Cooperatively Catalyzed Activation of Thioglycosides That Bypasses Intermediacy of Glycosyl Halides

Abstract

Reported herein is the development of a novel method for activating thioglycosides without a glycosyl halide intermediate. This has been achieved through the use of a silver salt coupled with an acid additive and molecular iodine. The enhanced stereocontrol was achieved via the H-bond mediated aglycone delivery (HAD) method, and the extended trisaccharide synthesis was achieved via iteration of deprotection and glycosylation steps.

Introduction

Many methods for chemical glycosylation and the synthesis of glycans are known. The first reactions were performed by Michael,^[1] Fischer,^[2] and Koenigs/Knorr.^[3] Those early methods relying on Cl, Br, or OH as the anomeric leaving group were mostly suitable for the synthesis of simple alkyl or aryl glycosides.^[4] In a perpetual quest for more efficient methods, many other leaving groups have been introduced and methods for their activation have been developed.^[5] Among these, thioglycosides, O-imidates, and halides are the most common. Thioglycosides are stable towards most protecting group manipulations, but can be readily activated in the presence of thiophilic reagents.^[6] A significant effort has been dedicated to the development of activators for their glycosidation.^[6c] NIS/TfOH system introduced by van Boom,^[7] remains a popular choice, but slow reaction rates and modest yields encountered in some applications prompted the discovery of other promoters that included metal salts,^[8] halogens,^[9] and organosulfur reagents,^[10] among others.^[11]

Although thioglycosides can be used as glycosyl donors directly, researchers often choose to invest into additional steps to introduce more reactive leaving groups such as O-imidates, phosphates, or halides. These syntheses still start with thioglycosides, which are first protected as needed for the purpose of the synthesis and then converted into more reactive donors. This multi-step synthesis is needed because highly reactive leaving groups are unable to withstand protecting group manipulations and need to be introduced directly prior to glycosylation.

Results and Discussion

Over the years, our lab has been studying methods for the activation of thioglycosides.^[12] Among approaches developed, the use of molecular bromine was particularly instrumental for the synthesis of 1,2-cis glycosides.^[13] However, this activation protocol lacks efficiency with unreactive (per-O-benzoylated or also known as disarmed) thioglycosides. For example, as depicted in Table 1, the activation of thioglycoside 1^[14] for reaction with glycosyl acceptor 3^[15] in the presence of 2.0 equiv. of bromine produced disaccharide 4^[16] in only 45 % yield in 16 h (Table 1, entry 1).^[13a] Since this reaction proceeds via intermediacy of the anomeric bromide, one of the ways of enhancing the outcome of this reaction is to add promoters that would be effective in the activation of the anomeric C–Br linkage.

Table 1.

Optimization of the cooperatively-catalyzed activation of thioglycosides 1 and 2.

Entry	Donor	Conditions	Product, yield [%], α/β ratio
1[13a]	1	Br2 (2.0 equiv.), rt, 16 h	4, 45, β-only
2[18]	1	1. Br2; 2. Ag2O (2.0 equiv.), TMSOTf (0.25 equiv.), 0 °C, 10 min	4, 99, β-only
3[19]	1	1. Br2; 2. Ag2SO4 (0.7 equiv.), TfOH (0.4 equiv.), 0 °C, 10 min[a]	4, 97, β-only
4[21]	2	I2 (3.0 equiv.), rt, 15 min	5, 97, α/β = 1.0/1
5	1	I2 (3.0 equiv.), rt, 24 h	No reaction
6	1	I2 (3.0 equiv.), Ag2SO4 (1.5 equiv.), rt, 30 min	4, 97, β-only
7	1	I2 (2.0 equiv.), Ag2SO4 (1.5 equiv.), rt, 45 min	4, 87, β-only
8	1	I2 (1.5 equiv.), Ag2SO4 (1.5 equiv.), rt, 90 min	4, 99, β-only
9	1	I2 (1.5 equiv.), Ag2SO4 (1.5 equiv.), TfOH (0.2 equiv), rt, 20 min	4, 99, β-only
10	1	I2 (1.5 equiv.), Ag2SO4 (1.0 equiv.), rt, 20 h	4, 94, β-only
11	1	I2 (1.5 equiv.), Ag2SO4 (1.0 equiv.), TfOH (0.2 equiv.), rt, 16 h	4, 87, β-only
12	1	I2 (1.5 equiv.), Ag2SO4 (1.0 equiv.), TfOH (0.5 equiv.), rt, 16 h	4, 89, β-only
13	1	I2 (1.5 equiv.), Ag2SO4 (0.5 equiv.), rt, 24 h	4, 81, β-only
14	1	I2 (1.2 equiv.), Ag2SO4 (1.5 equiv.), rt, 90 min	4, 91, β-only
15	1	I2 (1.2 equiv.), Ag2SO4 (1.5 equiv.), TfOH (0.2 equiv.), rt, 75 min	4, 82, β-only
16	1	I2 (1.2 equiv.), Ag2SO4 (1.0 equiv.), rt, 72 h	4, 74, β-only
17	2	I2 (1.5 equiv.), Ag2SO4 (1.5 equiv.), TfOH (0.2 equiv.), rt, 10 min	5, 88, α/β = 1.0/1
18	2	I2 (1.5 equiv.), Ag2SO4 (1.5 equiv.), rt, 15 min	5, 84, α/β = 1.0/1
19	2	I2 (1.5 equiv.), rt, 72 h	5, 81, α/β = 1.0/1

