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Nature Protocols 3, 97 - 113 (2008)
Published online: 10 January 2008 | doi:10.1038/nprot.2007.493

Subject Category: Synthetic chemistry

Regioselective one-pot protection of glucose

Cheng-Chung Wang1,2, Suvarn S Kulkarni3, Jinq-Chyi Lee1, Shun-Yuan Luo1 & Shang-Cheng Hung1,2,3

Detailed protocols for the regioselective protection of individual hydroxyls in monosaccharide units are described here. This expedient methodology incorporates up to seven reaction sequences, obviating the necessity to carry out intermittent tedious work-ups and time-consuming purifications. Using this TMSOTf-catalyzed one-pot protocol, the 2,3,4,6-tetra-O-trimethylsilylated hexopyranosides bearing an anomeric group could be transformed into a whole set of differentially protected 2-alcohols, 3-alcohols, 4-alcohols, 6-alcohols and fully protected monosaccharides in high yields. These tailor-made glycosyl donors and acceptors can then be used for stereoselective one-pot glycosylation for oligosaccharide synthesis. The total time for the preparation of a purified protected sugar unit ranges between 1 and 2 d. This process would otherwise take 1–2 weeks.

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Introduction

Carbohydrates play significant roles in a diverse set of biological processes1. Structurally, they are much more complex and diverse than proteins and nucleic acids. As most of the structural information of carbohydrate–protein, carbohydrate–nucleotide and carbohydrate–carbohydrate complex at molecular level remains obscure, homogeneous materials with well-defined configurations are essential for the determination of biological function and structure-activity relationship. These oligosaccharides, present in micro-heterogeneous forms, cannot be procured easily from natural sources in acceptable purity and amounts. Chemical methods to synthesize these function-oriented domains have, therefore, acquired immense importance. However, the very complexity that allows oligosaccharides and glycoconjugates to participate in specific biological functions renders their synthesis difficult. Here, we describe a straightforward protocol for one-pot synthesis of diversely protected monosaccharide building blocks that could be used for oligosaccharide assembly2. Conceptually, this is the first approach to discriminate sugar polyols in a single flask, using a single catalyst. Via the trimethylsilyl trifluoromethanesulfonate (TMSOTf)-catalyzed reaction, the desired building block with differential protective group scheme can be accessed easily after a work-up and single purification. This new protocol may offer a simple, practical and convenient tool to solve this long-standing problem in carbohydrate synthesis3.

Since its advent, carbohydrate synthesis has always been concerned with two major challenges, regioselective protection of individual hydroxyls and stereoselective glycosylation4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17. The former, which exists as a prerequisite of the latter, has been given relatively less attention and thus remained an arduous affair requiring multistep protocols and intermittent tedious purifications. This is due to the fact that all the secondary hydroxyls display comparable reactivity under various conditions leading to a mixture of regioisomers. Moreover, in the assembly of sugar-biopolymers, along with the stereoselectivity, it is also essential to control the regioselectivity of glycosylation so that only a specific hydroxyl group is coupled with the donor sugar. Consequently, two distinct types of building blocks are necessary: a semi-protected monosaccharide with a strategically positioned free hydroxyl group (a nucleophilic acceptor) and one bearing a labile leaving group at the anomeric carbon that acts as a glycosyl donor in the ensuing glycosylation reaction. Along with this, the installation of suitable protecting groups on the remaining hydroxyls, for tuning the overall electronic properties of donors and acceptors so as to 'match' the donor–acceptor pair and also for further deprotection and glycosylation or functional group modifications, is required4.

A traditional chemical approach for carbohydrate synthesis involves a prior protection of a hexose at the anomeric carbon (C1) to furnish the corresponding hexopyranoside tetraol via the formation of cyclic acetal. Transformation of the tetraol into either the fully protected monosaccharide or the individual alcohols with a free hydroxyl at C2, C3, C4 or C6 frequently encounters several difficulties such as (1) an independent and multi-step protection–deprotection sequence is needed to prepare each compound (4–6 steps), (2) a tedious work-up is often used in each synthetic step, (3) a time-consuming purification is required to separate different regioisomers and (4) low yield of the expected product is sometimes obtained due to the poor regioselectivity5.

We envisioned that the 2-, 3-, 4- and 6-hydroxyl groups on a monosaccharide bearing an anomeric group (XR, X = O or S) could be distinctively protected in a combinatorial, regioselective, orthogonal and sequential one-pot procedure, in such a way that the hydroxyl groups bear chemically differentiable protecting groups, to get either the fully protected monosaccharides or the individual alcohols. Thus, panoply of glycosyl donors and acceptors bearing various ether- and ester-type groups can be readily made available for one-pot glycosylation. Such a method would be of vital importance to expedite the overall synthetic process and reduce the labor involved in saccharide preparation.

For the successful development of a combinatorial, regioselective, orthogonal and one-pot protection scheme, the choice of the protecting groups was crucial. We opted for benzyl-type ethers, which have a larger family than esters or silyl ethers and can be selectively cleaved under appropriate conditions18, 19, 20, 21, 22. Our strategy was to first protect 4,6-diol groups with an arylidene acetal, carry out regioselective protection at O3 and subsequently at O2 to get fully protected building block. Arylidene acetals being amenable to selective cleavages would lead to O4 or O6 benzyl ether-type protected derivatives23. Benzyl groups are both acid and base resistant and usually serve as permanent protecting groups that can be reduced in the final step to yield free hydroxyl groups. Substituted benzyl ethers act as orthogonal protecting groups and their cleavages are advantageously carried out by using unique reagent combination. For example, para-methoxybenzyl (PMB) group can be cleaved by 2,3-dichlro-5,6-dicyano-1,4-benzoquinone (DDQ), ceric ammonium nitrite or trifluoroacetic acid, whereas 2-naphthylmethyl group (2-NAP), which is more stable than PMB in acidic condition, is susceptible to only DDQ, thus it can be differentiated by ceric ammonium nitrite or trifluoroacetic acid20. Likewise, halogen-substituted benzyl ethers21 can be converted to acid-labile amino-benzyl ethers by a Pd-catalyzed reaction in the following order of reactivity I > Br > Cl > F. Strategically, the protecting group at the O2 position of a monosaccharide unit to be used as a glycosyl donor plays a pivotal role in the stereocontrol of glycosylation. Generally, benzyl-type ethers at the C2 position of the donor favor the formation of axially oriented glycosidic bonds by the virtue of the anomeric effect, whereas installation of 1,2-trans-linkage is usually achieved by the neighboring group participation of an ester-type protecting group at C2. Thus, along with benzyl-type groups, the incorporation of acyl groups at O2, in the one-pot protection endeavor, was pertinent.

