Solid Support Oligosaccharide Synthesis

2.2 WHY GLYCAL ASSEMBLY? STRATEGIC CONSIDERATIONS 17

final purification. Moreover, and perhaps most importantly for the broader context of glycoconjugate synthesis, the completed oligosaccharide construct would likely require retrieval from the solid matrix before conjugation to the peptide or lipid, unless this portion were already present as part of the linker to the solid support.

In the other scenario, which we have found more novel, for reasons that will be explained shortly, the oligomer undergoing elongation is mounted to the solid support somewhere in the nonreducing region. In this case, the reducing end (i.e., glycosyl donor portion) of the molecule is available for coupling to a solution-based acceptor A (Scheme 2.1, case 2).

The use of A has two requirements. The first is that the precise acceptor site on A be properly identified. The second is that the reducing end of A be functionalized so that new donor capacity can be installed at the anomeric carbon of the elongated construct, in anticipation of the next coupling event. Not unlike the possible situation of case 1, a serious question of functional group compatibility must be anticipated during glycosylation in case 2. During the coupling step, acceptor A cannot possess a fully equipped, next-stage anomeric donor function at the same time as it carries a free hydroxyl group.

The reason for favoring the second scenario described above has to do with our development of the “glycal assembly” method, which appeared to offer several advantages for the solid-phase synthesis of oligosaccharides via this strategy (case 2, Scheme 2.1). Scheme 2.2, with the expression 1→2→4, captures the essence of the method and reveals the potential attractiveness of this approach. In this scheme, the nature of E+ and of the oniumlike species 2 has been left unspecified, as well as any indication of stereochemistry. Nevertheless, it is apparent that the glycal terminus of 1 could be converted to a donor function as represented in a general way with 2. For

C

a

se 1

OR2

OR2

R O

OR

s

R2O

OR2

s

2

2

*

O O

OP

O O

D

O

OH

OP

O

A

X

O

reiterate

*

R1O

OR1

R1O

OR1

deprotect

OR1

OR1

Case 2

OR

OR2

R

O

2

OR

R2O

OR2

s

2

A

2

s

O

O

O X HO

O

O

O

OP

O OP reiterate

D

*

*

R O

OR

1

R1O

OR1

1

deprotect,

OR

OR1

activate to X

1

Key: s , solid support and linker; P, unique protecting group; X, activating group; * indicates a uniquely differentiated hydroxyl group

Scheme 2.1 Strategies for building an oligosaccharide chain: glycosyl acceptor linked via its reducing end (case 1) and donor linked via a nonreducing end (case 2).

18 THE GLYCAL ASSEMBLY METHOD ON SOLID SUPPORTS

					HO	O	s
O	s		O	s		O
					PO		O
	O	E+		O	PO	PO	O
PO			PO			3		O
						PO	E PO
					E
PO	1		PO				4	PO
				2
s	= solid support		Oligosaccharide			1) reiterate
						2) retrieve
	and linker

Scheme 2.2 General strategy for the synthesis of oligosaccharides on a solid support using the glycal assembly method.

example, compound 2 could be an isolable entity such as a 1,2-anhydrosugar,7 in which case E+ corresponds to an epoxidizing agent. Moreover, through employment of a glycal as the solution based acceptor, the scheme benefits from relative simplicity in the identification of strategic hydroxyls for glycosylation. It is in fact a lot more straightforward to distinguish the hydroxyls of a pyranosidal glycal than those of a pyranose.

Compound 2 acts as a support bound glycosyl donor to yield 4 when treated with acceptor glycal 3, along with any necessary agents to promote the glycosylation (Scheme 2.2). The process can be repeated to assemble the desired oligosaccharide. This is followed by retrieval from the support and purification by chromatographic methods. Two major factors influence “reiterability” and therefore the success of this approach: (1) the polymer-bound glycal double bond must be effectively converted to a donor function and (2) the coupling conditions must be tolerant of the glycal double bond present in the acceptor, which, for example, is labile in acidic environments.The use of 3,3-dimethyldioxirane (DMDO) as the epoxidizing agent effectively converts 1 to a 1,2-anhydrosugar. This is the simplest of the possible cases and the most ideal situation. It is particularly successful when applied to an appropriately protected galactose series. As will be outlined in the following sections, some situations have demanded the elaboration of the glycal double bond to other donor functions, such as anomeric sulfides.

