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Synthesis 2024; 56(10): 1648-1656
DOI: 10.1055/s-0042-1751559
paper

Convenient Synthesis of Hexa- and Pentasaccharide Repeating Units Corresponding to the O-Polysaccharides of Acinetobacter baumannii O7 and Acinetobacter baumannii O10 Strains

Samim Sahaji
,
Puja Bag
,
Anup Kumar Misra∗

S.S. thanks the University Grants Commission (UGC-MANF), India for a senior research fellowship. P.B. thanks University Grants Commission, India for a junior research fellowship. This work was supported by the Bose Institute, Kolkata (A.K.M.).
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Abstract

The synthesis of the hexa- and pentasaccharide repeating units of the cell wall O-polysaccharides of Acinetobacter baumannii O7 and O10 strains, respectively, has been achieved in very good yield using [4+2] and [4+1] block glycosylation strategies. The p-methoxybenzyl (PMB) group was used as an in situ removable protecting group, which was removed after glycosylation in the same pot by tuning the reaction condition. A challenging β-l-rhamnopyranosyl linkage was constructed using ‘armed-disarmed glycosylation’ conditions by the influence of a remotely located, H-bond mediating, picolinoyl group in the glycosyl donor. A d-mannosamine intermediate was prepared from d-glucose using minimum reaction steps. The hexa- and pentasaccharide were achieved as their p-methoxyphenyl (PMP) glycosides.


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Key words

carbohydrates - glycosylation - glycoside - oligosaccharide - synthesis

Healthcare-acquired infections or nosocomial infections have become serious threats to the human population worldwide.[1] Inadequate waste disposal from healthcare systems as well as lack of hygienic conditions are the main reasons for these infections.[2] Since such infections are acquired during a stay in the healthcare system, they cause prolonged disability, antimicrobial resistance, increased mortality rate, and significant economic burden.[3] Most prevalent nosocomial infections include catheter-associated urinary tract infections, ventilator-associated pneumonia, central line associated bloodstream infections, etc.[4] A variety of pathogens are associated with nosocomial infections, which include bacteria, virus, and fungal parasites.[5] Among several pathogenic bacteria responsible for nosocomial infections, the Acinetobacter genre is predominant.[6] Acinetobacter baumannii (A. baumannii) are non-motile Gram negative opportunistic bacteria causing variety of healthcare acquired infections, such as urinary tract infections (UTI), pneumonia, and bacteremia.[7] Due to its high degree of resistance towards most of the commonly available antibiotics, A. baumannii is listed in the highest category of pathogens responsible for forthcoming threats to human health.[8] They are predominantly abundant in soil, water, and sewage.[9] A. baumannii are surrounded by thick layer of capsular polysaccharides (CPS) which protects them from disinfectants, immune system components, and antimicrobial agents.[10] In addition to CPS, a variety of lipopolysaccharides (LPS) are present in the cell wall of A. baumannii­, which play important roles in the virulent property of the bacteria similar to other Gram-negative bacteria.[11] LPSs contain O-polysaccharides or O-antigens, which are variable in their structures and allow the possibility for O-serotyping of the bacterial species.[12] O-Antigens of Gram negative bacteria have high level of antigenicity, which allows them to be used in the development of possible vaccine candidates. Since A. baumannii strains are highly resistant to antibiotics, it is quite pertinent to develop alternative therapies such as vaccination or immunotherapy to combat infections caused by them. In the 1990s, Haseley and Wilkinson reported the structures of the O-7[13] and O-10[14] antigens of A. baumannii, which are hexasaccharide and pentasaccharide repeating units, respectively, having a common tetrasaccharide chain. The O-7 hexasaccharide repeating unit is composed of one α-d-glucosamine (GlcpNAc), three α-l-rhamnose (Rhap), one β-d-glucosamine (GlcpNAc), and one β-l-rhamnose (Rhap) moiety and the O-10 pentasaccharide repeating unit is composed of one α-d-glucosamine (GlcpNAc), three α-l-rhamnose (Rhap), and one α-d-mannosamine (ManpNAc) moiety (Figure [1]). Although oligosaccharides can be obtained from natural sources by using fermentation techniques, it suffers from a number of shortcomings. such as handling of live strains of virulent bacteria, difficulties in obtaining pure oligosaccharides free from biological impurities (e.g., lipids, proteins, phenolic compounds), and lack of homogeneity and higher quantities. To overcome these difficulties, the development of chemical synthetic strategies to achieve the oligosaccharide repeating unit structures with adequate purity and structural integrity could be beneficial. The synthesized oligosaccharide or glycan can be conjugated with a suitable protein following standard reaction protocols to provide possible glycoprotein conjugate derivatives.[15] In the past, synthetic glycoprotein conjugate molecules showed promising results for their consideration as possible vaccine candidates.[16] However, it is essential to develop concise chemical synthetic strategies to achieve the oligosaccharide or glycan repeating units having conserved stereochemistry at the glycosylation linkages.[17] In this context, a straightforward synthetic strategy has been developed for the synthesis of hexa- and pentasaccharide repeating units of the O-7 and O-10 antigens of A. baumannii and presented herein (Figure [2]).

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Figure 1 (A) Structure of the repeating unit of the O-polysaccharide of A. baumannii O7[13] and (B) structure of the repeating unit of the O-polysaccharide of A. baumannii O10[14]
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Figure 2 Retrosynthetic analysis for the synthesis of penta- and hexasaccharides 1 and 2 and their synthetic intermediates

The desired hexa- and pentasaccharide were synthesized as p-methoxyphenyl (PMP) glycosides applying stereoselective [4+2] and [4+1] block glycosylation strategies. The PMP group was selected as the temporary anomeric protecting group, which can be removed under oxidative condition by treatment with ammonium cerium(IV) nitrate (CAN)[18] to furnish hexa- and pentasaccharide hemiacetal derivatives for their conjugation with a suitable protein under standard reaction conditions. The retrosynthetic analysis of hexasaccharide 1 and pentasaccharide 2 reveals that the desired compounds can be synthesized using a set of monosaccharide intermediates 3,[19] 4,[20] 5,[21] 6,[22] 7,[23] and 8 [24] (Figure [2]). The monosaccharide intermediates 3–8 were prepared from the commercially available reducing sugars using the reaction conditions reported earlier. The key feathers of the synthetic strategies include: (a) access to a hexa- and pentasaccharide with conserved stereochemistry at the glycosidic linkages; (b) stereoselective [4+2] and [4+1] block synthetic strategies; (c) use of the p-methoxybenzyl (PMB) group as the protecting group removable in situ under the glycosylation medium;[25] (d) construction of a β-l-rhamnosyl linkage in ‘armed-disarmed glycosyl­ation’[26] conditions by the influence of a remotely located H-bond mediating picolinoyl group[27] in the glycosyl donor; (e) preparation of d-mannosamine precursor from d-glucose;[28] and (f) use of a common tetrasaccharide derivative to obtain both compounds 1 and 2.

Stereoselective glycosylation of compound 3 (prepared from d-glucosamine hydrochloride in five steps) with thioglycoside derivative 4 (prepared from l-rhamnose in eight steps) mediated by a combination of N-iodosuccinimide (NIS) and trimethylsilyl trifluoromethanesulfonate (TMSOTf)[29] furnished disaccharide derivative 9 in 78% yield. The formation of compound 9 was confirmed by the spectroscopic analysis of compound 9. The presence of O-acetyl at C-2 of the glycosyl donor 4 led to the exclusive formation (1→3)-trans-glycosylated product due to a neighboring group participating effect. Compound 9 was subjected to a set of reactions involving direct transformation of the O-acetyl group into an O-benzyl group by treatment with benzyl bromide and sodium hydroxide[30] in the presence of tetrabutylammonium bromide (TBAB) and treatment of the benzylated product with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)[31] in a biphasic reaction medium to give compound 10 in 66% overall yield. Stereoselective glycosylation of disaccharide acceptor 10 with thioglycoside derivative 5 (prepared from l-rhamnose in eight steps) promoted by a combination of NIS and TMSOTf[29] furnished trisaccharide derivative 11 in 77% yield, which was treated with sodium methoxide to afford trisaccharide acceptor 12 in 91% yield after de-O-acetylation. The formation of compound 11 was unambiguously confirmed by the NMR spectroscopic analysis. The O-acetyl group present at C-2 of the glycosyl donor 5 provided a neighboring group participation effect to furnish exclusively (1→3)-trans-glycosylated product 11. Iodonium ion mediated glycosylation of compound 12 with compound 4 in the presence of a combination of NIS and TMSOTf[29] and in situ removal of the PMB group[25] from the glycosylation product by tuning the reaction condition after glycosylation led to the formation of the tetrasaccharide acceptor 13 in 66% yield. NMR spectroscopic analysis confirmed formation of compound 13 (Scheme [1], Table [1]).

