Orthogonal cleavage of the 2-naphthylmethyl group in the presence of the p-methoxy phenyl-protected anomeric position and its use in carbohydrate synthesis

Abstract

Orthogonal removal of naphthylmethyl (NAP) and anomeric O-p-methoxyphenyl (PMP) ethers using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and cerium(IV) ammonium nitrate, respectively, is described. These reactions were tested in the oligosaccharide assembly of biologically relevant motifs, such as α-D-Man-(1,3)-[α-D-(1,6)-Man]-α-D-Man, α-D-Fuc-(1,2)-α-D-Fuc and α-D-NeuNAc-(2,3)-β-D-Gal. The usefulness of these chemoselective deprotections was proven in the synthesis of high mannoses.

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Introduction

Aromatic protecting groups are largely used in carbohydrate chemistry, where typically building blocks need temporary protections to mask hydroxyls which are not involved in the formation of the glycosidic bond.¹ Protecting groups can aid the control of stereoselectivity and modulate the reactivity of the coupling partners during this step. The choice of the protecting groups is, therefore, crucial in these multi-step syntheses.² Aromatic ethers can be removed at the end of the oligosaccharide assembly by catalytic hydrogenation, which makes their use very popular.²

Electron-rich aromatic ethers are preferable to ester protecting groups as arm building blocks utilized in carbohydrate synthesis or when the control of stereoselectivity needs to exclude the anchimeric assistance, as in the formation of 1,2 cis-glycosidic linkages.²

In particular the NAP (2-naphthylmethyl) group is becoming very attractive for the possibility of easy removal in the presence of other aromatic protections, such as benzyl groups, by oxidative cleavage with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone),^3,4 catalytic hydrogenation⁵ or under acidic conditions.⁶

NAP and PMB (p-methoxybenzyl) can be orthogonally removed when used for the protection of non-anomeric hydroxyls by means of DDQ and CAN (cerium(IV) ammonium nitrate), respectively.^7,8 NAP can be cleaved while leaving PMB intact by fine tuning Pd/C-catalyzed hydrogenolysis.⁵ Conversely, taking advantage of PMB liability under acidic conditions, this group can be removed without impacting NAP by means of HCl and HFIP (hexafluoro-2-propanol).⁶

Due to its acid sensitivity, PMB is rarely used to protect the anomeric positions, and PMP (p-methoxyphenyl) is generally preferred. After the release of the PMP protection, the anomeric hydroxyl is usually transformed into an appropriate leaving group, such as imidate or phosphate (Scheme 1), to obtain a glycosyl donor ready for glycosylation.⁹ Direct conversion of glycosyl p-methoxyphenol into the corresponding thiol donor has also been described.¹⁰ Hence, PMP is generally used to protect the C-1 position in acceptors that are converted into donors at a later stage of the synthetic strategy.

Scheme 1 Usefulness of orthogonal removal of NAP and MP protections in carbohydrate synthesis.

Here we show that NAP can be selectively removed in the presence of the PMP-protected anomeric position, and vice versa. Next, this feature was proven useful in oligosaccharide assembly.

Results and discussion

Initially we observed that oxidative cleavage of NAP with DDQ in galactopyranoside 1a gave compound 1b without loss of the PMP anomeric protection, as demonstrated by the presence in the ¹H NMR spectrum of a singlet at 3.77 ppm assigned to the OCH₃ signal.

Conversely, treatment of 1a with CAN provided the product 1c, where the two α- and β-anomeric positions appeared in the ¹H NMR spectrum at 5.66 (J_1,2 3.6 Hz) and 4.78 (J_1,2 8.0 Hz), respectively.

To better explore these orthogonal deprotections, we synthesized the set of monosaccharides depicted in Table 1 (procedures are reported in the ESI†). Typically, treatment of the substrate with 5 equiv. of DDQ in a CH₂Cl₂–MeOH mixture for 5–6 h gave selective cleavage of the NAP protection in good yield. Of note, more prolonged reaction time (<6 h) for NAP removal in compound 7a led to the loss of the 4-O-benzyl group. This was confirmed by acetylation of the formed product and appearance in the ¹H NMR spectrum of a triplet (J 9.8 Hz) at 5.48 ppm, which was unambiguously assigned to H-4.

