Letter Cite This: Org. Lett. 2018, 20, 4833−4837
pubs.acs.org/OrgLett
Stereodivergent Mannosylation Using 2‑O‑(orthoTosylamido)benzyl Group Feiqing Ding, Akihiro Ishiwata, and Yukishige Ito* Synthetic Cellular Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
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S Supporting Information *
ABSTRACT: We report a novel strategy for obtaining both anomers from a single mannosyl donor equipped with a C2-oTsNHbenzyl ether (2-O-TAB) by switching reaction conditions. In particular, the formation of various β-mannosides was achieved with high selectivity by using a mannosyl phosphite in the presence of ZnI2.
T
of the anomeric (equatorial) and neighboring oxy-substituents (1,2-cis). In response to this problem, various methods have been developed for the stereoselective synthesis of βmannosides, by both direct6 and indirect7,8 reactions. The former typically use 4,6-O-benzylidene protected donors6a−h (eq 2), while the latter exploit either intramolecular aglycon delivery7 or stereochemical inversion of β-gluco8a−e or βgalacto8f,g glycosides. Recently, Demchenko and co-workers uncovered a facile hydrogen-bond-mediated aglycone delivery (HAD) approach to achieve β-stereoselective mannosylation with 3- and/or 6-O-picoloyl thiomannosyl donors (eq 3).9 In this study, we explored the utilization of the orthotosylamidobenzyl (TAB) group as a stereodirecting element. We envisaged that the hydrogen bond donating ability of the TsNH group would cause interaction with the incoming alcohol (ROH) and thereby lead to β-facial selectivity (eq 4).10 We deemed also that, under conditions that disrupt the intermolecular hydrogen bonding, selective formation of the αglycoside would be possible (eq 5). To test our hypothesis, a mannosyl donor (1a) equipped with the TAB group at the C2 position was prepared (see the Supporting Information (SI)) and subjected to screening of reaction conditions using 2a as an acceptor substrate (Scheme 1, Table 1 and Table S1 in SI). Reactions with various Lewis or protic acids were marginally selective in favor of the α-isomer (Table 1, entries 1, 2 and Table S1). In a nonparticipating solvent, conditions that include zinc salt uniquely gave the βglycoside predominantly (Table S1, entries 3−10), among which ZnI2 gave the most promising result (Table 1, entry 3). The β-selectivity was increased when the glycosyl phosphite (1d) was used as a donor (Table 1, entries 4−11), especially when an overstoichiometric amount of Zn2+ salt (2 equiv) was used (Table 1, entry 11). Eventually, the product 3a was obtained with high selectivity (α:β = 5:95) from 1d by using 2 equiv of ZnI2. However, when the promoter was switched to Cu(OTf)2, the same phosphite predominantly gave the α-
he addition of N-linked (asparagine-linked) glycans is one of the most widespread protein modifications, which exerts a variety of effects on the functions, structures, and stabilities of proteins.1 Despite their structural diversity, all of them contain a core pentasaccharide consisting of both α- and β-mannosides. In addition, a variety of oligo- or polysaccharides composed of α- or β-mannosyl linkages are known.2 Synthetic schemes for biologically active oligosaccharides are usually complicated, because formation of 1,2-cis and -trans glycosides requires distinct donors.3 In this context, the development of a common donor for the synthesis of both isomers would greatly simplify many oligosaccharide syntheses. Construction of α-mannosyl linkages is generally straightforward, as it can be achieved by neighboring group participation or through the exploitation of stereoelectronic effects (eq 1, Scheme 1).4 By contrast, β-mannosyl linkages are more challenging to synthesize,5 because of the unique alignment Scheme 1. Stereocontrol in Direct Mannosylation
