partners in these catalytic glycosylation re-
actions.
ASSOCIATED CONTENT
Supporting Information. Experimental and characterization
data of catalyst and substrate syntheses, details of computa-
tional studies (PDF). Crystallographic data for 4p (CIF). Crys-
tallographic data for S8 (CIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*jacobsen@chemistry.harvard.edu
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†Department of Discovery Chemistry, Merck & Co. Inc., 33 Av-
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Notes
The authors declare no competing financial interests.
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maceuticals has shown significant potential for enhancing biodis-
tribution by affecting molecular structure, hydrophilicity, stability,
and bioavailability. For a review, see: (a) Sola, R. J.; Griebenow, K.
Effects of Glycosylation on the Stability of Protein Pharmaceuticals
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(18) (a) Park, Y.; Harper, K. C.; Kuhl, N.; Kwan, E. E.; Liu, R. Y.;
Jacobsen, E. N. Macrocyclic Bis-thioureas Catalyze Stereospecific
Glycosylation Reactions. Science 2017, 355, 162−166. (b) Levi, S.
M.; Li, Q.; Rötheli, A. R.; Jacobsen, E. N. Catalytic Activation of Gly-
(25) (a) Huang, M.; Garrett, G. E.; Birlirakis, N.; Bohe, L.; Pratt, D.
A.; Crich, D. Dissecting the Mechanisms of a Class of Chemical Gly-
cosylation Using Primary 13C Kinetic Isotope Effects. Nat. Chem.
2012, 4, 663–667. (b) Crich, D.; Chandrasekera, N. S. Mechanism of
4,6-O-Benzylidene-Directed -Mannosylation as Determined by -
Deuterium Kinetic Isotope Effects. Angew. Chem. Int. Ed. 2004, 43,
5386–5389.
(26) (a) Sun, P.; Wang, P.; Zhang, Y.; Zhang, X.; Wang, C.; Liu, S.;
Lu, J.; Li, M. Construction of -Mannosidic Bonds via Gold(I)-Cata-
lyzed Glycosylations with Mannopyranosyl ortho-Hexynylbenzo-
ates and Its Application in Synthesis of Acremomannolipin A. J. Org.
Chem. 2015, 80, 4164–4175. (b) Heuckendorff, M.; Bendix, J.;
Pedersen, C. M.; Bols, M. -Selective Mannosylation with a 4,6-Si-
lylene-tethered Thiomannosyl Donor. Org. Lett. 2014, 16, 1116–
1119.
1
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3
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5
6
7
8
cosyl Phosphates for Stereoselective Coupling Reactions. Proc.
Natl. Acad. Sci. U. S. A. 2019, 116, 35−39. (c) Mayꢁield, A. B.; Metter-
nich, J. B.; Trotta, A. H.; Jacobsen, E. N. Stereospecific Furanosyla-
tions Catalyzed by Bis-thiourea Hydrogen-Bond Donors. J. Am.
Chem. Soc. 2020, 142, 4061−4069.
(19) A systematic evaluation of dimeric hydrogen-bond donors
led to the identification of 1 as the optimal catalyst for the -man-
nosylation reactions. Ent-1 and catalysts with less rigid structures
or different arylpyrrolidine components were found to be less -
selective and less reactive (see Figures S3–S5 for details).
(20) (a) A screen of phosphate leaving groups revealed the di-
phenyl derivatives as optimal. The more electron-withdrawing bis-
(4-cholorophenyl) phosphates were found to be more reactive but
less -selective in the presence of 1 (Figure S12). (b) Comparable
conversions and selectivities were obtained in di-n-butyl ether
(see Figures S6–S7). Diisopropyl ether was selected because of its
greater ability to solubilize acceptors.
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(27) (a) Moume-Pymbock, M.; Crich, D. Stereoselective C-Glyco-
side Formation with 2-O-Benzyl-4,6-O-benzylidene Protected 3-
Deoxy Gluco- and Mannopyranoside Donors: Comparison with O-
Glycoside Formation. J. Org. Chem. 2012, 77, 8905–8912. (b) Nu-
kada, T.; Bérces, A.; Whitfield, D. M. Can the Stereochemical Out-
come of Glycosylation Reactions Be Controlled by the Confor-
mational Preferences of the Glycosyl Donor? Carbohydr. Res.
