Catalytic Decarboxylative Glycosylation Promoted by Hafnium(IV) ­Trifluoromethanesulfonate: β-Selective Glycosylation via a Mixed Carbonate of an Acyl-Protected Donor Sugar

Abstract

Acyl-protected mixed carbonates were effectively decarboxylated by catalytic amounts (2-5 mol%) of hafnium(IV) tri­fluoromethanesulfonate in dichloromethane at room temperature to give the corresponding β-glycosides in excellent yields.

Key words

glycosylations - glycosides - Lewis acids - mixed carbonates - decarboxylation

References

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1

Present address: School of Pharmaceutical Sciences, Kitasato University, Shirokane, Minato-ku, Tokyo 108-8641, Japan.

12

Typical Experimental Procedure for the Decarboxylative Glycosylation: To a solution of a mixed β-carbonate (0.1 mmol) in CH₂Cl₂ (1 mL) with stirring at ambient temperature was added a metal triflate. After the reaction was completed, a saturated solution of NaHCO₃ was added and the mixture was extracted with EtOAc. The organic layer was washed with brine twice, dried over Na₂SO₄, filtered, and evaporated. The crude product was purified by silica gel column chromatography to give the corresponding glycosides.

13

Selected analytical data for the compounds 7b, 7c, 8a and 8d are shown. All the other glycosides are also fully characterized.
Compound 7b: [α]_D ²² +10.9 (c 0.50, CHCl₃). ¹H NMR (400 MHz, CDCl₃): δ = 1.11 (s, 9 H), 1.14 (s, 9 H), 1.15 (s, 9 H), 1.24 (s, 9 H), 3.33 (dd, J = 8.9 Hz, 10.1 Hz, 1 H), 3.37 (s, 3 H), 3.47 (dd, J = 3.7 Hz, 9.8 Hz, 1 H), 3.59 (dd, J = 6.4 Hz, 10.7 Hz, 1 H), 3.82 (ddd, J = 1.8 Hz, 6.1 Hz, 9.8 Hz, 1 H), 3.89 (m, 1 H), 3.95-4.04 (m, 3 H), 4.11 (dd, J = 6.4 Hz, 11.0 Hz, 1 H), 4.45 (anomeric, d, J = 7.9 Hz, 1 H), 4.54 (d, J = 3.7 Hz, 1 H), 4.57 (d, J = 11.3 Hz, 1 H), 4.65 (d, J = 12.2 Hz, 1 H), 4.77 (d, J = 11.9 Hz, 1 H), 4.79 (d, J = 10.7 Hz, 1 H), 4.87 (d, J = 11.0 Hz, 1 H), 4.98 (d, J = 11.0 Hz, 1 H), 5.05 (dd, J = 3.4 Hz, 10.4 Hz, 1 H), 5.23 (dd, J = 7.9 Hz, 10.4 Hz, 1 H), 5.37 (dd, J = 0.9 Hz, 3.4 Hz, 1 H), 7.25-7.36 (m, 15 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 27.0, 27.1, 27.1, 38.7, 38.7, 38.7, 39.0, 55.2, 61.1, 66.7, 68.4, 68.6, 69.9, 71.0, 71.1, 73.2, 74.8, 75.6, 76.7, 77.0, 77.2, 77.3, 78.1, 79.9, 81.9, 97.7, 101.5, 127.5, 127.7, 127.8, 127.9, 128.1, 128.3, 128.4, 138.1, 138.2, 138.8, 176.5, 176.9, 177.3, 177.8 ppm. MS (FAB-NBA + NaI): m/z = 986 (M + Na⁺ + 1). HRMS (FAB-NBA + NaI): calcd for C₅₄H₇₄O₁₅Na: 985.4925; found: 985.4937.
Compound 7c: [α]_D ²² -1.3 (c 0.50, CHCl₃). ¹H NMR (400 MHz, CDCl₃): δ = 1.07 (s, 9 H), 1.10 (s, 9 H), 1.14 (s, 9 H), 121 (s, 9 H), 3.37 (s, 3 H), 3.41 (dd, J = 3.7 Hz, 9.5 Hz, 1 H), 3.50-3.56 (m, 2 H), 3.60 (dd, J = 1.8 Hz, 10.4 Hz, 1 H), 3.78-3.84 (m, 2 H), 3.93-4.03 (m, 3 H), 4.22 (anomeric, d, J = 7.9 Hz, 1 H), 4.34 (d, J = 11.9 Hz, 1 H), 4.59 (d, J = 3.7 Hz, 1 H), 4.60 (d, J = 12.5 Hz, 1 H), 4.69 (dd, J = 3.4 Hz, 10.4 Hz, 1 H), 4.70 (d, J = 11.3 Hz, 1 H), 4.74 (d, J = 12.5 Hz, 1 H), 4.82 (d, J = 12.2 Hz, 1 H), 5.07 (d, J = 11.0 Hz, 1 H), 5.10 (dd, J = 7.9 Hz, 10.4 Hz, 1 H), 5.22 (dd, J = 0.6 Hz, 3.1 Hz, 1 H), 7.19-7.45 (m, 15 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 27.0, 27.1, 27.1, 27.2, 38.7, 38.7, 38.9, 55.4, 61.1, 67.0, 67.6, 69.3, 69.6, 70.6, 71.3, 73.5, 73.7, 75.1, 75.3, 76.7, 77.0, 77.2, 77.3, 78.0, 79.7, 98.7, 99.4, 127.1, 127.3, 127.7, 128.0, 128.1, 128.3, 128.5, 128.6, 128.8, 137.5, 138.4, 139.4, 176.3, 176.9, 177.2, 177.8 ppm. MS (FAB-NBA + NaI): m/z = 986 (M + Na⁺ + 1). HRMS (FAB-NBA + NaI): calcd for C₅₄H₇₄O₁₅Na: 985.4926; found: 985.4915.