To test this strategy, D-glucose was selected as a model substrate. First, methyl alpha-D-glucopyranoside and p-methylphenyl (toluenyl, Tol) beta-D-thioglucopyranoside, available from commercial sources, were trimethylsilylated using chlorotrimethylsilane and triethylamine (Et3N) to afford the corresponding ethers 1a and 1b in near-quantitative yields, respectively. The per-O-silylation of sugars not only provided the requisite O-trimethylsilylated functionalities for regioselective protection, but also dramatically improved the solubility as compared to the unprotected monosaccharides in common organic solvents like dichloromethane (DCM). The reaction conditions and the results of their differential one-pot protection, to generate a vast array of building blocks, are summarized in Figure 1. Our general protocols involved (1) selective protection of oxygen atoms at O4 and O6 as an arylidene acetal followed by highly regioselective reductive arylmethylation at O3 to furnish the 2-alcohols, (2) O4,O6-arylidenation, O3-arylmethylation and subsequent etherification or acylation at O2 to get the fully protected monosaccharides, (3) O4,O6-arylidenation, O3-PMB or O3-2-NAP protection, O2-etherification or O2-acylation and removal of PMB or 2-NAP group to yield the 3-alcohols, (4) O4,O6-arylidenation, O3-arylmethylation, O2-acylation and regioselective ring opening of arylidene acetals at O4 and O6 to provide the 4-alcohols and 6-alcohols, respectively. The novelty and success of this approach lies in tuning of the reaction conditions in such a way so as to generate a single regioisomer at each stage that allows sequential addition of reagents in the same pot.


Accordingly, as shown in Figure 2, compound 1a or 1b was first treated with 1.05 equivalent of aryl aldehyde in the presence of TMSOTf as a catalyst in DCM at ice-bath temperature. After ring formation of arylidene acetal at O4 and O6 was completed (monitored by thin-layer chromatography (TLC)), a different aryl aldehyde or another equivalent of the same aldehyde was added to the mixture at - 86 °C, followed by treatment with triethylsilane (Et3SiH) (refs. 24,25). The reaction was highly regioselective, giving a single O3-ether26. The reaction mixture was then treated with a 1 M solution of tetra-n-butylammonium fluoride (TBAF) in tetrahydrofuran to remove the TMS group at O2, and the corresponding 2-alcohols 2a1-2a18 and 2b1-2b4, as listed in Table 1, were obtained in high overall yields after recrystallization or column chromatography. Various combinations of arylaldehydes, including benzaldehyde (PhCHO), anisaldehyde (4-OMePhCHO), 3,4-dimethoxybenzaldehyde (3,4-diOMe-PhCHO), 2-naphthaldehyde, 4-nitrobenzaldehyde (4-NO2PhCHO), 4-chlorobenzaldehyde (4-ClPhCHO) and 4-bromobenzaldehyde (4-BrPhCHO), worked well under the conditions affording the corresponding 2-alcohols.



In another set of experiments (Fig. 3), the first three steps were repeated as such, and the appropriate electrophile was added under basic conditions to get the fully protected glycosides 3a1-3a63 and 3b1-3b8 in good yields and in a one-pot manner (Table 2), including the introduction of various benzyl-, allyl- and acyl-type groups at the O2 position. For O2 acylation, acetic anhydride (Ac2O), chloroacetic anhydride [(ClAc)2O] and benzoyl chloride (BzCl) were used as electrophiles together with Et3N as a base. For O2-benzylation or allylation, the corresponding halides such as benzyl bromide (BnBr), 4-methoxybenzyl chloride (PMBCl), 2-naphthylmethyl bromide (2-C11H9Br), 4-chlorobenzyl chloride (4-ClBnCl), 4-bromobenzyl bromide (4-BrBnBr) and allyl bromide (AllylBr) were used as electrophiles, 60% (wt/wt) sodium hydride (NaH) was introduced as base and dimethylformamide was added to form the co-solvent with the preexisting CH2Cl2 to create a suitable condition for the Williamson's etherification.



For the preparation of 3-alcohols (Fig. 4), PMB was used as a temporary protecting group at O3. The same operation, as described for the fully protected glycosides, was repeated, DDQ was finally added to the reaction mixture to selectively remove the PMB group and the 3-OH compounds 4a1-4a5 and 4b1-4b2 were obtained (Table 3). It was possible to carry out O2-acylation of the 2-OTMS intermediate, generated after first two steps, under the prevailing acidic conditions of TMSOTf as well, using acyl anhydride (acyl = benzoyl or acetyl) as a reagent to afford the fully protected monosaccharides (Fig. 5, Table 4). Likewise, the 3-OH derivatives 4a1, 4a6-4a9 and 4b3 were isolated in good yields via subsequent oxidative cleavage of the 2-NAP group at O3 with DDQ in one flask (Fig. 6, Table 5). The 4-alcohols (Fig. 7, Table 6, 5a1-5a4 and 5b1-5b4) and 6-alcohols (Fig. 8, Table 7, 6a1-6a8 and 6b1-6b4) were, in turn, accessed via regioselective O4- and O6-ring opening of arylidene acetals, formed by tandem transformations, O4,O6-arylidenation, O3-arylmethylation and O2-acylation, on 1a or 1b, in the same pot using HCl(g)/NaBH3CN and BH3/THF as reductants, respectively. Thus, D-glucopyranosides could be efficiently converted into various glycosyl donors and acceptors bearing chemically differentiable protective groups. The methodology worked equally well on large scale (up to 40 g).











In this way, hundreds of building blocks starting from D-glucose have been efficiently prepared, and it is expected that the protocol is equally accepted to other sugars as well. For the success of the one-pot reaction, the experimenter should bear in mind that it is very important to maintain anhydrous reaction conditions throughout. A slight amount of moisture could deprotect silyl groups and lead to a mixture of products. Care should be taken while checking the TLC of the reaction by not allowing much exposure to air. It is also necessary to follow the temperature conditions to get the desired regioselectivity, especially in the reductive etherification step. The temperature of this step should not be raised to >- 78 °C. One should also pay attention to the prior activation of molecular sieves.