2002, 337, 765–774.
(28) The bis-acetonide-protected oxocarbenium intermediate
5e was found to be destabilized by 2.8 kcal/mol relative to 5d, sug-
gesting that the high -selectivities obtained with donor 2f even
with relatively unreactive, hindered nucleophiles result from a
combination of acceleration of -selective and deceleration of -
selective pathways arising from 5e.
(21) Higher reactivity of 2f was achieved by increasing reaction
temperature to 50 °C, albeit with slightly lower -selectivity (85%
conversion, 1:19 :). - and -Configurations were assigned
based on 3JH1-H2 (0 Hz for -products and 2.5–3.0 Hz for -products)
1
and JC1-H1 (ca. 170 Hz for -products and ca. 160 Hz for -prod-
ucts).
(22) For selective hydrolysis of 4,6-acetonides, see (a)
Combemale, S.; Assam-Evoung, J.-N.; Houaidji, S.; Bibi, R.; Barragan-
Montero, V. Gold Nanoparticles Decorated with Mannose-6-phos-
phate Analogues. Molecules 2014, 19, 1120–1149. (b) Neogi, A.;
Majhi, T. P.; Achari, B.; Chattopadhyay, P. Palladium-Catalyzed In-
tramolecular C–O Bond Formation: An Approach to the Synthesis
of Chiral Benzodioxocines. Eur. J. Org. Chem. 2008, 2008, 330–336.
For deprotection of bis-acetonide, see (c) Cocinero, E. J.; Stanca-Ka-
posta, E. C.; Scanlan, E. M.; Gamblin, D. P.; Davis, B. G.; Simons, J. P.
Conformational Choice and Selectivity in Singly and Multiply Hy-
drated Monosaccharides in the Gas Phase. Chem. Eur. J. 2008, 14,
8947–8955.
(29) Chromatographic separation of 4b from 3b was challeng-
ing, so accurate isolated yield could not be determined. Thus, 3b
was not included in Figure 4.
(30) The more accessible C32 position of FK-506 has been selec-
tively functionalized previously. For examples, see (a) Chamni, S.;
He, Q.-L.; Dang, Y.; Bhat, S.; Liu, J. O.; Romo, D. Diazo Reagents with
Small Steric Footprints for Simultaneous Arming/SAR Studies of
Alcohol-Containing Natural Products via O–H Insertion. ACS Chem.
Biol. 2011, 6, 1175–1181. (b) McPherson, M.; Yang, Y.; Hammond,
P. W.; Kreider, B. L. Drug Receptor Identification from Multiple Tis-
sues Using Cellular-Derived mRNA Display Libraries. Chem. Biol.
2002, 9, 691–698.
(31) The anomeric configuration of 4k was assigned based on
1JC1-H1 (158.2 Hz) of deprotected compound 8b. No other glycosyl-
ated FK-506 derivatives were detectable in the crude NMR spectra
or isolated upon chromatographic purification.
(32) The TMSOTf-promoted glycosylation reactions were car-
ried out using the procedures reported by Seeberger and Hash-
imoto: (a) Plante, O. J.; Palmacci, E. R.; Andrade, R. B.; Seeberger, P.
H. Oligosaccharide Synthesis with Glycosyl Phosphate and Dithio-
phosphate Triesters as Glycosylating Agents. J. Am. Chem. Soc.
2001, 123, 9545−9554. (b) Tsuda, T.; Nakamura, S.; Hashimoto, S.
A Highly Stereoselective Construction of 1,2-trans-β-Glycosidic
Linkages Capitalizing on 2-azido-2-deoxy-D-Glycosyl Diphe-
nylphosphates as Glycosyl Donors. Tetrahedron 2004, 60 10711–
10737.
