Synthesis of 2-Amino-2-deoxy-β-d-mannosides via Stereoselective Anomeric O-Alkylation of 2N,3O-Oxazolidinone-Protected d-Mannosamine: Synthesis of the Trisaccharide Repeating Unit of Streptococcus pneumoniae 19F Polysaccharide

Abstract

Cesium carbonate-mediated stereoselective anomeric O-alkylation of a 2N,3O-oxazolidinone-protected d-mannosamine with sugar-derived primary or secondary alkyl triflates afforded the corresponding 2-amino-2-deoxy-β-d-mannosides in moderate to good yields and excellent stereoselectivity. The oxazolidinone ring can be opened with aqueous alkali hydroxide to liberate the amine functionality. This method has been successfully applied to the synthesis of the trisaccharide repeating unit of Streptococcus pneumoniae 19F Polysaccharide.

Graphical Abstract

Bacterial surface glycans often consist of structurally unusual and complex monosaccharides as well as challenging glycosidic linkages. They are well-known as the virulence factors and the targets for development of effective therapeutics. For instance, bacterial capsular polysaccharides (CPS)-based glycoconjugate vaccines have been extensively studied and demonstrated great success in preventing bacterial infections.¹ Due to their highly heterogeneous nature, it is exceedingly difficult to isolate large quantities of pure and structurally well-defined microbial glycan antigens from natural sources. Alternatively, these carbohydrate molecules may be obtained in bulk with good purity by chemical synthesis. Despite remarkable achievements in the development of glycosylation methods, the complexity of bacterial glycans demands the development of new efficient synthetic methods and strategies.

2-Acetamido-2-deoxy-β-mannosides, i.e. β-ManNAc, are a type of microbial glycosides found in the CPS and lipopolysaccharides (LPS) of invasive bacteria strains, e.g. a trisaccharide repeating unit 1 from Streptococcus pneumoniae CPS² and a hexasaccharide repeating unit 2 from the vegetative cell wall polysaccharide of Bacillus anthracis (Figure 1).³ In addition, a trisaccharide repeating unit →6)-α-d-GlcNAc-(1→4)-β-d-ManNAc-(1→4)-β-d-GlcNAc-(1→ was discovered in Bacillus anthracis secondary cell wall polysaccharide (SCWP).⁴ Furthermore, 2-acetamido-2-deoxy-β-d-mannuronic acids (β-ManNAcA) exist in methicillin-resistant Staphylococcus aureus (MRSA)⁵ and Micrococcus luteus.⁶ It is well-known that stereoselective synthesis of β-mannosides,⁷ one type of 1,2-cis-glycosides,⁸ is exceptionally challenging in carbohydrate chemistry, due to the steric effect as well as the absence of anomeric effect. Stereoselective synthesis of 2-acetamido-2-deoxy-β-mannosides poses additional challenges over β-mannosides, mainly because the C2-AcNH group may rearrange into a relatively unreactive oxazoline intermediate during the glycosylation reaction.⁹ Nevertheless, a substantial number of studies on the synthesis of β-ManNAcwere reported including: 1) silver silicate-promoted direct glycosidation of 3,4,6-tri-O-acetyl-2-azido-2-deoxy-α-d-mannopyranosyl bromide;¹⁰ 2) S_N2 inversion of the C2 equatorial triflate of β-glucopyranosides using an azide anion;¹¹ 3) stereoselective reduction of β-glucopyranosides-derived C2 oxime¹² or O-acyloxime;¹³ 4) application of 4,6-O-benzylidene protecting group,¹⁴ a well-known strategy for β-mannoside synthesis,¹⁵ or (S)-4,6-O-pyruvyl ketal,¹⁶ for the synthesis 2-azido-2-deoxy-β-mannosides; 5) use of N-benzyl-2N,3O-oxazolidinone-protected mannosamine thioglycoside donors.¹⁷ In addition, stereoselective synthesis of 2-azido-2-deoxy-β-d-mannuronic acids (ManN₃A) was achieved by using mannosazide uronate donors.¹⁸

Figure 1.

Representative microbial glycans containing 2-acetamino-2-deoxy-β-d-mannoside.

Recently, our laboratory disclosed an approach for stereoselective synthesis of β-mannopyranosides 5 via Cs₂CO₃mediated anomeric O-alkylation of partially protected mannose 3 with sugar-derived electrophiles (a, Scheme 1).¹⁹ In addition, similar strategy was successfully applied to the construction of β-(1→6)-linked 2-azido-2-deoxy-mannosides 8 involving 2-azido-2-deoxy-mannose 6 and primary triflates (b, Scheme 1).²⁰ However, less reactive sugar-derived secondary electrophiles were found not suitable for anomeric O-alkylation of 2-azido-2-deoxy-mannose 6. For instance, only trace amount of 2-azido-2-deoxy-d-mannoside 14 was obtained employing 2-azido-2-deoxy-d-mannose 6 and d-galactose-derived C4-triflate 13 under optimal condition (entry 1, Table 1). In this communication, we wish to report stereoselective synthesis of 2-amino-2-deoxy-β-mannosides 10 via anomeric O-alkylation of a 2N,3O-oxazolidinone-protected d-mannosamine lactol 9^17,21,22 with secondary triflates and its application to the synthesis of the trisaccharide repeating unit of Streptococcus pneumoniae 19F polysaccharide (cf. 1, Figure 1).

Scheme 1.

Stereoselective construction of β-mannoside-type linkages via anomeric O-alkylation.