Table 1 Orthogonal cleavage of NAP in the presence of anomeric PMP^a

Entry	Substrate	NAP removal	PMP removal
a For procedures see the Experimental section and the ESI.
1
2
3
4
5
6
7
8
9
10

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Occurrence of debenzylation with DDQ either in the presence or absence of water has been well documented,¹¹ hence it needs to be taken into account as a possible side reaction.

For PMP removal, the use of 5–6 equiv. of CAN in 4 : 1 acetonitrile–water for 6 h provided the 1-OH products in moderate to good yields for all the tested compounds (Table 1),¹² without impairing the NAP protection. The moderate yield of some examples could be ascribed to the formation of a quinone adduct as a by-product, which has been reported in this reaction.¹³ As shown in Table 2 for compound 2a, the reaction occurred giving a higher yield when acetonitrile–water was used as the solvent mixture instead of acetone–water.

Table 2 Optimization of the reactions

Entry	Substrate	Conditions	Product	Yield
1	2a	DDQ (3 eq.), CH2Cl2–MeOH, 6 h, r.t.	2b	35%
2	2a	DDQ (5 eq.), CH2Cl2–MeOH, 6 h, r.t.	2b	78%
3	2a	CAN (3 eq.), CH3CN–H2O, 6 h, r.t.	2c	28%
4	2a	CAN (5 eq.), (CH3)2CO–H2O, 6 h, r.t.	2c	45%
5	2a	CAN (5 eq.), CH3CN–H2O, 12 h, r.t.	2c	53%
6	2a	CAN (5 eq.), CH3CN–H2O, 6 h, 0 °C	2c	68%
7	7a	CAN (5 eq.), CH3CN–H2O, 6 h, r.t.	7c	15%
8	7a	CAN (5 eq.), CH3CN–H2O, 6 h, 0 °C	7c	58%
9	8a	CAN (5 eq.), CH3CN–H2O, 6 h, r.t.	8c	28%
10	8a	CAN (5 eq.), CH3CN–H2O, 6 h, 0 °C	8c	77%

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Furthermore a reaction temperature of 0 °C avoided the formation of by-products. Particularly for substrates 7a and 8a concomitant loss of the NAP protection was observed as a side reaction (Table 2, entries 7–10). After acetylation of the by-products the ¹H NMR signals related to the H-3 positions were shifted to a region between 5.45 and 5.30 ppm, clearly indicating that NAP deprotection had taken place.

Under the optimized conditions esters (Ac, Lev, Table 1, entries 1–4), benzyl ethers (Table 1, entries 5–9), benzylidene (Table 1, entries 4–6) and silyl groups (Table 1, entries 7 and 8) were untouched. In contrast, isopropylidene protection was proven labile with both DDQ and CAN because of the intrinsic acidity of the reagents (Table 1, entry 10), as manifested in the ¹H NMR spectrum by the disappearance of the two signals related to the CH₃ groups at 1.32 and 1.31 ppm, respectively.

Having established the orthogonality of the two protective groups, we assessed the applicability of the method to generate useful glycosyl donors.

We focused our attention to the 3-ONAP mannoside 7a, which was used as the acceptor to form the α-D-Man-(1,3)-[α-D-(1,6)-Man]-α-D-Man trisaccharide motif overexpressed in cancer cells,^14–16 and which is contained in high mannoses, a family of oligomannose containing glycans highly expressed in mammalian cells and HIV.^17–19 To obtain acceptor 11 with the two OH groups at positions 2 and 6 available for glycosylation, compound 7a was first subjected to TBDPS removal (Scheme 2) and then to NAP cleavage. The attempt to remove the TBDPS group from 7b caused migration of the 2-O-acetyl group to position 3 during the treatment with TBAF. Glycosylation of the acceptor with trichloroacetimidate 12²⁰ in the presence of TMSOTf as the promoter afforded the desired trisaccharide 13 uneventfully.

