Synthesis of Two Tetrasaccharide Pentenyl Glycosides Related to the Pectic Rhamnogalacturonan I Polysaccharide

The synthesis of two protected tetrasaccharide pentenyl glycosides with diarabinan and digalactan branching related to the pectic polysaccharide rhamnogalacturonan I is reported. The strategy relies on the coupling of N-phenyl trifluoroacetimidate disaccharide donors to a common rhamnosyl acceptor. The resulting trisaccharide thioglycosides were finally coupled to an n-pentenyl galactoside acceptor to access the two protected branched tetrasaccharides.


Introduction
Pectins are the most structurally complex polysaccharides found in plant cell walls. These highly heterogeneous polysaccharides are fundamental components of the primary cell wall of plants where they modulate plant cell functions such as support, defense, signaling, and cell adhesion [1,2]. They also play a vital role in the food industry, serving as stabilizing and thickening agents in products such as jams, yogurt, and jellies [3]. Furthermore, it has been documented that they are useful in the production of biodegradable films, materials for biomedical implantation, and in drug delivery [4][5][6]. Rhamnogalacturonan I (RG-I) is one of the structural classes of pectic polysaccharides, along with homogalacturonan, rhamnogalacturonan II, xylogalacturonan, and apiogalacturonan [7]. The chemical structure of RG-I consists of a backbone with alternating units of α-linked L-rhamnose and D-galacturonic acid. The backbone has numerous branches at the C-4 position of the L-rhamnose residues including galactans, arabinans, or arabinogalactans. Galactans are mostly linear chains of β-(1→4)-linked D-galactose residues. Arabinans are chains of α-(1→5)-linked L-arabinofuranosides that are frequently branched at C-3 and sometimes at C-2. The most frequent form of arabinogalactan side chains is arabinogalactan I, which is a β-(1→4)-galactan with arabinan branches [8,9]. To the best of our knowledge, except for the synthesis of tri-and tetrasaccharide building blocks containing a single galactose unit as a side chain by Vogel and co-workers [10,11], branched RG-I fragments have not previously been addressed by chemical synthesis [12]. These structures are of great interest because they are useful for the study of pectin and the enzymes involved in both the biosynthesis and degradation of RG-I [1, 2,7,13,14]. Herein, we report the synthesis of two protected tetrasaccharides with diarabinan and digalactan branching, designed for the assembly of larger RG-I oligosaccharides.
The retrosynthetic analysis of targets 1 and 2 is depicted in Scheme 1. We invisioned that both targets could be obtained in a linear synthetic strategy with the last glycosylation involving the galactosyl acceptor 3. We have previously succesfully used galactosides in the assembly of an unprotected hexasaccharide backbone fragment of RG-I using a post-glycosylation-oxidation strategy, which has also been successfully applied by others [15][16][17]. The two trisaccharides 4 and 5 could be assembled by employing the common acceptor 6 forming the disaccharidic backbone unit for both targets. The putative side chain consisting of digalactan 7 can be assembled by glycosylating galactose acceptor 9 with donor 10. Likewise, diarabinan 8 can be derived from 11. Scheme 1. Retrosynthetic analysis of branched RG-I oligosaccharides 1 and 2.

Results and Discussion
Starting from β-D-galactose pentaacetate, galactosyl acceptor 3 was synthesized according to our previous work [18,19] and the diol 12 was accessed as reported earlier [20]. After masking the free hydroxyl groups of compound 12 as benzoyl esters a regioselective opening [21][22][23] of the benzylidene acetal resulted in acceptor 9 in 70% yield over two steps (Scheme 2). Scheme 1. Retrosynthetic analysis of branched RG-I oligosaccharides 1 and 2.

