Synthesis and in vitro cytotoxicity of acetylated 3-fluoro, 4-fluoro and 3,4-difluoro analogs of D-glucosamine and D-galactosamine

Abstract

Background: Derivatives of D-glucosamine and D-galactosamine represent an important family of the cell surface glycan components and their fluorinated analogs found use as metabolic inhibitors of complex glycan biosynthesis, or as probes for the study of protein-carbohydrate interactions. This work is focused on the synthesis of acetylated 3-deoxy-3-fluoro, 4-deoxy-4-fluoro and 3,4-dideoxy-3,4-difluoro analogs of D-glucosamine and D-galactosamine via 1,6-anhydrohexopyranose chemistry. Moreover, the cytotoxicity of the target compounds towards selected cancer cells is determined.

Results: Introduction of fluorine at C-3 was achieved by the reaction of 1,6-anhydro-2-azido-2-deoxy-4-O-benzyl-β-D-glucopyranose or its 4-fluoro analog with DAST. The retention of configuration in this reaction is discussed. Fluorine at C-4 was installed by the reaction of 1,6:2,3-dianhydro-β-D-talopyranose with DAST, or by fluoridolysis of 1,6:3,4-dianhydro-2-azido-β-D-galactopyranose with KHF2. The amino group was introduced and masked as an azide in the synthesis. The 1-O-deacetylated 3-fluoro and 4-fluoro analogs of acetylated D-galactosamine inhibited proliferation of the human prostate cancer cell line PC-3 more than cisplatin and 5-fluorouracil (IC50 28 ± 3 μM and 54 ± 5 μM, respectively).

Conclusion: A complete series of acetylated 3-fluoro, 4-fluoro and 3,4-difluoro analogs of D-glucosamine and D-galactosamine is now accessible by 1,6-anhydrohexopyranose chemistry. Intermediate fluorinated 1,6-anhydro-2-azido-hexopyranoses have potential as synthons in oligosaccharide assembly.

Keywords: amino sugars; cytotoxicity; fluorinated carbohydrates; fluorine; hexosamines.

Figure 1

Examples of deoxofluorinated hexosamines.

Scheme 1

Retrosynthetic plan.

Scheme 2

Preparation of starting 2-azido compounds. Reagents and conditions: (a) NaN₃, NH₄Cl, MeOC₂H₄OH, 79%; (b) i) BzCl, py; ii) NaBrO₃, Na₂S₂O₄, AcOEt, H₂O, 47% over 2 steps; (c) Pd/C, H₂, EtOH, 96%; (d) Tf₂O, then (Bu)₄NNO₂, 69%; (e) (i) NaH, BnBr, THF, 79%; (ii) LiN₃, NH₄Cl, MeOC₂H₄OH, 100 °C, 81%; (f) (i) TsCl, py, (ii) AcOH, H₂O; (iii) IRA 410 (OH⁻), MeOH, 78% over 3 steps; (g) (i) Tf₂O, py, CH₂Cl₂; (ii) LiN₃, DMF, 82% over 2 steps.

Scheme 3

Preparation of mono and difluoro analogs of 2-azido-2-deoxy-1,6-anhydro-β-D-gluco- and galactopyranoses. Reagents and conditions: (a) DAST, benzene, 75–81 °C, 88%; (b) NaBrO₃, Na₂S₂O₄, EtOAc, H₂O, 75%; (c) Tf₂O, py, CH₂Cl₂; then (Bu)₄NNO₂, DMF, 67%; (d) DAST, benzene, 75–81 °C; or DAST, CH₂Cl₂, −25 °C to rt; (e) DAST, CH₂Cl₂, −25 °C to rt, 78%; (f) DAST, CH₂Cl₂, −25 °C to rt, 65%; (g) KHF₂, C₂H₄(OH)₂, 175 °C, 40–60% (26); (h) DAST, benzene, 75–81 °C, 46% (28), 12% (29); (i) DAST, CH₂Cl₂, −50 °C to rt, 52 %, (j) LiN₃, NH₄Cl, MeOC₂H₄OH, 100 °C, 74% (31).

Scheme 4

Suggested mechanisms for deoxofluorination at C-3 of 1,6-anhydro-β-D-glucohexopyranose derivatives. A) participation of the O-benzyl group; B) participation of the azide group; C) internal fluorine attack, concerted mechanism; D) internal fluorine attack, contact ion pair mechanism.

Scheme 5

Formation of oxazoline 41 from 19.

Scheme 6

1-O-Deacetylation of monofluorinated hexosamines. Reagents and conditions: (a) BnNH₂, THF, 62%; (b) C₅H₁₀NH, THF, 74%; (c) C₅H₁₀NH, THF, 60%; (d) silica gel, MeOH, 30 days, 40%.