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Scheme 1 Reagents and conditions: (a) NIS, TMSOTf, CH2Cl2, MS-4Å, –30 °C, 1 h, 9: 78%, 11: 77%; (b) BnBr, NaOH, TBAB, DMF, r.t., 2 h; (c) DDQ, CH2Cl2/H2O (5:1), r.t., 4 h, 66% (2 steps); (d) CH3ONa, CH3OH, r.t., 2 h, 91%; (e) NIS, TMSOTf, CH2Cl2, MS-4Å, –20 °C, 1 h, then r.t., 30 min, 66%.

In another experiment, treatment of thioglycoside acceptor 6 (prepared from d-glucosamine hydrochloride in seven steps) with thioglycoside donor 7 (prepared from l-rhamnose in eight steps) in the presence of a combination of NIS and TMSOTf[29] under ‘armed-disarmed’ glycosyl­ation[26] conditions furnished disaccharide thioglycoside 14 with a newly formed (1→4)-β-glycosidic linkage in 65% yield together with a small quantity (~5–8%) of (1→4)-α-glycosylated product, which was separated by column chromatography. The presence of a picolinoyl group at C-3 of compound 7 led to H-bond mediated aglycon delivery[27] during glycosylation resulting in the formation of a β-glycosidic linkage in the product. The NMR spectroscopic analysis confirmed the formation of compound 14 (Scheme [2]). After obtaining the tetrasaccharide acceptor 13 and disaccharide thioglycoside 14, attempts were made to carry out iodonium mediated stereoselective glycosylation between them in the presence of NIS and TMSOTf.[29] Unfortunately after several attempts, synthesis of the desired hexasaccharide could not be achieved. Probably, the presence of the deactivating picolinoyl group in the glycosyl donor 14 made it unreactive towards the glycosylation leading to the formation of the desired glycosylated product. Such kind of failure in the glycosylation using a glycosyl donor containing a picolinoyl group was observed in our earlier studies also.[32] To overcome the hurdle, it was decided to replace the picolinoyl group with an O-acetyl group. Removal of picolinoyl group from compound 14 by treatment with copper(II) acetate[33] followed by acetylation of the free hydroxyl group using acetic anhydride and pyridine afforded disaccharide thioglycoside derivative 15 in 75% overall yield (Scheme [2], Table [1]).

Table 1 1H and 13C NMR Signals of Glycosidic Linkages of the Newly Formed Compounds

Compound

Significant 1H NMR signals (δ)

Significant 13C NMR signals (δ)

9

5.41 (br s, H-1A), 5.02 (br s, H-1B)

98.5 (C-1B), 98.0 (C-1A)

11

5.38 (d, J = 3.5 Hz, H-1A), 5.04 (br s, H-1B), 5.00 (br s, H-1C)

99.5 (C-1C), 98.5 (C-1B), 97.7 (C-1A)

13

5.49 (d, J = 3.5 Hz, H-1A), 5.15 (br s, H-1B), 5.07 (br s, H-1C), 4.96 (br s, H-1D)

100.9 (C-1D), 98.8 (C-1C), 98.3 (C-1B), 97.7 (C-1A)

14

5.47 (d, J = 10.5 Hz, H-1E), 4.64 (br s, H-1F)

102.2 (C-1F), 80.7 (C-1E)

16

5.70 (d, J = 10.5 Hz, H-1E), 5.48 (d, J = 3.5 Hz, H-1A), 5.13 (br s, H-1B), 5.06 (br s, H-1C), 5.04 (br s, H-1D), 4.68 (br s, H-1F)

102.6 (C-1F), 101.0 (C-1C), 98.6 (C-1D), 98.4 (C-1B), 98.1 (C-1E), 97.7 (C-1A)

17

5.50 (d, J = 3.5 Hz, H-1A), 5.22 (br s, H-1B), 5.18 (br s, H-1C), 5.05 (br s, H-1D), 4.96 (br s, H-1E)

100.9 (C-1D), 99.1 (C-1E), 98.3 (C-1C), 97.7 (C-1A) and 94.5 (C-1B)

1

5.46 (d, J = 3.0 Hz, H-1A), 5.18 (br s, H-1C), 4.99 (br s, H-1D), 4.92 (br s, H-1B), 4.89 (br s, H-1F), 4.73 (d, J = 8.0 Hz, H-1E)

102.6 (C-1E), 101.8 (C-1D), 101.3 (C-1B), 100.8 (C-1C), 100.7 (C-1F), 96.9 (C-1A)

2

5.46 (d, J = 3.5 Hz, H-1A), 5.22 (br s, H-1C), 5.02 (br s, 2 H, H-1D, H-1E), 4.92 (br s, H-1B)

102.1 (C-1C), 101.3 (C-1B), 100.7 (C-1D), 96.9 (C-1A), 95.1 (C-1E)
Zoom Image
Scheme 2 Reagents and conditions: (a) NIS, TMSOTf, CH2Cl2, MS-4Å, –10 °C, 3 h, 65%; (b) (i) Cu(OAc)2, CH2Cl2/CH3OH (4:1), r.t., 30 min; (ii) Ac2O, pyridine, r.t., 1 h, 75% (2 steps).

To our satisfaction, stereoselective glycosylation of compound 13 with compound 15 in the presence of a combination of NIS and TMSOTf[29] led to the formation of hexasaccharide derivative 16 in 71% yield. The formation of the desired compound 16 was unambiguously confirmed by NMR spectroscopic analysis. The presence of a phthalimido group at C-2 of the glycosyl donor directed the formation of the β-glycosylated linkage in compound 16. In a similar approach, stereoselective glycosylation of tetrasaccharide acceptor 13 with 2-azido-d-mannose thioglycoside donor 8 (prepared from d-glucose in five steps) in the presence of a combination of NIS and TMSOTf[29] led to the formation of pentasaccharide derivative 17 in 68% yield. The NMR spectroscopic analysis of compound 17 confirmed its formation. After achieving the hexasaccharide derivative 16 and pentasaccharide derivative 17, it was planned to deprotect them to obtain the hexa- and pentasaccharide repeating units similar to their native forms. Compound 16 was subjected to a set of transformations that included (a) treatment with hydrazine hydrate to remove the phthaloyl group;[34] (b) acetylation of the product using acetic anhydride and pyridine; (c) transformation of the azido group to an acetamido group by treatment with thioacetic acid and pyridine;[35] (d) removal of the O-acetyl groups using sodium methoxide, and (e) removal of the O-benzyl groups and benzylidene group by hydrogenolysis[36] to give hexasaccharide 1 as p-methoxyphenyl glycoside in 31% yield in five steps. Similarly, compound 17 was subjected to a set of reactions including (a) treatment with thioacetic acid to convert azido groups into acetamido groups[35] and (b) de-O-acetylation using sodium methoxide and removal of O-benzyl groups and benzylidene group by hydrogenolysis[36] to furnish compound 2 as p-methoxyphenyl glycoside in 34% yield in three steps. The formation of compounds 1 and 2 was unambiguously confirmed by their NMR spectroscopic analysis (Scheme [3], Table [1]).

Zoom Image
Scheme 3 Reagents and conditions: (a) NIS, TMSOTf, CH2Cl2, MS-4Å, –20 °C, 1 h, 71%; (b) NIS, TMSOTf, CH2Cl2, MS-4Å, –30 °C, 1 h, 68%; (c) NH2NH2·H2O, EtOH, 80 °C, 10 h; (d) Ac2O, pyridine, r.t., 2 h; (e) CH3COSH, pyridine, r.t., 12 h; (f) 0.1 M CH3ONa, CH3OH, r.t., 3 h; (g) H2, 20% Pd(OH)2-C, CH3OH, r.t., 18 h, 31% (5 steps); (h) CH3COSH, pyridine, r.t., 24 h; (i) CH3ONa, CH3OH, r.t., 3 h; (j) H2, 20% Pd(OH)2-C, CH3OH, r.t., 8 h, 34% (3 steps).