Results and Discussion
Starting from β-D-galactose pentaacetate, galactosyl acceptor 3 was synthesized according to our previous work [18,19] and the diol 12 was accessed as reported earlier [20]. After masking the free hydroxyl groups of compound 12 as benzoyl esters a regioselective opening [21][22][23] of the benzylidene acetal resulted in acceptor 9 in 70% yield over two steps (Scheme 2). Scheme 2. Synthesis of the n-pentenyl galactoside 9.
The N-phenyl trifluoroacetimidate donor 10 was prepared from commercially available Dgalactose in four steps [24]. Rhamnosyl acceptor 6 was obtained in two steps from L-rhamnose (Scheme 3). Triol 13 [25] was transformed into the diol 14 [26] by a tin-mediated regioselective benzylation of the C-3 hydroxyl group. The higher acidity of the C-2 alcohol was exploited to protect it as a NAP-ether. In the literature [27], there is an example of a selective benzylation of a C-2 hydroxyl group in a similar rhamnose derivative under phase-transfer conditions, however, we observed low yields using the same reaction conditions due to the formation of the regioisomer. The NAP-protected rhamnoside 6 was instead synthesized by reacting the diol 14 with NAPBr in the presence of sodium hydride and TBAI resulting in 6 in 65% yield (Scheme 3). The synthesis of the digalactan N-phenyl trifluoroacetimidate donor 7 started by TMSOTfpromoted glycosylation of acceptor 9 with the perbenzoylated donor 10. The reactants were mixed at −40 °C and then immediately warmed to 0 °C. The reaction mixture was left stirring at this temperature for 3 hours resulting in disaccharide 15 in 76% yield. The hydrolysis of the n-pentenyl group was achieved in two steps: first 15 was titrated with bromine at 0 °C, and then the resulting bromide was treated directly with silver(I) carbonate in a mixture of acetone and water. This approach afforded the corresponding hemiacetal, which was further reacted with N-phenyl trifluoroacetimidoyl chloride in the presence of cesium carbonate to give the digalactan donor 7 in 60% yield over three steps (Scheme 4). Rhamnose acceptor 6 was then glycosylated with donor 7 followed by a Zemplén deacylation yielding the trisaccharide 16 in 77% yield over two steps. Global benzylation gave trisaccharide donor 4 in 79% yield (Scheme 5). The N-phenyl trifluoroacetimidate donor 10 was prepared from commercially available D-galactose in four steps [24]. Rhamnosyl acceptor 6 was obtained in two steps from L-rhamnose (Scheme 3). Triol 13 [25] was transformed into the diol 14 [26] by a tin-mediated regioselective benzylation of the C-3 hydroxyl group. The higher acidity of the C-2 alcohol was exploited to protect it as a NAP-ether. In the literature [27], there is an example of a selective benzylation of a C-2 hydroxyl group in a similar rhamnose derivative under phase-transfer conditions, however, we observed low yields using the same reaction conditions due to the formation of the regioisomer. The NAP-protected rhamnoside 6 was instead synthesized by reacting the diol 14 with NAPBr in the presence of sodium hydride and TBAI resulting in 6 in 65% yield (Scheme 3). The N-phenyl trifluoroacetimidate donor 10 was prepared from commercially available Dgalactose in four steps [24]. Rhamnosyl acceptor 6 was obtained in two steps from L-rhamnose (Scheme 3). Triol 13 [25] was transformed into the diol 14 [26] by a tin-mediated regioselective benzylation of the C-3 hydroxyl group. The higher acidity of the C-2 alcohol was exploited to protect it as a NAP-ether. In the literature [27], there is an example of a selective benzylation of a C-2 hydroxyl group in a similar rhamnose derivative under phase-transfer conditions, however, we observed low yields using the same reaction conditions due to the formation of the regioisomer. The NAP-protected rhamnoside 6 was instead synthesized by reacting the diol 14 with NAPBr in the presence of sodium hydride and TBAI resulting in 6 in 65% yield (Scheme 3). The synthesis of the digalactan N-phenyl trifluoroacetimidate donor 7 started by TMSOTfpromoted glycosylation of acceptor 9 with the perbenzoylated donor 10. The reactants were mixed at −40 °C and then immediately warmed to 0 °C. The reaction mixture was left stirring at this temperature for 3 hours resulting in disaccharide 15 in 76% yield. The hydrolysis of the n-pentenyl group was achieved in two steps: first 15 was titrated with bromine at 0 °C, and then the resulting bromide was treated directly with silver(I) carbonate in a mixture of acetone and water. This approach afforded the corresponding hemiacetal, which was further reacted with N-phenyl