2.3 LINKER DESIGN

Following adoption of the glycal paradigm for the solid-phase assembly of oligosaccharides, the next strategic consideration involved the choice of a solid support and implementation of a method of linkage. Our first preference for the insoluble support was polystyrene 1% divinylbenzene copolymer, which is commonly used for the solid-supported synthesis of peptides. This polymer has high loading capacity, is compatible with a wide range of reaction conditions, and is cheap.

2.3

LINKER DESIGN 19

OH

O

O

O

1) BuLi

OSiR2-

O

TMEDA

SiR2Cl

8

O O

cyclohexane

O

O

2) R2SiCl2

(i-Pr)2NEt

5

benzene

6: R = Ph

CH2Cl2

9: R = Ph

3: R = i-Pr

10: R = i-Pr

= Polymer support backbone

Scheme 2.3 Glycal attachment to a polystyrene resin via a silyl linker.

Launching of the program required attachment of the first glycal monomer, through its nonreducing end, to a polystyrene-based matrix. A covalent attachment through a silicon–oxygen bond would mimic one of the most common protecting groups used for alcohols. Using chemistry developed by Chan and coworkers,8 lithiation of polystyrene 5 at aromatic sites, followed by silylation with a dihalosilane, such as diphenyldichlorosilane, resulted in polymer-bound silyl halide 6 (Scheme 2.3). This silicon–chlorine linkage now became the attachment site for glycal 8 to create polymer bound glycal 9. Unreacted sites were capped by reaction with methanol. The extent of glycal loading was on average 0.3 mmol of saccharide per gram of polymer.

The success of this approach depended on the ability to load the monomer through the silylation reaction and, most of all, on the robustness of the silyl ether linkage during the coupling events. A significant improvement in stability was subsequently achieved through the use of a diisopropyl linker (bound glycal 10) in place of the diphenyl arrangement (Scheme 2.3).

This loading approach is successful for the relatively unhindered 6-hydroxyl, but proved to be inefficient for more sterically encumbered alcohols. A modification of the abovementioned method has been developed to enable attachments through these more hindered sites.9 This process is accomplished with inexpensive materials, and the linker is compatible with a variety of reaction conditions. Moreover, it allows for facile recycling of the polymer support for further use once the final carbohydrate assemblage is cleaved from the resin.

The success of this modification, exemplified by the use of 3,6-dibenzyl-glucal (Scheme 2.4), relies on the enhanced reactivity of a dialkyldihalosilane relative to its

	OBn	iPr	OBn
	Cl2Si(iPr)2	iPr			imidazole	iPr		OBn
		X Si		O			iPr
HO	O	O			O	Si		O
BnO		BnO				O
	imidazole				OH
						BnO
	11		12		13	14

Scheme 2.4 Polymer support silylene linking method for hindered hydroxyl-bearing glycals.

20 THE GLYCAL ASSEMBLY METHOD ON SOLID SUPPORTS

	O		O
O	O			iPr	OBz
		O
				O Si iPr	O
	Si			O
	O		iPr
		iPr		BzO
		15		16
				Pri	iPr
				Si
	iPr		OTIPS	O	O
				O
O	Si	iPr		O	O
		O	O
				O
	TIPSO
		17		18

Scheme 2.5 Glycals loaded via silylene linkage.

mono-halogenated counterpart. This differential in reactivity allows for a single linkage of more sterically demanding systems in the first stage, while discouraging 2-fold silylation (see 11→12). A more reactive nucleophile, such as a primary alcohol, even if polymer bound, can now join at the less active remaining silylating site. The commercially available hydroxymethyl-modified Merrifield10 or Wang11 resins perform well in the second-stage silylation, which effectively constitutes the loading step to furnish 14. The unreacted sites on 13 are capped by reversing the order of additions (imidazole and diisopropydichlorosilane are added to the polymer followed by methanol). Scheme 2.5 shows a variety of other glycals, which were successfully loaded via this method. Construct 17 was fashioned from extremely hindered hydroxyl groups. Typical loading capacities were in the range of 0.12–0.19 mmol/g.

