30.1.3 Carbohydrate Derivatives (Including Nucleosides)

T. Nokami

General Introduction

O,N-Acetals are ubiquitous structures in carbohydrate derivatives. Nucleosides are one of these structures, having an oxygen atom in a furan ring (O-containing five membered ring) and a nitrogen atom in a nucleoside base. N-Linked glycopeptides are also an important class of compounds, having an oxygen atom in a pyran ring (O-containing six membered ring) and a nitrogen atom in a peptide.^[1] Tremendous efforts have been made to construct these biologically important structures; however, it is still difficult to construct the C—N bonds of nucleosides and N-linked glycosides because of the low reactivity of nucleoside bases and asparagine derivatives. In the case of nucleoside synthesis, a nucleoside base is converted into the corresponding silylated base, e.g. 2, in situ to raise its nucleophilicity toward glycosyl donor 1 and afford product 3 in high yield (▶ Scheme 1).^[2] In contrast, for the preparation of glycopeptides, the number of direct methods to form C—N glycosidic bonds is quite limited. It has been reported that silylated acetamide 5 can be used as a glycosyl acceptor in the glycosylation of glycosyl sulfoxide 4 as a glycosyl donor; however, this is the only example of the combination of glycosyl donor and acceptor (▶ Scheme 2).^[3] Therefore, indirect methods to form an amide linkage between a glycosylamine with a peptide are commonly used to prepare N-linked glycopeptides.

Scheme 1 Nucleoside Synthesis^[2]

Scheme 2 Direct Glycopeptide Synthesis^[3]

30.1.3.1 Glycosyl Asparagine Derivatives

One of the major target molecules containing the anomeric C—N bond of pyranosides are N-linked glycopeptides. Analogous to formation of the anomeric C—O bond of pyranosides via the glycosylation between the anomeric carbon and a hydroxy group, glycosylation with a primary amide as a glycosyl acceptor is a possible retrosynthesis of the anomeric C—N bond (▶ Scheme 3, route a); however, because of the low nucleophilicity of the nitrogen atom in primary amides, there are few examples of glycopeptide synthesis via this bond-forming reaction. Therefore, amidation of aspartic acid derivatives with glycosylamines or their equivalents is one of the most reliable methods to prepare glycosyl asparagine derivatives (▶ Scheme 3, route b). In this section, chemical glycosylations using nitrogen nucleophiles are summarized, and then conventional amidation reactions using glycosylamines and their equivalents are featured.

Scheme 3 Retrosynthetic Approaches to Glycopeptide Synthesis

Among various glycosyl donors for glycosylation, glycosyl trichloroacetimidates and their derivatives are some of the most reactive. The yields of N-glycosylation of glycosyl imidates 6 with acetamide in the presence of a catalytic amount of trimethylsilyl trifluoromethanesulfonate highly depend on the substituents of the imidate (▶ Scheme 4).^[4] Although glycosyl trichloroacetimidate 6 (R¹ = H; X = Cl) affords the corresponding glycosylacetamide 7 in only moderate yield (42%), glycosyl N-phenyltrifluoroacetimidate 6 (R¹ = Ph; X = F) gives product 7 quantitatively. The latter glycosyl imidate is useful for Nglycosylation of nonactivated primary amides 8, including a tripeptide (▶ Scheme 5). Lower solubility of the tripeptide in nitromethane is suggested as a reason for the low efficiency of the N-glycosylation in this case.

Scheme 4 N-Glycosylation of Glycosyl Imidates with Acetamide^[4]

Scheme 5 N-Glycosylation with Nonactivated Primary Amides^[4]

R1	Yield (%)	Ref
	98	[4]
	94	[4]
	39	[4]

A microfluidic/batch system significantly improves yields of N-glycosylation (▶ Scheme 6).^[5] Mixing of a solution containing both glycosyl donor 10 (43 mM) and amide 11 (86 mM) as an acceptor with a solution of trimethylsilyl trifluoromethanesulfonate (43 mM) at room temperature is carried out using a micromixer and the resulting solution is stirred for an additional 12 hours in a flask to produce glycopeptide 12 in 84% yield.