trifluoroacetimidoyl chloride in the presence of cesium carbonate to give the digalactan donor 7 in 60% yield over three steps (Scheme 4). Rhamnose acceptor 6 was then glycosylated with donor 7 followed by a Zemplén deacylation yielding the trisaccharide 16 in 77% yield over two steps. Global benzylation gave trisaccharide donor 4 in 79% yield (Scheme 5). The synthesis of the digalactan N-phenyl trifluoroacetimidate donor 7 started by TMSOTf-promoted glycosylation of acceptor 9 with the perbenzoylated donor 10. The reactants were mixed at −40 • C and then immediately warmed to 0 • C. The reaction mixture was left stirring at this temperature for 3 h resulting in disaccharide 15 in 76% yield. The hydrolysis of the n-pentenyl group was achieved in two steps: first 15 was titrated with bromine at 0 • C, and then the resulting bromide was treated directly with silver(I) carbonate in a mixture of acetone and water. This approach afforded the corresponding hemiacetal, which was further reacted with N-phenyl trifluoroacetimidoyl chloride in the presence of cesium carbonate to give the digalactan donor 7 in 60% yield over three steps (Scheme 4). The N-phenyl trifluoroacetimidate donor 10 was prepared from commercially available Dgalactose in four steps [24]. Rhamnosyl acceptor 6 was obtained in two steps from L-rhamnose (Scheme 3). Triol 13 [25] was transformed into the diol 14 [26] by a tin-mediated regioselective benzylation of the C-3 hydroxyl group. The higher acidity of the C-2 alcohol was exploited to protect it as a NAP-ether. In the literature [27], there is an example of a selective benzylation of a C-2 hydroxyl group in a similar rhamnose derivative under phase-transfer conditions, however, we observed low yields using the same reaction conditions due to the formation of the regioisomer. The NAP-protected rhamnoside 6 was instead synthesized by reacting the diol 14 with NAPBr in the presence of sodium hydride and TBAI resulting in 6 in 65% yield (Scheme 3). The synthesis of the digalactan N-phenyl trifluoroacetimidate donor 7 started by TMSOTfpromoted glycosylation of acceptor 9 with the perbenzoylated donor 10. The reactants were mixed at −40 °C and then immediately warmed to 0 °C. The reaction mixture was left stirring at this temperature for 3 hours resulting in disaccharide 15 in 76% yield. The hydrolysis of the n-pentenyl group was achieved in two steps: first 15 was titrated with bromine at 0 °C, and then the resulting bromide was treated directly with silver(I) carbonate in a mixture of acetone and water. This approach afforded the corresponding hemiacetal, which was further reacted with N-phenyl trifluoroacetimidoyl chloride in the presence of cesium carbonate to give the digalactan donor 7 in 60% yield over three steps (Scheme 4). Rhamnose acceptor 6 was then glycosylated with donor 7 followed by a Zemplén deacylation yielding the trisaccharide 16 in 77% yield over two steps. Global benzylation gave trisaccharide donor 4 in 79% yield (Scheme 5). Rhamnose acceptor 6 was then glycosylated with donor 7 followed by a Zemplén deacylation yielding the trisaccharide 16 in 77% yield over two steps. Global benzylation gave trisaccharide donor 4 in 79% yield (Scheme 5). Having prepared the trisaccharide thiophenyl donor 4, we investigated the coupling with galactose acceptor 3. We examined different promoters: NIS/TESOTf, NIS/Yb(OTf)3, MeOTf, Ph2SO/Tf2O, DMTST, Me2S2/Tf2O, but in all cases, the yield of the desired product was unsatisfactory. Furthermore, screening of these reaction conditions with monomeric building blocks mainly generated a C-glycoside through an intramolecular cyclization of the NAP moiety, as also observed by Crich et al. [28]. Clearly, the NAP-group had to be replaced to avoid the formation of the cyclized by-product, and the chloroacetyl (ClAc) ester was chosen as it can be removed selectively in the presence of the C-6 acetyl group.
Treatment of 4 with DDQ in the presence of water in a mixture of CH2Cl2 and methanol afforded the corresponding alcohol. The free hydroxyl group was then protected as a chloroacetyl ester by treating it with chloroacetic anhydride under basic conditions (59% over two steps). A mixture of donor 17 and galactose acceptor 3 in CH2Cl2 was treated with Ph2SO/Tf2O and giving the target tetrasaccharide 1 in 42% yield (Scheme 6). Scheme 6. Synthesis of tetrasaccharide 1.
Next, we turned our attention to the synthesis of tetrasaccharide 2. Diarabinan N-phenyl trifluoroacetimidate donor 8 was prepared in six steps starting from the arabinose donor 11 [29] similar to previous work by Varrot and coworkers [30] (Scheme 7).

