Dehydrative glycosylation with cyclic phosphonium anhydrides

Abstract

Cyclic phosphonium anhydrides generated from bis-phosphine oxides and trifluoromethanesulfonic anhydride are shown as general coupling reagents in a dehydrative glycosylation reaction of C1-hemiacetals. This reaction protocol is characterized by a broad substrate scope and high yields, including reactions of O-, C-, N-, and S-based nucleophiles with furanose, pyranose, and deoxysugar donors.

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The glycosidic bond is one of the most abundant structural elements found in natural biopolymers,^1–3 and efficient and general methods for the synthesis of oligosaccharides and glycoconjugates are critical for realizing the full potential of saccharides in biology and medicine.⁴ Among a gamut of methodologies for stereoselective preparation of saccharides, dehydrative glycosylation⁵ plays a special role due to the unique properties of anomeric alcohols, such as the ease of preparation^6,7 and the mild activation protocols often compatible with other glycosyl donors.^8–16 Despite these seemingly useful properties of C1-hemiacetals, established dehydrative methods often lead to variable yields and selectivities. Direct conversion of the hydroxyl functionality using Brønsted or Lewis acid conditions (Fischer glycosylation, Scheme 1) or in situ activation with electrophilic reagents such as pyridinium salts,^17–19 sulfoxonium triflates,^20–24 carboxylic acid anhydrides,^25–27 triflic anhydride,^28,29 Appel reagents,³⁰ or transition metals (Ti,^31,32 Sn,^33,34 Cu,³⁵ Bi³⁶) result in variable yields and selectivities (Scheme 1). Moreover, use of in situ activating agents is often limited to O-based nucleophiles. With the above considerations in mind, we explored the utility of electrophilic phosphorus reagents³⁷ in dehydrative glycosylation reactions. Herein, we describe an efficient method applicable to the preparation of O-, C-, N-, and S-substituted glycosides using cyclic phosphonium anhydrides as coupling reagents. These reagents promoted the synthesis of glycosides in high yields and selectivities, and, unlike previous methods, the glycosyl donors and the coupling reagent can be recovered and reused.

Scheme 1 Selected dehydrative glycosylation reactions with C1-hemiacetals.

Our interest in phosphonium salts was sparked by a report from Mukaiyama,³⁸ who investigated the use of the Hendrickson's ‘POP’ reagent (Ph₃P⁺OP⁺Ph₃·2TfO⁻)^39–42 in a glycosylation reaction of furanoses. More recently, the use of the Hendrickson's reagent was reported in reactions of glucopyranoses, indicating that the utility of the phosphonium anhydrides can be extended to pyranose donors.⁴³ Yet, there were limitations to this protocol including a major side reaction from dimerization of the glycosyl donor and diminished reactivity of substrates substituted with deactivating protective groups. Moreover, the Hendrickson's reagent failed to promote glycosylation of a ribose donor with a moderately reactive acceptor.⁴⁴ We reasoned that cyclic phosphonium anhydrides⁴⁵ could be more suitable for glycosylations due to their increased electrophilic character (as measured by the Gutman–Beckett method)^46,47 and the ease of activation of the anomeric alcohol. Because of their broad functional group tolerance, convenient methods of formation, and facile modulation of structural and electronic properties, this class of reagents offers advantages over other dehydrative reagents. Furthermore, activation of the anomeric alcohol by formation of a mixed anhydride would lead to an intermediate with a cationic nature, thus diminishing its propensity to undergo unwanted dimerization because exchange at the phosphonium centre with a glycosyl acceptor would be disfavored.⁴⁸

To test the above hypothesis, we prepared cyclic phosphonium anhydrides (Scheme 2). Bis-phosphine oxides were treated with Tf₂O at 0 °C, cleanly generating the corresponding cyclic anhydrides 7–16 within minutes, as determined by ³¹P NMR measurements.⁴⁵ The anhydrides 7 and 9 were also isolated by precipitation with Et₂O and analysed by NMR spectroscopy, and the material obtained via isolation was spectroscopically identical to the intermediates generated in situ. For practical reasons, however, the phosphonium salts in this study were freshly prepared and used as a solution in CH₂Cl₂.

Scheme 2 Generation of cyclic phosphonium anhydrides from bis-phosphine bis-oxides.