^[a]reaction in toluene.

Under classical Koenigs–Knorr reaction conditions,^[3,17] a glycosyl bromide donor is coupled with a glycosyl acceptor in the presence of silver oxide or carbonate. This reaction can be slow too, particularly with unreactive, benzoylated bromides. Recently, we discovered that the addition of a catalytic amount of a Lewis acid to the Ag₂O-promoted glycosylation dramatically increases the rate of the reactions and enhances the yields.^[18] For example, when per-O-benzoylated glucosyl bromide, freshly prepared from thioglycoside 1 by reaction with bromine, was glycosidated with acceptor 3 in the presence of Ag₂O (3.0 equiv.) and 25 mol % of TMSOTf, disaccharide 4 was rapidly (10 min) obtained in 99 % yield (Table 1, entry 2).^[18] After preliminary screening of the silver salts and acidic additives we identified Ag₂SO₄ (0.7 equiv.) along with TfOH (0.4 equiv) as a promising new promoter system for the activation of glycosyl bromides. Thus, under these reaction conditions, disaccharide 4 was obtained in 10 min in 97 % yield (Table 1, entry 3).^[19] While these results for glycosidation of bromides were quite impressive, the two-step conversion from thioglycosides is less attractive. In particular, the interim evaporation and drying hampers this otherwise very efficient process. Therefore, we continued a quest for identifying mild activators, which would ensure smooth conversion of thioglycosides into O-glycosides and glycans.

Upon investigating a variety of other known thioglycoside promoters, we have gained a renewed interest in molecular iodine, which was originally investigated by Field as a mild promoter for the activation of methyl thioglycosides.^[9c] Our own study with this promoter was very instrumental for creating the basis for differentiating the reactivity levels of per-O-benzylated (armed) ethylthio glycoside 2^[20] and its 2-O-benzoyl-3,4,6-tri-O-benzyl (superarmed) counterpart at low temperature (−25 °C).^[21] Reactions at the ambient temperature were relatively fast with both of these substrates. For example, glycosidation of per-O-benzylated donor 2 with glycosyl accept-or 3 in the presence of 3.0 equiv. of iodine produced disaccharide 5^[22] in 97 % in 15 min (Table 1, entry 4).^[21] However, when we attempted a similar reaction with less reactive per-O-benzoylated donor 1^[14] no activation was seen and no products were obtained even after 24 h (Table 1, entry 5). Assuming that the mechanism of activation with I₂ would be similar to that of reactions in the presence of bromine, we performed this activation in the presence of 1.5 equiv. of Ag₂SO₄ and, to our delight, disaccharide 4 was rapidly produced (30 min) in 97 % yield (Table 1, entry 6).