Scheme 2 Application of PMP protected sugars as acceptors. Reaction conditions: a. TBAF, THF; DDQ, MeOH–CH₂Cl₂, 71%; b. TMSOTf, CH₂Cl₂, −20 °C, 78%; c. Cl₃CCN, DBU, CH₂Cl₂, 91%; d. CH₂Cl₂, TMSOTf, CH₂Cl₂, −20 °C, 82%; e. NIS–TfOH, CH₂Cl₂, −40 °C, 65%.

Next, we utilized the fucoside building block 9c to generate the α-D-Fuc-(1,2)-α-D-Fuc motif, found in the Schistosoma mansoni complex glycan.²¹ After removing the PMP protection with CAN, trichloroacetimidate 14 was prepared in high yield (Scheme 2) by a classic reaction with Cl₃CCN and DBU. TMSOTf promoted glycosylation of PMP protected acceptor 9b with the attained donor 14 provided disaccharide 15 with exclusive α-stereoselectivity, as proved by the J_1,2 = 3.6 Hz at 4.89 ppm for the newly generated glycosidic linkage in the ¹H NMR spectrum.²² The non-participating NAP group exhibited a favourable effect on the stereocontrol of the reaction.

In another example, 3-OH galactoside 5b prepared from 5a (Table 1) was used as the acceptor of the N-acetyloxazolidinone sialic donor 16 to synthesize the α-D-NeuNAc-(2,3)-β-D-Gal disaccharide 17 which is present in a variety of mammalian and microbial glycans (Scheme 2). The NIS–TfOH promoted reaction gave the α-anomer stereoselectively. The H-3e signal at 2.89 ppm in the ¹H NMR spectrum was compared with a similar disaccharide synthesized by Hsu et al.,²³ where H-3e was at 2.89 ppm. Furthermore, the C-1 chemical shift of 169 ppm and ³J(C-1, H-3eq) of 6.0 were in perfect agreement with the assigned configuration.

Finally we tested the deprotection in the context of the multistep synthesis of high mannoses (Scheme 3). First, the PMP deprotected mannoside 8c was converted into the corresponding trichloroacetimidate donor 18 by treatment of the 1-OH sugar with Cl₃CCN and DBU. After coupling the linker through a TMSOTf promoted reaction in good yield (→19), removal of the NAP protection gave the acceptor 20 ready for glycosylation with donor 12.²⁰

Scheme 3 Application of NAP deprotection in multistep synthesis of Man6. Reaction conditions: a. Cl₃CCN, DBU, CH₂Cl₂, 67%; b. TMSOTf, CH₂Cl₂, −30 °C, 73% from linker, 91% from 12; c. DDQ, MeOH–CH₂Cl₂, 78%; d. NaOMe, MeOH, 94%; e. NIS–TfOH, CH₂Cl₂, −30 °C, 88% from 22, 81% from 25; f. TBAF, THF, 91%; g. NaOMe, MeOH, then H₂, Pd–C, 74%.

Subsequent methanolysis of the acetyl ester from the synthesized disaccharide 21 made the 2-OH position of 22 available for further elongation. Glycosylation of 22 with the known donor 23,²⁴ using NIS–TfOH as the promoter, gave the tetramannoside 24. Desilylation of the latter compound with TBAF in THF gave acceptor 25, which was again glycosylated with 23 ²⁴ under the aforementioned conditions to afford the hexamannosides 26. Deacetylation under Zemplen conditions and Pd–C catalyzed hydrogenolysis yielded the so called Man6 glycan 27. The NMR spectra of the final compound were in agreement with those reported in the literature.²⁵

