JP3268299B2 - Sugar donor having trichloroacetimidate as a leaving group useful for production of sulfated / phosphorylated trisaccharide serine, its production method and its intermediate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to β-Gal (1 → 3) β-Gal (6), a sulfated / phosphorylated trisaccharide serine.
—SO ₃ Na) (1 → 4) -β-Xyl (2-OPO ₃ N)
a ₂ ) Ser [Gal is galactose, Xyl is xylose, and Ser is serine. A Gal-Gal sugar donor useful for the production of More specifically, the compound of formula (1), which is a receptor,

[0002]

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The present invention relates to a sugar donor capable of forming a trisaccharide capable of forming the sulfated / phosphorylated trisaccharide serine by coupling and regioselectively introducing a sulfate group.

[0004]

2. Description of the Related Art Proteoglycan (PG), which is a major component of the extracellular matrix, has been attracting attention in recent years because it has an important role in transmitting information between cells. Structurally PG
Is composed of a core protein and a linear glycosaminoglycan (GAG) extending in a branch from the core protein. GA
G chains are classified into heparin, heparan sulfate, chondroitin sulfate, dermatan sulfate, and keratan sulfate by repeating units having 0 to 3 sulfate groups per disaccharide.
GAG is a "common tetrasaccharide moiety" bound to a core protein
(Xyl-Gal-Gal-GlcA) and a “repeating disaccharide region” in which a disaccharide unit of uronic acid and an amino sugar follows the non-reducing side. In biosynthesis, GAGs correspond to the corresponding UD starting from the serine (Ser) hydroxyl group of the core protein.
It is considered that the sugar chain is sequentially extended by a monosaccharide unit to the non-reducing side by P sugar and glycosyltransferase. The repeating disaccharide region is hexosamine (α-GlcNAc and β-GalNAc)
Are classified into heparin type and chondroitin type according to their types, and their sorting mechanism in the biosynthesis process (regulation of two types of glycosyl transfer at the fifth reducing terminal) has not yet been elucidated.

[0005] Biochemical information is mainly carried by the disaccharide region. In particular, the steric configuration of the hydroxyl group and amino group of the sugar chain, the binding position of the sugar, and the variation in the number and position of the sulfate groups that modify them can transmit a vast amount of information. I have to. Sulfate groups are mainly used for sugar chain modification, and studies on the relationship between the positional specificity and the expression of functions have been actively conducted mainly on heparin, a kind of GAG. The discovery of the presence of phosphate groups is relatively recent (see, for example, TROegema Jr, ELKraft, GW Jourdian and T. A .;
R. Van Valen, J. Biol. Chem., 259, 1720 (1984); K. Sugahar
a, Y.Ohi, T.Harada, P.de Waard and JFVliegenthart,
J. Biol. Chem., 267, 6027 (1992).). This means
It is known that the phosphate group was easily dropped off when the sugar was isolated from nature. In addition, the presence of the phosphate group in GAG is determined by detecting the presence of the phosphate group at the position of xylose (Xyl) 2 at the reducing end (bonded to the serine hydroxyl group).
However, its biochemical significance has not yet been elucidated.

Regarding the biochemical significance of phosphorylated sugars,
From the report of Franssson et al., It is speculated that it has some influence on the sugar chain elongation including the sorting mechanism [J. Moses, Å, Oldberg, F. Cheng and L.Å,
Fransson, Eur. J. Biochem., 248, 521 (1997)]. Under these circumstances, pure sulfated saccharides extracted from nature are already marketed as drugs having the anticoagulant effect of heparin, and there is a great demand in the field of elucidating various biochemical mechanisms of GAG. . If the biosynthesis mechanism of GAG sugar chain and its control mechanism become clear, it will be possible to contribute to the development of many new products including the above-mentioned medicines by making full use of biotechnology. It is clear that this will lead to the development of. However, as described above, phosphate groups are easily dropped during isolation from nature, and pure phosphorylated saccharide obtained by extraction from nature alone is not sufficient to meet the demand. Therefore, if the phosphorylated saccharide can be chemically synthesized, it will greatly contribute to elucidation of the mechanism of sugar chain elongation by the reaction of the enzyme, and as a result, efficient drug production utilizing biotechnology and a large number of Clearly, it can contribute to the development of new products.