We are now poised to proceed with glycal assembly on solid support, and the synthesis of oligosaccharides will be described in the following sections. As will be evident, the synthesized structures became increasingly complex as our confidence increased in the applicability of the glycal assembly method to solid-phase synthesis. This confidence goes hand in hand with our ability to manipulate glycal double bonds, in the context of the solid matrix, into becoming other appropriate donors. The synthesis of glycopeptides will be described in Section 2.10.

2.4 SOLID SUPPORT GLYCAL ASSEMBLY VIA 1,2-ANHYDROSUGAR DONORS

Reduction to practice was first realized in the context of the synthesis of a linear tetrasaccharide, outlined in Scheme 2.6.12 Polymer-bound galactal 9 was converted to the 1,2-anhydrosugar 19 by epoxidation with DMDO.13 Polymer-bound 19 acted as a glycosyl donor when reacted with a solution of 8 in the presence of zinc chloride, resulting in disaccharide 20a. The glycosylation procedure was reiterated using galactal 8 as acceptor, to afford 21. One further reiteration using glucal 22 yielded

2.4

SOLID SUPPORT GLYCAL ASSEMBLY VIA 1,2-ANHYDROSUGAR DONORS 21

OH

OR

OSiPh2-

O

O

O

O

O

OSiPh

-

O

2

O

1) DMDO

O O

O

O

O

O

O

CH2Cl2

O

DMDO

O

8

HO

O

O

ZnCl2 THF

O

CH2Cl2

O

O

2) 8

9

19

O

ZnCl2 THF

TBAF

20a: R = SiPh2-

OR

AcOH

20b: R = H

THF

O

O

O

O

O

OSiPh2-

HO O

O

O

O

O

O

1) DMDO

O

O

O

O

CH2Cl2

HO

HO

O

O

O

O

2)

OH

O

O

O

O

O

O

O

BnO

HO

HO

O

BnO

BnO

O

O

O

22

O

BnO

21

ZnCl2

TBAF

23a: R = SiPh2-

THF

AcOH

23b: R = H

THF

Scheme 2.6 Solid-phase synthesis of a tetrasaccharide using the glycal assembly method.

tetrasaccharide 23a. The cleavage of the product from the polymeric support could be performed at any stage of the sequence as a means of checking the progress of each new coupling. This was done by stirring polymer bound saccharide in a 1:1 mixture of 0.1 M acetic acid in THF and 0.1 M TBAF in THF for 3–4 h, followed by filtration and rinsing with THF. The saccharides were purified by column chromatography on silica gel using methanol/ether eluent mixtures. Cleavage after the final coupling gave linear tetrasaccharide 23b in 32% overall yield from 9. This first success translated to an average yield of 70% per coupling cycle (consisting of epoxidation and glycosylation), assuming quantitative retrieval during fluoride-mediated cleavage.

The method results in little or no “interior deletion” products. The final oligosaccharide is purified from highly polar byproducts with very straightforward chromatography. Most likely, any 1,2-anhydrosugar that has failed to couple to the acceptor is hydrolyzed during the intervening rinsing procedure. This hydrolysis has the dual benefit that it results in a permanently terminated sequence, which is also highly polar and therefore easily separable from the desired final product.

The versatility of this approach was demonstrated in the synthesis of a variety of oligosaccharides (Scheme 2.7). The synthesis of hexasaccharide 30 (Scheme 2.8) exemplified the use of a glucal acceptor with a secondary alcohol (27) as well as the use of a disaccharide acceptor such as 29. These typical, varied systems can be included in the protocol without complications.