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Materials

Reagents

Equipment

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Procedure

Overview

  1. Synthesis of 2-alcohol (2a and 2b)Place a 25-ml round-bottomed flask under vacuum and flame-dry it using a propane torch, then allow the flask to cool to room temperature (RT; 23–27 °C) while still under vacuum.
  2. Fill the flask with nitrogen gas, remove from vacuum manifold and cap with a rubber septum.
  3. Remove the rubber septum, weigh 207 mumol of starting material compound 1a or 1b and add 218 mumol of ArCHO (1.05 equiv.) (weigh and pour solid ArCHO, such as 2-naphthaldehyde, 4-chlorobenzaldehyde, 4-bromobenzaldehyde, 4-nitrobenzaldehyde, directly or inject liquid ArCHO, such as benzaldehyde, using a micro-volume syringe) into the flask and recap the flask with the same rubber septum immediately.
  4. Weigh 100 mg of powdered molecular sieves 3 Å into another 25-ml round-bottomed flask containing a Teflon-coated magnetic stir bar.
  5. Put this second flask under vacuum and flame-dry it using a propane torch, then allow the flask to cool to RT (23–27 °C) while still under vacuum.
    Caution Gas may liberate vigorously during the process.
  6. Fill the flask with nitrogen gas, remove from vacuum manifold and cap with a rubber septum.
  7. Remove the rubber septum of both flasks, pour the freshly dried molecular sieves 3 Å and Teflon-coated magnetic stir bar into the flask containing the starting material and recap with the septum immediately.
  8. Attach the flask to a 1-atm nitrogen atmosphere using a nitrogen balloon connected to a needle through the rubber septum.
  9. Transfer 1.5-ml freshly dried DCM into the flask with a dry glass syringe fitted with a 20-gauge hypodermic needle.
  10. Turn the magnetic stirrer on.
  11. Pre-dry the reaction mixture for 1 h at - 86 °C in an acetone bath contained in the Dewar flask cooled by an ultra-low temperature chiller.
  12. Inject 31.7 mumol of TMSOTf (0.15 equiv.) into the flask through the septum using a micro-volume syringe at - 86 °C, while maintaining the nitrogen balloon attached to the flask. Maintain the nitrogen balloon through Steps 12–16.
    Critical step Make sure that TMSOTf is added into the mixture of the reaction without adhesion on the wall of the flask.
  13. Keep stirring at - 86 °C for 2 h (Fig. 9).
  14. After starting material compound 1a or 1b is consumed (as monitored by TLC, developing with hexane/ethyl acetate (9/1) and visualizing by UV absorbance at 254 nm or by spraying with a solution of Ce(NH4)2(NO3)6, (NH4)6Mo7O2 as well as H2SO4 in water and subsequent heating on a hot plate), add 228 mumol of Et3SiH (1.1 equiv.), 248 mumol of RCHO (1.2 equiv.) (for solid ArCHO, such as 2-naphthaldehyde, 4-chlorobenzaldehyde, 4-bromobenzaldehyde and 4-nitrobenzaldehyde, remove the septum and directly pour into the mixture and recap immediately, and for the liquid ArCHO, such as benzaldehyde, 4-methoxybenzaldehyde, 3,4-dimethoxybenzaldehyde and 2-butenal, inject through the septum using a micro-volume syringe) and 15.9 mumol of TMSOTf (0.075 equiv.) sequentially into the flask using micro-volume syringes at - 86 °C. Samples for TLC analysis can be obtained without interrupting the reaction, by inserting a glass capillary through a disposable syringe needle passed through the septum and dipping the capillary into the solution.
    Critical step Pour the solid ArCHO as rapidly as possible, and make sure that TMSOTf and Et3SiH are added into the mixture of the reaction without adhesion on the wall of the flask.Troubleshooting
  15. Keep stirring at - 86 °C for another 5 h.
  16. Add 1 M TBAF solution in 0.41 mmol of THF (2.0 equiv.), remove the flask from the - 86 °C acetone bath, allow the reaction mixture to warm up to RT and stir at RT for 2 h.Pause Point The reaction mixture can be left at RT overnight.
  17. Remove the nitrogen balloon and the septum. Dilute the reaction mixture with 3 ml of ethyl acetate, remove the molecular sieves 3 Å through a filter funnel packed with celite into a 50-ml round-bottomed flask, and further wash the filter with 5 ml of ethyl acetate.
  18. Transfer the filtrate into a 50-ml separatory funnel and wash it with saturated aqueous sodium hydrogen carbonate (2 times 5 ml).
  19. Extract the aqueous layer with ethyl acetate (3 times 10 ml).
  20. Combine the organic layer, wash it with brine 5 ml, and dry it by adding approximately 1 g of anhydrous magnesium sulfate, shake mildly for 30 s, filter the mixture under gravity through a fluted filter paper on a funnel to remove the magnesium sulfate, and collect the filtrate in a 100-ml round-bottomed flask.
  21. Evaporate the solvent using a rotary evaporator at RT under aspirator vacuum (10–20 mbar).
  22. Dissolve the crude product with 3 ml of DCM and transfer the solution to a 25-ml glass sample vial.
  23. Evaporate the DCM using a rotary evaporator at RT under aspirator vacuum.Pause Point The crude product can be stored at 4 °C for days.
  24. Pack a chromatography column (1.5 cm internal diameter (i.d.) times 40 cm length) with silica gel using a mixture of hexane and ethyl acetate 1:1 (vol/vol) for 2a and 3:1 (vol/vol) for 2b.
  25. Dissolve the crude mixture obtained from Step 23 with approximately 0.5 ml of DCM and load it on top of the silica bed.
  26. Elute the product with a mixture of hexane and ethyl acetate 1:1 (vol/vol) for 2a and 2:1 (vol/vol) for 2b, and collect fractions of 5–10 ml.
  27. Identify the fractions containing 2-alcohol compound 5a or 5b by silica gel TLC, developed with a mixture of hexane and ethyl acetate 1:1 (vol/vol) for 2a (Rf approx 0.3) and 3:1 (vol/vol) for 2b (Rf approx 0.3). Products are visualized by UV absorbance at 254 nm or by spraying with a solution of Ce(NH4)2(NO3)6, (NH4)6Mo7O2 as well as H2SO4 in water and subsequent heating on a hot plate.
  28. Collect the fractions and evaporate the solvent using a rotary evaporator.
  29. Dry the residue under reduced pressure to give 2-alcohol compound 2a or 2b as white solids in the yields as listed in Table 1. The products can be further recrystallized from hot ethanol.
  30. Characterize the product 2a or 2b from Step 29 by NMR spectroscopy using deuterated chloroform as solvent.
  31. Synthesis of fully protected monosaccharides (3a and 3b)Perform Steps 1–16.
  32. For 2-O-esterification, inject 3.11 mmol of Et3N (15 equiv.) using micro-volume syringes and add 1.04 mmol of acid anhydride (5.0 equiv.) (for liquid acid anhydride, such as acetic anhydride and chloroacetic anhydride, inject through the septum using a micro-volume syringe, or for solid acid anhydride, such as benzoic anhydride, remove the septum and directly pour into the mixture and recap immediately) to the reaction mixture at 0 °C in an ice bath contained in the Dewar flask. For 2-O-etherification, transfer 3 ml of anhydrous dimethylformamide into the flask with a dry glass syringe fitted with a 20-gauge hypodermic needle, add 0.62 mmol of alkylhalide (3.0 equiv.) (for liquid alkylhalide, such as benzyl bromide, 3-bromo-1-propene and 4-methoxybenzyl chloride, inject through the septum using a micro-volume syringe, or for solid alkylhalide, such as 2-(bromomethyl)naphthalene, 4-chlorobenzyl chloride and 4-bromobenzyl bromide) and pour 0.62 mmol of 60% NaH powder (3.0 equiv.) into the flask by removing and recap the septum immediately at 0 °C in an ice bath contained in the Dewar flask.
  33. Remove the ice bath and stir the reaction mixture at RT for another 12 h.Pause Point The reaction mixture can be left at RT overnight.
  34. After compound 2a or 2b is consumed (as monitored by TLC), remove the nitrogen balloon and the septum. Remove the molecular sieves 3 Å through a filter funnel packed with celite into a 50-ml round-bottomed flask, and further wash the filter with 5 ml of DCM.
  35. Transfer the filtrate into a 50-ml separatory funnel and wash it with water (20 ml).
  36. Perform Steps 19–23.
  37. Pack a chromatography column (1.5 cm i.d. times 40 cm length) with silica gel using a mixture of hexane and ethyl acetate 4:1 (vol/vol) for 3a and 9:1 (vol/vol) for 3b.
  38. Dissolve the crude mixture of Step 36 with approximately 0.5 ml of DCM and load it on top of the silica bed.
  39. Elute the product with a 1:1 (vol/vol) mixture of hexane and ethyl acetate, and collect fractions of 5–10 ml.
  40. Identify the fractions containing fully protected compound 3a or 3b by silica gel TLC, developed with a mixture of hexane and ethyl acetate 4:1 (vol/vol) for 3a and 9:1 (vol/vol) for 3b. Products are visualized by UV absorbance at 254 nm or by spraying with a solution of Ce(NH4)2(NO3)6, (NH4)6Mo7O2 as well as H2SO4 in water and subsequent heating on a hot plate.
  41. Collect the fractions and evaporate the solvent using a rotary evaporator.
  42. Dry the residue under reduced pressure to give the fully protected compound 3a or 3b as white solids in the yields as listed in Table 2. The products can be further recrystallized from hot ethanol.
  43. Characterize the product 3a or 3b from Step 42 by NMR spectroscopy using deuterated chloroform as solvent.
  44. Synthesis of 3-alcohol (4a and 4b)Perform Steps 1–13.
  45. After starting material compound 1a or 1b is consumed, add 228 mumol of Et3SiH (1.1 equiv.), 248 mumol of 4-methoxybenzaldehyde (1.2 equiv.) and 15.9 mumol of TMSOTf (0.075 equiv.) sequentially into the flask using micro-volume syringes at - 86 °C.
    Critical step Make sure that TMSOTf and Et3SiH are added into the mixture of the reaction without adhesion on the wall of the flask.
  46. Keep stirring at - 86 °C for another 5 h.
  47. Perform Steps 32 and 33.