Subsequent reduction of the amount of iodine to 2.0 and 1.5 equiv. still resulted in smooth and efficient reactions, and disaccharide 4 was obtained in 87 % and 99 % yields, respectively (entries 7 and 8). The rate of the reaction also decreased to 45 and 90 min, respectively. To enhance the rate, we performed the reaction in the presence of 20 mol % of TfOH. This three-component cooperative promoter system led to a smooth and efficient synthesis of disaccharide 4 that was obtained in 99 % in 20 min (Table 1, entry 9). Subsequent attempts to reduce the amounts of reagents did not result in notable improvements. Thus, reducing the amount of Ag₂SO₄ to 1.0 equiv. slowed the reaction dramatically (20 h, Table 1, entry 10). Even adding TfOH (0.2 or 0.5 equiv.) did not increase the rates: the reactions produced commendable yields (87–89 %), but still required 16 h to complete (entries 11 and 12). An attempt to reduce the amount of iodine to 1.2 equiv. led to decreased yields (74–91 %) and increased reaction times (75 min–72 h, entries 14–16). All reactions with glycosyl donor 1 proceeded with complete β-stereoselectivity due to participation of the neighboring 2-O-benzoyl group.

We then applied our most promising reaction conditions (Table 1, entry 9), I₂ (1.5 equiv.), Ag₂SO₄ (1.5 equiv.), TfOH (0.2 equiv.), to glycosidation of benzylated donor 2.^[20] This reaction was very swift, and disaccharide 5 was produced in 10 min in 88 % yield (Table 1, entry 17). Even in the absence of TfOH, we saw an excellent rate (15 min) and a commendable yield (84 %, Table 1, entry 18). Excluding Ag₂SO₄ from the promoter system was particularly detrimental for this reaction. In the presence of 1.5 equiv. of iodine, product 5 was obtained in 81 % yield, but the reaction required 72 h (Table 1, entry 19). All reactions with donor 2 were non-stereoselective due to the lack of stereodirecting handles such as a participating group at C-2.

Previously, we suggested the reaction mechanism by which the cooperatively catalyzed activation of glycosyl halides takes place in the presence of Ag₂O and TMSOTf (Scheme 1A).^[18] The process originates by bromide complexation with halophilic Ag₂O resulting in intermediate A that also forms in the classic Koenigs–Knorr reaction. Intermediate A will ultimately dissociate, but Ag₂O is too weak a promoter to effectively pull the leaving group, especially in the case of unreactive glycosyl halides. When catalytic TMSOTf (or TfOH) is added, a strongly ionized species B are formed. The latter will readily dissociate leading to the production of reactive intermediates that will subsequently react with the acceptor to form the desired glycoside product.

Scheme 1.

Previously suggested mechanistic pathway for glycosyl halides (A) and the anticipated mechanism for the activation of thioglycosides (B).

In case of the I₂/Ag₂SO₄/TfOH-catalyzed reaction developed herein, interaction of the thioglycoside starting material with thiophilic iodine will result in intermediate C (Scheme 1B), which was also proposed in previous studies by Field^[23] and by us.^[21] It should be noted that neither Ag₂SO₄ nor TfOH prompt the leaving group departure in thioglycosides. Even when used in cooperation, Ag₂SO₄/TfOH do not activate thioglycosides. Intermediate C will dissociate only in the case of reactive (armed) glycosyl donors. However, with unreactive thioglycosides it will remain intact until the halophilic silver salt is added. When the halophilic silver salt is added to intermediate C, a complex D will be formed. Based on our preliminary experimentation depicted in Table 1, we hypothesize that even complex D would not yet prompt the leaving group departure with the unreactive substrates (or will do so very slowly). Upon addition of catalytic TfOH, a strongly ionized species E are formed. The latter will readily dissociate leading to the production of the reactive intermediate F along with AgI and AgHSO₄. The formation of insoluble AgI implies that there is no clear path by which glycosyl iodide can be formed in this reaction because iodide anion is removed from the reaction medium. The reaction monitoring by 1H NMR showed no formation of glycosyl iodides. Intermediate F will then dissociate to form glycosyl cation G. Depending on the nature of the protecting group at C–2, intermediate G will be stabilized either via acyloxonium or oxacarbenium intermediate. The latter will react with the glycosyl acceptor (ROH) to form a protonated glycoside, which, upon deprotonation, will lead to the desired glycoside product. Also produced at this stage is TfOH that can be used in the next catalytic cycle.