Conclusions

In summary, we developed a general protocol for orthogonal removal of the NAP protection in the presence of an anomeric PMP group using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and cerium(IV) ammonium nitrate, respectively. We demonstrated that these chemoselective reactions are useful for sequential oligosaccharide assembly of biologically relevant motifs, such as α-D-Man-(1,3)-[α-D-(1,6)-Man]-α-D-Man, α-D-Fuc-(1,2)-α-D-Fuc and α-D-NeuNAc-(2,3)-β-D-Gal. In the case of Fuc-α-(1,2)-Fuc disaccharide the 2-ONAP protection governed the stereocontrol of the reaction. The usefulness of orthogonal cleavage of NAP and PMP protection was proved in the synthesis of a hexamannose (Man6), belonging to high mannose glycans.^17–19 We expect that the reported finding will find applications in synthetic strategies leading to variously substituted glycans.

Experimental

General procedure for NAP removal

To a solution of sugar (0.1 mmol) in 4 : 1 CH₂Cl₂–MeOH (5 ml) DDQ (5 equiv.) was added. The mixture was stirred for 6 h, when TLC (cyclohexane–EtOAc) showed disappearance of the starting material. The crude mixture was diluted with CH₂Cl₂ (15 ml) and extracted three times with aq. NaHCO₃. Combined organic layers were evaporated under vacuum and purified on silica gel (cyclohexane–EtOAc) to provide the deprotected compound.

General procedure for PMP removal

To a solution of sugar (0.1 mmol) in 4 : 1 (or 9 : 1) CH₃CN–H₂O (5 ml), CAN (5 equiv.) was added. The mixture was stirred for 6 h at 0 °C or room temperature, then TLC (cyclohexane–EtOAc) showed the completion of the reaction. The crude mixture was diluted with CH₂Cl₂ (15 ml) and extracted three times with aq. NaCl. Combined organic layers were evaporated under vacuum and purified on silica gel (cyclohexane–EtOAc) to provide the deprotected compound.

4-Methoxyphenyl 2-O-acetyl-3,4,6-tri-O-benzyl-α-D-mannopyranosyl-(1,3)-[2-O-acetyl-3,4,6-tri-O-benzyl-α-D-(1,6)-mannopyranosyl]-2-O-acetyl-4-O-benzyl-α-D-mannopyranoside 13

To a solution of mannoside donor 12 (365 mg, 2.5 mmol) and acceptor 11 (100 mg, 0.23 mmol) in dry CH₂Cl₂ (5 ml) containing 4 Å MS (250 mg), TMSOTf (2 μl, 0.01 mmol) was added at −20 °C under nitrogen. The mixture was stirred for 30 min, and then TLC (4 : 1 cyclohexane–EtOAc) showed the completion of the reaction. The crude was quenched with TEA, concentrated and chromatographed on silica gel to give 13 (245 mg, 78%) as a transparent oil. [α]²⁵_D = +48.7° (c 0.33, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): 7.41–6.80 (m, 39H, H–Ar), 5.54 (t, J = 1.7 Hz, 1H, H-2^C), 5.47 (dd, J = 1.6, 2.4 Hz, 1H, H-2^B), 5.45 (d, 1H, H-1^B), 5.38 (dd, J = 1.4, 3.1 Hz, 1H, H-2^A), 5.29 (d, 1H, H-1^A), 5.00 (d, 1H, H-1^C), 4.93–4.51 (m, 14H, 7 × CH₂Ph), 4.48–4.47 (m, 1H), 4.05–3.98 (m, 3H), 3.95–3.83 (m, 6H), 3.82–3.71 (m, 8H), 3.66 (s, 3H, OCH₃), 2.22, 2.20 (2 s, 3H each, 2 × CH₃CO). ¹³C NMR (CDCl₃): δ 170.4, 170.3, 170.2 (3 × CO), 155.1–114.5 (C–Ar), 102.0 (C-1^A), 97.9 (C-1^B), 95.8 (C-1^C), 80.0, 77.9, 77.7, 75.3, 75.2, 75.0, 74.3, 74.0, 74.5, 73.4, 72.5, 71.8, 71.7, 71.6, 71.5, 68.9, 68.7, 68.5, 68.4, 55.5 (OCH₃), 21.1, 21.0 (3 × CH₃CO). HR ESI MS C₈₀H₈₆O₂₀: [M + H]⁺ calc. 1367.5642; found 1367.5791.