  48. After compound 2a or 2b is consumed (as monitored by TLC), remove the nitrogen balloon and the septum. Add 15 ml of H2O to the reaction mixture; stir the mixture vigorously for 5 min.
  49. Remove the aqueous layer roughly with a Pasteur pipette.
  50. Pour 621 mumol of DDQ (3.0 equiv.) into the reaction mixture and stir for another 3 h.
  51. After the cleavage of 4-methoxybenzyl group (as monitored by TLC, developed with a mixture of hexane and ethyl acetate 4:1 (vol/vol) for 4a and 9:1 (vol/vol) for 4b), remove the molecular sieves 3 Å through a filter funnel packed with celite into a 50-ml round-bottomed flask, and further wash the filter with 5 ml of DCM.
  52. Perform Steps 18–23.
  53. Pack a chromatography column (1.5 cm i.d. times 40 cm length) with silica gel using a mixture of hexane and ethyl acetate 2:1 (vol/vol) for 4a and 3:1 (vol/vol) for 4b.
  54. Dissolve the crude mixture obtained from Step 52 with approximately 0.5 ml of DCM and load it on top of the silica bed.
  55. Elute the product with a mixture of hexane and ethyl acetate 2:1 (vol/vol) for 4a and 3:1 (vol/vol) for 4b, and collect fractions of 5–10 ml.
  56. Identify the fractions containing 3-alcohol compound 4a or 4b by silica gel TLC, developed with a mixture of hexane and ethyl acetate 2:1 (vol/vol) for 4a (Rf approx 0.3) and 3:1 (vol/vol) for 4b (Rf approx 0.3). Products are visualized by UV absorbance at 254 nm or by spraying with a solution of Ce(NH4)2(NO3)6, (NH4)6Mo7O2 as well as H2SO4 in water and subsequent heating on a hot plate.
  57. Collect the fractions and evaporate the solvent using a rotary evaporator.
  58. Dry the residue under reduced pressure to give 3-alcohol compound 4a or 4b as white solids in the yields as listed in Table 3. The products can be further recrystallized from hot ethanol.
  59. Characterize the product 4a or 4b from Step 58 by NMR spectroscopy using deuterated chloroform as solvent.
  60. Synthesis of fully protected monosaccharides (3a and 3b)Perform Steps 1–15.
  61. Move the flask from the - 86 °C acetone bath to a 0 °C ice bath contained in the Dewar flask.
  62. Add 248 mumol of acetic anhydride (1.2 equiv.) into the flask using a micro-volume syringe or pour benzoic anhydride 248 mumol (1.2 equiv.) into the flask by removing and recap the septum immediately at 0 °C. Maintain the nitrogen balloon through Steps 60–63.
    Critical step The addition of benzoic anhydride should be carried out quickly.
  63. Add 82.8 mumol of TMSOTf (0.4 equiv.) sequentially into the flask using micro-volume syringes at 0 °C.
  64. Move the flask into a 4 °C refrigerator, and stir the reaction mixture at 4 °C for another 12 h.Pause Point The reaction mixture can be left at 4 °C overnight.
  65. After compound 2a or 2b is consumed (as monitored by TLC), perform Steps 17–23.Troubleshooting
  66. Perform Steps 37–40.
  67. Collect the fractions and evaporate the solvent using a rotary evaporator.
  68. Dry the residue under reduced pressure to give the fully protected compound 3a or 3b as white solids in the yields as listed in Table 4. The products can be further recrystallized from hot ethanol.
  69. Characterize the product 3a or 3b from Step 68 by NMR spectroscopy using deuterated chloroform as solvent.
  70. Synthesis of 3-alcohol (4a and 4b)Perform Steps 1–13.
  71. After starting material compound 1a or 1b is consumed, add 228 mumol of Et3SiH (1.1 equiv.), 248 mumol of 2-naphthaldehyde (1.2 equiv.) and 15.9 mumol of TMSOTf (0.075 equiv.) sequentially into the flask using micro-volume syringes at - 86 °C.
    Critical step Make sure that TMSOTf and Et3SiH are added into the mixture of the reaction without adhesion on the wall of the flask.
  72. Keep stirring at - 86 °C for another 5 h.
  73. Perform Steps 61–64.
  74. Remove ice bath, nitrogen balloon and septum; pour 621 mumol of DDQ (3.0 equiv.) to the reaction, and stir the reaction for another 5 h.
  75. After the cleavage of 2-NAP (as monitored by TLC, developed with a mixture of hexane and ethyl acetate 4:1 (vol/vol) for 4a and 9:1 (vol/vol) for 4b), remove the molecular sieves 3 Å through a filter funnel packed with celite into a 50-ml round-bottomed flask, and further wash the filter with 5 ml of DCM.
  76. Perform Steps 18–23.
  77. Perform Steps 53–57.
  78. Dry the residue under reduced pressure to give 3-alcohol compound 4a or 4b as white solids in the yields as listed in Table 5. The products can be further recrystallized from hot ethanol.
  79. Characterize the product 4a or 4b from Step 78 by NMR spectroscopy using deuterated chloroform as solvent.
  80. Synthesis of 4-alcohol (5a and 5b)Perform Steps 1–15.
  81. Perform Steps 61–64.
  82. After compound 2a or 2b is consumed (as monitored by TLC), move the flask from 4 °C refrigerator into an ice bath.
  83. Pour 3.11 mmol of NaCNBH3 (15 equiv.) into the flask by removing and recapping the septum quickly at 0 °C.
    Critical step This step should be carried out quickly.
  84. Stir the mixture vigorously at 0 °C for 30 min.
  85. Add 4 M HCl to 1,4-dioxane in the flask with a dry glass syringe fitted with a 20-gauge hypodermic needle at 0 °C in a dropwise manner until the pH value reaches 1approx2. While adding the HCl solution, monitor the pH with pH test paper every five drops. Samples for pH test paper analysis can be obtained by inserting a glass capillary through a disposable syringe needle passed through the septum and dipping the capillary into the solution without interrupting the reaction.
    Critical step The success of this reaction depends on the pH and the dryness of the reaction. Be sure the pH reaches 1approx2, and avoid the interference of moisture.Troubleshooting
  86. Remove the ice bath and allow the reaction mixture to warm up to RT.
  87. Stir the mixture at RT for 1 h.
  88. Perform Steps 17–23.
  89. Pack a chromatography column (1.5 cm i.d. times 40 cm length) with silica gel using a mixture of hexane and ethyl acetate 2:1 (vol/vol) for 5a and 4:1 (vol/vol) for 5b.
  90. Dissolve the crude mixture obtained from Step 88 with approximately 0.5 ml of DCM and load it on top of the silica bed.
  91. Elute the product with a mixture of hexane and ethyl acetate 2:1 (vol/vol) for 5a and 4:1 (vol/vol) for 5b, and collect fractions of 5–10 ml.
  92. Identify the fractions containing 2-alcohol compound 5a or 5b by silica gel TLC, developed with a mixture of hexane and ethyl acetate 2:1 (vol/vol) for 5a (Rf approx 0.3) and 4:1 (vol/vol) for 5b (Rf approx 0.3). Products are visualized by UV absorbance at 254 nm or by spraying with a solution of Ce(NH4)2(NO3)6, (NH4)6Mo7O2 as well as H2SO4 in water and subsequent heating on a hot plate.
  93. Collect the fractions and evaporate the solvent using a rotary evaporator.
  94. Dry the residue under reduced pressure to give 4-alcohol compound 5a or 5b as colorless syrups in the yields as listed in Table 6.
  95. Characterize the product 5a or 5b from Step 93 by NMR spectroscopy using deuterated chloroform as solvent.
  96. Synthesis of 6-alcohol (6a and 6b)Perform Steps 1–15.
  97. Perform Steps 61–64.
  98. Remove 1 M solution of borane in THF from 4 °C refrigerator and allow it to warm to RT.
  99. After compound 2a or 2b is consumed (as monitored by TLC), move the flask from 4 °C refrigerator to an ice bath.
  100. Add 1 M solution of borane in 1.04 mmol of THF (5.0 equiv.) into the flask with a dry glass syringe fitted with a 20-gauge hypodermic needle at 0 °C in a dropwise manner.
    Caution The rapid addition of borane in THF may cause vigorous liberation of gases.
  101. Add 0.1 mmol of TMSOTf (0.5 equiv.) sequentially into the flask using micro-volume syringes at 0 °C.
  102. Stir the mixture at 0 °C for 6 h.
  103. Remove the nitrogen balloon and the septum. Quench the reaction by adding 10 ml of MeOH into the flask with a dry glass syringe fitted with a 20-gauge hypodermic needle at 0 °C in a dropwise manner.
    Caution The rapid addition of MeOH may cause vigorous liberation of gases.
  104. Evaporate the solvent carefully using a rotary evaporator at RT under aspirator vacuum.
    Caution Gas may liberate vigorously during the processes.
  105. Coevaporate the mixture with MeOH (3 times 5 ml) carefully using a rotary evaporator at RT under aspirator vacuum.
  106. Dissolve the mixture with 15 ml of ethyl acetate, and transfer the solution into a 50-ml separatory funnel and wash it with water (10 ml).
  107. Perform Steps 19–23.
  108. Pack a chromatography column (1.5 cm i.d. times 40 cm length) with silica gel using a mixture of hexane and ethyl acetate 2:1 (vol/vol) for 6a and 4:1 (vol/vol) for 6b.
  109. Dissolve the crude mixture obtained from Step 106 with approximately 0.5 ml of DCM and load it on top of the silica bed.
  110. Elute the product with a mixture of hexane and ethyl acetate 2:1 (vol/vol) for 6a and 4:1 (vol/vol) for 6b, and collect fractions of 5–10 ml.
  111. Identify the fractions containing 2-alcohol compound 6a or 6b by silica gel TLC, developed with a mixture of hexane and ethyl acetate 2:1 (vol/vol) for 6a (Rf approx 0.3) and 4:1 (vol/vol) for 6b (Rf approx 0.3). Products are visualized by UV absorbance at 254 nm or by spraying with a solution of Ce(NH4)2(NO3)6, (NH4)6Mo7O2 as well as H2SO4 in water and subsequent heating on a hot plate.
  112. Collect the fractions and evaporate the solvent using a rotary evaporator.
  113. Dry the residue under reduced pressure to give 6-alcohol compound 6a or 6b in the yields as listed in Table 7.
  114. Characterize the product 6a or 6b from Step 113 by NMR spectroscopy using deuterated chloroform as solvent.
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Timing