Scheme 6 N-Glycosylation Using a Microfluidic/Batch System^[5]

N²-(Benzyloxycarbonyl)-N⁴-(2-O-benzoyl-3,4,6-tri-O-benzyl-β-D-galactopyranosyl)-L-asparagine Allyl Ester {9, R¹ =(S)-3-(Allyloxy)-2-[(benzyloxycarbonyl)amino]-3-oxopropyl}; Typical Procedure:^[4]

A mixture of 2-O-benzoyl-3,4,6-tri-O-benzyl-β-D-galactopyranosyl N-phenyl-2,2,2-trifluoroacetimidate (55.4 mg, 0.072 mmol), N-(benzyloxycarbonyl)-L-asparagine allyl ester {8; R¹ =(S)-3-(allyloxy)-2-[(benzyloxycarbonyl)amino]-3-oxopropyl; 15.6 mg, 0.051 mmol}, and activated 4-Å molecular sieves in dry MeNO₂ (1.0 mL) was stirred at rt for 1 h under argon to remove a trace of H₂O, and then cooled to 0°C. TMSOTf (1.80 μL, 10.0 μmol) was added. After being stirred at the same temperature for 30 min, the mixture was allowed to warm to rt. After a further 30 min of stirring, the mixture was quenched by the addition of Et₃N (0.5 equiv), filtered through a Celite pad, and concentrated under reduced pressure. The clear oil obtained was purified with an automatic column machine (hexane/EtOAc 1:1) and gel-permeation chromatography to obtain the title compound; yield: 42.1 mg (98%).

30.1.3.1.2 Method 2: Synthesis from Pent-4-enyl Glycosides

Pent-4-enyl glycosides such as 13, which are stable and easy to handle, are used to prepare α-glycosidic linkages with peptides (▶ Scheme 7).^[6,7] The reaction seems to be a conventional direct C—N bond formation via glycosylation; however, the nitrogen atom attached to the anomeric carbon of N-glycosylation product 14 comes from the solvent acetonitrile. In this case, an α-acetonitrilium ion is proposed as an intermediate and the observed α-selectivity stems from the stereochemistry of the intermediate. On the other hand, the neighboring-group participation of the N-phthaloyl group at the 2-position of glycosyl donor 15 plays the crucial role to control the β-selectivity (▶ Scheme 8).^[8]

Scheme 7 α-Selective N-Glycosylation of a Pent-4-enyl Glycoside^[6]

Scheme 8 β-Selective N-Glycosylation of a Pent-4-enyl Glycoside^[8]

30.1.3.1.3 Method 3: Synthesis from Thioglycosides

Thioglycosides also work as coupling partners for amino acids (▶ Scheme 9).^[9] Although the same α-acetonitrilium ion intermediate as in the case of the corresponding pent-4-enyl glycoside 13 (see ▶ Section 30.1.3.1.2) is proposed to form from thioglycoside 16, the yield of the glycopeptide 14 is higher than that observed with the pent-4-enyl glycoside.

Scheme 9 N-Glycosylation of a Thioglycoside^[9]

30.1.3.1.4 Method 4: Synthesis from Glycals

Glycals, which have a C=C bond between the C2 and C3 positions of the tetrahydropyran ring, are useful building blocks for constructing glycopeptides (▶ Scheme 10).^[10] In this case, glycals are used as precursors of glycosylamines. Glycosyl azide 19 is prepared in two steps from glycal 17 via 2-iodo N-phenylsulfonyl glycosylamine 18 and ultimately reduced to the corresponding glycosylamine 20 in the presence of neutral Raney nickel (W-2). Additional manipulations of the hydroxy-protecting groups are necessary to increase stability of the glycosylamine prior to the reduction of the anomeric azido group. This method is applicable for the solid-phase synthesis of glycopeptides (▶ Scheme 11).^[11–13] In the solid-phase synthesis, the use of anthracene-9-sulfonamide in the azasulfonamidation sequence is crucial for the conversion of the polymer-bound glycal 21 into the corresponding β-aminoglycoside 22, which is a precursor for conjugation with peptides.

Scheme 10 Synthesis of a Glycopeptide from a Glycal^[10]

Scheme 11 Solid-Phase Synthesis^[12]

Glycals are useful as precursors of 2-deoxyglycosylamines (▶ Scheme 12).^[14] The three-step synthesis of 2-deoxy-N-glycopeptide 27 starts from the conversion of 3,4,6-tri-O-benzyl-D-glucal (23) into the corresponding 2-deoxy-1-azido sugar 24. Glycosylamine 25, which is generated by the reduction of glycosyl azide 24, can be coupled with 1-succinimidyl-N-(tert-butoxycarbonyl) dipeptide 26 to afford the β-linked glycodipeptide 27 in 50% yield (2 steps).