[0007] There have been reports from two groups on the synthesis of oligosaccharides with phosphorylated GAG reducing end. 1. Jacquinet et al. Report on the synthesis of GAG reducing end phosphorylated disaccharide and tetrasaccharide serylglycine. However, this is not completely natural because the amino group of serine is acetylated (S.Rio, J.-M.Beau and J.-
C. Jacquinet, Carbohydr. Res., 255, 103 (1994)]. 2. Nilsson et al. Synthesize 2-4 saccharides, which are methyl glycosides and are unnatural [M. Nilsson, J. Westman
and C.-M, Svahn, J. Carbohydr. Chem. 12, 23 (1993)]. None of these reports has obtained a sugar chain having a phosphate group and a sulfate group at the same time.

The present inventor has found that the general formula 2

[0009]

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[0010] The natural type monosaccharide serine (1) of the natural type
And a method for synthesizing the disaccharide serine (2) and (3) [Bioorganic & Medicinal Chemistry Let
ters, 1911-1914, 9 (1999): Reference A]. There, Gal-Xy substituted with a phosphate group and further substituted with a sulfate group is used.
In order to obtain the l-Ser product, a galactosyl donor, which is a sugar donor appropriately designed for the purpose, is synthesized, and the desired compound is obtained by a coupling technique. In general, the effects and effects of protecting groups, leaving groups, phosphate groups, etc. on sugar chain elongation can be obtained only by actually performing synthesis by trial and error. There is difficulty. Therefore, although the above-mentioned literature discloses a method of obtaining a Gal-Xyl-Ser product substituted with a phosphate group and further substituted with a sulfate group, in the synthesis of a sulfated / phosphorylated trisaccharide serine, a disaccharide chain is not used. Not only that the known synthesis method cannot be applied as it is until the synthesis of, but also by establishing a method for producing a phosphorylated trisaccharide serine by revising the chemical structure of a sugar donor and an acceptor, In addition, it is necessary to establish a production method that can obtain a phosphorylated trisaccharide serine having a chemical structure such that a sulfate group is introduced regioselectively by going back to the point where the chemical structures of the sugar donor and acceptor are devised. It is.
Therefore, β-Gal (1 → 3) β-Gal (6-SO ₃
Na) (1 → 4) -β-Xyl (2-OPO ₃ Na ₂ ) S
The above is not an exception in obtaining the trisaccharide serine of er, and the above trisaccharide serine can be obtained. In particular, a phosphorylated trisaccharide serine having a chemical structure in which a sulfate group is introduced regioselectively can be used. It is necessary to devise the chemical structures of the sugar donor and the sugar acceptor that can obtain the following.

[0011]

An object of the present invention is to provide β-Gal (1 → 3) β which is a sulfated / phosphorylated trisaccharide serine.
-Gal (6-SO ₃ Na) (1 → 4) -β-Xyl
_{(2-OPO 3 Na 2)} Ser ( But, Gal is galactose, Xyl xylose, Ser is serine.)
Which is useful for the production of, particularly, a phosphorylated trisaccharide serine having a chemical structure in which a sulfate group is introduced regioselectively.
To provide a l-Gal disaccharide donor and a method for synthesizing the disaccharide donor, and to provide a sugar derivative which is an intermediate having a protective group useful for producing the disaccharide donor. That is.

[0012]

A first aspect of the present invention is that (2,
3,4,6-tetra-O-acetyl-β-D-galactopyranosyl)-(1 → 3) -2-O-acetyl-6-O
-T-butyldiphenylsilyl-4-O- (4-methylbenzoyl) -D-galactopyranosyltrichloroacetimidate (hereinafter referred to as "Gal-Gal disaccharide donor"). In the second to sixth aspects of the present invention, the Gal-
It is a novel intermediate useful for synthesizing Gal disaccharide donors. In the seventh aspect of the present invention, the Ga
The present invention relates to a method for synthesizing a 1-Gal disaccharide donor.