(33) (a) Crich, D.; Picione, J. Direct Synthesis of the -L-Rhamno-
pyranosides. Org. Lett. 2003, 5, 781–784. (b) Crich, D.; Vinod, A. U.;
Picione, J. The 3,4-O-Carbonate Protecting Group as a -Directing
Group in Rhamnopyranosylation in Both Homogeneous and Heter-
ogeneous Glycosylations As Compared to the Chameleon-like 2,3-
O-Carbonates. J. Org. Chem. 2003, 68, 8453–8458. (c) Zhu, Y.; Shen,
Z.; Li, W.; Yu, B. Stereoselective Synthesis of -Rhamnopyranosides
via Gold(I)-Catalyzed Glycosylation with 2-Alkynyl-4-nitro-benzo-
ate Donors. Org. Biomol. Chem. 2016, 14, 1536–1539. (d) Heucken-
dorff, M.; Pedersen, C. M.; Bols, M. Rhamnosylation: Diastereoselec-
tivity of Conformationally Armed Donors. J. Org. Chem. 2012, 77,
5559–5568. (e) Elferink, H.; Pedersen, C. M. L-Rhamnosylation: The
Solvent is the Solution. Eur. J. Org. Chem. 2017, 53–59. For a recent
(23) Mannosylations with bis-acetonide donors under Lewis
acidic conditions were generally -selective or unselective. For ex-
amples, see (a) Mattson, A. L.; Michel, A. K.; Cloninger, M. J. Using
In(III) as a Promoter for Glycosylation. Carbohydr. Res. 2012, 347,
142–146. (b) Yang, J.; Cooper-Vanosdell, C.; Mensah, E. A.; Nguyen,
H. M. Cationic Palladium(II)-Catalyzed Stereoselective Glycosyla-
tion with Glycosyl Trichloroacetimidates. J. Org. Chem. 2008, 73,
794–800. (c) Crich, D.; Sun, S. Direct Chemical Synthesis of -Man-
nopyranosides and Other Glycosides via Glycosyl Triflates. Tetra‐
hedron 1998, 54, 8321–8348. Mildly -selective, silver-salt-pro-
moted mannosylations with bis-acetonide donors have been re-
ported. For examples, see (d) Garegg, P. J.; Iversen, T.; Johansson,
R. Synthesis of Disaccharides Containing -D-Mannopyranosyl
Groups. Acta Chem. Scand. 1980, B34, 505–508. (e) Garegg, P. J.;
Iversen, T.; Norberg, T. A Practical Synthesis of p-Nitrophenyl -D-
Mannopyranoside. Carbohydr. Rer, 1979, 73, 313. (f) Silver-salt-
promoted rhamnosylations with 2,3-acetonide donors with mod-
erate -selectivity and limited acceptor scope have been reported.
Iversen, T.; Bundle, D. R. A New and Efficient Synthesis of -L-
Rhamnopyranosides. Carbohydr. Res. 1980, 84, C13–C15.
(24) (a) Whitfield, D. M. Computational Studies of the Role of
Glycopyranosyl Oxacarbenium Ions in Glycobiology and Gly-
cochemistry. Adv. Carbohydr. Chem. Biochem. 2009, 62, 83–159. (b)
Ionescu, A. R.; Whitfield, D. M.; Zgierski, M. Z.; Nukada, T. Investiga-
tions into the Role of Oxacarbenium Ions in Glycosylation Reac-
tions by ab initio Molecular Dynamics. Carbohydr. Res. 2006, 341,
2912–2920. (c) Barnett, C. B.; Wilkinson, K. A.; Naidoo, K. J. Molec-
ular Details from Computational Reaction Dynamics for the Cello-
biohydrolase I Glycosylation Reaction. J. Am. Chem. Soc. 2011, 133,
19474–19482.
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review, see: (f) Rai, D.; Kulkarni, S. S. Recent Advances in β-L-Rham-
(35) The stereoselective synthesis utilized 4,6-benzylidene ace-
tal strategy for -mannosylations. Crich, D.; Banerjee, A.; Yao, Q. Di-
rect Chemical Synthesis of the -D-Mannans: The  -(1→2) and  -
(1→4) Series. J. Am. Chem. Soc. 2004, 126, 14930–14934.
nosylation. Org. Biomol. Chem. 2020, 18, 3216–3228.
(34) The anomeric configuration of 7k was assigned based on
1JC1-H1 (158.2 Hz). We did not isolate any other glycosylated FK-506
derivatives after chromatographic purification, and no discernible
peaks corresponding to other glycosylated FK-506 derivatives
were present in the crude NMR. We therefore reported the selec-
tivity as .
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(36) The reaction proceeded to 94% conversion after 22 h.
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