Received: June 25, 2018 Published: July 27, 2018 © 2018 American Chemical Society
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DOI: 10.1021/acs.orglett.8b01979 Org. Lett. 2018, 20, 4833−4837
Letter
Organic Letters Table 1. Glycosylation of 1a−d with 2a under Various Conditionsa,b
entry
1
promoter (equiv)
1
a
TfOH (0.1)
2
a
3 4
a a
TMSOTf (0.1) ZnI2 (0.1)e ZnI2 (0.1)e
5 6
a a
ZnI2 (0.1)e ZnI2 (0.1)e
7
a
ZnI2 (1)e
8
b
ZnI2 (1)e
9
c
ZnI2 (1)e
10
d
ZnI2 (1)e
11
d
ZnI2 (2)e
12
d
13
d
14
d
15
d
16
d
Cu(OTf)2 (0.1) Cu(OTf)2 (0.1) Cu(OTf)2 (0.1) Cu(OTf)2 (0.1) Cu(OTf)2 (0.1)
solvent (M)
temp/t (h)
3a (%)c
α:βd
benzene (0.05) benzene (0.05) DCM (0.05) MeCN (0.05) Et2O (0.05) toluene (0.05) toluene (0.05) toluene (0.05) toluene (0.05) toluene (0.05) toluene (0.05) MeCN (0.1)
rt/3
75
71:29
rt/3
73
73:27
rt/3 rt/3
79 65
24:76 57:43
rt/3 rt/3
63 82
26:74 19:81
0 °C/24
42
14:86
0 °C/24
63
17:83
0 °C/24
45
11:89
0 °C/24
55
8:92
−10 °C/72
81f
5:95
rt/1
58
72:28
Et2O (0.1)
rt/1
48
69:31
toluene (0.1) toluene (0.1) toluene (0.1)
rt/1
67
76:24
50 °C/1
70
93:7
80 °C/1
76g
96:4
Figure 1. Glycosylation of TAB-protected donor 1d with various alcohols.
Bu (3f and 3g), acetonide (3i and 3p), and benzylidene acetal (3n) were tolerant under both conditions A and B. To show the utility of the “single donor” strategy, we conducted a synthesis of all possible stereoisomers of the trisaccharide D-Man-(1→2)-D-Man-(1→6)-D-Glc (Scheme 2).12 To execute this, selective deprotection of the TAB group was carried out as previously described.13 Disaccharides α- and β-3a were subjected to the four-step process that included (1) Boc protection, (2) deprotection of tosylamide group by Mg treatment, (3) Boc deprotection, and (4) treatment with DDQ to provide α- and β-8, respectively. While a more facile procedure would be desirable, our protocol for the selective cleavage of the TAB ether can be completed in 24 h and is practical enough for the current purpose. Subsequent glycosylation of β-8 with donor 1d under the conditions A produced trisaccharides β-3q (α/β = 3/97). On the other hand, conditions B produced the isomeric trisaccharide α-3q in 63% yield (α/β = 98/2). Similarly, α-8 was led to the trisaccharides β-3r and α-3r upon glycosylation under conditions A (55% yield of pure β isomer, α/β = 3/97) and B (64% yield of pure α isomer, α/β = 98/2), respectively. Two isomers of D-Man-(1→2)-D-Man-(1→6)-α-D-Glc-OMe (9 and 10) were obtained after global deprotection of the protected trisaccharides, showing that the TAB group can be removed under the standard debenzylation conditions (Scheme 2). In order to understand the origin of stereoselectivity, several model experiments were performed (Scheme 3). Under conditions A, perbenzylated donor 1e gave the corresponding glycoside 3s with drastically diminished stereoselectivity (α/β = 24/76). Stereoselectivity was similar between 1e and 1d (α/ β = 95/5 and 96/4, respectively) under conditions B. Interestingly, a donor equipped with an o-Ts2Nbenzyl group 1f exclusively gave the α-glycoside 3t under both conditions,
a
A complete list of examined reaction conditions is provided in the SI (Table S1). bDonor 1 (2 equiv), acceptor 2 (1 equiv), and MS 3 Å (100 mg/mL) were used. cIsolated yield of the anomeric mixture based on the acceptor. dDetermined by the integration ratio obtained from 1H NMR of crude mixture. e1 M solution in Et2O was used. f Isolated yield of corresponding β-glycoside. gIsolated yield of corresponding α-glycoside.
