Synthesis of Precursor Glucosamine Building Blocks of Chitin

Abstract

The acylation reaction is an important process for biological and chemical applications. Biologically, this reaction is used in a mechanism critical to numerous cellular processes, such as protein assembly and regulation. This article reviews the state of the art of microwave-assisted reactions and the influence of microwaves on mass and heat transfer. The heating behaviour of representative test reactions and single substances is compared for heating with microwaves and thermal energy.

Keywords

Homogenous reactions, Acylation, Beckmann rearrangements, NMR.

Introduction

Differentially protected glucosamines are very important structural units of many bioactive products which are synthesized in human body. β–(1-4) linked N-acetyl glucosamine moiety is a frequently occurring structural unit in various naturally and biologically important oligosaccharides and their conjugates.

Chitin is a structural polysaccharide in crab shells, and chitobiose is usually found at the reducing end of Nglycan residues of glycoproteins. Of these biologically active chitooligosaccharides, chitotetraose has the highest affinity among chitooligosaccharides to bind to rat NKR-PL antigen, a carbohydrate-binding protein in rat natural killer (NK) cells [1-16].

Chitosan is one of the most abundant renewable polysaccharides prepared from chitin by deacetylation. Chitosan and chitosanase are attracting a wide attention in their potential application in medicine, industry and agriculture. Also, chitooligosaccharides have received much more interest, because they are not only water-soluble but also possess distinctive biological activity [16-24], such as antitumor, antifungal and antibacterial activities, immuno-enhancing effects [24-29], and promote host defense against infection of certain pathogens in mice.

Objective

The main objective of the project is to synthesize the precursor monomer building block from glucosamine hydrochloride as starting material.

Results and Discussions

Synthesis of differentially protected glucosamines

Glucosamine derivatives are most important building blocks in carbohydrate chemistry for synthesizing oligosaccharides [29-38]. Synthesis of our monosaccharide building block starts with insitu preparation of triflic azide from triflic anhydride and sodium azide for the conversion of 2-amino group in the glucosamine to azide followed by peracetylation and anomeric deacetyation furnishes the common precursor.

Experimental Section

General methods

¹H NMR and ¹³C NMR were recorded on Bruker Avance-400MHz NMR machine using solution in CDCl₃. ¹H NMR referred respectively to TMS used as an internal standard and the central line for CDCl₃. Chemical shifts were reported in (δ) ppm and coupling constants (J) reported in Hz. Pyridine was purchased from Merck. Triflic anhydride and other chemicals used were purchased from Aldrich. DCM was freshly distilled from CaH₂ [39-53]. Methanol was distilled from MgSO₄. Column Chromatography was performed over silica gel from SISCO, using hexanes and ethyl acetate mixture as eluent. Solvents were removed under reduced pressure on rotovap. Organic extracts were dried with anhydrous Na₂SO₄. The visualization of spots on TLC plates was effected by exposure to 5%MeOH in H₂SO₄.

Preparation of -2-azido-2-deoxy-D-glucose

1. Preparation of TfN₃:

To a solution of NaN₃ (15g, 230mmol) in water (30mL) CH₂Cl₂ (40mL) was added at 0°C. The mixture is stirred vigorously and treated with Tf₂O (7.8mL, 46.36mmol). The reaction mixture was stirred at 0°C for 2.5h. The aqueous phase was extracted with CH₂Cl₂. Combined organic layers were washed with saturated Na₂CO₃, dried over Na₂SO₄ and concentrated in vacuo then immediately used for the reaction [54-69].

2. Conversion of 2-amino group to azide:

To a solution of glucosamine hydrochloride (5g, 23.18mmol) in water (74mL), CuSO₄ 5H₂O (55mg, 0.22mmol) and K₂CO₃ (4.8g, 36mmol) were added. Then methanol (50mL) was added to the reaction mixture followed by addition of TfN₃ solution (prepared from triflic anhydride and sodium azide) [70-84]. Methanol was added until the solution become homogenous (~25mL). The light blue color solution was allowed to stir for 24h at room temperature. After completion of the reaction, solvents were removed to obtain compound 2 which was used directly for the next step.

Preparation of 1, 3, 4, 6-tetra-O-acetyl-2-azido-2-deoxy α/β-D-glucopyranose:

To the azido compound 2 in pyridine (25mL), Ac₂O (25mL) and catalytic DMAP were added and the reaction mixture was allowed to stir for 12h [85-89]. After completion of the reaction by TLC monitoring, it was worked up with EtOAc and dried over Na₂SO₄ followed by removal of solvent on rotovap and purified by column chromatography with Hexane and Ethyl acetate as eluents (3:1) to afford desired compound 3. Yield = 35%

¹H NMR (400 MHz, CDCl₃): δ 5.55 (d, J = 8.6 Hz, 1H), 5.09-5.05 (m, 2H), 4.3 (dd, J=12.5, 4.4 Hz, 1H), 4.08 (dd, J=1.9, 12.5 Hz, 1H), 3.75 (m, 1H), 2.2 (s, 3H), 2.10 (s, 3H), 2.08 (s, 1H), 2.03 (s, 3H).

Preparation of 3, 4, 6-tetra-O-acetyl-2-azido-2-deoxy α/β-D-glucopyranose:

Peracetylated compound 3 (3g, 8.1mmol) was dissolved in dry DMF (30mL), to this ammonium acetate (1.25g, 16.2mmol) was added. The reaction mixture was stirred at room temperature overnight [90-96]. After completion of the reaction, solvent was removed and pure compound 4 (958mg, 8.04mmol) was obtained by column chromatography (Hexane/EtoAc 3:1) [97-100].

Yield: 36%

¹H NMR (400 MHz, CDCl3): δ 5.48 (m, 1H), 5.4 (d, J=6.4 Hz, 1H), 5.27 (m, 1H),

4.29-4.41(m, 2H), 4.12 (m, 1H), 3.17(m, 1H), 2.2-2.02(m, 9H).

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