Scheme 5. Synthesis of digalactan-containing trisaccharide donor 4.
Having prepared the trisaccharide thiophenyl donor 4, we investigated the coupling with galactose acceptor 3. We examined different promoters: NIS/TESOTf, NIS/Yb(OTf) 3 , MeOTf, Ph 2 SO/Tf 2 O, DMTST, Me 2 S 2 /Tf 2 O, but in all cases, the yield of the desired product was unsatisfactory. Furthermore, screening of these reaction conditions with monomeric building blocks mainly generated a C-glycoside through an intramolecular cyclization of the NAP moiety, as also observed by Crich et al. [28]. Clearly, the NAP-group had to be replaced to avoid the formation of the cyclized by-product, and the chloroacetyl (ClAc) ester was chosen as it can be removed selectively in the presence of the C-6 acetyl group.
Treatment of 4 with DDQ in the presence of water in a mixture of CH 2 Cl 2 and methanol afforded the corresponding alcohol. The free hydroxyl group was then protected as a chloroacetyl ester by treating it with chloroacetic anhydride under basic conditions (59% over two steps). A mixture of donor 17 and galactose acceptor 3 in CH 2 Cl 2 was treated with Ph 2 SO/Tf 2 O and giving the target tetrasaccharide 1 in 42% yield (Scheme 6). Having prepared the trisaccharide thiophenyl donor 4, we investigated the coupling with galactose acceptor 3. We examined different promoters: NIS/TESOTf, NIS/Yb(OTf)3, MeOTf, Ph2SO/Tf2O, DMTST, Me2S2/Tf2O, but in all cases, the yield of the desired product was unsatisfactory. Furthermore, screening of these reaction conditions with monomeric building blocks mainly generated a C-glycoside through an intramolecular cyclization of the NAP moiety, as also observed by Crich et al. [28]. Clearly, the NAP-group had to be replaced to avoid the formation of the cyclized by-product, and the chloroacetyl (ClAc) ester was chosen as it can be removed selectively in the presence of the C-6 acetyl group.
Treatment of 4 with DDQ in the presence of water in a mixture of CH2Cl2 and methanol afforded the corresponding alcohol. The free hydroxyl group was then protected as a chloroacetyl ester by treating it with chloroacetic anhydride under basic conditions (59% over two steps). A mixture of donor 17 and galactose acceptor 3 in CH2Cl2 was treated with Ph2SO/Tf2O and giving the target tetrasaccharide 1 in 42% yield (Scheme 6). Scheme 6. Synthesis of tetrasaccharide 1.
Next, we turned our attention to the synthesis of tetrasaccharide 2. Diarabinan N-phenyl trifluoroacetimidate donor 8 was prepared in six steps starting from the arabinose donor 11 [29] similar to previous work by Varrot and coworkers [30] (Scheme 7). Scheme 6. Synthesis of tetrasaccharide 1.
Next, we turned our attention to the synthesis of tetrasaccharide 2. Diarabinan N-phenyl trifluoroacetimidate donor 8 was prepared in six steps starting from the arabinose donor 11 [29] similar to previous work by Varrot and coworkers [30] (Scheme 7). We also explored an alternative route to the disaccharide 18 (Scheme 7). Triol 19 [30] was transformed into the fully protected arabinose derivative through a two-step, one-pot protocol. Subsequently, the TBDPS-group was removed by treatment with TBAF (62% over two steps). The resulting alcohol 20 was coupled with donor 11 to give 18 in 92% yield. The benzyl group was used as temporary protection of the anomeric position in 18. Catalytic hydrogenolysis provided the corresponding hemiacetal, which was transformed into the target disaccharide donor 8 in 80% over two steps. The synthesis of the diarabinan-containing trisaccharide 5 is shown in Scheme 8. TMSOTfmediated coupling of 8 with rhamnose acceptor 6 afforded trisaccharide 21 in 84% yield. The presence of the participating benzoyl group at the C-2 position of donor 8 ensured the formation of the α-glycosidic linkage. The benzoyl esters of 21 were exchanged with benzyl protective groups in two steps (68% yield) giving trisaccharide donor 5. For the synthesis of the diarabinan-containing trisaccharide donor 5, an alternative approach to the one described above also gave excellent results, as shown in Scheme 9. The rhamnose acceptor 6 was glycosylated with arabinose donor 11 followed by deacylation under Zemplén conditions providing triol 22 (70% yield over two steps). Selective glycosylation of the primary alcohol using donor 11 afforded the partially protected trisaccharide 23 in 68% yield. Protecting group manipulations gave 5 in 75% yield over two steps. This approach allowed us to reach trisaccharide 5 in five steps from 6 and 11. We also explored an alternative route to the disaccharide 18 (Scheme 7). Triol 19 [30] was transformed into the fully protected arabinose derivative through a two-step, one-pot protocol. Subsequently, the TBDPS-group was removed by treatment with TBAF (62% over two steps). The resulting alcohol 20 was coupled with donor 11 to give 18 in 92% yield. The benzyl group was used as temporary protection of the anomeric position in 18. Catalytic hydrogenolysis provided the corresponding hemiacetal, which was transformed into the target disaccharide donor 8 in 80% over two steps. The synthesis of the diarabinan-containing trisaccharide 5 is shown in Scheme 8. TMSOTf-mediated coupling of 8 with rhamnose acceptor 6 afforded trisaccharide 21 in 84% yield. The presence of the participating benzoyl group at the C-2 position of donor 8 ensured the formation of the α-glycosidic linkage. The benzoyl esters of 21 were exchanged with benzyl protective groups in two steps (68% yield) giving trisaccharide donor 5. We also explored an alternative route to the disaccharide 18 (Scheme 7). Triol 19 [30] was transformed into the fully protected arabinose derivative through a two-step, one-pot protocol. Subsequently, the TBDPS-group was removed by treatment with TBAF (62% over two steps). The resulting alcohol 20 was coupled with donor 11 to give 18 in 92% yield. The benzyl group was used as temporary protection of the anomeric position in 18. Catalytic hydrogenolysis provided the corresponding hemiacetal, which was transformed into the target disaccharide donor 8 in 80% over two steps. The synthesis of the diarabinan-containing trisaccharide 5 is shown in Scheme 8. TMSOTfmediated coupling of 8 with rhamnose acceptor 6 afforded trisaccharide 21 in 84% yield. The presence of the participating benzoyl group at the C-2 position of donor 8 ensured the formation of the α-glycosidic linkage. The benzoyl esters of 21 were exchanged with benzyl protective groups in two steps (68% yield) giving trisaccharide donor 5. For the synthesis of the diarabinan-containing trisaccharide donor 5, an alternative approach to the one described above also gave excellent results, as shown in Scheme 9. The rhamnose acceptor 6 was glycosylated with arabinose donor 11 followed by deacylation under Zemplén conditions providing triol 22 (70% yield over two steps). Selective glycosylation of the primary alcohol using donor 11 afforded the partially protected trisaccharide 23 in 68% yield. Protecting group manipulations gave 5 in 75% yield over two steps. This approach allowed us to reach trisaccharide 5 in five steps from 6 and 11. For the synthesis of the diarabinan-containing trisaccharide donor 5, an alternative approach to the one described above also gave excellent results, as shown in Scheme 9. The rhamnose acceptor 6 was glycosylated with arabinose donor 11 followed by deacylation under Zemplén conditions providing triol 22 (70% yield over two steps). Selective glycosylation of the primary alcohol using donor 11 afforded the partially protected trisaccharide 23 in 68% yield. Protecting group manipulations gave 5 in 75% yield over two steps. This approach allowed us to reach trisaccharide 5 in five steps from 6 and 11. Scheme 9. Synthesis of trisaccharide 5 by an alternative approach.
As discussed previously, the NAP-group was exchanged with a chloroacetyl ester to give 24 in 69% yield over two steps. Final glycosylation of galactosyl acceptor 3 with trisaccharide donor 24 was performed, using Ph2SO/Tf2O as a promoter at −60 °C, resulting in the desired tetrasaccharide 2 in 42% yield (Scheme 10).