Synthesis of 2-alcohol (2a and 2b)
Steps 1–3, 30 min; Steps 4–8, 30 min; Steps 9 and 10, 5 min; Steps 12 and 13, 2 h; Step 14, 15 min; Step 15, 5 h; Step 16, 2 h; Step 17, 5 min; Steps 18–20, 30 min; Steps 21–23, 20 min; Step 24, 1 h; Steps 25 and 26, 30 min; Steps 27 and 28, 1h; Step 29, 12 h; Step 30, 10 min. Total for Steps 1–16, 10 h (should be performed in a single day);Steps 17–30, 16 h (should be performed in a single day).
Synthesis of fully protected monosaccharides (3a and 3b)
Step 31, 9 h; Steps 32 and 33, 12.5 h; Steps 34 and 35, 15 min; Step 36, 40 min; Step 37, 1 h; Steps 38 and 39, 30 min; Steps 40 and 41, 1 h; Step 42, 12 h; Step 43, 10 min. Total for Steps 31–33, 21.5 h (should be performed in a single day); Steps 34–43, 15.5 h (should be performed in a single day).
Synthesis of 3-alcohol (4a and 4b)
Step 44, 4 h; Steps 45 and 46, 5 h; Step 47, 12.5 h; Steps 48 and 49, 10 min; Step 50, 3 h; Steps 51 and 52, 1 h; Step 53, 1 h; Steps 54 and 55, 30 min; Steps 56 and 57, 1 h; Step 58, 12 h; Step 59, 10 min. Total for Steps 44–47, 21.5 h (should be performed in a single day); Steps 48–59, 19 h (should be performed in a single day).
Synthesis of fully protected monosaccharides (3a and 3b)
Step 60, 9 h; Step 61, 1min; Steps 62 and 63, 10 min; Step 64, 12 h; Step 65, 40 min; Steps 66 and 67, 2.5 h; Step 68, 12 h; Step 69, 10 min. Total for Steps 60–64, 21 h (should be performed in a single day); Steps 65–69, 15 h (should be performed in a single day).
Synthesis of 3-alcohol (4a and 4b)
Steps 70–72, 9 h; Step 73, 12 h; Step 74, 5 h; Step 75, 20 min; Step 76, 1 h; Step 77, 2.5 h; Step 78, 12 h; Step 79, 10 min. Total for Steps 70–73, 21 h (should be performed in a single day); Steps 74–79, 21 h (should be performed in a single day).
Synthesis of 4-alcohol (5a and 5b)
Step 80, 9 h; Step 81, 12 h; Steps 82 and 83, 30 min; Step 84, 30 min; Step 85, 20 min; Steps 86 and 87, 1 h; Step 88, 1 h; Step 89, 1 h; Steps 90 and 91, 30 min; Steps 92 and 93, 1 h; Step 94, 12 h; Step 95, 10 min. Total for Steps 80 and 81, 21 h (should be performed in a single day); Steps 82–95, 18 h (should be performed in a single day).
Synthesis of 6-alcohol (6a and 6b)
Step 96, 9 h; Step 97, 12 h; Steps 98–101, 20 min; Step 102, 6 h; Step 103, 5 min; Step 104, 20 min; Step 105, 10 min; Steps 106 and 107, 30 min; Step 108, 1 h; Steps 109 and 110, 30 min; Steps 111 and 112, 1 h; Step 113, 12 h; Step 114, 10 min. Total for Steps 96 and 97, 21 h (should be performed in a single day); Steps 98–114, 22 h (should be performed in a single day).