In summary, syntheses of hexa- and pentasaccharide repeating units as PMP glycosides corresponding to the cell wall O-polysaccharides of Acinetobacter baumannii O7 and Acinetobacter baumannii O10 strains respectively have been achieved in very good yield. The synthetic strategies consist of stereoselective [4+2] and [4+1] block glycosylations using a common tetrasaccharide acceptor and di- and monosaccharide thioglycosides as donors. The PMB group has been used as a temporary protecting group, which was removed in situ after the glycosylation by tuning the reaction condition. A synthetically challenging β-l-rhamnose containing disaccharide thioglycoside donor 15 was prepared in very good yield using a remotely placed O-picolinoyl group containing thioglycoside donor to direct H-bond mediated aglycone delivery under an ‘armed-disarmed’ glycosylation of the thioglycoside. All intermediate glycosylation steps were high yielding with satisfactory stereochemical outcome.

All reactions were monitored by TLC over silica gel coated TLC plates. The spots on TLC were visualized by warming ceric sulfate (2% Ce(SO4)2 in 2 N H2SO4) sprayed plates on a hot plate. Silica gel 230–400 mesh was used for column chromatography. 1H NMR were recorded on a 700 MHz NMR spectrometer (Bruker Avance 700) and 13C NMR spectra were recorded on a 175 MHz NMR spectrometer (Bruker Avance 700) using CDCl3 as solvent and D2O/TMS as internal reference unless otherwise stated. MS were recorded on a Bruker mass spectrometer. Optical rotations were recorded on a Jasco P-2000 spectrometer. Commercially available grades of organic solvents of adequate purity are used in all reactions.


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p-Methoxyphenyl {2-O-Acetyl-4-O-benzyl-3-O-(p-methoxybenzyl)-α-l-rhamnopyranosyl}-(1→3)-2-azido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranoside (9)

To solution of 3 (2 g, 5.00 mmol) and 4 (2.5 g, 5.42 mmol) in anhyd CH2Cl2 (25 mL) was added MS-4Å (2 g) and the mixture was cooled to –30 °C under argon. To the cooled mixture was added solid NIS (1.3 g, 5.78 mmol) under argon followed by TMSOTf (50 μL, 0.27 mmol) and it was stirred at same temperature for 1 h. The mixture was filtered through a Celite bed and washed with CH2Cl2 (50 mL). The combined filtrates were successively washed with 5% Na2S2O3 (50 mL), sat. aq NaHCO3 (50 mL), and H2O (50 mL). The collected organic layer was dried (anhyd Na2SO4), filtered, and concentrated. The crude product was purified by column chromatography (silica gel, hexane/EtOAc 4:1) to give pure 9 (3.1 g, 78%) as a yellow oil.

[α]D 25 +26 (c 1.0, CHCl3).

1H NMR (700 MHz, CDCl3): δ = 7.40–6.74 (m, 18 H, Ar-H), 5.45 (s, 1 H, PhCH), 5.41 (br s, 1 H, H-1A), 5.40 (br s, 1 H, H-2B), 5.02 (br s, 1 H, H-1B), 4.78 (d, J = 11.2 Hz, 1 H, PhCH), 4.58 (d, J = 11.2 Hz, 1 H, PhCH), 4.45 (d, J = 11.2 Hz, 1 H, PhCH), 4.38 (d, J = 11.2 Hz, 1 H, PhCH), 4.25 (t, J = 9.8 Hz, 1 H, H-4A), 4.19–4.16 (m, 1 H, H-6aA), 4.03–3.99 (m, 1 H, H-5A), 3.96–3.93 (m, 1 H, H-5B), 3.84 (dd, J = 9.8, 3.5 Hz, 1 H, H-2A), 3.70 (s, 6 H, 2 OCH 3), 3.66 (t, J = 10.5 Hz, 1 H, H-6bA), 3.53 (t, J = 9.8 Hz, 1 H, H-4B), 3.38 (dd, J = 9.8, 3.5 Hz, 1 H, H-3B), 3.29 (t, J = 9.5Hz, 1 H, H-3A), 2.07 (s, 3 H, COCH 3), 0.8 (d, J = 6.5 Hz, 3 H, CCH 3).

13C NMR (175 MHz, CDCl3): δ = 170.3 (COCH3), 159.2–113.8 (Ar-C), 102.1 (PhCH), 98.5 (C-1B), 98.0 (C-1A), 80.1 (C-3A), 80.0 (C-4B), 77.6 (C-2A), 75.2 (PhCH2), 74.3 (C-4A), 71.4 (PhCH2), 68.9 (C-2B), 68.7 (C-6A), 68.1 (C-5B), 64.1 (C-5A), 63.6 (C-3B), 55.7 (OCH3), 55.2 (OCH3), 21.1 (COCH3), 17.3 (CCH3).

HRMS: m/z [M + H]+ calcd for C43H48N3O12: 798.3232; found: 798.3214.


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p-Methoxyphenyl (2,4-Di-O-benzyl-α-l-rhamnopyranosyl)-(1→3)-2-azido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranoside (10)

To a solution of 9 (2.5 g, 3.13 mmol) in DMF (10 mL) were added powdered NaOH (0.25 g, 6.25 mmol), TBAB (50 mg, 0.16 mmol), and BnBr (0.7 mL, 5.89 mmol) and the mixture was briskly stirred at r.t. for 2 h. The reaction was quenched by adding H2O (50 mL) and extracted with CH2Cl2 (50 mL). The organic layer was washed with sat. aq NaHCO3 (50 mL) and H2O (50 mL), dried (anhyd Na2SO4), and concentrated. To a solution of the product in CH2Cl2 (30 mL) was added a solution of DDQ (1.0 g, 4.40 mmol) in H2O (5 mL) and the biphasic mixture was stirred vigorously at r.t. for 4 h. The reaction was diluted by the addition of sat. NaHCO3 (50 mL) and extracted with CH2Cl2 (50 mL). The organic layer was washed with H2O (50 mL), dried (anhyd Na2SO4), and concentrated. The crude product was purified by column chromatography (silica gel, hexane/EtOAc 3:1) to give pure 10 (1.5 g, 66%) as a yellow oil.

[α]D 25 +8 (c 1.0, CHCl3).

1H NMR (700 MHz, CDCl3): δ = 7.40–7.17 (m, 15 H, Ar-H), 6.95 (d, J = 8.4 Hz, 2 H, Ar-H), 6.78 (d, J = 8.4 Hz, 2 H, Ar-H), 5.42 (s, 1 H, PhCH), 5.41 (d, J = 4.0 Hz, 1 H, H-1A), 5.13 (br s, 1 H, H-1B), 4.73 (d, J = 11.2 Hz, 1 H, PhCH), 4.71 (d, J = 11.8 Hz, 1 H, PhCH), 4.52 (J = 11.8 Hz, 1 H, PhCH), 4.48 (J = 11.2 Hz, 1 H, PhCH), 4.25 (t, J = 9.8 Hz, 1 H, H-4A), 4.17–4.15 (m, 1 H, H-6aA), 4.01–3.98 (m, 1 H, H-5A), 3.88–3.83 (m, 2 H, H-2A, H-5B), 3.79–3.78 (m, 1 H, H-2B), 3.71 (s, 3 H, OCH 3), 3.64 (t, J = 10.5 Hz, 1 H, H-6bA), 3.50 (t, J = 9.8 Hz, 1 H, H-4B), 3.39 (dd, J = 9.8, 3.5 Hz, 1 H, H-3B), 3.18 (t, J = 9.8 Hz, 1 H, H-3A), 2.24 (d, J = 9.1 Hz, 1 H, OH), 0.72 (d, J = 6.5 Hz, 3 H, CCH 3).