General Information
Starting materials, reagents, and solvents were purchased from commercial suppliers and have been used without further purification. All solvents were HPLC-grade. Anhydrous solvents were obtained from Innovative Technology PS-MD-7 Pure-solv solvent purification system (Innovative Technology, Pewaukee, WI, USA). The reactions were carried out in flame-dried glassware under an inert atmosphere unless stated otherwise. Thin-layer chromatography (TLC) was performed on Merck (Merck, Darmstadt, Germany) Aluminium Sheets pre-coated with silica, C-60 F254 plates. Compounds were visualized by charring after dipping in CAM stain (Ce(SO4)2 (1.6 g) and (NH4)6Mo7O24 (4 g) in 10% sulphuric acid (200 mL). Eluent systems are specified for each Rf value, and ratios are given as volume ratios. Evaporation of solvents was performed with a VWR International Laborota 400 under reduced pressure (in vacuo) at temperatures ranging between 35-55 °C. Trace solvent was removed under reduced pressure using a vacuum pump (Edwards, Burgess Hill, UK). Flash chromatography was performed using Matrex 60 Å silica gel (35-70 μm, Merck, Darmstadt, Germany) as the stationary phase by the general procedure developed by Still et al. [31]. The eluent system is specified by the protocol for each synthesis. Eluent ratios are given as volume Scheme 9. Synthesis of trisaccharide 5 by an alternative approach.
As discussed previously, the NAP-group was exchanged with a chloroacetyl ester to give 24 in 69% yield over two steps. Final glycosylation of galactosyl acceptor 3 with trisaccharide donor 24 was performed, using Ph 2 SO/Tf 2 O as a promoter at −60 • C, resulting in the desired tetrasaccharide 2 in 42% yield (Scheme 10). As discussed previously, the NAP-group was exchanged with a chloroacetyl ester to give 24 in 69% yield over two steps. Final glycosylation of galactosyl acceptor 3 with trisaccharide donor 24 was performed, using Ph2SO/Tf2O as a promoter at −60 °C, resulting in the desired tetrasaccharide 2 in 42% yield (Scheme 10).