4-Methoxyphenyl 3,4-O-benzyl-α-D-fucopyranosyl-(1,2)-3,4-O-benzyl-2-O-naphtylmethyl-α-D-fucopyranoside 15

Fucoside donor 14 (85 mg, 0.13 mmol) and acceptor 9b (50 mg, 0.1 mmol) were dissolved in dry CH₂Cl₂ (5 ml) containing 4 Å MS (250 mg). After stirring for 15 min under nitrogen at −20 °C, TMSOTf (1 μl, 0.005 mmol) was added. Stirring was prolonged for 30 min, and then TLC (9 : 1 cyclohexane–EtOAc) showed the completion of the reaction. The crude was quenched with TEA, concentrated and chromatographed on silica gel to give 15 (79 mg, 85%) as a syrup. [α]²⁵_D = +72.5° (c 0.60, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): 7.39–6.63 (m, 32H, H–Ar), 5.51 (d, J = 3.6 Hz, 1H, H-1^A), 4.99 (d, J = 3.3 Hz, 1H, H-1^B), 4.96–4.50 (m, 10H, 5 × CH₂Ar), 4.56 (dd, J = 3.4, 10.5 Hz, H-2^A), 4.15 (dd, J = 2.7, 9.9 Hz, 1H, H-3^A), 4.11–4.07 (m, 1H, H-5^B), 4.02–3.98 (m, 1H, H-5^A), 4.02 (dd, J = 2.3, 9.3 Hz, H-2^B), 3.91 (dd, J = 7.0, 8.9 Hz, 1H, H-3^B), 3.70–3.55 (d, 1H, H-4^B), 3.63 (s, 3H, OCH₃), 1.09 (d, J = 6.0 Hz, 3H, H-6^B), 0.90 (d, J = 6.3 Hz, 3H, H-6^A). ¹³C NMR (CDCl₃, 100 MHz): δ 155.8–114.4 (C–Ar), 95.4 (C-1^A), 94.4 (C-1^B), 83.8 (C-5^A,B), 82.2 (C-3^B), 82.5 (C-3^A), 78.2 (C-4^A), 77.4 (C-4^B), 76.0, 74.7, 73.0, 72.6, 70.7 (CH₂Ar), 67.0 (C-2^B), 63.3 (C-2^A), 55.6 (OCH₃), 16.6, 16.3 (C-6^A,B). HR ESI MS C₅₈H₆₀O₁₀: [M + H]⁺ calc. 939.4084; found 939.4056.

4-Methoxyphenyl 2-O-benzyl-4,6-O-benzylidene-3-O-(methyl 5 acetamido-7,8,9-tetra-5-N,4-O-carbonyl-3,5-dideoxy-D-glycero-α-D-galacto-non-2-ulopyranosylonate)-D-galactopyranoside 17