13C NMR (175 MHz, CDCl3): δ = 155.5–114.7 (Ar-C), 102.3 (PhCH), 97.9 (C-1B), 97.6 (C-1A), 82.2 (C-3A), 80.1 (C-4B), 78.4 (C-2A), 74.8 (PhCH2), 74.4 (C-4A), 72.9 (PhCH2), 71.2 (C-2B), 68.7 (C-6A), 67.6 (C-5B), 64.3 (C-5A), 63.7 (C-3B), 55.7 (OCH3), 17.2 (CCH3).

HRMS: m/z [M + H]+ calcd for C40H44N3O10: 726.3020; found: 726.3001.


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p-Methoxyphenyl (2-O-Acetyl-3,4-di-O-benzyl-α-l-rhamnopyranosyl)-(1→3)-(2,4-di-O-benzyl-α-l-rhamnopyranosyl)-(1→3)-2-azido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranoside (11)

To solution of 10 (1.2 g, 1.65 mmol) and 5 (0.78 g, 1.81 mmol) in anhyd CH2Cl2 (15 mL) was added MS-4Å (1 g) and the mixture was cooled to –30 °C under argon. To the cooled mixture was added solid NIS (425 mg, 1.89 mmol) under argon followed by TMSOTf (20 μL, 0.11 mmol) and it was stirred at same temperature for 1 h. The mixture was filtered through a Celite bed and washed with CH2Cl2 (50 mL). The combined filtrates were successively washed with 5% Na2S2O3 (25 mL), sat. aq NaHCO3 (25 mL), and H2O (25 mL). The collected organic layer was dried (anhyd Na2SO4), filtered, and concentrated. The crude product was purified by column chromatography (silica gel, hexane/EtOAc 3:1) to give pure 11 (1.4 g, 77%) as a yellow oil.

[α]D 25 +18 (c 1.0, CHCl3).

1H NMR (700 MHz, CDCl3): δ = 7.39–7.14 (m, 25 H, Ar-H), 6.94 (d, J = 8.4 Hz, 2 H, Ar-H), 6.77 (d, J = 8.4 Hz, 2 H, Ar-H), 5.44 (s, 1 H, PhCH), 5.43 (br s, 1 H, H-2C), 5.38 (d, J = 3.5 Hz, 1 H, H-1A), 5.04 (br s, 1 H, H-1B), 5.00 (br s, 1 H, H-1C), 4.86 (d, J = 11.2 Hz, 1 H, PhCH), 4.66 (d, J = 11.2 Hz, 1 H, PhCH), 4.61–4.58 (m, 3 H, 3 PhCH), 4.52 (d, J = 11.2 Hz, 1 H, PhCH), 4.44 (d, J = 11.2 Hz, 1 H, PhCH), 4.38 (d, J = 11.2 Hz, 1 H, PhCH), 4.22 (t, J = 9.8 Hz, 1 H, H-4A), 4.18–4.14 (m, 1 H, H-6aA), 4.01–3.97 (m, 2 H, H-3C, H-5A), 3.88 (dd, J = 9.8, 3.5 Hz, 1 H, H-2A), 3.87–3.84 (m, 1 H, H-5B), 3.79–3.76 (m, 1 H, H-5C), 3.75–3.74 (m, 1 H, H-2B), 3.70 (s, 3 H, OCH 3), 3.64 (t, J = 10.5 Hz, 1 H, H-6bA), 3.51 (t, J = 9.8 Hz, 1 H, H-4B), 3.45 (t, J = 9.8 Hz, 1 H, H-4C), 3.36 (dd, J = 9.8, 3.5 Hz, 1 H, H-3B), 3.33 (t, J = 9.8 Hz, 1 H, H-3A), 2.00 (s, 3 H, COCH 3), 1.19 (d, J = 6.2 Hz, 3 H, CCH 3), 0.69 (d, J = 6.2 Hz, 3 H, CCH 3).

13C NMR (175 MHz, CDCl3): δ = 170.0 (COCH3), 155.5–114.8 (Ar-C), 102.3 (PhCH), 99.5 (C-1C), 98.5 (C-1B), 97.7 (C-1A), 80.8 (C-4C), 80.1 (C-4B), 80.0 (C-3A), 77.9 (C-2A), 77.6 (2 C, C-2B, C-3C), 75.1 (2 C, 2 PhCH2), 74.6 (C-4A), 72.6 (PhCH2), 71.8 (PhCH2), 69.0 (C-5C), 68.7 (C-6A), 68.5 (C-5B), 68.3 (C-2C), 64.4 (C-3B), 63.7 (C-5A), 55.7 (OCH3), 21.0 (COCH3), 17.9 (CCH3), 17.1 (CCH3).

HRMS: m/z [M + H]+ calcd for C62H68N3O15: 1094.4644; found: 1094.4623.


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p-Methoxyphenyl (3,4-Di-O-benzyl-α-l-rhamnopyranosyl)-(1→3)-(2,4-di-O-benzyl-α-l-rhamnopyranosyl)-(1→3)-2-azido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranoside (12)

A solution of 11 (1.2 g, 1.09 mmol) in 0.1 M CH3ONa in CH3OH (25 mL) was stirred at r.t. for 2 h. The solution was neutralized with Dowex 50W X8 (H+) resin and filtered and the filtrate was passed through a short pad of silica gel and concentrated under reduced pressure to give 12 (1.05 g, 91%) as a yellow oil.

[α]D 25 +32 (c 1.0, CHCl3).

1H NMR (700 MHz, CDCl3): δ = 7.49–7.26 (m, 25 H, Ar-H), 7.05 (d, J = 8.4 Hz, 2 H, Ar-H), 6.88 (d, J = 8.4 Hz, 2 H, Ar-H), 5.55 (s, 1 H, PhCH), 5.49 (d, J = 3.5 Hz, 1 H, H-1A), 5.18 (br s, 1 H, H-1C), 5.15 (br s, 1 H, H-1B), 4.93 (d, J = 11.2 Hz, 1 H, PhCH), 4.76–4.63 (m, 6 H, 6 PhCH), 4.55 (d, J = 11.2 Hz, 1 H, PhCH), 4.31 (t, J = 9.8 Hz, 1 H, H-4A), 4.27–4.25 (m, 1 H, H-6aA), 4.10–4.08 (m, 2 H, H-3C, H-5A), 4.03–4.02 (m, 1 H, H-2C), 3.98–3.95 (m, 1 H, H-5B), 3.89–3.85 (m, 3 H, H-2A, H-2B, H-5C), 3.81 (s, 3 H, OCH 3), 3.75 (t, J = 10.5 Hz, 1 H, H-6bA), 3.64 (t, J = 9.8 Hz, 1 H, H-4B), 3.52 (t, J = 9.8 Hz, 1 H, H-4C), 3.50–3.46 (m, 2 H, H-3A, H-3B), 1.28 (d, J = 6.2 Hz, 3 H, CCH 3), 0.80 (d, J = 6.2 Hz, 3 H, CCH 3).

13C NMR (175 MHz, CDCl3): δ = 155.5–114.8 (Ar-C), 102.3 (PhCH), 101.0 (C-1C), 98.6 (C-1B), 97.7 (C-1A), 80.9 (C-4C), 80.1 (C-4B), 80.0 (C-3A), 79.8 (C-2A), 77.7 (2 C, C-2B, C-3C), 75.1 (2 C, 2 PhCH2), 74.5 (C-4A), 72.7 (PhCH2), 72.1 (PhCH2), 68.9 (C-5C), 68.7 (C-6A), 68.5 (C-5B), 67.9 (C-2C), 64.4 (C-3B), 63.7 (C-5A), 55.7 (OCH3), 17.9 (CCH3), 17.1 (CCH3).

HRMS: m/z [M + H]+ calcd for C60H66N3O14: 1052.4539; found: 1052.4522.