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Troubleshooting

Troubleshooting advice can be found in Table 8.


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Anticipated results

The combination of 'regioselective one-pot protection' and 'stereoselective one-pot glycosylation' may offer an efficient and convenient protocol to prepare the functional oligosaccharides of active domains and open a new door to lead carbohydrate chemistry and glycobiology research into a new era.

Analytic data

Compound 2a4. mp 198–199 °C

[alpha] D +105.0 (c 0.7 in CHCl3 at 24 °C)

1H NMR (400 MHz, CDCl3): delta 7.81–7.73 (m, 4H, Ar-H), 7.52–7.50 (m, 3H, Ar-H), 7.48–7.43 (m, 2H, Ar-H), 7.40–7.39 (m, 3H, Ar-H), 5.59 (s, 1H, PhCH), 5.12 (d, J = 11.9 Hz, 1H, CH2-2-Naph), 4.97 (d, J = 11.9 Hz, 1H, CH2-2-Naph), 4.82 (d, J = 3.8 Hz, 1H, H-1), 4.31 (dd, J = 9.7, 4.3 Hz, 1H, H-6a), 3.87 (t, J = 9.0 Hz, 1H, H-3), 3.86–3.75 (m, 3H, H-2, H-5, H-6b), 3.68 (t, J = 9.0 Hz, 1H, H-4), 3.44 (s, 3H, OCH3), 2.33 (bs, 1H, OH-2)

13C NMR (100 MHz, CDCl3): delta 137.5 (C), 136.1 (C), 133.4 (C), 133.1 (C), 129.1 (CH), 128.4 (CH), 128.2 (CH), 128.1 (CH), 127.8 (CH), 126.8 (CH), 126.3 (CH), 126.2 (CH), 126.1 (CH), 125.9 (CH), 101.5 (CH), 100.0 (CH), 82.0 (CH), 78.9 (CH), 74.9 (CH2), 72.7 (CH), 69.2 (CH2), 62.7 (CH), 55.5 (CH3)

IR (CHCl3, cm- 1): 3,405, 2,923, 2,854, 1,647, 1,366, 1,079, 1,031

High Resolution Mass Spectrometry (HRMS) (FAB, M+): calculated for C25H26O6 422.1729, found to be 422.1721

Compound 3a1. mp 89–90 °C (see ref. 28; 91–91.5 °C)

[alpha] D +65.9 (c 1.0 in CHCl3 at 18 °C) (see ref. 28; [alpha] D +81.2 (c 0.5 in CH2Cl2 at 23 °C))

1H NMR (400 MHz, CDCl3): delta 7.50–7.46 (m, 2H, Ar-H), 7.40–7.34 (m, 3H, Ar-H), 7.30–7.22 (m, 5H, Ar-H), 5.57 (s, 1H, PhCH), 4.91 (dd, J = 9.2, 3.8 Hz, 1H, H-2), 4.88 (d, J = 3.8 Hz, 1H, H-1), 4.87 (d, J = 11.8 Hz, 1H, CH2Ph), 4.69 (d, J = 11.8 Hz, 1H, CH2Ph), 4.29 (dd, J = 10.0, 4.6 Hz, 1H, H-6a), 4.03 (t, J = 9.2 Hz, 1H, H-3), 3.86 (ddd, J = 10.0, 9.2, 4.6 Hz, 1H, H-5), 3.76 (t, J = 10.0 Hz, 1H, H-6b), 3.70 (t, J = 9.2 Hz, 1H, H-4), 3.38 (s, 3H, OCH3), 2.07 (s, 3H, OAc).