22 THE GLYCAL ASSEMBLY METHOD ON SOLID SUPPORTS

OH*

O

OH*

Ph

O

O

O

O

O

O

O

O

O

*HO

O

O

O

O

HO

HO

HOBnO

O

Ph

O

BnO

O

O

O

O

BnO

O

BnO

O

O

HO

O

O

O

BnO

HO

BnO

26

24

25

Scheme 2.7 Oligosaccharides assembled on solid phase via glycal assembly. The asterisk denotes the achorage site of the first glycal to the solid support via SiPh2 linker.

The method can also be applied to the synthesis of branched oligosaccharides by capitalizing on the fact that the opening of a 1,2-anhydrosugar during glycosylation results in a newly exposed C2-hydroxyl group. This uniquely exposed hydroxyl function can serve in turn as a glycosyl acceptor and will be a key factor in the synthesis of blood group determinants.

			OR
	O		O
			O
1) DMDO
			O	O					1) DMDO
21	CH2Cl2		HO O						CH2Cl2
			O
2) ZnCl2, THF			O	O	O				2) ZnCl2, THF
Ph O									OH		OBn
		O		HO
	O				O	Ph	O		BnO
	HO			O				O
						O	O		O	O	O
	27			O			O		BnO
										BnO
						HO			OBn
	OR		TBAF		28a R = SiPh2-					29
O	O		AcOH		28b R = H
			THF
	O
	O		O
	HO	O
	O		O
	O
			O
			HO O	Ph O		O		O
			O
				O	O				OBn
			O		O		BnO
						HO
				HO						O
						BnO		O	O
TBAF									BnO
			30a R = SiPh2-					OBn
AcOH
THF			30b R = H

Scheme 2.8 Solid-phase synthesis of a linear hexasaccharide.

2.5 SOLID-PHASE SYNTHESIS OF THE BLOOD GROUP H DETERMINANT 23

High-resolution magic-angle spinning NMR (HR-MAS) experiments proved to be an ideal way of monitoring the solid support synthesis by obtaining 1H NMR, 13C NMR, and 1H13C-NMR “on resin” spectra of high quality.14 This capability was first exemplified with the monitoring of the formation of crude product from the multistep synthesis of a trisaccharide similar to 21 (Scheme 2.6). This technique is less time-consuming and less wasteful of material than the cleavage method in the analysis phase. Since their introduction, on-resin NMR techniques have greatly facilitated the development of novel synthetic schemes of oligosaccharides and glycopeptides on solid support. We refer the reader to Chapter 8 in this book, which is dedicated to this topic.

2.5 SOLID-PHASE SYNTHESIS OF THE BLOOD GROUP H DETERMINANT

The synthesis of carbohydrate domains having blood group determining specificities15 was the first striking demonstration of the power of glycal assembly on solid support. These branched structures are naturally expressed as glycoproteins or glycolipids and play key roles in cell adhesion and other binding phenomena.16 Furthermore, glycoconjugates related to blood group substances have been recognized as markers for the onset of various tumors. These tumor-associated antigens are currently being studied in vaccines for cancer immunotherapy.17

The H-type 2 determinant (Scheme 2.9) is found largely on the surface of erythrocytes and the epidermis of type O persons, at the termini of membrane associated glycoproteins.18 Persons of blood types A and B also possess this determinant, which is further glycosylated at its galactose nonreducing terminus with a galactosamine (type A) or galactose moiety (type B). The solid phase assembly of

OSi(iPr)2-

F

OSi(iPr)2-

OBn

O

OBn

1) DMDO

O

O

O

O

CH2Cl2

O

BnO

OBn

32

O

O

O

O

O

2)

BnO

BnO

Sn(OTf)2

HO

10

11

HO

O

31

DTBP Et2O

BnO

ZnCl2 THF

OR

OBn

O

O

OAc

OAc

O

OAc

O

O

AcO

AcO

O

O

BnO

O

O

O

AcO

O

O

O

OBn

O AcO

NHAc

BnO OBn

O

OAc

TBAF

33a: R = Si(iPr)

-

AcO OAc

AcOH

2

34

33b: R = H

THF

Scheme 2.9 Solid-phase synthesis of an H-type 2 blood group determinant.