[0013]

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[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail. For the synthesis of sulfated / phosphorylated trisaccharide serine, the method for producing a disaccharide serine disclosed by the present inventor has been reduced to the production of phosphorylated and sulfated / phosphorylated trisaccharide serine. It was not applicable to the manufacturing method. Therefore, as described above, the present invention provides a Gal-Gal disaccharide donor useful for producing sulfated / phosphorylated trisaccharide serine, and further provides a novel Gal-Gal disaccharide donor useful for synthesizing the Gal-Gal disaccharide donor. It is to provide an intermediate, and to provide a method for synthesizing the Gal-Gal disaccharide donor. The present invention provides a monosaccharide for the synthesis of the sulfated / phosphorylated trisaccharide serine,
It had to go back to the point where the chemical structures of the sugar donor and acceptor of the disaccharide were devised.

The foregoing will be appreciated from the following unsuccessful attempts. That is, in order to produce the above-mentioned sulfated / phosphorylated trisaccharide serine, the sequential coupling reaction of sugar chains described in the above Reference A is performed according to the reaction formula Z.

[0016]

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Attempts have been made to synthesize the phosphorylated trisaccharide serine represented by the chemical formula (A) and obtain a chemical formula (B) that can be regioselectively sulfated by debenzylidene conversion. Not realized because the reaction does not progress.
Via reaction formula X or X ′

[0018]

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Attempts to synthesize sugar chains by successive coupling reactions could not be realized because the desired compound (C) could not be obtained. Still further, an attempt via synthesis of an acceptor according to Scheme Y leads to Scheme Y '

[0020]

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Therefore, it cannot be realized. The features of the present invention and the device will be specifically understood from the reaction steps described in Examples.

[0022]

EXAMPLE 1 Example I. The method for producing the compounds of the formulas 1α and β, which are the target compounds of the present invention, will be described according to Reaction Scheme 1.

[0023]

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From the compound of formula S _{2 to} the compound of formula 2 (4-methoxyphenyl 2,4,6-tri-O-acetyl-3
-O-allyl-β-D-galactopyranoside) from commercially available 1,2,3,4,6-pentaacetyl-β-D-galactopyranose (S ₁ ) [F. Goto and T. Og
awa, Tetrahedron Lett., 33 ( 35) 5099 (1992) the compound of formula S ₂ obtained according to] (54.1 g, 165 mmol) was suspended in THF (400 ml) and toluene (400 ml), oxidation n-
Dibutyltin (49.6 g, 199 mmol) was added, and the mixture was heated and refluxed for 6 hours with a water separator. After cooling, the solvent was distilled off, and THF (400 ml), tetra n-butylammonium bromide (5.31 g, 16.5 mmol), allyl bromide (210 m
1, 2.48 mol) and heated under reflux for 30 minutes. After allowing to cool, triethylamine (350 ml) was added, the solvent was distilled off, and pyridine (300 ml) and acetic anhydride (300 ml) were added to the residue, followed by stirring at room temperature overnight. Crushed ice (100 g) was added to this solution and stirred for several hours. The reaction solution was diluted with chloroform, and the organic layer was washed successively with a saturated aqueous solution of sodium hydrogen carbonate and saturated saline, dried over anhydrous magnesium sulfate, filtered, and the solvent was distilled off under reduced pressure. The residue was subjected to silica gel column chromatography (C-200, 1200 g, hexane-ethyl acetate 1: 1 to 1
1: 5) to give the compound of formula 2 (58.4 g, 72%). Compound of formula 2 Rf 0.64 (toluene-ethyl acetate 1: 3) Melting point 130-131 ° C. (recrystallized from hexane-ethyl acetate) [α] _D +23.60 (c 1.52, chloroform) Elemental analysis Calculated value _{(C 22 H 28 0 10)} C: 58.39, H: 6.26 Found C: 58.38, H: 6.25 H -NMR data: shift values ppm, coupling constants (J) are expressed in Hz. Standard is tetramethylsilane [(CH ₃ ) ₄ Si = 0 pp
m]. The measurement was performed at 25 ° C. in deuterated chloroform (CDCl ₃ ). 4.86 (H-1), 5.34 (H-2, J _1,2 = 8.
05, J _2,3 = 10.01), 3.59 (H-3, J _3,4
= 3.66), 5.46 (H-4), 3.90-3.9.
6 (H-5, 6a), 4.13 to 4.24 (H-6b,
_{OCH 2 CHCH 2), 5.75~5.85 (} OCH 2 C
HCH ₂ ), 5.17 to 5.28 (OCH ₂ CHC)
H _2), 2.08,2.11,2.17 (3OCOC
_{H 3), 3.77 (OCH 3} ), 6.79~6.83,
6.93-6.97 (Ph).

