Michael-Type Addition to 2-Nitrogalactal - A Simple Method to Access 1,1-Linked Oligosaccharides

Abstract

Anomeric O-unprotected sugars add to 3,4,6-tri-O-benzyl-2-nitro-d-galactal to accomplish nitro group-containing 1,1-linked oligosaccharides in respectable yields with good selectivities. A 1:1 mixture of toluene and n-heptane has been found as the appropriate solvent system for these Michael-type additions. The nitro group-containing 1,1-linked oligosaccharides are easily convertible into interesting trehalosamine analogues.

Key words

Michael-type addition - 2-nitrogalactal - 1,1-linked ­oligosaccharides - O-unprotected sugars - nitro group

References

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8

General Procedure for the Michael-Type Addition:
To a solution of anomeric O-unprotected sugar (0.16 mmol) in toluene (1 mL), t-BuOK (0.16 mmol) was added at 0 °C. After 10 min, a solution of 1 (0.19 mmol) in toluene-n-heptane mixture (1:2, 3 mL) was added to it. The reaction mixture was stirred at the same temperature for the specific period of time given in Table [1] . Few drops of HOAc were added to quench the reaction. It was taken in EtOAc, washed with H₂O, and sat. brine, dried over anhyd MgSO₄, and concentrated. Purification of the crude residue by chromatography on silica gel afforded the 1,1-linked oligosaccharide. Selected ¹H NMR data (250 MHz, CDCl₃): Compound 2a: δ = 5.58 (J _1,2 = 4.3 Hz, 1-H_a), 5.25 (J _1,2 = 3.7 Hz, 1-H_b). Compound 2b: δ = 5.50 (J _1,2 = 4.1 Hz, 1-H_a), 4.56 (J _1,2 = 8.3 Hz, 1-H_b). Compound 3a: δ = 5.56 (J _1,2 = 4.2 Hz, 1-H_a), 5.25 (J _1,2 = 3.9 Hz, 1-H_b). Compound 3b: δ = 5.49 (J _1,2 = 4.2 Hz, 1-H_a), 4.60 (J _1,2 = 8.1 Hz, 1-H_b). Compound 4a: δ = 5.55 (J _1,2 = 4.1 Hz, 1-H_a), 5.16 (J _1,2 = 3.8 Hz, 1-H_b), 5.36 (J _1,2 = 3.3 Hz, 1-H_c). Compound 4b: δ = (J _1,2 = 4.0 Hz, 1-H_a), 4.63 (J _1,2 = 8.1 Hz, 1-H_b), 5.41 (J _1,2 = 3.8 Hz, 1-H_c). Compound 5a: δ = 5.63 (J _1,2 = 4.2 Hz, 1-H_a), 5.11 (J _1,2 = 1.1 Hz, 1-H_b). Compound 6a: δ = 5.60 (J _1,2 = 4.2 Hz, 1-H_a), 5.21 (J _1,2 = 3.9 Hz, 1-H_b). Compound 6b: δ = 5.61 (J _1,2 = 4.1 Hz, 1-H_a), 4.42 (J _1,2 = 8.2 Hz, 1-H_b). Compound 7a: δ = 5.63 (J _1,2 = 4.1 Hz, 1-H_a), 5.41 (J _1,2 = 4.1 Hz, 1-H_b). Compound 7b: δ = 5.34 (J _1,2 = 4.0 Hz, 1-H_a), 5.35 (J _1,2 = 8.7 Hz, 1-H_b). Compound 8a: δ = 5.50 (J _1,2 = 4.1 Hz, 1-H_a), 5.01 (J _1,2 = 3.6 Hz, 1-H_b), 4.36 (J _1,2 = 8.3 Hz, 1-H_c). Compound 8b: δ = 5.29 (J _1,2 = 4.1 Hz, 1-H_a), 5.08 (J _1,2 = 8.2, 1-H_b), 4.51 (J _1,2 = 8.0 Hz, 1-H_c). Compound 9a: δ = 5.23 (J _1,2 = 3.0 Hz, 1-H_a), 5.32 (J _1,2 = 3.5 Hz, 1-H_b).