Based on the preliminary experimentation, we chose the activation conditions comprising I₂ (1.5 equiv.), Ag₂SO₄ (1.5 equiv.), and TfOH (0.2 equiv.) in 1,2-DCE at rt (entry 9, Table 1) for all subsequent glycosylations. For expanding the scope of this reaction, we chose standard glycosyl acceptors 6–9^[15,24] depicted in Figure 1.

Figure 1.

Structures of glycosyl acceptors 6–9.

The results of this study are summarized in Table 2. Thus, glycosidation of per-O-benzoylated glucosyl donor 1 with 4-OH acceptor 6 under the standard reaction conditions produced disaccharide 10^[15–16] in 1.5 h in 83 % yield (Table 2, entry 1). Glycosidation of donor 1 with 3-OH acceptor 7 produced disaccharide 11^[15–16] in 1.25 h in 87 % yield (Table 2, entry 2). Glycosidation of glucosyl donor 1 with 2-OH acceptor 8 afforded disaccharide 12^[15–16] in 40 min in 89 % yield (Table 2, entry 3). Glycosidation of glucosyl donor 1 with deactivated 6-OH acceptor 9 produced disaccharide 13^[25] in 30 min in 99 % yield (Table 2, entry 4). These reactions proceeded with complete β-stereoselectivity due to participation of the neighboring 2-O-benzoyl group.

Table 2.

Expanding the cooperatively catalyzed glycosylation to other donor-acceptor combinations.

To further explore the scope of this reaction, we investigated other thioglycosides as glycosyl donors. Glycosidation of α-thioglycoside 14^[14,26] with 6-OH acceptor 9 was very efficient, and disaccharide 13 was obtained in 45 min in 96 % yield (Table 2, entry 5). This result was practically identical to that obtained with β-thioglycoside 1 depicted in entry 4. Glycosidation of per-O-benzoylated phenylthio and tolylthio glycosides 15^[27] and 16^[28] with 6-OH acceptor 3 under standard reaction conditions produced disaccharide 4 in 30 min −2 h in 87 % and 96 % yield, respectively (Table 2, entries 6 and 7). The glycosidation of S-phenyl glycosides was faster, but S-tolyl donor produced a higher yield.

In expansion to other common sugar series, we investigated glycosyl donors of the galacto and manno series. Thus, glycosidation of per-O-benzoylated galactosyl donor 17^[14] with the deactivated 6-OH acceptor 9 under standard reaction conditions, swiftly (15 min) produced disaccharide 18^[29] in 96 % yield (Table 2, entry 8). Glycosidation of per-O-benzylated galactosyl donor 19^[30] with acceptor 9 was even faster (10 min). The resulting disaccharide 20^[29] was obtained in 82 % yield as a mixture of anomers (α/β=1/2.5, Table 2, entry 9). The loss of stereoselectivity encountered here is due to the lack of the neighboring participating group in donor 19. Glycosidation of mannosyl donors 21^[31] and 23^[32] with acceptor 9 under standard reaction conditions produced the respective disaccharides 22^[25] and 24^[25] in 67–70 % yields (Table 2, entries 11 and 12) with complete α-selectivity.

So far, our studies have demonstrated that I₂/Ag₂SO₄/TfOH-catalyzed reactions are swift and high yielding. However, they entirely lack stereocontrol in case of 2-O-benzylated glycosyl donors. To gain a stereocontrolling mode we moved on to investigating whether these new reaction conditions would be compatible with the H-bond-mediated Aglycone Delivery (HAD) pathway.^[33] The HAD reaction was based on discovery that glycosyl donors equipped with 3-, 4-, or 6-O-picoloyl (Pico) protecting group provide high syn-selectivities in respect to Pico.^[33] We acquired experimental evidence to explain that the glycosyl acceptor forms an H-bond with the Pico nitrogen of the donor. Upon the leaving group activation, the acceptor (aglycone) is delivered to form the glycosidic bond with high syn-selectivity in respect to the remote Pic/Pico group (illustrated for 1,4- and 1,6-syn in Scheme 2A).^[33] Using our reaction conditions comprising I₂/Ag₂SO₄/TfOH, glycosidation of 4-O-picoloylated glucosyl donor 25^[33] with 6-OH acceptor 9 produced the corresponding disaccharide 26^[34] in 86 % with complete α-selectivity (Scheme 2B).