From the compound of formula 2 to the compound of formula 3
A compound of formula 2 (590.6 mg, 1.31 mmol) was obtained in methanol (4 ml) to obtain triethylamine (2 m
l) and water (2 ml) were added and the mixture was stirred at room temperature for 16 hours. The residue obtained by distillation under reduced pressure was dissolved in dimethylformamide (8 ml), and imidazole (216.6 mg, 3.18 mmol) and t-butylchlorodiphenylsilane (0.55 ml, 2.11 mmol) were added.
Stir for 9 hours. The reaction solution was diluted with ethyl acetate, an aqueous sodium hydrogen carbonate solution was added, and the mixture was extracted with ethyl acetate. The organic layer was washed successively with a saturated aqueous solution of sodium bicarbonate and a saturated saline solution, dried over anhydrous magnesium sulfate, filtered, and the solvent was distilled off under reduced pressure. The residue was subjected to silica gel column chromatography (C-200, 40 g, toluene-ethyl acetate 2
0: 1 to 1: 1) to give the compound of formula 3 (670.6 mg, 8
5%) and the compound of formula 4 (77.0 mg, 10%). Compound of formula 3 Rf 0.65 (toluene-ethyl acetate 2: 1) Melting point 116 ° C. (recrystallized from hexane-ethyl acetate) [α] _D +3.960 (c 0.91, chloroform) Elemental analysis Calculated value (C ₃₄ H ₄₂ SiO ₈ ) C: 67.29, H: 6.69 Found C: 67.41, H: 7.02 Compound of formula 4 Rf 0.39 (toluene-ethyl acetate 2: 1) [α] _D -13.20 (c 1. 145, chloroform) analysis: calculated _{(C 32 H 40 SiO 8)} C: 68.04, H: 7.15 Found C: 67.77, H: 7.18

Formula 3 to obtain a compound of Formula 5 from a compound of Formula 3
Was dissolved in pyridine (5 ml) and p-methylbenzoyl chloride (0.32 ml, 2.46 mmol) was dissolved in pyridine (5 ml).
l) and a catalytic amount of dimethylaminopyridine were added and stirred overnight at room temperature. To this solution was added methanol (1 ml) and 4
Stirred for hours. The reaction solution was diluted with chloroform, an aqueous solution of sodium hydrogen carbonate was added, and the organic layer was washed successively with a saturated aqueous solution of sodium bicarbonate and brine, dried over anhydrous magnesium sulfate, and filtered, and the solvent was distilled off under reduced pressure. The residue was subjected to silica gel column chromatography (C-200, 50
g, hexane-hexane-ethyl acetate 2: 1) to give the compound of formula 5 (576.4 mg, 97%). Compound of formula 5 Rf 0.50 (hexane-ethyl acetate 2: 1) [α] _D +33.50 (cl. 55, chloroform) Elemental analysis Calculated (C ₄₂ H ₄₈ SiO ₉ ) C: 69.58, H: 6.69 Found: C: 69.66, H: 6.69

A catalytic amount of an iridium complex [Ir (COD) (Ph ₂ MeP) ₂ PF ₆ ] for obtaining a compound of formula 6 from a compound of formula 5 is converted to T
The suspension was suspended in HF (5 ml), replaced with hydrogen, and stirred until the red color disappeared. afterwards. Degas, replace with argon several times,
Compound 5 (346.2 mg, 0.478 mmol) in THF (6 mL) was added, and the mixture was stirred at room temperature overnight. After cooling on ice, water (7.2 ml), sodium hydrogen carbonate (1.52 g, 18.1 mmol), iodine (242 mg,
0.95 mmol), and the mixture was stirred. After 30 minutes, the reaction solution was diluted with chloroform, and the organic layer was washed with hypo and saturated saline, dried over anhydrous magnesium sulfate, filtered, and the solvent was distilled off under reduced pressure. The residue was subjected to silica gel column chromatography (C-200, 12 g, hexane-ethyl acetate 3
0: 1 to 1: 1) to give the compound of formula 6 (322.7 mg, 9
9%). Compound of formula 6 Rf 0.48 (hexane-ethyl acetate 1: 1) [α] _D -0.230 (c 1.72, chloroform) Elemental analysis Calculated value (C ₃₉ H ₄₄ SiO ₉ .0.1H ₂₀ ) C: 67.89, H: 6.47 Actual value C: 67.89, H: 6.45