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p-Methoxyphenyl (2-O-Acetyl-4-O-benzyl-α-l-rhamnopyranosyl)-(1→2)-(3,4-di-O-benzyl-α-l-rhamnopyranosyl)-(1→3)-(2,4-di-O-benzyl-α-l-rhamnopyranosyl)-(1→3)-2-azido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranoside (13)

To solution of 12 (800 mg, 0.76 mmol) and 4 (385 mg, 0.84 mmol) in anhyd CH2Cl2 (10 mL) was added MS-4Å (1 g) and the mixture was cooled to –20 °C under argon. To the cooled mixture was added solid NIS (200 mg, 0.89 mmol) under argon followed by TMSOTf (10 μL, 0.03 mmol) and it was stirred at same temperature for 1 h. After consumption of the starting materials, the mixture was stirred at r.t. for 30 min. The mixture was filtered through a Celite bed and washed with CH2Cl2 (50 mL). The combined filtrates were successively washed with 5% Na2S2O3 (20 mL), sat. aq NaHCO3 (20 mL), and H2O (20 mL). The collected organic layer was dried (anhyd Na2SO4), filtered, and concentrated. The crude product was purified by column chromatography (silica gel, hexane/EtOAc 3:1) to give pure 13 (670 mg, 66%) as a yellow oil.

[α]D 25 +10 (c 1.0, CHCl3).

1H NMR (700 MHz, CDCl3): δ = 7.48–7.23 (m, 30 H, Ar-H), 7.04 (d, J = 8.4 Hz, 2 H, Ar-H), 6.88 (d, J = 8.4 Hz, 2 H, Ar-H), 5.56 (s, 1 H, PhCH), 5.49 (d, J = 3.5 Hz, 1 H, H-1A), 5.31–5.30 (m, 1 H, H-2D), 5.15 (br s, 1 H, H-1B), 5.07 (br s, 1 H, H-1C), 4.96 (br s, 1 H, H-1D), 4.92 (d, J = 11.2 Hz, 1 H, PhCH), 4.83 (d, J = 11.2 Hz, 1 H, PhCH), 4.75–4.59 (m, 7 H, 7 PhCH), 4.46 (d, J = 11.2 Hz, 1 H, PhCH), 4.29 (t, J = 9.8 Hz, 1 H, H-4A), 4.27–4.25 (m, 1 H, H-6aA), 4.20–4.17 (m, 1 H, H-5D), 4.10–4.07 (m, 1 H, M-5A), 4.02 (dd, J = 9.8, 3.5 Hz, 1 H, H-3C), 4.00 (br s, 1 H, H-2C), 3.96–3.91 (m, 2 H, H-2A, H-3D), 3.88–3.87 (m, 1 H, H-2B), 3.86–3.84 (m, 1 H, H-5C), 3.81 (s, 3 H, OCH 3), 3.80–3.78 (m, 1 H, H-5B), 3.75 (t, J = 10.5 Hz, 1 H, H-6bA), 3.66 (t, J = 9.8 Hz, 1 H, H-4B), 3.52–3.46 (m, 3 H, H-3A, H-3B, H-4C), 3.33 (t, J = 9.8 Hz, 1 H, H-4D), 2.15 (s, 3 H, COCH 3), 1.27 (d, J = 6.2 Hz, 3 H, CCH 3), 1.17 (d, J = 6.2 Hz, 3 H, CCH 3), 0.72 (d, J = 6.2 Hz, 3 H, CCH 3).

13C NMR (175 MHz, CDCl3): δ = 170.7 (COCH3), 155.5–114.8 (Ar-C), 102.2 (PhCH), 100.9 (C-1D), 98.8 (C-1C), 98.3 (C-1B), 97.7 (C-1A), 81.7 (C-4C), 80.5 (C-3A), 80.2 (C-4D), 80.0 (C-4B), 79.5 (C-2D), 77.9 (C-2A), 77.2 (C-2B), 75.2 (PhCH2), 75.1 (C-3C), 75.0 (PhCH2), 74.9 (PhCH2), 74.5 (C-4A), 72.6 (C-3D), 72.5 (PhCH2), 72.2 (PhCH2), 70.2 (C-5D), 68.7 (C-6A), 68.6 (C-5B), 68.5 (C-5C), 68.0 (C-2C), 64.4 (C-3B), 63.7 (C-5A), 55.7 (OCH3), 21.1 (COCH3), 18.0 (CCH3), 17.9 (CCH3), 17.1 (CCH3).

HRMS: m/z [M + H]+ calcd for C75H84N3O19: 1330.5693; found: 1330.5675.


#

Ethyl (2,4-Di-O-benzyl-3-O-picolinoyl-β-l-rhamnopyranosyl)-(1→4)-3-O-acetyl-6-O-benzyl-2-deoxy-2-phthalimido-1-thio-β-d-glucopyranoside (14)

To solution of 6 (500 mg, 1.03 mmol) and 7 (510 mg, 1.03 mmol) in anhyd CH2Cl2 (10 mL) was added MS-4Å (1 g) and the mixture was cooled to –10 °C under argon. To the cooled mixture was added solid NIS (235 mg, 1.04 mmol) under argon followed by TMSOTf (10 μL, 0.06 mmol) and it was stirred at same temperature for 3 h. The mixture was filtered through a Celite bed and washed with CH2Cl2 (30 mL). The combined filtrates were successively washed with 5% Na2S2O3 (20 mL), sat. aq NaHCO3 (20 mL), and H2O (20 mL). The collected organic layer was dried (anhyd Na2SO4), filtered, and concentrated. The crude product was purified by column chromatography (silica gel, hexane/EtOAc 2:1) to give pure 14 (615 mg, 65%) as a yellow oil.

[α]D 25 –22 (c 1.0, CHCl3).

1H NMR (700 MHz, CDCl3): δ = 8.69–6.90 (m, 23 H, Ar-H), 5.73 (t, J = 9.1 Hz, 1 H, H-3E), 5.47 (d, J = 10.5 Hz, 1 H, H-1E), 4.85 (dd, J = 9.2, 3.0 Hz, 1 H, H-3F), 4.75–4.71 (m, 2 H, 2 PhCH), 4.64 (br s, 1 H, H-1F), 4.60–4.52 (m, 3 H, 3 PhCH), 4.40 (d, J = 11.2 Hz, 1 H, PhCH), 4.24 (t, J = 10.5 Hz, 1 H, H-2E), 4.02–3.99 (m, 1 H, H-6aE), 3.90 (br s, 1 H, H-2F), 3.87 (t, J = 10.5 Hz, 1 H, H-4E), 3.74–3.69 (m, 3 H, H-4F, H-5E, H-6bE), 3.34–3.31 (m, 1 H, H-5F), 2.66–2.59 (m, 2 H, SCH 2CH3), 1.90 (s, 3 H, COCH 3), 1.29 (d, J = 6.2 Hz, 3 H, CCH 3), 1.16 (t, J = 7.4 Hz, 3 H, SCH2CH 3).

13C NMR (175 MHz, CDCl3): δ = 170.2 (COCH3), 168.8 (PhthCO), 167.5 (PhthCO), 163.9 (COPic), 149.9–123.7 (Ar-C), 102.2 (C-1F), 80.7 (C-1E), 78.7 (C-4F), 77.9 (C-5E), 77.1 (2 C, C-3F, C-4E), 75.8 (C-2F), 75.2 (PhCH2), 74.9 (PhCH2), 74.1 (C-3E), 73.3 (PhCH2), 72.1 (C-5F), 69.4 (C-6E), 54.1 (C-2E), 24.4 (SCH2CH3), 20.7 (COCH3), 17.9 (CCH3), 15.1 (SCH2 CH3).

HRMS: m/z [M + H]+ calcd for C51H53N2O12S: 917.3313; found: 917.3290.


#

Ethyl (3-O-Acetyl-2,4-di-O-benzyl-β-l-rhamnopyranosyl)-(1→4)-3-O-acetyl-6-O-benzyl-2-deoxy-2-phthalimido-1-thio-β-d-glucopyranoside (15)

To a solution of 14 (500 mg, 0.54 mmol) in CH2Cl2/CH3OH (10 mL; 4:1) was added Cu(OAc)2 (150 mg, 0.83 mmol) and the mixture was stirred at r.t. for 30 min. The solvents were removed under reduced pressure and a solution of the crude product in a mixture of Ac2O/pyridine (2 mL; 1:1) was stirred at r.t. for 1 h. The solvents were removed under reduced pressure and the crude product was purified by column chromatography (silica gel hexane/EtOAc 3:1) to give pure 15 (345 mg, 75%) as a colorless oil.

[α]D –34 (c 1.0, CHCl3).