Galactoside acceptor 5b (40 mg, 0.08 mmol) and silayl donor 16 (57 mg, 0.1 mmol) were stirred in CH₂Cl₂ (4 ml) containing 4 Å MS (200 mg) at −60 °C for 15 min. NIS (23 mg, 0.1 mmol) and TfOH (7 μl, 0.08 mmol) were added, and stirring was continued for 30 min at −30 °C then TLC (1 : 1 toluene–EtOAc) showed the completion of the reaction. The mixture was neutralized with TEA, filtered and concentrated. Chromatography of the residue (cyclohexane–EtOAc) gave 50 mg of product 17 (65%). [α]²⁵_D = +34.0° (c 0.22, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): 7.54–6.82 (m, 14H, H–Ar), 5.70 (s, 1H, PhCH), 5.58–5.54 (m, 1H, H-8^B), 4.96–4.89 (m, 3H, PhCHa, H-6^B, incl. d, 4.88, J = 8.6 Hz, H-1^A), 4.80–4.55 (m, 3H, PhCHb, H-6a^A, H-4^B), 4.49–4.43 (m, 1H, H-7^B), 4.39–4.19 (m, 2H, H-3^A, 6b^A), 4.39–4.19 (dd, J = 2.7, 9.9 Hz, 1H, H-3^A), 4.11–4.07 (m, 1H, H-5^B), 3.98–3.78 (m, 1H, H-2^A,9B, incl. s, 3.79, OCH₃), 3.66–3.30 (m, 2H, H-5^A,B), 2.89 (dd, J = 3.3, 12.1, H-3e^B), 2.20–2.05 (m, 14H, H-3a^B, 4 × CH₃CO). ¹³C NMR (CDCl₃): δ 171.9, 171.4, 171.0, 169.8, 165.7 (5 × CO), 138.5–126.2 (C–Ar), 102.9 (C-1^B), 101.8 (PhCH), 76.9, 76.4, 76.0, 75.5 (PhCH₂), 76.4, 74.2, 73.5, 70.4, 69.2 (C-6^A), 66.3, 63.2 (C-9^B), 60.4, 59.2 (OCH₃), 53.7 (C-5^C), 52.8 (COOCH₃), 36.6 (C-3^B), 24.5, 21.1, 20.8, 20.7 (4 × CH₃CO). HR ESI MS C₄₆H₅₁NO₁₉: [M + H]⁺ calc. 922.3134; found 922.3166.

3-(Benzyloxycarbonyl)aminopropyl 2-O-acetyl-3,4,6-tri-O-benzyl-α-D-mannopyranosyl-(1,3)-2,4-di-O-benzyl-6-O-tertbutyldiphenyl silyl-α-D-mannopyranoside 21

To a solution of acceptor 20 (1 g, 1.26 mmol) and donor 12 (1.04 g, 1.64 mmol) in CH₂Cl₂ (20 ml) containing preactivated 4 Å MS, TMSOTf (14 μl, 0.08 mmol) was added at −30 °C. The mixture was stirred for 30 min, when TLC (7 : 3 cyclohexane–EtOAc) showed the completion of the reaction. The mixture was neutralized with TEA, filtered and concentrated. Purification of the residue on silica gel (cyclohexane–EtOAc) gave the product 21 (2.1 g, 94%). [α]²⁵_D = +30.0° (c 2.4, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): 7.87–6.25 (m, 39H, H–Ar), 5.52 (br. s, 1H, H-2^B), 5.25 (s, 1H, H-1^B), 5.13, 5.09 (2 d, J = 11.7 Hz, 2H, CH₂^Cbz), 4.95–4.51 (m, 11H, 5 × CH₂Ph, incl. s, 4.88, H-1^A), 4.16–3.77 (m, 8H), 3.76–3.66 (m, 4H), 3.47–3.42 (s, 1H, H-1′b), 3.27–3.19 (m, 2H, H-3′), 2.11 (s, 3H, CH₃CO), 1.77–1.72 (m, 2H, H-2′), 1.12 (s, 9H, tBuSi). ¹³C NMR (CDCl₃): δ 163.5, 156.3 (2 × CO), 138.6–127.8 (C–Ar), 99.7 (C-1^B), 97.3 (C-1^A), 78.8, 78.0, 75.1, 74.9, 74.8, 74.3, 73.7, 73.2, 72.9, 72.3, 72.1, 69.2, 68.9, 66.5 (C-6^A), 65.1 (C-6^B), 63.1 (C-1′), 38.5 (C-3′), 29.5 (C-2′), 26.8 (tBuSi), 21.0 (CH₃CO). HR ESI MS C₇₆H₈₃NO₁₆Si: [M + Na]⁺ calc. 1268.5531; found 1268.5543.