A compound of the formula 7 is obtained from a compound of the formula 6 and a compound of the formula 7 to obtain a compound of the formula 8 [known compound: PHAMvam-Zoll]
o and P. Sinay, Carbohydr. Res., 150, 199 (1986)] (195.
9 mg, 0.398 mmol) and the compound of formula 6 (169.5 mg, 0.247 mmol)
To a solution of 1) in dichloromethane (10 ml) was added dry molecular sieves AW300 (1.2 g), and the mixture was stirred at room temperature for 35 minutes. The suspension was cooled to -200 <0> C and trimethylsilyl trifluoromethanesulfonate (50 [mu] L, 0.28 mmol)
Was added. The temperature was continuously raised to 80 ° C. over 2 hours, the reaction solution was diluted with chloroform, and an appropriate amount of an aqueous saturated sodium hydrogen carbonate solution was added. The insoluble material was separated by filtration and extracted with chloroform. The organic layer was washed successively with a saturated aqueous solution of sodium bicarbonate and a saturated saline solution, dried over anhydrous magnesium sulfate, filtered, and the solvent was distilled off under reduced pressure. The residue was separated by gel filtration (S-X1, 2.8φ × 89cm, toluene).
Silica gel column chromatography (C-300,1
0 g, hexane-ethyl acetate 4: 1 to 1: 3) to give the compound of formula 8 (108.8 mg, 43%). Compound of formula 8 Rf 0.26 (hexane-ethyl acetate 1: 1) [α] _D +26.40 (c 1.12, chloroform) Elemental analysis Calculated value (C ₅₃ H ₆₂ SiO ₈ .0.5H ₂₀ ) C : 62.15, H: 6.21 Actual value C: 62,24, H: 6.16

Formula 8 to obtain compound of Formula 9 from compound of Formula 8
Compound (312.0 mg, 0.307 mmol) in acetonitrile (20
cerium nitrate (843 mg, 1.54 mmol) was added to a solution of water (5 mL) and water (5 mL), and the mixture was stirred at 0 ° C. for 50 minutes. Chloroform and an appropriate amount of saturated saline were added to this solution, and extracted with chloroform. The organic layer was washed with saturated saline, dried over anhydrous magnesium sulfate, filtered, and the solvent was distilled off under reduced pressure. The residue was subjected to silica gel column chromatography (C-200, 25 g, hexane-ethyl acetate 4:
1-1: 5) to give compound 9 (260.5 mg, 93%). The compound of formula 9 was used in the next reaction without further purification. Compound of formula 9 Rf 0.18 (hexane-ethyl acetate 1: 1)

Preparation of Compounds of Formulas 1β and 1α from Compound of Formula 9 A solution of the compound of formula 9 (260.5 mg, 0.287 mmol) in dichloromethane (6 mL) was treated with trichloroacetonitrile (1.6 m
L) was added and the mixture was cooled to 0 ° C., and 1 drop of 1,8-diazabicyclo [5.4.0] -7-undecene was added thereto with stirring, followed by stirring at the same temperature for 30 minutes and at room temperature for 20 minutes. The reaction solution was directly subjected to silica gel column chromatography (C-200, 60
g, hexane-ethyl acetate 10: 1 to 1: 1) to obtain Compound 1 (279.3 mg, 93%) [a mixture of β and α (integral ratio: about 7: 3)]. The mixture of formulas 1β and 1α could not be separated from one another and was used without further purification in the next reaction. Compound of formula 1 (β) Rf 0.39, 0.29 (hexane-ethyl acetate 1: 1)