Scheme 2.

H-bond-mediated Aglycone Delivery (A) and the cooperatively catalyzed HAD reaction (B).

Having achieved high efficiency in the preliminary experimentation, we turned our attention to investigating the multi-step glycan synthesis. For this purpose, we reacted 4-Pico donor 25 with glycosyl acceptor 3 to afford disaccharide 27^[13b] in 88 % yield with complete α-selectivity (Scheme 3). We then subjected disaccharide 27 to chemoselective removal of the Pico group in the presence of Cu(OAc)₂–H₂O in a mixture of DCM/MeOH (3/1, v/v)^[35] that was achieved in 94 % yield. The resulting 4’-OH disaccharide acceptor 28 was glycosylated with donor 1 to afford trisaccharide 30 in 97 % yield with complete β-selectivity. Disaccharide acceptor 28 was also glycosylated with thioglycoside 29^[36] equipped with the superarming protecting group pattern. As a result, trisaccharide 31 was obtained in 87 % yield with complete β-selectivity.

Scheme 3.

Cooperatively catalyzed iterative synthesis of trisaccharides 30 and 31.

The stepwise synthesis of (1–4, 1–6)-patterned trisaccharides.

We then investigated 6-Pico donor 32^[33] that was expected to be capable of directing glycosylation towards β-linked products. Indeed, glycosylation of acceptor 3 with 6-Pico donor 32 produced disaccharide 33^[33] in 90 % yield with complete β-selectivity (Scheme 4). Disaccharide 33 was then subjected to chemoselective removal of the Pico group. This was achieved in the presence of Cu(OAc)₂–H₂O in a mixture of DCM/MeOH (3/1, v/v) in 92 % yield.

Scheme 4.

Cooperatively catalyzed iterative synthesis of trisaccharides 35–37.

The stepwise synthesis of (1–6)-linked trisaccharides.

The resulting 6’-OH glycosyl acceptor 34^[37] was glycosylated with donors 1, 29 or 32 to afford the respective trisaccharides 35–37 in 72–94 % yields with complete (35 and 36) or high (37, α/β = 1/13.6) β-selectivity. Subsequently, we coupled superarmed donor 29 with acceptor 3 to afford disaccharide 38^[38] in 89 % yield with complete β-selectivity (Scheme 5). The latter was deacylated under Zemplen conditions to afford 2’-OH acceptor 39^[39] in 99 % yield. Reactions of donors 1 or 29 with disaccharide acceptor 39 gave the respective trisaccharides 40 or 41 in 86–87 % yield and complete β-selectivity.

Scheme 5.

Cooperatively catalyzed iterative synthesis of trisaccharides 40 and 41.

The stepwise synthesis of (1–2, 1–6)-patterned trisaccharides.

Conclusions

Our previous studies of the cooperatively catalyzed Koenigs-Knorr reaction showed that the activation of glycosyl bromides and chlorides can be swift and efficient.^[18–19,40] Excellent yields have been achieved with many substrates. However, previously employed multistep process that required conversion of thioglycoside precursors to halides, work-up and/or drying, followed by glycosylation was less desirable. Developed herein is a new glycosylation reaction for the direct activation of conventional thioglycosides using cooperative catalysis. We have made first steps to demonstrate that the direct cooperatively catalyzed activation of thioglycosides that does not proceed via intermediacy of glycosyl halides can yield a superior method for glycosylation. As demonstrated by several substrates and targets, this method offers new synthetic capabilities and helps to revisit cases where glycosylations previously showed slow rates, gave low yields, or did not work at all. We have also made first steps towards showcasing how α-selectivity of the cooperatively catalyzed glycosylations can be enhanced by the HAD method. Previously, high α-selectivity was only achieved in cooperatively catalyzed galactosylations using other stereocontrolling handles.^[41] In the study reported herein we combined the cooperative catalysis with the HAD method in one platform. The acquired results indicate that this may yield a superior method for efficient and stereocontrolled glycosylation that will be applicable to the synthesis of challenging linkages and glycan targets.