3-(Benzyloxycarbonyl)aminopropyl 3,4,6-tri-O-benzyl-α-D-mannopyranosyl-(1,3)-2,4-di-O-benzyl-6-O-tertbutyldiphenylsilyl-α-D-mannopyranoside 22

Compound 21 (1 g, 0.8 mmol) was dissolved in MeOH (10 ml) and NaOMe was added until the pH was 9–10. The mixture was stirred overnight, then neutralized with Dowex H+ and filtered. The filtrate was concentrated and purified on silica gel (cyclohexane–EtOAc) to provide the 2-OH compound 22 (910 mg, 84%). [α]²⁵_D = +15.8° (c 2.05, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): 7.85–6.27 (m, 39H, H–Ar), 5.32 (s, 1H, H-1^B), 5.20, 5.11 (2 d, J = 11.7 Hz, 2H, CH₂^Cbz), 4.97–4.57 (m, 11H, 5 × CH₂Ph, incl. s, 4.91, H-1^A), 4.19–3.90 (m, 9H), 3.80–3.71 (m, 4H), 3.47–3.41 (s, 1H, H-1′b), 3.31–3.21 (m, 2H, H-3′), 2.56 (br. s, 1H, OH), 1.80–1.73 (m, 2H, H-2′), 1.17 (s, 9H, tBuSi).¹³C NMR (CDCl₃): δ 156.4 (CO), 138.6–126.5 (C–Ar), 101.6 (C-1^B), 97.5 (C-1^A), 80.4, 78.2, 75.0, 74.9, 74.8, 74.5, 73.4, 73.3, 72.4, 72.0, 71.9, 69.4, 68.8, 66.5 (C-6^A), 65.3 (C-6^B), 63.1 (C-1′), 38.6 (C-3′), 29.6 (C-2′), 26.8 (tBuSi). HR ESI MS C₇₄H₈₁NO₁₂Si: [M + Na]⁺ calc. 1226.5461; found 1226.5433.

3-(Benzyloxycarbonyl)aminopropyl 2-O-acetyl-3,4,6-tri-O-benzyl-α-D-mannopyranosyl-(1,2)-3,4,6-tri-O-benzyl-α-D-manno pyranosyl-(1,2)-3,4,6-tri-O-benzyl-α-D-mannopyranosyl-(1,3)-2,4-di-O-benzyl-6-O-tertbutyldiphenylsilyl-α-D-mannopyranoside 24

Disaccharide acceptor 22 (1 g, 0.98 mmol) and donor 23 (1 g, 0.81 mmol) were stirred in CH₂Cl₂ (40 ml) containing 4 Å MS (2 g) at −40 °C for 15 min. After addition of NIS (275 mg, 1.2 mmol) and TfOH (10 μl, 0.11 mmol) stirring was continued for 30 min at −30 °C when TLC (7 : 3 cyclohexane–EtOAc) showed the completion of the reaction. The mixture was neutralized with TEA, filtered and concentrated. Chromatography of the residue (cyclohexane–EtOAc) gave 1.5 g of 24 (88%). [α]²⁵_D = +18.9° (c 2.15, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): 7.77–7.03 (m, 70H, H–Ar), 5.63 (br. s, 1H, H-2^D), 5.34 (s, 1H, H-1^B), 5.28 (s, 1H, H-2^C), 5.23–5.11 (m, 3H, CH₂^Cbz, NH), 4.98–4.32 (m, 22H, 11 × CH₂Ph, incl. s, 4.86, H-1^A), 4.16–3.47 (m, 21H), 3.47–3.41 (s, 1H, H-1′b), 3.24–3.09 (m, 2H, H-3′), 2.21 (s, 3H, CH₃CO), 1.77–1.71 (m, 2H, H-2′), 1.12 (s, 9H, tBuSi). ¹³C NMR (CDCl₃): δ 170.2, 156.3 (2 × CO), 138.8–126.5 (C–Ar), 101.2 (C-1^B), 100.5 (C-1^C), 99.4 C-1^D), 97.2 (C-1^A), 80.3, 79.4, 78.3, 75.2, 75.0, 74.5, 74.7, 74.2, 73.4, 73.0, 72.7, 72.4, 72.0, 71.9, 69.8, 69.2, 68.7, 66.5, 65.2, 63.2 (C-1′), 38.4 (C-3′), 29.6 (C-2′), 26.8 (tBuSi), 21.2 (CH₃CO). HR ESI MS C₁₃₀H₁₄₁NO₂₄Si: [M + Na]⁺ calc. 2150.9511; found 2150.9525.