Compound 8a: [α]_D ²³ +11.8 (c 1.00, CHCl₃). ¹H NMR (400 MHz, CDCl₃): δ = 1.00-1.90 (m, 10 H), 1.87 (s, 3 H), 2.03 (s, 9 H), 2.11 (s, 3 H), 3.60 (m, 1 H), 3.86 (ddd, J = 2.4 Hz, 4.9 Hz, 10.1 Hz, 1 H), 4.16 (dd, J = 2.3 Hz, 10.9 Hz, 1 H), 4.37 (m, 2 H), 5.17 (t, J = 9.8 Hz, 1 H), 5.46 (anomeric, d, J = 8.2 Hz, 1 H), 5.80 (dd, J = 8.9 Hz, 10.7 Hz, 1 H), 7.74 (m, 2 H), 7.86 (m, 2 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 14.1, 20.4, 20.6, 20.7, 23.5, 23.7, 25.3, 31.4, 33.1, 54.8, 60.3, 62.2, 69.2, 70.8, 71.6, 76.7, 77.0, 77.3, 77.6, 96.6, 123.5, 131.4, 134.2, 169.4, 170.1, 170.7 ppm. MS (FAB-NBA + NaI): m/z = 541 (M + Na⁺). HRMS (FAB-NBA + NaI): calcd for C₂₆H₃₁O₁₀Na: 540.1846; found: 540.1840.
Compound 8d: [α]_D ²² -23.7 (c 0.37,CHCl₃). ¹H NMR (400 MHz, CDCl₃): δ = 1.87 (s, 3 H), 2.01 (s, 3 H), 2.05 (s, 3 H), 3.18 (s, 3 H), 3.37-3.46 (m, 2 H), 3.66 (dd, J = 3.8 Hz, 10.1 Hz, 1 H), 3.81-3.85 (m, 2 H), 3.91 (ddd, J = 2.5 Hz, 4.9 Hz, 10.1 Hz, 1 H), 3.98 (br, 1 H), 4.08 (dd, J = 2.9 Hz, 10.4 Hz, 1 H), 4.19 (dd, J = 2.5 Hz, 12.5 Hz, 1 H), 4.19 (t, J = 12.5 Hz, 1 H), 4.22 (d, J = 3.7 Hz, 1 H), 4.34 (dd, J = 5.2 Hz, 12.2 Hz, 1 H), 4.34 (d, J = 12.2 Hz, 1 H), 4.41 (dd, J = 8.7 Hz, 10.8 Hz, 1 H), 4.43 (d, J = 11.3 Hz, 1 H), 4.54 (d, J = 11.6 Hz, 1 H), 4.92 (d, J = 11.6 Hz, 1 H), 5.18 (dd, J = 9.2 Hz, 10.1 Hz, 1 H), 5.65 (anomeric, d, J = 8.2 Hz, 1 H), 5.93 (dd, J = 9.2 Hz, 10.7 Hz, 1 H), 6.99-7.36 (m, 15 H), 7.61-7.88 (m, 4 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 14.2, 20.4, 20.6, 20.6, 21.0, 29.6, 55.1, 55.1, 60.3, 62.0, 68.9, 69.1, 69.1, 70.4, 71.5, 73.2, 73.4, 74.9, 75.8, 76.7, 77.0, 77.2, 77.3, 78.8, 98.5, 99.4, 123.5, 123.6, 127.5, 127.6, 127.6, 127.6, 127.7, 127.7, 127.7, 127.8, 127.8, 128.1, 128.2, 128.2, 128.3, 128.3, 128.4, 128.4, 128.4, 128.5, 128.6, 131.5, 134.2, 134.2, 138.0, 138.3, 138.8, 167.3, 169.6, 170.0, 170.5 ppm. MS (FAB-NBA + NaI): m/z = 905 (M + Na⁺). HRMS (FAB-NBA + NaI): calcd for C₄₈H₅₁O₁₅NNa: 904.3157; found: 904.3129.

14

Competition Experiment: To a solution of 9 (32.4 mg, 0.050 mmol) and 10 (33.3 mg, 0.050 mmol) in CH₂Cl₂ (1 mL) with stirring at ambient temperature was added Hf(OTf)₄ (1.3 mg, 0.02 equiv). After 20 min, a saturated solution of NaHCO₃ was added and the mixture was extracted with EtOAc. The organic layer was washed with brine twice, dried with Na₂SO₄, filtered, and evaporated. The crude product was carefully purified by silica gel column chromatography so as not to lose any of the generated glycosides. The mixture of glycosides and the authentic glycosides were analyzed by HPLC (eluent: 0.5% IPA in n-hexane).

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Preparation of Authentic Glycosides for the Competition Experiment: Authentic glycosides (13-16) were synthesized by decarboxylative glycosylation promoted by trimethylsilyl trifluoromethanesulfonate [5c] from the corresponding mixed β-carbonates (9-12).

16

Evolution of carbon dioxide (90% yield) was confirmed by measuring the volume of generated gas with a buret filled with water. Spaces in the reaction vessel and the buret were dynamically separated by silicon oil that cannot dissolve carbon dioxide.