24 THE GLYCAL ASSEMBLY METHOD ON SOLID SUPPORTS

the H-type 2 tetrasaccharide 34 is depicted in Scheme 2.9.19 Galactal 10, bound to polymer via the diisopropysilyl linker, was treated with a solution of DMDO, followed by glucal acceptor 11 and zinc chloride, thereby providing disaccharide 31. With its unique C2-hydroxyl newly uncovered, compound 31 now acted as a polymer-bound acceptor vis-à-vis the solution-based fucosyl donor 32. Polymer-bound trisaccharide 33a was thus obtained, through the agency of tin triflate. The addition of the nonnucleophilic base di-tert-butylpyridine (DTBP) was necessary in this instance to protect the glycal double bond. Treatment of 33a with TBAF provided trisaccharide glycal 33b in 50% overall yield from 10.

When this synthesis was undertaken, no solid support methodology existed to construct glycosidic linkages bearing C2-acylamino functions. We therefore had to take recourse to solution-phase chemistry in preparing the H type 2 blood group determinant tetrasaccharide glycal 34. The trisaccharide glycal was cleaved from the solid support and extended via iodosulfonamidation in solution. Protecting group manipulations eventually led to 34 (Scheme 2.9), whose glycal functionality provided a handle for further functionalization at the reducing end. As will be shown later, a protocol for the azaglycosylation on the solid phase has been developed, so that the abovementioned 1-sugar homologation can now be made directly on solid support.

2.6 SOLID SUPPORT GLYCAL ASSEMBLY VIA THIOETHYL GLYCOSYL DONORS

The use of glycals on the solid support has led to the efficient construction of β-galactosyl linkages, even with hindered acceptors such as C4 hydroxyls flanked by protecting groups at C3 and C6. We have taken advantage of the stability of the 1,2-anhydrogalactose donor to very mild Lewis acids such as anhydrous zinc chloride. This stability may be a result of the constraints imposed on the molecule by the carbonate protecting group bridging the C3 and C4 hydroxyls (Scheme 2.10, compound 35).

The analogous β-glucosidic linkages cannot be prepared as efficiently as the galactosidic linkages with the methodology developed thus far. This may be because no similarly constrained glucosyl epoxy donor is available. The glucosyl system 36 (Scheme 2.10) is highly reactive in the presence of zinc chloride and degradation of the donor is competitive with glycosylation, especially with hindered acceptors.

O OR		OR
O	BnO	O
O
O	BnO	O
O
35		36

Scheme 2.10 Galactosyl and glucosyl 1,2-anhydrosugar donors.

2.6 SOLID SUPPORT GLYCAL ASSEMBLY VIA THIOETHYL GLYCOSYL DONORS 25

An approach has therefore been introduced to overcome this problem, which involves the conversion of glycals into thioethyl glycosyl donors.20 On activation with thiophilic reagents, thioethyl donors bearing a participatory protecting group at C2, such as a pivaloyl group, become very powerful β-glycosylating agents.21 Pivaloyl groups are chosen because they have been shown to prevent orthoester formation during the coupling step.22

Polymer-supported glucal 37 was converted to the protected thioethyl glucosyl donor 39 as outlined in Scheme 2.11. Compound 37 was first epoxidized by the action of DMDO. The resulting 1,2-anhydrosugar was opened by a mixture of ethanethiol and dichloromethane (1:1) in the presence of a trace of trifluoroacetic acid. Polymer-bound 38 was thus obtained in 91% yield. This was a substantial improvement over the 78% yield obtained by the same protocol in solution. Protection by reaction with pivaloyl chloride occurred in quantitative yield to furnish 39a.