General Information
Starting materials, reagents, and solvents were purchased from commercial suppliers and have been used without further purification. All solvents were HPLC-grade. Anhydrous solvents were obtained from Innovative Technology PS-MD-7 Pure-solv solvent purification system (Innovative Technology, Pewaukee, WI, USA). The reactions were carried out in flame-dried glassware under an inert atmosphere unless stated otherwise. Thin-layer chromatography (TLC) was performed on Merck (Merck, Darmstadt, Germany) Aluminium Sheets pre-coated with silica, C-60 F254 plates. Compounds were visualized by charring after dipping in CAM stain (Ce(SO4)2 (1.6 g) and (NH4)6Mo7O24 (4 g) in 10% sulphuric acid (200 mL). Eluent systems are specified for each Rf value, and ratios are given as volume ratios. Evaporation of solvents was performed with a VWR International Laborota 400 under reduced pressure (in vacuo) at temperatures ranging between 35-55 °C. Trace solvent was removed under reduced pressure using a vacuum pump (Edwards, Burgess Hill, UK). Flash chromatography was performed using Matrex 60 Å silica gel (35-70 μm, Merck, Darmstadt, Germany) as the stationary phase by the general procedure developed by Still et al. [31]. The eluent system is specified by the protocol for each synthesis. Eluent ratios are given as volume