1H NMR (700 MHz, CDCl3): δ = 7.89–7.27 (m, 19 H, Ar-H), 5.82 (t, J = 9.8 Hz, 1 H, H-3E), 5.54 (d,J = 10.5 Hz, 1 H, H-1E), 4.84 (d, J = 11.2 Hz, 1 H, PhCH), 4.70 (d, J = 11.2 Hz, 1 H, PhCH), 4.66 (dd, J = 9.8, 3.0 Hz, 1 H, H-3F), 4.65 (br s, 1 H, H-1F), 4.64–4.59 (m, 3 H, 3 PhCH), 4.47 (d, J = 11.2 Hz, 1 H, PhCH), 4.32 (t, J = 10.5 Hz, 1 H, H-2E), 4.08–4.06 (m, 1 H, H-6aE), 3.94 (t, J = 9.8 Hz, 1 H, H-4E), 3.84–3.78 (m, 3 H, H-2F, H-5E, H-6bE), 3.59 (t, J = 9.8 Hz, 1 H, H-4F), 3.37–3.31 (m, 1 H, H-5F), 2.77–2.67 (m, 2 H, SCH 2CH3), 1.93 (s, 3 H, COCH 3), 1.82 (s, 3 H, COCH 3), 1.35 (d, J = 6.2 Hz, 3 H, CCH 3), 1.27 (t, J = 7.4 Hz, 3 H, SCH2CH 3).

13C NMR (175 MHz, CDCl3): δ = 170.3 (COCH3), 170.1 (COCH3), 167.8 (COPhth), 167.4 (COPhth), 102.1 (C-1F), 80.7 (C-1E), 78.7 (C-4F), 78.0 (C-5E), 76.7 (C-3F), 76.2 (C-4E), 75.7 (C-2F), 75.1 (PhCH2), 75.0 (PhCH 2 ), 74.1 (C-3E), 73.2 (PhCH2), 72.0 (C-5F), 69.4 (C-6E), 54.1 (C-2E), 24.4 (SCH2CH3), 20.8 (COCH3), 20.6 (COCH3), 17.8 (CCH3), 15.1 (SCH2 CH3).

HRMS: m/z [M + H]+ calcd for C47H52NO12S: 854.3204; found: 854.3185.


#

p-Methoxyphenyl (3-O-Acetyl-2,4-di-O-benzyl-β-l-rhamnopyranosyl)-(1→4)-(3-O-acetyl-6-O-benzyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl)-(1→3)-(2-O-acetyl-4-O-benzyl-α-l-rhamnopyranosyl)-(1→2)-(3,4-di-O-benzyl-α-l-rhamnopyranosyl)-(1→3)-(2,4-di-O-benzyl-α-l-rhamnopyranosyl)-(1→3)-2-azido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranoside (16)

To solution of 13 (300 mg, 0.22 mmol) and 15 (200 mg, 0.23 mmol) in anhyd CH2Cl2 (8 mL) was added MS-4Å (0.5 g) and the mixture was cooled to –20 °C under argon. To the cooled mixture was added solid NIS (60 mg, 0.26 mmol) under argon followed by TMSOTf (3 μL, 0.02 mmol) and it was stirred at same temperature for 1 h. The mixture was filtered through a Celite bed and washed with CH2Cl2 (30 mL). The combined filtrates were successively washed with 5% Na2S2O3 (10 mL), sat. aq NaHCO3 (10 mL), and H2O (10 mL). The collected organic layer was dried (anhyd Na2SO4), filtered, and concentrated. The crude product was purified by column chromatography (silica gel, hexane/EtOAc 2:1) to give pure 16 (336 mg, 71%) as a yellow oil.

[α]D 25 +42 (c 1.0, CHCl3).

1H NMR (700 MHz, CDCl3): δ = 7.45–6.87 (m, 53 H, Ar-H), 5.76 (t, J = 9.8 Hz, 1 H, H-3E), 5.70 (d, J = 10.5 Hz, 1 H, H-1E), 5.54 (s, 1 H, PhCH), 5.48 (d, J = 3.5 Hz, 1 H, H-1A), 5.44 (br s, 1 H, H-2D), 5.13 (br s, 1 H, H-1B), 5.06 (br s, 1 H, H-1C), 5.04 (br s, 1 H, H-1D), 4.94 (d, J = 11.2 Hz, 1 H, PhCH), 4.85 (d, J = 11.2 Hz, 1 H, PhCH), 4.72–4.70 (m, 2 H, 2 PhCH), 4.68 (br s, 1 H, H-1F), 4.67–4.58 (m, 10 H, H-3D, H-3F, 8 PhCH), 4.43–4.40 (m, 3 H, 3 PhCH), 4.35 (dd, J = 10.5 Hz, 1 H, H-2E), 4.30 (t, J = 9.8 Hz, 1 H, H-4A), 4.27–4.25 (m, 1 H, H-6aA), 4.22 (d, J = 11.2 Hz, 1 H, PhCH), 4.18–4.12 (m, 2 H, H-4E, H-5D), 4.10–4.05 (m, 1 H, H-5A), 4.02–3.96 (m, 3 H, H-2C, H-3C, H-6aE), 3.94 (dd, J = 9.8, 3.5 Hz, 1 H, H-2A), 3.92–3.85 (m, 4 H, H-2F, H-5B, H-5C, H-6bE), 3.81 (s, 3 H, OCH 3), 3.79 (d, J = 2.8 Hz, 1 H, H-2B), 3.73 (t, J = 10.5 Hz, 1 H, H-6bA), 3.70–3.66 (m, 1 H, H-5E), 3.64 (t, J = 9.8 Hz, 1 H, H-4B), 3.58 (t, J = 9.8 Hz, 2 H, H-4C, H-4F), 3.48–3.44 (m, 2 H, H-3A, H-3B), 3.33 (t, J = 9.8 Hz, 1 H, H-4D), 3.31–3.29 (m, 1 H, H-5F), 2.08 (s, 3 H, COCH 3), 1.98 (s, 3 H, COCH 3), 1.78 (s, 3 H, COCH 3), 1.32 (d, J = 6.2 Hz, 3 H, CCH 3), 1.29 (d, J = 6.2 Hz, 3 H, CCH 3), 0.96 (d, J = 6.2 Hz, 3 H, CCH 3), 0.70 (d, J = 6.2 Hz, 3 H, CCH 3).

13C NMR (175 MHz, CDCl3): δ = 170.2 (COCH3), 170.1 (COCH3), 170.0 (COCH3), 168.0 (COPhth), 167.7 (COPhth), 155.5–114.7 (Ar-C), 102.6 (C-1F), 102.2 (PhCH), 101.0 (C-1C), 98.6 (C-1D), 98.4 (C-1B), 98.1 (C-1E), 97.7 (C-1A), 80.3 (2 C, C-3A, C-4C), 80.0 (C-4B), 79.5 (C-4D), 78.8 (C-2A), 78.0 (2 C, C-3C, C-4E), 77.9 (C-5D), 77.0 (C-4A), 76.5 (2 C, C-2F, C-4F), 75.7 (C-2B), 75.1 (PhCH2), 75.0 (2 C, C-3F, PhCH2), 74.9 (2 C, 2 PhCH2), 74.7 (PhCH2), 74.5 (C-3D), 74.2 (C-5E), 73.4 (C-3E), 73.2 (PhCH2), 72.5 (PhCH2), 72.0 (C-5F), 71.9 (PhCH2), 71.5 (C-2D), 68.7 (C-6A), 68.6 (C-5C), 68.5 (C-2C), 68.2 (C-6E), 67.8 (C-5B), 64.4 (C-3B), 63.7 (C-5A), 55.7 (OCH3), 55.2 (C-2E), 21.0 (COCH3), 20.8 (COCH3), 20.6 (COCH3), 18.0 (CCH3), 17.7 (CCH3), 17.6 (CCH3), 17.1 (CCH3).

HRMS: m/z [M + H]+ calcd for C120H129N4O31: 2121.8635; found: 2121.8613.