With a series of phosphonium salts in hand, we started our methodological study by investigating glycosylation reactions of 2,3,4,6-tetra-O-benzyl-D-glucopyranose 17 (α/β 2 : 1) with benzyl or cyclohexyl alcohol as a model system (Table 1). Glycosyl donor 17 was treated with cyclic phosphonium anhydrides 7–16 in the presence of 2,4,6-tri-tert-butylpyridine (TTBP), and, after 20 minutes at 0 °C, reacted with an alcohol acceptor. We found that the cyclic alkyl chain reagents successfully promoted the glycosylation reactions, and salts 7 and 9 gave the best overall yields. As the size of the phosphonium salt had little impact on the diastereoselectivity, we wondered if the use of chiral phosphonium anhydrides might lead to preferential formation of a given glycosyl anomer. To test this hypothesis, we chose two phosphonium salts 15 and 16, which were prepared under the standard conditions from the corresponding bis-oxides (entries 12–15). The yields for the glycosylation reactions were largely dependent on the type of a reagent used (either enantiomer of 16 gave better yields than 15) rather than their absolute configuration, and the chiral phosphonium salts had little impact on the diastereoselectivity of these reactions. To compare the reactivity of the cyclic phosphonium anhydrides with an acyclic alkyl analogue, we generated a salt from n-Bu₃PO/Tf₂O, but this salt failed to provide 18 under the standard conditions. Based on these optimization studies we elected reagent 9 (generated from 1,4-bis(diphenylphosphino)butane bis-oxide, DPPBO₂) as the optimal reagent. Reaction of 1 equiv. of glycosyl donor with 1.5 equiv. of DPPBO₂, 1.25 equiv. of Tf₂O, and 3 equiv. of TTBP cleanly afforded the glycoside 18b in an excellent yield (91%) and 1 : 1.2 α/β diastereselectivity (entry 5).²¹ In order to improve the anomeric selectivity, we hypothesized that the putative phosphonium intermediate can undergo anomerization in the presence of an iodide or bromide salt and can result in the formation of a more reactive β anomeric halide, similar to the protocol by Lemieux.⁴⁹

Table 1 Development of dehydrative glycosylation with phosphonium anhydrides^a

Entry	Anhydride^b	ROH	Yield^c	α : β^d
a General reaction optimization protocol: 17 (1 equiv.), ROH (1.5 equiv.), bis-phosphine oxide (1.5 equiv.), Tf2O (1.25 equiv.), TTBP (3 equiv.), CH2Cl2, 4 Å MS, 0 °C. b Phosphonium anhydrides were generated in situ. c Yields refer to isolated, analytically pure material. d Determined by ^1H NMR of the crude reaction mixture. e TBAI (1 equiv.) was added and (CH2Cl)2 was used as a solvent.
1	7	CyOH	56%	1.8 : 1
2	7	BnOH	69%	1.2 : 1
3	8	CyOH	<5%	N.D.
4	9	CyOH	83%	1 : 1.9
5	9	BnOH	91%	1 : 1.2
6^e	9	CyOH	50%	α only
7	10	CyOH	56%	1 : 2
8	11	CyOH	72%	1 : 2
9	12	CyOH	<5%	N.D.
10	13	CyOH	<5%	N.D.
11	14	CyOH	<5%	N.D.
12	(R)-15	CyOH	36%	1 : 1.5
13	(S)-15	CyOH	23%	1 : 2.2
14	(R)-16	CyOH	54%	1 : 1.8
15	(S)-16	CyOH	75%	1 : 1.7

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Thus, to fine-tune the reaction conditions, a solution of 1 equiv. of TBAI (tetra-n-butylammonium iodide) was added to the activated anomeric phosphonium intermediate together with the alcohol acceptor. Although the reaction yield was diminished, exclusive α selectivity of 18a was observed (entry 6). Other activation modes of DPPBO₂ (e.g., TMSOTf, Ms₂O) as well as the use of bis-phosphine monoxide (DPPE monoxide) in combination with Tf₂O and TTBP resulted in trace amounts of the expected glycosides 18. Due to the limited solubility of the phosphonium anhydrides, solvents such as PhMe, THF, Et₂O, and MeCN were not compatible with the glycosylation reactions, and 18 was formed in significantly lower yields. The nature of the base is also important – exclusion of the base altogether as well as the use of smaller bases (Et₃N, pyridine) led to diminished yields of 18, rendering TTBP the reagent of choice for this reaction. The progress of the glycosylation reaction can be easily monitored visually because a very polar phosphine bis-oxide by-product (DPPBO₂) precipitates once the reaction is complete. Furthermore, to demonstrate the scalability of the dehydrative protocol, a glycosylation reaction of 17 with Cy–OH was conducted with 1.2 g of DPPBO₂ and 2,3-di-tert-butyl-4-methylpyridine as an inexpensive alternative to TTBP to afford the product 18a in 83% isolated yield using the optimized conditions. The phosphine bis-oxide DPPBO₂ was recovered from the reaction mixture in 91% yield via simple precipitation with hexane and filtration prior to chromatographic purification and reused in subsequent reactions without any loss of activity.

Having established a general glycosylation protocol, the scope of the reaction was investigated (Table 2). We found that deoxysugars (fucose, entry 1; arabinose, entries 2 and 3) and ribofuranose⁵⁰ (entry 4) are excellent substrates for the glycosylation reactions with primary and secondary alcohol acceptors. Furthermore, when the benzylidene mannose donor 27 was activated under the optimized conditions, preferential formation of the β-anomer (entry 5) was observed,^51–58 whereas 2-O-benzoyl-mannose gave the expected α-anomer (entries 7 and 8). Similarly, phenolic acceptor was also tolerated in this reaction giving rise to the α anomer 33 in 98% yield. However, 2-O-benzyl mannose resulted in a mixture of anomers (entry 10). Curiously, the sulfoxide donor was also smoothly activated by the phosphonium salt (entry 11),⁵⁹ whereas the anomeric thioether was tolerated in this protocol (entry 7) opening a possibility of a sequential one-pot glycosylation via orthogonal activation with C1-hemiacetals.⁶⁰ In all examples tested under the dehydrative conditions, we were unable to detect homocoupling products, and the excess of the acceptor could be recovered.