General Information
Starting materials, reagents, and solvents were purchased from commercial suppliers and have been used without further purification. All solvents were HPLC-grade. Anhydrous solvents were obtained from Innovative Technology PS-MD-7 Pure-solv solvent purification system (Innovative Technology, Pewaukee, WI, USA). The reactions were carried out in flame-dried glassware under an inert atmosphere unless stated otherwise. Thin-layer chromatography (TLC) was performed on Merck (Merck, Darmstadt, Germany) Aluminium Sheets pre-coated with silica, C-60 F254 plates. Compounds were visualized by charring after dipping in CAM stain (Ce(SO 4 ) 2 (1.6 g) and (NH 4 ) 6 Mo 7 O 24 (4 g) in 10% sulphuric acid (200 mL). Eluent systems are specified for each R f value, and ratios are given as volume ratios. Evaporation of solvents was performed with a VWR International Laborota 400 under reduced pressure (in vacuo) at temperatures ranging between 35-55 • C. Trace solvent was removed under reduced pressure using a vacuum pump (Edwards, Burgess Hill, UK). Flash chromatography was performed using Matrex 60 Å silica gel (35-70 µm, Merck, Darmstadt, Germany) as the stationary phase by the general procedure developed by Still et al. [31]. The eluent system is specified by the protocol for each synthesis. Eluent ratios are given as volume ratios. NMR-spectra were recorded on a Varian Mercury 300 B (Varian, Palo Alto, CA, USA), Bruker Ascend 400, Bruker Avance 800 MHz, Bruker DQX 400 or Bruker AC 500 spectrometer (Bruker, Billerica, MA, USA). Chemical shifts (δ) are reported in ppm and coupling constants in Hz. Solvents used for NMR experiments were CDCl 3 , CD 3 OD, D 2 O or d 6 -DMSO and their resonances were used as internal standards. Most of the compounds have been characterized by NMR.