This polymer bound thioglucoside 39a reacted with solution-based glycal acceptors as outlined in Scheme 2.12. Activation was performed by using methyl triflate, in the presence of one equivalent of the nonnucleophilic base di-tert-butylpyridine (DTBP). This base was added to prevent decomposition of the glycal bond resident in the acceptors. The formation of β-glucosyl (1→4) and β-glucosyl (1→3)-linked disaccharides 41a and 43a was almost free of contaminating side products and provided the disaccharides in good yields (Scheme 2.12). Only the formation of the β-glucosyl (1→6)-linked disaccharide 40a was accompanied by formation of detectable side products.23

The solid-phase synthesis of systems with branching from C2 is also accessible through this methodology, as demonstrated by the synthesis of 46b outlined in Scheme 2.13. The C2-pivaloyl protecting group of the β-glycosyl (1→4)-linked disaccharide 41a was removed by treatment with DIBAL. The exposed C2 hydroxyl group could now function as the polymer-bound glycosyl acceptor. Formation of the synthetically challenging β-(1→2) glycosidic linkage was accomplished in 59% yield when glucosyl donor 45 was used (Scheme 2.13).23

The synthesis of a tetrasaccharide containing exclusively β-(1→4) glucosidic linkages (Scheme 2.14) further demonstrated the efficiency of this solid-phase methodology. Transformation of polymer-bound disaccharide glycal 41a into the C2-pivaloyl thioglycosyl donor 47 was followed by coupling to provide the trisaccharide 48a in 45% overall yield based on 37 as determined after cleavage from the solid support to furnish 48b. The procedure was reiterated to convert 48a to a

OSi(iPr)2-

OSi(iPr)2-

OR

O

1) DMDO, CH Cl

PivCl, DMAP

BnO

O

SEt

BnO

2 2

BnO

O SEt

BnO

BnO

2) EtSH, CH2Cl2

BnO

CH2Cl2

OPiv

OH

37

cat.TFAA

38

TBAF

39a: R = Si(iPr)2-

AcOH

38b: R = H

THF

Scheme 2.11 Synthesis of polymer-bound thioethyl glucosyl donor.

26 THE GLYCAL ASSEMBLY METHOD ON SOLID SUPPORTS

OR

OH

BnO

O

O

BnO

1) MeOTf, DTBP

OPiv

39a

O

+

BnO

O

BnO

4 A MS, CH2Cl2

BnO

BnO

22

TBAF

40a: R = Si(iPr)2

AcOH

40b: R = H

THF

OBn

OR

OBn

1) MeOTf, DTBP

O

BnO

O

O

O

39a

+

HO

4 A MS, CH2Cl2

BnO

OPiv BnO

BnO

11

TBAF

41a: R = Si(iPr)2

AcOH

41b: R = H

THF

PMP

OR PMP

O

O

O

O

1) MeOTf, DTBP

O

O

39a

+

O

BnO

O

HO

4 A MS, CH2Cl2

BnO

OPiv

42

TBAF

43a: R = Si(iPr)2

AcOH

43b: R = H

THF

Scheme 2.12 Synthesis of disaccharides using a polymer-bound thioethyl glucosyl donor.

trisaccharyl thioethyl donor and to couple again to glycal acceptor 11. Cleavage from the solid support at the conclusion of the sequence furnished the desired tetrasaccharide 49b in 20% yield over 12 steps from 37 as determined after cleavage from the support by fluoridolysis. This overall result corresponds to an average yield of 84% per step (Scheme 2.14).23

BnO

BnO

O

SEt

OR

OBn

Dibal-H

OSi(iPr)2-

OBn BnO

OPiv

41a

O

45

BnO

O O

O

CH2Cl2

BnO

O

O

BnO

BnO

BnO

BnO

OH

MeOTf, DTBP

BnO

O

BnO

O

4A MS, CH2Cl2

44

BnO

OPiv

TBAF

46a: R = Si(iPr)2-

AcOH

46b: R = H

THF

Scheme 2.13 Synthesis of a branched trisaccharide via thioethyl donors.