3-(Benzyloxycarbonyl)aminopropyl 2-O-acetyl-3,4,6-tri-O-benzyl-α-D-mannopyranosyl-(1,2)-3,4,6-tri-O-benzyl-α-D-manno pyranosyl-(1,2)-3,4,6-tri-O-benzyl-α-D-mannopyranosyl-(1,3)-2,4-di-O-benzyl-α-D-mannopyranoside 25

Tetrasaccharide 24 (1.5 g, 0.7 mmol) was dissolved in 30 ml of 1 : 4 pyridine–dioxane and 70% HF·pyridine was added. After stirring overnight, the mixture was concentrated and purified on silica gel (cyclohexane–EtOAc) to give 25 (1.17 g, 91%). [α]²⁵_D = +29.4° (c 0.46, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): 7.44–7.17 (m, 60H, H–Ar), 5.64 (br. s, 1H, H-2^D), 5.33 (s, 1H, H-1^B), 5.28 (s, 1H, H-2^C), 5.19–5.13 (m, 3H, CH₂^Cbz, NH), 4.92–4.32 (m, 22H, 11 × CH₂Ph, incl. s, 4.88, H-1^A), 4.18–3.51 (m, 21H), 3.39–3.39 (s, 1H, H-1′b), 3.24–3.13 (m, 2H, H-3′), 2.20 (s, 3H, CH₃CO), 1.74–1.60 (m, 2H, H-2′). ¹³C NMR (CDCl₃): δ 170.1, 156.4 (2 × CO), 138.8–126.5 (C–Ar), 101.0 (C-1^B), 100.6 (C-1^C), 99.5 C-1^D), 97.6 (C-1^A), 79.4, 79.3, 78.3, 77.9, 75.2, 74.7, 74.2, 73.4, 73.1, 72.8, 72.5, 72.3, 72.2, 72.0, 69.7, 69.9, 68.5, 66.6, 65.2, 62.2 (C-1′), 39.3 (C-3′), 29.6 (C-2′), 21.1 (CH₃CO). HR ESI MS C₁₁₄H₁₂₃NO₂₄: [M + Na]⁺ calc. 1912.8333; found 1912.8351.

3-(Benzyloxycarbonyl)aminopropyl 2-O-acetyl-3,4,6-tri-O-benzyl-α-D-mannopyranosyl-(1,2)-3,4,6-tri-O-benzyl-α-D-manno pyranosyl-(1,2)-3,4,6-tri-O-benzyl-α-D-mannopyranosyl-(1,3)-6-O-[2-O-acetyl-3,4,6-tri-O-benzyl-α-D-manno pyranosyl-(1,2)-3,4,6-tri-O-benzyl-α-D-mannopyranosyl-(1,2)]-2,4-di-O-benzyl-α-D-mannopyranoside 26

Glycosylation of acceptor 25 (430 mg, 0.35 mmol) and donor 23 (507 mg, 0.46 mmol) as described for 24 yielded hexasaccharide 26 (490 mg, 81%). The product was similar to that described in the literature,²⁵ except for the signals of the linker 3.75–3.72 (m, 1H, H-1′a), 3.47–3.41 (s, 1H, H-1′b), 3.17–3.04 (m, 2H, H-3′), 1.71–1.68 (m, 2H, H-2′). HR ESI MS C₁₇₀H₁₃₁NO₃₅: [M + Na]⁺ calc. 2819.2312; found 2819.2331.

Man6

NMR data for this compound were in agreement with those reported in the literature for a similar hexamannoside.²⁵

Acknowledgements

We gratefully acknowledge the support of Dr Daniela Proietti with ESI MS analysis.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6qo00144k

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