glycoside (Table 1, entries 12−16). The selectivity was highest in toluene especially at elevated temperature (Table 1, entry 16).11 The anomeric configuration of the products was confirmed through undecoupled 13C NMR spectroscopy, the 1 JC1−H1 values being 175 and 162 Hz, for the α- and βmannosides respectively (see SI). In all cases, the majority of excess donor was converted to the intramolecularly Nglycosylated product 4 (Table S1). By using various alcohols, including primary, secondary, and tertiary alcohols, as well as amino acids, and naturally occurring steroids, the scope of the reaction was assessed under conditions A (Table 1, entry 11) and B (Table 1, entry 16) (Figure 1). In most cases, βglycosides were dominant under conditions A. Of particular note, disaccharides 3k and 3l that correspond to the core structure (β-Man-(1→4)-GlcNAc) of N-linked glycans was formed with excellent selectivity. Acid-labile groups such as t4834
DOI: 10.1021/acs.orglett.8b01979 Org. Lett. 2018, 20, 4833−4837
Letter
Organic Letters Scheme 2. Assembly of All Four Isomers of the Trisaccharides D-Man-(1→2)-D-Man-(1→6)-D-Glc from “Single Donor”
Scheme 4. (a) Comparison of Sulfonamide Chemical Shifts and (b) Proposed Mechanism of Stereoselective Mannosylation
ZnI2 is required for highest selectivity, the involvement of Zn2+ in directing the entering nucleophile to the β-face of the donor is indicated. Based on these results, the mechanism of the β-directing effect of the TAB group is proposed, as depicted in Scheme 4b. Namely, 2-O-TAB protected donor 1 predominantly exists in an internally hydrogen-bonded form H-1, which predominantly gives α-glycosides. Under the conditions A, oxygens at the 2- and 3-position coordinate Zn2+, cleaving the intramolecular hydrogen bonding. The liberated NH group is now able to interact with an incoming nucleophile in an intermolecular fashion, reversing the stereocontrolling effect of the TAB group. In summary, a novel mannosyl donor equipped with a 2-OTAB ether allows both α- and β-selective glycosylation to be achieved simply by switching the reaction conditions. This approach was applied for the assembly of all four isomers of the naturally occurring trisaccharides D-Man-(1→2)-D-Man(1→6)-D-Glc. Further study to more precisely understand the stereodirecting effects of TAB group and expansion of the concept to a wider range of oligosaccharides is ongoing.
Scheme 3. Model Experiments Conducted by Using Perbenzylated (1e) and Di-N-tosylated (1f) Donors
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01979. Experimental and spectral data (PDF)
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suggesting that the presence of NH plays a role in β-selective glycosylation. NMR experiments indicate the presence of an intramolecular hydrogen bonding in the TAB group (Scheme 4a). Namely, NH signals of the donor (1d) and glycosides (3) were markedly shifted to the downfield region (8.5−9.2 ppm) compared to the unsubstituted tosylamide 11 (6.5 ppm)14 and similar to the value of the methyl ether 12, indicating that the tosylamide NH is in contact intramolecularly with the benzylic oxygen via hydrogen bonding. As more than stoichiometric
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Feiqing Ding: 0000-0002-0211-7101 Akihiro Ishiwata: 0000-0002-5542-2214 Yukishige Ito: 0000-0001-6251-7249 Notes
The authors declare no competing financial interest. 4835
DOI: 10.1021/acs.orglett.8b01979 Org. Lett. 2018, 20, 4833−4837
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Organic Letters
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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Specially Promoted Research (No. 16H06290, Y.I.) and a Grant-in-Aid for Scientific Research (No. 26350966, 15H02443) from the Japan Society for the Promotion of Science, and the RIKEN Special Postdoctoral Program (F.D.). We thank Dr. H. Koshino and Dr. T. Nakamura (RIKEN CSRS) for NMR and HR-Mass analysis and A. Takahashi (RIKEN) for technical assistance.
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