Table 1.

Anomeric O-alkylation of various protected 2-amino-2-deoxy-D-mannoses with D-galactose-derived C4-secondary triflate 13.^a

^aUnless otherwise noted, all reactions involved lactol (1.0 eq.), triflate 13 (2.5 eq.), and Cs₂CO₃ (3.0 eq.) in 1,2-dichloroethane (DCE) at 40 °C for 24 hours.

^bIsolated yields.

^cRb₂CO₃ (3.0 eq.) were used instead of Cs₂CO₃.

^dTrifluorotoluene was used as solvent.

^eTHF was used as solvent.

^fDCE/CH₃CN(4/1, v/v) was used as solvent.

^gTriflate 13 (3.0 eq.) and Cs₂CO₃ (3.5 eq.) were used.

Initially, we thought replacing the strong electron-withdrawing azido group in lactol 6 to the naturally occurring acetamido group, e.g. 11, may improve its reactivity in anomeric O-alkylation. To our disappointment, when 11 was subjected to the standard cesium carbonate-mediated anomeric O-alkylation with secondary triflates, e.g. 13, only trace amount of 2-acetamido-2-deoxy-β-mannoside 15 was obtained (entry 2, Table 1). Further changing the C2-acetamido (AcNH) to Alloc-NH carbamate group (cf. 12) did not improve the outcome (entry 3). Gratifyingly, it was found that anomeric O-alkylation of 2N,3O-oxazolidinone-protected D-mannosamine lactol 9 with secondary triflate 13 (2.5 eq.) using cesium carbonate (3.0 eq.) in 1,2-dichloroethane (DCE) at 40 °C for 24 hours afforded desired 2-amino-2-deoxy-β-mannoside 17 in 54% yield (β only, entry 4). Use of Rb₂CO₃ instead of Cs₂CO₃ or changing solvents produced 2-amino-2-deoxy-β-mannoside 17 in inferior yields (entries 5-8). Finally, the yield of 2-amino-2-deoxy-β-mannoside 17 was improved to 66% when triflate 13 (3.0 eq.) and Cs₂CO₃ (3.5 eq.) were used (β only, entry 9).

Under this optimal condition, anomeric O-alkylation of lactol 9 with D-galactosamine-derived secondary triflate 18 and D-allose-derived C3-triflate 19²³ afforded corresponding 2-amino-2-deoxy-β-mannoside 23 and 24 in 56% and 54% yield (β only), respectively (Table 2). When more reactive deoxy sugar-derived secondary triflates 20 and 21 were used for O-alkylation of lactol 9, higher yields of 2-amino-2-deoxy-β-mannoside 25 and 26 were obtained with less amounts of the electrophiles. Not surprisingly, this 2N,3O-oxazolidinone-protected D-mannosamine lactol 9 also reacts efficiently with primary triflate 22 (1.5 eq.) in the presence of Cs₂CO₃ (2.0 eq.) to afford 2-amino-2-deoxy-β-mannoside 27 in good yield and excellent stereoselectivity.

Table 2.

Synthesis of 2-amino-2-deoxy-β-D-mannosides via stereoselective anomeric O-alkylation of 2N,3O-oxazolidinone-protected D-mannosamine lactol.^a,b

^aUnless otherwise noted, all reactions involved lactol (1.0 eq.), triflate (3.0 eq.), and Cs₂CO₃ (3.5 eq.) in 1,2-dichloroethane (DCE) at 40 °C for 24 hours.

^bIsolated yields.

^cTriflate 20 (2.5 eq.) and Cs₂CO₃ (3.0 eq.) were used.

^dTriflate 21 (1.5 eq.) and Cs₂CO₃ (2.0 eq.) were used.

^eTriflate 22 (1.5 eq.) and Cs₂CO₃ (2.0 eq.) were used.

Next, we applied this method to the synthesis of the trisaccharide repeating unit (32) of Streptococcus pneumoniae 19F polysaccharide.^11a,13,24 As depicted in Scheme 2, under the optimal condition, anomeric O-alkylation of 2N,3O-oxazolidinone-protected D-mannosamine lactol 9 with D-galactose-derived secondary triflate 28 afforded desired 2-amino-2-deoxy-β-mannoside 29 (63% yield, β only). This reaction was also performed at 1 mmol scale and 2-amino-2-deoxy-β-mannoside 29 was obtained in 60% yield (β only), indicating the robustness of this method. This disaccharide thioglycoside donor 29 was then subjected to a traditional glycosidation with an L-rhamnoside acceptor 30 to furnish the trisaccharide 31 in 69% yield (α only). Finally, aqueous sodium hydroxide-mediated ring opening of the oxazolidinone functionality followed by Birch reduction and a chemoselective acetylation of the free amine provided the final target 32. The spectroscopic data match exactly with those reported for the same compound.²⁵

Scheme 2.

Synthesis of the trisaccharide repeating unit (32) of Streptococcus pneumoniae 19F polysaccharide.

In conclusion, we have successfully developed an approach for stereoselective construction of 2-amino-2-deoxy-β-D-mannosides via cesium carbonate-mediated anomeric O-alkylation of a 2N,3O-oxazolidinone-protected D-mannosamine lactol involving primary and secondary electrophiles. The 2N,3O-oxazolidinone ring can be cleaved with aqueous alkali hydroxide to free the amine group. Synthesis of the trisaccharide repeating unit of Streptococcus pneumoniae 19F polysaccharide has been accomplished employing this methodology.

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.