13C NMR (100 MHz, CDCl3): delta 170.3 (C), 138.5 (C), 137.3 (C), 128.9 (CH), 128.24 (CH times 2), 128.21 (CH times 2), 127.61 (CH times 2), 127.56 (CH), 126.0 (CH times 2), 101.3 (CH), 97.7 (CH), 82.0 (CH), 76.1 (CH), 74.8 (CH2), 73.0 (CH), 68.9 (CH2), 62.3 (CH), 55.2 (CH3), 20.9 (CH3)

IR (CHCl3, cm- 1): 3,405, 2,923, 2,854, 1,647, 1,366, 1,079, 1,031

HRMS (FAB, MH+): calculated for C23H27O7 415.1757, found to be 415.1762

Analysis (calculated, found for C23H26O7) C (66.65, 66.39) H (6.32, 6.07)

Compound 4b1. mp 129–130 °C

[alpha]D - 22.6 (c 0.6 in CHCl3 at 25 °C)

1H NMR (400 MHz, CDCl3): delta 7.49–7.42 (m, 6H, Ar-H), 7.40–7.32 (m, 6H, Ar-H), 7.15 (d, J = 8.0 Hz, 2H, Ar-H), 5.54 (s, 1H, PhCH), 4.96 (d, J = 10.9 Hz, 1H, CH2Ph), 4.82 (d, J = 10.9 Hz, 1H, CH2Ph), 4.71 (d, J = 9.7 Hz, 1H, H-1), 4.37 (dd, J = 10.3, 5.0 Hz, 1H, H-6a), 3.92 (td, J = 9.1, 2.4 Hz, 1H, H-3), 3.78 (t, J = 10.3 Hz, 1H, H-6b), 3.52 (t, J = 9.2 Hz, 1H, H-4), 3.48–3.40 (m, 2H, H-2, H-5), 2.48 (d, J = 2.4 Hz, 1H, OH-3), 2.39 (s, 3H, CH3)

13C NMR (100 MHz, CDCl3): delta 138.1 (C), 138.1 (C), 136.9 (C), 132.8 (CH), 129.7 (CH), 129.2 (CH), 128.5 (CH), 128.3 (CH), 128.2 (CH), 127.9 (CH), 126.2 (CH), 101.7 (CH), 88.1 (CH), 80.7 (CH), 80.2 (CH), 75.4 (CH2), 75.3 (CH), 70.0 (CH), 68.6 (CH2), 21.1 (CH3)

IR (CHCl3, cm- 1): 3,466, 2,884, 1,646, 1,209, 1,088, 754, 704

HRMS (FAB, MH+): calculated for C27H29O5S 465.1736, found to be 465.1725

Analysis (calculated, found for C27H28O5S) C (69.80, 69.64) H (6.07, 6.58)

Compound 5a1. [alpha]D20 +73.7 (c 1.7 in CHCl3 at 20 °C)

1H NMR (400 MHz, CDCl3): delta 7.35–7.25 (m, 10H, Ar-H), 4.90 (d, J = 3.6 Hz, 1H, H-1), 4.83 (dd, J = 9.9, 3.6 Hz, 1H, H-2), 4.79 (d, J = 11.8 Hz, 1H, CH2Ph), 4.71 (d, J = 11.8 Hz, 1H, CH2Ph), 4.60 (d, J = 12.1 Hz, 1H, CH2Ph), 4.54 (d, J = 12.1 Hz, 1H, CH2Ph), 3.82 (dd, J = 9.6, 8.4 Hz, 1H, H-3), 3.77–3.67 (m, 4H, H-4, H-5, H-6a, H-6b), 3.37 (s, 3H, OCH3), 2.50 (s, 1H, OH-4), 2.05 (s, 3H, OAc)

13C NMR (100 MHz, CDCl3): delta 170.3 (C), 138.5 (C), 137.9 (C), 128.5 (CH), 128.4 (CH), 127.8 (CH), 127.7 (CH), 127.6 (CH), 127.5 (CH), 97.1 (CH), 79.7 (CH), 75.1 (CH2), 73.6 (CH2), 73.3 (CH), 71.4 (CH), 69.8 (CH), 69.7 (CH2), 55.1 (CH3), 20.9 (CH3)

IR (CHCl3, cm- 1): 3,474, 3,030, 2,922, 1,742, 1,371, 1,238, 1,053, 739

HRMS (FAB, MH+): calculated for C23H29O7 417.1913, found to be 417.1921

Compound 6b1. mp 138–139 °C

[alpha]D +39.0 (c 0.5 in CHCl3 at 29 °C)

1H NMR (400 MHz, CDCl3): delta 7.35–7.24 (m, 12H, Ar-H), 7.10 (d, J = 7.9 Hz, 2H, Ar-H), 4.95 (dd, J = 10.0, 9.4 Hz, 1H, H-2), 4.80 (d, J = 10.9 Hz, 1H, CH2Ph), 4.79 (d, J = 11.4 Hz, 1H, CH2Ph), 4.66 (d, J = 11.4 Hz, 1H, CH2Ph), 4.61 (d, J = 10.9 Hz, 1H, CH2Ph), 4.57 (d, J = 10.0 Hz, 1H, H-1), 3.86 (d, J = 11.8 Hz, 1H, H-6a), 3.70–3.65 (m, 2H, H-3, H-6b), 3.59 (t, J = 9.4 Hz, 1H, H-4), 3.39 (ddd, J = 9.4, 4.7, 2.6 Hz, 1H, H-5), 2.32 (s, 3H, CH3), 1.99 (s, 3H, OAc), 1.91 (t, J = 4.2 Hz, 1H, OH-6)

13C NMR (100 MHz, CDCl3): delta 169.5 (C), 138.4 (C), 138.0 (C), 137.7 (C), 133.1 (CH), 129.7 (CH), 128.6 (C), 128.5 (CH), 128.5 (CH), 128.1 (CH), 128.0 (CH), 127.8 (C), 127.8 (CH), 86.2 (CH), 84.2 (CH), 79.4 (CH), 77.5 (CH), 75.3 (CH2), 75.2 (CH2), 71.9 (CH), 62.0 (CH2), 21.1 (CH3), 21.0 (CH3)

IR (CHCl3, cm- 1): 3,420, 2,886, 1,732, 1,642, 1,239, 1,017, 803, 740, 695

HRMS (FAB, MH+): calculated for C29H33O6S 509.2000, found to be 509.2004.