General Procedure I for Glycosylation with N-Phenyl Trifluoroacetimidate Glycosyl Donors
A mixture of the donor (1.2 equiv.) and the acceptor (1.0 equiv.) was co-evaporated with toluene and subjected to high vacuum for 2 h. The mixture was dissolved in anhydrous CH 2 Cl 2 (c. 0.1-0.01 M) and cooled to −40 • C. TMSOTf (0.1 equiv.) was added, and the reaction mixture was stirred at −40 • C until TLC showed completion of the reaction (10-30 min). The reaction mixture was quenched by addition of Et 3 N, evaporated and purified by flash column chromatography.

General Procedure II for Preparation of the N-Phenyl Trifluoroacetimidate Glycosyl Donors
Hemiacetal (1.0 equiv.) was dissolved in CH 2 Cl 2 (c. 0.1-0.01 M). PhNC(Cl)CF 3 (2.0 equiv.) and Cs 2 CO 3 (2.0 equiv.) were added and the reaction mixture was stirred at 20 • C until TLC showed completion of the reaction (2-5 h). The reaction mixture was filtered through a plug of Celite, concentrated, and the residue was purified by flash column chromatography.

General Procedure III for Removal of Acyl Protective Groups (Zemplén Conditions)
The starting material was dissolved in MeOH (c. 0.1-0.01 M) or, in a 1:1 mixture of MeOH and THF. A freshly prepared 1 M NaOMe solution (0.1 equiv.) in MeOH was added. The reaction mixture was stirred at 20 • C until TLC showed the full conversion (1-24 h). The reaction mixture was then quenched by addition of Amberlite IR-120 (H + ). The resin was filtered off, and the filtrate was concentrated, and purified by flash column chromatography.

General Procedure IV for Benzylation of Hydroxyl Groups
To a solution of the starting material (1.0 equiv.) in DMF, BnBr (1.2 equiv. for 1-OH) and TBAI (0.01 equiv.) were added, and the mixture was cooled in an ice bath. NaH (1.2 equiv. for 1-OH) was added and the mixture was stirred at 20 • C for 15 h and then quenched by addition of MeOH. The reaction mixture was partially concentrated, diluted with EtOAc and washed with water and brine. The organic phase was dried over Na 2 SO 4 , concentrated and purified by flash column chromatography.

General Procedure V for Removal of the 2-Naphthylmethyl (NAP) Group
The protected saccharide (1.0 equiv.) was dissolved in a 4:1 mixture of CH 2 Cl 2 and MeOH. Water (2%) was added followed by addition of DDQ (1.4 equiv.). The reaction mixture was stirred at 20 • C until TLC showed completion of the reaction (2-5 h). The reaction mixture was diluted with CH 2 Cl 2 and washed with sat. NaHCO 3 . The combined aqueous phases were extracted with CH 2 Cl 2 . The combined organic phases were dried over Na 2 SO 4 , filtered, concentrated and purified by flash chromatography.

General Procedure VI for Introducing a Chloroacetyl (ClAc) Group
Alcohol (1.0 equiv.) was dissolved in anhydrous CH 2 Cl 2 and cooled in an ice bath. Et 3 N (2.0 equiv.) was added followed by addition of ClAc 2 O (1.1 equiv.). The reaction mixture was stirred at 0 • C for 2 h, then warmed up to 20 • C, diluted with CH 2 Cl 2 and washed with 0.1 M HCl. The organic phase was dried over Na 2 SO 4 , concentrated and purified by flash column chromatography.

General Procedure VII the Ph 2 SO/Tf 2 O-Promoted Glycosylation
A mixture of the donor (1.2 equiv.), Ph 2 SO (1.2 equiv.) and TTBP (1.2 equiv.) was co-evaporated with toluene and subjected to high vacuum for 2 h. The mixture was dissolved in anhydrous CH 2 Cl 2 and cooled to −60 • C. Tf 2 O (1.3 equiv.) was added, and the reaction mixture was stirred at −60 • C for 5 min. Afterwards, a solution of the acceptor (1.0 equiv.) in anhydrous CH 2 Cl 2 was added. The mixture was warmed to −40 • C over 2 h and Et 3 N was added. The mixture was diluted with CH 2 Cl 2 and washed with brine, dried over Na 2 SO 4 , concentrated and purified by flash chromatography.

Phenyl 3-O-benzyl-2-O-(2-naphthylmethyl)-1-thio-α-L-rhamnopyranoside 6
To a solution of diol 14 (700 mg, 2.0 mmol) in DMF (6 mL) NAPBr (490 mg, 2.2 mmol) and TBAI (75 mg, 0.2 mmol) were added and the mixture was cooled in an ice bath. NaH (90 mg, 2.2 mmol, 60% in oil) was added and the mixture was stirred at 20 • C for 12 h and then quenched by addition of MeOH. The reaction mixture was partially concentrated, diluted with EtOAc and washed with water and brine. The combined water phase was extracted with EtOAc. The combined organic phase was dried over Na 2 SO 4 , concentrated and purified by flash chromatography (19:1 toluene/EtOAc) to afford 6 as a colorless oil. Yield 640 mg (65%). R