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p-Methoxyphenyl (3,4,6-Tri-O-acetyl-2-azido-2-deoxy-α-d-mannopyranosyl)-(1→3)-(2-O-acetyl-4-O-benzyl-α-l-rhamnopyranosyl)-(1→2)-(3,4-di-O-benzyl-α-l-rhamnopyranosyl)-(1→3)-(2,4-di-O-benzyl-α-l-rhamnopyranosyl)-(1→3)-2-azido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranoside (17)

To solution of 13 (300 mg, 0.22 mmol) and 8 (100 mg, 0.27 mmol) in anhyd CH2Cl2 (5 mL) was added MS-4Å (0.3 g) and the mixture was cooled to –30 °C under argon. To the cooled mixture was added solid NIS (65 mg, 0.29 mmol) under argon followed by TMSOTf (3 μL, 0.02 mmol) and it was stirred at same temperature for 1 h. The mixture was filtered through a Celite bed and washed with CH2Cl2 (30 mL). The combined filtrates were successively washed with 5% Na2S2O3 (10 mL), sat. aq NaHCO3 (10 mL), and H2O (10 mL). The collected organic layer was dried (anhyd Na2SO4), filtered, and concentrated. The crude product was purified by column chromatography (silica gel, hexane/EtOAc 2:1) to give pure 17 (246 mg, 68%) as a yellow oil.

[α]D 25 +37 (c 1.0, CHCl3).

1H NMR (700 MHz, CDCl3): δ = 7.48–7.24 (m, 30 H, Ar-H), 7.05 (d, J = 9.0 Hz, 2 H, Ar-H), 6.88 (d, J = 9.0 Hz, 2 H, Ar-H), 5.55 (s, 1 H, PhCH), 5.50 (d, J = 3.5 Hz, 1 H, H-1A), 5.44 (br s, 1 H, H-2D), 5.36–5.33 (m, 2 H, H-3E, H-4E), 5.22 (br s, 1 H, H-1B), 5.18 (br s, 1 H, H-1C), 5.05 (br s, 1 H, H-1D), 4.96 (br s, 1 H, H-1E), 4.94 (d, J = 11.5 Hz, 1 H, PhCH), 4.81 (d, J = 11.5 Hz, 1 H, PhCH), 4.76–4.65 (m, 6 H, 6 PhCH), 4.58 (d, J = 11.5 Hz, 1 H, PhCH), 4.49 (d, J = 11.5 Hz, 1 H, PhCH), 4.32 (t, J = 9.5 Hz, 1 H, H-4A), 4.28–4.25 (m, 1 H, H-6aA), 4.18–4.13 (m, 2 H, H-3A, H-3B), 4.11–4.07 (m, 1 H, H-5A), 4.05–4.02 (m, 3 H, H-2B, H-3C, H-3D), 3.98–3.95 (m, 1 H, H-6aE), 3.92 (dd, J = 9.5, 3.0 Hz, 1 H, H-2A), 3.90–9.86 (m, 4 H, H-2C, H-2E, H-5C, H-6bE), 3.85–3.82 (m, 1 H, H-5B), 3.81 (s, 3 H, OCH 3), 3.75 (t, J = 10.5 Hz, 1 H, H-6bA), 3.64 (t, J = 9.5 Hz, 1 H, H-4B), 3.55–3.47 (m, 3 H, H-4C, H-5D, H-5E), 3.36 (t, J = 9.5 Hz, 1 H, H-4D), 2.17 (s, 3 H, COCH 3), 2.13 (s, 3 H, COCH 3), 2.07 (s, 3 H, COCH 3), 1.93 (s, 3 H, COCH 3), 1.30 (d, J = 6.0 Hz, 3 H, CCH 3), 1.15 (d, J = 6.0 Hz, 3 H, CCH 3), 0.74 (d, J = 6.0 Hz, 3 H, CCH 3).

13C NMR (175 MHz, CDCl3): δ = 170.8 (COCH3), 170.3 (COCH3), 170.2 (COCH3), 169.3 (COCH3), 155.5–114.8 (Ar-C), 102.2 (PhCH), 100.9 (C-1D), 99.1 (C-1E), 98.3 (C-1C), 97.7 (C-1A), 94.5 (C-1B), 80.5 (C-4C), 80.4 (C-5E), 80.1 (C-4B), 79.7 (C-2C), 79.2 (C-4D), 78.6 (C-3B), 77.9 (C-3D), 75.5 (PhCH2), 75.2 (PhCH2), 75.1 (C-5C), 75.0 (PhCH2), 74.6 (C-3C), 73.3 (C-3A), 72.3 (2 C, 2 PhCH2), 71.0 (C-3E), 68.7 (2 C, C-5B, C-6A), 68.5 (2 C, C-2A, C-2B), 68.4 (C-4A), 67.8 (C-2D), 65.2 (C-4E), 64.5 (C-5D), 63.7 (C-5A), 61.4 (C-6E), 61.2 (C-2E), 55.7 (OCH3), 21.1 (COCH3), 20.7 (COCH3), 20.6 (COCH3), 20.5 (COCH3), 17.8 (CCH3), 17.7 (CCH3), 17.1 (CCH3).

HRMS: m/z [M + H]+ calcd for C87H99N6O26: 1643.6603; found: 1643.6582.


#

p-Methoxyphenyl (β-l-Rhamnopyranosyl)-(1→4)-(2-acetamido-2-deoxy-β-d-glucopyranosyl)-(1→3)-(α-l-rhamnopyranosyl)-(1→2)-(α-l-rhamnopyranosyl)-(1→3)-(α-l-rhamnopyranosyl)-(1→3)-2-acetamido-2-deoxy-α-d-glucopyranoside (1)

To a solution of 16 (200 mg, 0.09 mmol) in EtOH (15 mL) was added NH2NH2·H2O (0.1 mL, 2.0 mmol) and the solution was stirred at 80 °C for 10 h. The solvents were removed under reduced pressure and a solution of the crude product in a mixture of Ac2O/pyridine (4 mL, 1:1) was stirred at r.t. for 2 h. The solvents were removed under reduced pressure and the crude product was passed through a short pad of silica gel (EtOAc). To a solution of the crude product in pyridine (2 mL) was added CH3COSH (40 mg, 0.52 mmol) and the mixture was stirred at r.t. for 12 h. The solvents were removed under reduced pressure and co-evaporated with toluene (3 × 5 mL). A solution of the acetylated product in 0.1 M CH3ONa in CH3OH (15 mL) was stirred at r.t. for 3 h, neutralized with Dowex 50W X8 (H+) resin, filtered, and concentrated. To a solution of the de-O-acetylated product in CH3OH (10 mL) were added 20% Pd(OH)2-C (50 mg) and the mixture was stirred at r.t. for 18 h under a positive pressure of H2. The mixture was filtered through a Celite bed, washed with CH3OH/H2O (15 mL, 5:1) and concentrated under reduced pressure. The crude product was passed through Sephadex LH-20 column (CH3OH/H2O 3:1) to give pure 1 (32 mg, 31%) as a white powder; mp 55–58 °C.

[α]D 25 –26 (c 1.0, H2O).

1H NMR (500 MHz, D2O): δ = 7.15 (d, J = 9.0 Hz, 2 H, Ar-H), 7.00 (d, J = 9.0 Hz, 2 H, Ar-H), 5.46 (d, J = 3.0 Hz, 1 H, H-1A), 5.18 (br s, 1 H, H-1C), 4.99 (br s, 1 H, H-1D), 4.92 (br s, 1 H, H-1B), 4.89 (br s, 1 H, H-1F), 4.73 (d, J = 8.0 Hz, 1 H, H-1E), 4.32 (br s, 1 H, H-2D), 4.24 (dd, J = 9.5, 3.0 Hz, 1 H, H-2A), 4.12–4.03 (m, 3 H, H-2B, H-2C, H-5A), 4.02–3.92 (m, 4 H, H-2F, H-3A, H-6aA, H-6аE), 3.90–3.83 (m, 4 H, H-3C, H-5C, H-6bA, H-6bE), 3.82 (s, 3 H, OCH 3), 3.81–3.77 (m, 2 H, H-2E, H-3D), 3.76–3.67 (m, 5 H, H-3B, H-3E, H-4E, H-5B, H-5D), 3.63–3.48 (m, 5 H, H-3F, H-4A, H-4B, H-4D, H-5E), 3.44–3.35 (m, 3 H, H-4C, H-4F, H-5F), 2.09 (s, 3 H, COCH 3), 2.06 (s, 3 H, COCH 3), 1.34 (d, J = 6.2 Hz, 6 H, 2 CCH 3), 1.33 (d, J = 6.2 Hz, 6 H, 2 CCH 3).