Scheme 12 Glycopeptide Synthesis via a 2-Deoxyglycosylamine^[14]

1-Azido-2-deoxy-3,4,6-tri-O-benzyl-D-glucose (24); Typical Procedure:^[14]

To a stirred soln of TMSONO₂ (bulk soln made from TMSCl and AgNO₃; 20 mol%) in freshly distilled dry MeCN (2 mL) at 0 °C was added TMSN₃ (37 μL, 0.288 mmol). A soln of 3,4,6-tri-O-benzyl-D-glucal (23; 0.240 mmol) in MeCN (2 mL) was then added dropwise to the mixture. The cooling bath was removed and stirring was continued for 14 h. The mixture was then extracted with Et₂O (2 × 20 mL) followed by standard workup to give a residue, which was purified by column chromatography; yield: 75%; (α/β) 2:1.

Palladium-catalyzed glycosylation using glycals as glycosyl donors is a powerful method to prepare the desired glycosidic linkages stereoselectively (▶ Scheme 13).^[15] Glycal derivative 28 is used as a precursor of a π-allyl palladium complex, which reacts with isatin derivative 29 to form the anomeric C—N bond. The high β-selectivity observed in product 30 stems from retention of the configuration of the starting material 28 via the π-allyl palladium complex as an intermediate.

Scheme 13 Palladium-Catalyzed Glycosylation of a Glycal^[15]

30.1.3.1.5 Method 5: Synthesis from Glycosyl Halides

One of the most reliable processes to prepare glycosylamines involves the corresponding glycosyl halides as intermediates (▶ Scheme 14).^[16–18] The anomeric halides are good leaving groups and glycosyl halides have α-configuration because of the anomeric effect. Therefore, the nucleophilic addition of sodium azide to glycosyl halide 32, which can be prepared from glycosyl acetate 31, gives β-glycosyl azide 33 stereoselectively. The subsequent reduction of the azide group affords the corresponding glycosylamine 34, which is a coupling partner for aspartic acid derivative 35. N-Chitobiosylasparagine peptide 36 can thus be synthesized in four steps from chitobiosyl acetate 31 as a starting material.

Scheme 14 Synthesis of a Glycosylamine from a Glycosyl Chloride^[18]

N⁴-[2-Acetamido-4-O-(2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-β-D-glucopyranosyl)-3,6-di-O-acetyl-2-deoxy-β-D-glucopyranosyl]-N²-(tert-butyloxycarbonyl)-L-asparagine Allyl Ester (36); Typical Procedure:^[18]

To a stirred soln of chitobiosylamine 34 (0.76 g, 1.2 mmol) and aspartic acid derivative 35 (0.33 g, 1.2 mmol) in CH₂Cl₂ (20 mL) at rt was added EEDQ (0.44 g, 1.8 mmol). During stirring at the same temperature for 3 d, the product started to crystallize. After the removal of solvent under reduced pressure, the residue was purified by column chromatography. Further separation of impurity with mixed solvent (EtOAc/petroleum ether 1:1) and subsequent dissolution of the product with EtOAc afforded the title compound; yield: 0.9 g (91%).

30.1.3.1.6 Method 6: Synthesis from Glycosyl Isothiocyanates

Synthesis of glycopeptides via glycosyl isothiocyanates is useful as an alternative to the preparation via glycosyl azides. Glycosyl isothiocyanates of monosaccharides are easily prepared in a single step from the corresponding glycosyl halides as precursors.^[19] This method is successfully applied for the synthesis of β-mannosyl-chitobiosyl-asparagine derivative 40 (▶ Scheme 15).^[20] Dihydrooxazole 37, which is prepared from the corresponding glycosyl acetate, serves as a precursor of glycosyl isothiocyanate 38. Although glycosyl isothiocyanates are reactive enough to couple with carboxylic acid 39 in the absence of a condensation reagent, it is important to perform the reaction under strictly anhydrous conditions. Otherwise, formation of byproducts such as the N,N′-bisglycosylthiourea cannot be avoided completely.

Scheme 15 Synthesis of a Glycopeptide from a Glycosyl Isothiocyanate^[20]

Stay updated, free articles. Join our Telegram channel

Join