Here, the H-NMR data ( ¹ H NMR using 400 MHz) of the target compound will be shown. Shift value is pp
m and the coupling constant (J) are expressed in Hz. The standard is tetramethylsilane [(CH ₃ ) Si ₄ = 0 ppm]. Deuterated chloroform (C
DCL ₃ ) at 25 ° C. Protons are indicated, for example, as Gal ^2-3 , which is 2
It means the proton at position 3 of the second galactose residue. Compound ^{1β 5.87 (Gal 1 -1, J} 1,2 = 8.29), 5.60
(Gal ^1-2 , J _2,3 = 10.00), 4.08 (Ga
l ¹ -3), 5.75 (Gal ¹ -4, J _3,4 = 3.1
^{7), 3.99 (Gal 1 -5} ), 3.74 (Gal 1 -
6a, J _5,6a = 6.59), 3.83 (Gal ¹ -6)
b, J _5,6b = 5.61, J _{6a, 6b} = 10.73), 4.
65 (Gal ² -1, J _1,2 = 7.81), 5.03 (G
_{^{al 2 -2, J 2,3 = 10.49}} ), 4.92 (Gal 2
-3, J _3,4 = 3.42), 5.32 (Gal ² -4),
_{^{3.87 (Gal 2 -5, J 5,6a}} = J 5,6b = 7.5
^{7), 4.04~4.19 (Gal 2 -6} ), 8.68
(NH). Compound ^{1α 6.62 (Gal 1 -1, J} 1,2 = 3.66), 5.44
(Gal ^1-2 , J _2,3 = 10.49), 4.38 (Ga
l ¹ -3, J _3,4 = 3.42), 5.88 (Gal ¹ −
^{4), 4.34 (Gal 1 -5} ), 3.68 (Gal 1 -
6a, J _5,6a = 6.83, J _{6a, 6b} = 10.73),
_{^{3.77 (Gal 1 -6b, J 5,6b}} = 6.10), 4.
72 (Gal ² -1, J _1,2 = 7.81), 5.09 (G
_{^{al 2 -2, J 2,3 = 10.49}} ), 4.96 (Gal 2
-3, J _3,4 = 3.42), 5.33 (Gal ² -4),
_{^{3.91 (Gal 2 -5, J 5,6a}} = J5,6b = 7.2
^{0), 4.04~4.19 (Gal 2 -6} ), 8.65
(NH). The following peaks, which cannot be determined as either 1β or 1α, were also observed. 0.99 [C (C
H _{3) 3],} 1.92,1.93,1.94,2.05,
2.05, 2.08, 2.08, 2.08, 2.10,
2.11 (OCOCH ₃ ), 2.42 (PhCH ₃ ),
7.23 to 7.40, 7.54 to 5.61, 7.88,
7.90 (Ph).

[0032]

As described above, the present invention provides a sugar donor and an acceptor useful for obtaining a novel oligosaccharide, and is useful for enriching an oligosaccharide library. It is effective in the fields of biochemistry, medicine, and pharmacy, including elucidation of the GAG biosynthesis mechanism and development of new pharmaceuticals.

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Claims (7)

(57) [Claims] (1) (2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl)-(1 → 3) -2-
O-acetyl-6-O-t-butyldiphenylsilyl-
4-O- (4-methylbenzoyl) -D-galactopyranosyltrichloroacetimidate. 2. 4-Methoxyphenyl 2-O-acetyl-3-O-allyl-6-O-t-butyldiphenylsilyl-β-D-galactopyranoside. 3. 4-Methoxyphenyl 2-O-acetyl-3-O-allyl-6-O-t-butyldiphenylsilyl-4-O- (4-methylbenzoyl) -β-D-galactopyranoside. 4. 4-Methoxyphenyl 2-O-acetyl-6-O-t-butyldiphenylsilyl-4-O-
(4-Methylbenzoyl) -β-D-galactopyranoside. 5. A method for preparing 4-methoxyphenyl (2,3,4)
6-tetra-O-acetyl-β-D-galactopyranosyl)-(1 → 3) -2-O-acetyl-6-O-t-butyldiphenylsilyl-4-O- (4-methylbenzoyl)- β-D-galactopyranoside. 6. (2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl)-(1 → 3) -2-
O-acetyl-6-O-t-butyldiphenylsilyl-
4-O- (4-methylbenzoyl) -D-galactopyranose 7. The compound of formula 3 is obtained from the compound of formula 2 by the reaction formula 1, and the compound of formula 5 is obtained from the compound of formula 3;
The compound of formula 6 is obtained from the compound of formula 6, the compound of formula 8 is obtained from the compound of formula 6 and the compound of formula 7, the compound of formula 9 is obtained from the compound of formula 8, and the compound of formula 9 is described in claim 1. A method for synthesizing a compound of the formula: Embedded image