13C NMR (125 MHz, D2O): δ = 174.9 (COCH3), 173.9 (COCH3), 154.6–115.0 (Ar-C), 102.6 (C-1E), 101.8 (C-1D), 101.3 (C-1B), 100.8 (C-1C), 100.7 (C-1F), 96.9 (C-1A), 79.9 (C-3A), 79.5 (C-3D), 77.6 (C-2D), 77.2 (C-3B), 76.7 (C-2C), 74.3 (C-4E), 73.4 (C-2B), 72.8 (C-4D), 72.6 (C-4B), 72.2 (C-4C), 72.1 (C-3E), 71.9 (C-5E), 71.6 (C-3F), 70.8 (C-5F), 70.6 (C-4F), 70.58 (C-5D), 69.9 (C-5A), 69.6 (C-2F), 69.4 (C-4A), 69.1 (C-3C), 68.9 (C-5C), 68.1 (C-5B), 60.7 (C-6E), 60.3 (C-6A), 55.8 (2 C, C-2E, OCH3), 53.1 (C-2A), 22.1 (COCH3), 21.9 (COCH3), 16.7 (CCH3), 16.6 (CCH3), 16.5 (CCH3), 16.4 (CCH3).

HRMS: m/z [M + H]+ calcd for C47H75N2O28: 1115.4500; found: 1115.4478.


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p-Methoxyphenyl (2-Acetamido-2-deoxy-α-d-mannopyranosyl)-(1→3)-(α-l-rhamnopyranosyl)-(1→2)-(α-l-rhamnopyranosyl)-(1→3)-(α-l-rhamnopyranosyl)-(1→3)-2-acetamido-2-deoxy-α-d-glucopyranoside (2)

To a solution of 17 (200 mg, 0.12 mmol) in pyridine (5 mL) was added CH3COSH (100 mg, 1.31 mmol) and the mixture was stirred at r.t. for 24 h. The solvents were removed under reduced pressure and co-evaporated with toluene (3 × 5 mL). A solution of the product in 0.1 M CH3ONa in CH3OH (15 mL) was stirred at r.t. for 3 h, neutralized with Dowex 50W X8 (H+) resin, filtered, and concentrated. To a solution of the de-O-acetylated product in CH3OH (10 mL) was added 20% Pd(OH)2-C (50 mg) and the mixture was stirred at r.t. for 10 h under a positive pressure of H2. The mixture was filtered through a Celite bed, washed with CH3OH/H2O (15 mL, 5:1) and concentrated under reduced pressure. The crude product was passed through Sephadex LH-20 column (CH3OH/H2O 3:1) to give pure 2 (40 mg, 34%) as a white powder; mp 60–65 °C.

[α]D 25 –38 (c 1.0, H2O).

1H NMR (500 MHz, D2O): δ = 7.14 (d, J = 9.0 Hz, 2 H, Ar-H), 7.00 (d, J = 9.0 Hz, 2 H, Ar-H), 5.46 (d, J = 3.5 Hz, 1 H, H-1A), 5.22 (br s, 1 H, H-1C), 5.02 (br s, 2 H, H-1D, H-1E), 4.92 (br s, 1 H, H-1B), 4.41 (d, J = 4.0 Hz, 1 H, H-2E), 4.31 (br s, 1 H, H-2D), 4.24 (dd, J = 9.5, 3.5 Hz, 1 H, H-2A), 4.15–4.07 (m, 3 H, H-2C, H-3C, H-3D), 4.02–3.92 (m, 3 H, H-3A, H-3B, H-5E), 3.90–3.82 (m, 8 H, H-2B, H-3E, H-4E, H-5A, H-6abA, H-6abE), 3.80 (s, 3 H, OCH 3), 3.79–3.73 (m, 2 H, H-5C, H-5D), 3.70 (t, J = 9.5 Hz, 2 H, H-4A, H-4D), 3.58 (t, J = 9.5 Hz, 1 H, H-4B), 3.54 (t, J = 9.5 Hz, 1 H, H-4C), 3.52–3.48 (m, 1 H, H-5B), 2.08 (s, 6 H, 2 COCH 3), 1.35–1.27 (3 d, J = 6.0 Hz, 9 H, 3 CCH 3).

13C NMR (125 MHz, D2O): δ = 174.8 (COCH3), 173.9 (COCH3), 154.6–115.6 (Ar-C), 102.1 (C-1C), 101.3 (C-1B), 100.7 (C-1D), 96.9 (C-1A), 95.1 (C-1E), 79.5 (C-3B), 78.3 (C-2B), 77.1 (C-3E), 74.2 (C-4E), 72.8 (C-2C), 72.2 (C-3A), 72.0 (C-5B), 71.7 (C-4C), 70.6 (C-4B), 70.2 (C-5A), 69.9 (C-5E), 69.1 (C-3D), 68.9 (C-3C), 68.8 (2 C, C-5C, C-5D), 68.1 (C-4A), 66.5 (C-4D), 65.9 (C-2D), 60.3 (C-6E), 60.2 (C-6A), 55.9 (OCH3), 53.1 (C-2A), 52.8 (C-2E), 21.9 (COCH3), 21.8 (COCH3), 16.7 (CCH3), 16.6 (CCH3), 16.4 (CCH3).

HRMS: m/z [M + H]+ calcd for C41H65N2O24: 969.3921; found: 969.3900.


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Conflict of Interest

The authors declare no conflict of interest.

Supporting Information

    Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/s-0042-1751559.
  • Supporting Information

Corresponding Author

Anup Kumar Misra
Bose Institute, Department of Chemical Sciences
Block EN-80, Sector-V, Salt Lake, Kolkata 700091
India   

Publication History

Received: 01 January 2024

Accepted after revision: 26 January 2024

Article published online:
14 February 2024

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Figure 1 (A) Structure of the repeating unit of the O-polysaccharide of A. baumannii O7[13] and (B) structure of the repeating unit of the O-polysaccharide of A. baumannii O10[14]
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Figure 2 Retrosynthetic analysis for the synthesis of penta- and hexasaccharides 1 and 2 and their synthetic intermediates
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Scheme 1 Reagents and conditions: (a) NIS, TMSOTf, CH2Cl2, MS-4Å, –30 °C, 1 h, 9: 78%, 11: 77%; (b) BnBr, NaOH, TBAB, DMF, r.t., 2 h; (c) DDQ, CH2Cl2/H2O (5:1), r.t., 4 h, 66% (2 steps); (d) CH3ONa, CH3OH, r.t., 2 h, 91%; (e) NIS, TMSOTf, CH2Cl2, MS-4Å, –20 °C, 1 h, then r.t., 30 min, 66%.
Zoom Image
Scheme 2 Reagents and conditions: (a) NIS, TMSOTf, CH2Cl2, MS-4Å, –10 °C, 3 h, 65%; (b) (i) Cu(OAc)2, CH2Cl2/CH3OH (4:1), r.t., 30 min; (ii) Ac2O, pyridine, r.t., 1 h, 75% (2 steps).
Zoom Image
Scheme 3 Reagents and conditions: (a) NIS, TMSOTf, CH2Cl2, MS-4Å, –20 °C, 1 h, 71%; (b) NIS, TMSOTf, CH2Cl2, MS-4Å, –30 °C, 1 h, 68%; (c) NH2NH2·H2O, EtOH, 80 °C, 10 h; (d) Ac2O, pyridine, r.t., 2 h; (e) CH3COSH, pyridine, r.t., 12 h; (f) 0.1 M CH3ONa, CH3OH, r.t., 3 h; (g) H2, 20% Pd(OH)2-C, CH3OH, r.t., 18 h, 31% (5 steps); (h) CH3COSH, pyridine, r.t., 24 h; (i) CH3ONa, CH3OH, r.t., 3 h; (j) H2, 20% Pd(OH)2-C, CH3OH, r.t., 8 h, 34% (3 steps).