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Acknowledgments

This work was supported by the National Science Council of Taiwan (NSC 94-2113-M-007-021, NSC 94-2627-M-007-002, NSC 95-2113-M-007-028-MY3, NSC 95-2627-M-007-002 and NSC 95-2752-B-007-002-PAE) and the Academia Sinica (AS-92-TP-A04, 94C007 and AS-95-TP-AB1). S.S.K. thanks Academia Sinica for a postdoctoral fellowship.

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References

  1. Varki, A. et al. (eds.) Essentials of Glycobiology (Cold Spring Harbor Laboratory Press, New York, 1999).
  2. Wang, C.-C. et al. Regioselective one-pot protection of carbohydrates. Nature 446, 896–899 (2007). | Article | PubMed | ChemPort |
  3. Wang, P.G. Sugars synthesized in a snap. Nat. Chem. Biol. 3, 309–310 (2007). | Article | PubMed | ChemPort |
  4. Paulsen, H. Advances in selective chemical syntheses of complex oligosaccharides. Angew. Chem. Int. Ed. 21, 155–224 (1982). | Article |
  5. Ernst, B., Hart, G.W. & Sinaÿ, P. (eds.) Carbohydrates in Chemistry and Biology Vol. 1 (Wiley-VCH Verlag, Weinheim, 2000).
  6. Bertozzi, C.R. & Kiessling, L.L. Chemical glycobiology. Science 291, 2357–2364 (2001). | Article | PubMed | ISI | ChemPort |
  7. Schofield, L. et al. Synthetic GPI as a candidate anti-toxin vaccine in model of malaria. Nature 418, 785–789 (2002). | Article | PubMed | ISI | ChemPort |
  8. Yamada, H., Harada, T., Miyazaki, H. & Takahashi, T. One-pot sequential glycosylation: a new method for the synthesis of oligosaccharides. Tetrahedron Lett. 35, 3979–3982 (1994). | Article | ChemPort |
  9. Douglas, N.L., Ley, S.V., Lücking, U. & Warriner, S.L. Tuning glycoside reactivity: new tool for efficient oligosaccharide synthesis. J. Chem. Soc., Perkin Trans. 1, 51–65 (1998). | Article |
  10. Zhang, Z. et al. Programmable one-pot oligosaccharide synthesis. J. Am. Chem. Soc. 121, 734–753 (1999). | Article | ISI | ChemPort |
  11. Sears, P. & Wong, C.-H. Toward automated synthesis of oligosaccharides and glycoproteins. Science 291, 2344–2350 (2001). | Article | PubMed | ISI | ChemPort |
  12. Plante, O.J., Palmacci, E.R. & Seeberger, P.H. Automated solid-phase synthesis of oligosaccharides. Science 291, 1523–1527 (2001). | Article | PubMed | ISI | ChemPort |
  13. Danishefsky, S.J., McClure, K.F., Randolph, J.T. & Ruggeri, R.B. A strategy for the solid-phase synthesis of oligosaccharides. Science 260, 1307–1309 (1993). | Article | PubMed | ChemPort |
  14. Kim, J.-H., Yang, H., Park, J. & Boons, G.-J. A general strategy for stereoselective glycosylations. J. Am. Chem. Soc. 127, 12090–12097 (2005). | Article | PubMed | ChemPort |
  15. Flitsch, S.L. Glycosylation with a twist. Nature 437, 201–202 (2005). | Article | PubMed | ChemPort |
  16. Demchenko, A.V. Stereoselective chemical 1,2-cis-O-glycosylation: from 'sugar ray' to modern techniques of the 21st century. Synlett, 1225–1240 (2003).
  17. Pellissier, H. Use of O-glycosylation in total synthesis. Tetrahedron 61, 2947–2993 (2005). | Article | ChemPort |
  18. Kocienski, P.J. Protecting Groups 3rd ed. (Georg Thieme Verlag, Stuttgart, 2005).
  19. Wuts, P.G.M. Greene's Protective Groups in Organic Synthesis 4th ed. (John Wiley & Sons, New York, 2007).
  20. Wright, J.A., Yu, J. & Spencer, J.B. Sequential removal of the benzyl-type protecting groups PMB and NAP by oxidative cleavage using CAN and DDQ. Tetrahedron Lett. 42, 4033–4036 (2001). | Article | ChemPort |
  21. Plante, O.J., Buchwald, S.L. & Seeberger, P.H. Halobenzyl ethers as protecting groups for organic synthesis. J. Am. Chem. Soc. 122, 7148–7149 (2000). | Article | ChemPort |
  22. Jobron, L. & Hindsgaul, O. Novel para-substituted benzyl ethers for hydroxyl group protection. J. Am. Chem. Soc. 121, 5835–5836 (1999). | Article | ChemPort |
  23. Shie, C.-R. et al. Cu(OTf)2 as an efficient and dual-purpose catalyst in the regioselective reductive ring opening of benzylidene acetals. Angew. Chem. Int. Ed. 44, 1665–1668 (2005). | Article | ChemPort |
  24. Tsunoda, T., Suzuki, M. & Noyori, R. A facile procedure for acetalization under aprotic conditions. Tetrahedron Lett. 21, 1357–1358 (1980). | Article | ChemPort |
  25. Hatakeyama, S. et al. Efficient reductive etherification of carbonyl compounds with alkoxytrimethylsilanes. Tetrahedron Lett. 35, 4367–4370 (1994). | Article | ChemPort |
  26. Wang, C.-C. et al. Synthesis of biologically potent alpha1,2-linked disaccharide derivatives via regioselective one-pot protection glycosylation. Angew. Chem. Int. Ed. 41, 2360–2362 (2002). | Article | ChemPort |
  27. Pangborn, A.B., Giardello, A., Grubbs, R.H., Rosen, R.K. & Timmers, F.J. Safe and convenient procedure for solvent purification. Organometallics 15, 1518–1520 (1996). | Article | ISI | ChemPort |
  28. Wulff, G. & Wichelhaus, J. Zur synthese von beta-D-mannopyranosiden. Chem. Ber. 112, 2847–2853 (1979). | Article | ChemPort |
  29. Söderberg, E., Westman, J. & Oscarson, S. Rapid carbohydrate protecting group manipulations assisted by microwave dielectric heating. J. Carbohydr. Chem. 20, 397–410 (2001). | Article |
  1. Department of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan.
  2. Chemical Biology and Molecular Biophysics Program, Taiwan International Graduate Program, Academia Sinica, Taipei 115, Taiwan
  3. Genomics Research Center, Taiwan International Graduate Program, Academia Sinica, Taipei 115, Taiwan.

Correspondence to: Shang-Cheng Hung1,2,3 e-mail: hung@mx.nthu.edu.tw

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