Table 2 Reaction scope with O-based nucleophiles^a

Entry	Donor	Acceptor	Product^c
a Optimized protocol from Table 1 (entry 5). b Optimized protocol from Table 1 (entry 6). c Yields refer to isolated material.
1		Cy–OH
2		Cy–OH
3
4^b		Cy–OH
5		Cy–OH
6
7
8
9		PhOH
10		CyOH
11

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Table 3 lists reactions of C1-hemiacetals with sulfur-, nitrogen- and carbon-based nucleophiles promoted by dicationic phosphonium salt 9. Excellent to good selectivities were observed even without a participating group at C2 (entries 1 and 2), giving primarily the α-anomers for small nucleophiles and β-anomers for aromatic acceptors (entries 3 and 4). These reactions represent a rare example where anomeric alcohols can be directly converted into unnatural glycosides in excellent yields and selectivities. The reaction with a thiol is of interest as the majority of chemical methods to install the anomeric thioether rely on a participating group at C2 resulting in predominantly β anomers.

Table 3 Reaction scope of dehydrative glycosylation^a

Entry	Donor	Product	Yield^b	α : β
a Reactions were carried out using the optimized protocol from Table 1 (entry 5). b Yields refer to isolated material. c TolSH used as a nucleophile. d TMSN3 used as a nucleophile. e Indole used as a nucleophile. f Benzotriazole used as a nucleophile.
1^c			96%	6 : 1
2^d			94%	12 : 1
3^e			88%	β only
4^f			75%	β only

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To ascertain the reactivity of cyclic phosphonium salts, the progress of the dehydrative glycosylation reaction was monitored by ³¹P NMR spectroscopy.⁶¹ Addition of alcohol 17 to the cyclic salt 9 resulted in the formation of two species, which were assigned as the phosphonium anomers (see, ESI†). The ratio of α/β anomers in this reaction remained unchanged (CD₂Cl₂, rt, 24 h), and external additives such as phosphine oxides (Ph₃PO) had no influence on the anomer distribution. Addition of cyclohexanol led to immediate formation of the expected glycoside 18a and phosphine oxide (DPPBO₂). Based on the above observations, the proposed mechanism of glycosylation with cyclic phosphonium anhydrides is presented in Scheme 3. Addition of a glycoside hemiacetal 41 to cyclic phosphonium anhydride 42 generates a mixture of anomeric intermediates 43. The ³¹P NMR also supports the notion that 43 exists in a cyclic form as we were unable to detect any signals corresponding to the putative phosphine oxide 45. Addition of the alcohol acceptor R′-OH leads then to the formation of the glycoside 44. The formation of the glycosidic bond can occur through a direct displacement at the anomeric carbon atom and release of DPPBO₂ (low temperature experiments did not reveal the presence of anomeric triflate). Alternatively, in the presence of a C2 participating substituent, the delivery of nucleophile will be directed by the neighbouring group according to an established mechanism. On the other hand, reactions of C1-hemiacetals with acyclic phosphonium anhydrides 46 proceed via formation of the intermediate 47, which can react with alcoholic acceptor in a fashion similar to the cyclic analogs to furnish glycoside 44. However, the activated glycosidic acceptor 47 may undergo a competitive reaction with R′-OH to generate 48 and 41, which can engage in a reaction with 47 to form a glycoside dimer product. The use of a cyclic salt is therefore advantageous as it prevents undesired glycoside homocoupling reactions by minimizing the exchange reactions at the phosphonium centre in the activated glycosyl donor.

Scheme 3 Proposed mechanism of dehydrative glycosylation.

To summarize, we have developed a dehydrative glycosylation method using phosphonium anhydrides and demonstrated its utility in a range of anomeric alcohol substrates. In contrast to previous methods, cyclic phosphonium anhydrides are versatile reagents for the preparation of O-, N-, C-, and S-glycosides in excellent yields, can tolerate deactivated glycosyl donors and the undesired dimerization of glycosyl substrates previously observed with Hendrickson's reagent was suppressed. This first application of cyclic phosphonium anhydrides in a glycosylation reaction indicates that the dicationic nature of these reagents suppresses undesired exchange reactions and opens up possibilities for the future use of the phosphonium salts in the synthesis of oligosaccharides. Moreover, unlike previous dehydrative methods, the glycosyl donor can be recovered unreacted and the coupling reagent (phosphine bis-oxide) reused. Further studies on the mechanism and scope of the dehydrative method will be reported in due course.

This work was generously supported by the University of Colorado at Boulder.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures, characterization data for new compounds and copies of NMR spectra. See DOI: 10.1039/c6ob01812b

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