Experimental Section

General:

Column chromatography was performed on silica gel 60 (70–230 mesh), reactions were monitored by TLC on Kieselgel 60 F254. The compounds were detected by examination under UV light and by charring with 10 % sulfuric acid in methanol. Solvents were removed under reduced pressure at < 40 °C. CH₂Cl₂ and ClCH₂CH₂Cl (1,2-DCE) were distilled from CaH₂ directly prior to application. Molecular sieves (3 Å or 4 Å), used for reactions, were crushed and activated in vacuo at 390 °C during 8 h in the first instance and then for 2–3 h at 390 °C directly prior to application. Optical rotations were measured at ‘Jasco P-2000’ polarimeter. ¹H NMR spectra were recorded in CDCl₃ at 300, 400, or 700 MHz. ¹³C {¹H} NMR spectra were recorded in CDCl₃ at 75 or 175 MHz. The ¹H NMR chemical shifts were referenced to tetramethyl silane (TMS, δ_H = 0 ppm) or CDCl₃ (δ_H = 7.26 ppm) for ¹H NMR spectra for solutions in CDCl₃. The ¹³C{¹H} NMR chemical shifts were referenced to the central signal of CDCl₃ (δ_C = 77.00 ppm) for solutions in CDCl₃. Structural assignments were made with additional information from COSY and HSQC experiments. Compound purity or compound ratios were accessed or calculated by comparing the integration intensities of the relevant signals in their ¹H NMR spectra. Accurate mass spectrometry determinations were performed using Agilent 6230 ESI TOF LCMS mass spectrometer.

Synthesis of Disaccharides

General method for glycosylation in the presence of I₂, Ag₂SO₄, and TfOH:

A mixture of a glycosyl donor (0.05 mmol), glycosyl acceptor (0.045 mmol), and freshly activated molecular sieves (3 Å, 150 mg) in 1,2-dichloroethane (1.0 mL) was stirred under argon for 1 h at rt. I₂ (0.0675 mmol), Ag₂SO₄ (0.0675 mmol), and TfOH (0.009 mmol) were added, and the resulting mixture was stirred under argon at rt for the time specified in tables. After that, the solids were filtered off through a pad of Celite and washed successively with CH₂Cl₂. The combined filtrate (~ 40 mL) was washed with 10 % aq. Na₂S₂O₃ (10 mL) and brine (2 × 10 mL). The organic phase was separated, dried with magnesium sulfate, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (ethyl acetate–hexane or toluene gradient elution) to afford the respective disaccharides in yields and stereoselectivites listed in tables. Anomeric ratios (or anomeric purity) were determined by comparison of the integral intensities of relevant signals in ¹H NMR spectra.

Synthesis of trisaccharides

General method for removal of O-Pico groups:

Cu(OAc)₂–H₂O (10 mg, 0.055 mmol) was added to a solution of O-Pico derivative (0.037 mmol) in CH₂Cl₂/MeOH (1.0 mL, 3/1, v/v), and the resulting mixture was stirred for 20 min at rt. After that, the reaction mixture was diluted with CH₂Cl₂ (~ 10 mL) and washed with water (5 mL), sat. aq. NaHCO₃ (5 mL), and water (2 × 5 mL). The organic phase was separated, dried with Na₂SO₄, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (ethyl acetate–hexane gradient elution) to afford the unprotected compound in a yield specified in Schemes.

General method for removal of O-benzoyl groups:

A benzoylated compound (0.095 mmol) was dissolved in MeOH/THF (10 mL, 9/1, v/v), 1 N solution of NaOMe in MeOH was added until pH ~ 14, and the resulting mixture was stirred for 1–48 h at rt. The reaction mixture was neutralized with Dowex H⁺, the resin was filtered off and rinsed successively with MeOH. The combined filtrate was concentrated under reduced pressure, and the residue was dried in vacuo for 2 h to afford the unprotected compound in a yield specified in Schemes.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.