Protection of an alcohol as a carboxylic ester; acetate (1,2,3,4-tetra-O-acetyl-6-deoxy-D-galactose)SyntheticPage 75 (2001)
Submitted 16th Aug 2001, published 17th Aug 2001
A contribution from the Caddick Group, Sussex
alcohol (D-fucose) (prepared, 1 equiv.),
acetic anhydride (Avocado, 1.5 equiv./hydroxyl group),
pyridine (Avocado, 0.2 equiv.),
dichloromethane (GPR, 4 mL/mmol),
saturated bicarbonate solution (2 x 1.5 mL/mmol),
water (1 mL/mmol),
brine (1 mL/mmol)
To a suspension of the alcohol (D-fucose) (6.11 mmol) in acetic anhydride (35 mL, 37.41 g, 36.64 mmol, 6 equiv.) was added a catalytic amount of pyridine (1.0 mL, 0.97 g, 12.21 mmol, 0.2 equiv.). Additional sugar (54.96 mmol) was added to the resulting white suspension in portions keeping the internal temperature between 30 and 40 °C (if necessary by cooling in a cold water or ice-bath). After addition was complete the reaction mixture was allowed to stir at room temperature. After 24 hours, when the reaction mixture had turned clear and colourless, dichloromethane (250 mL) was added, followed by extraction with saturated sodium bicarbonate solution (2 x 100 mL), water (50 mL) and brine (50 mL). The organic fraction was dried over sodium sulfate and the solvent removed in vacuo to give the crude product consisting of both anomers of the protected sugar (20.2 g, > 99 %, beta : alpha (2 : 1)). Separation of the 2 anomers can be achieved by flash column chromatography (petroleum ether/ethyl acetate, 5 : 1).
This transformation has been carried out numerous times on 1 to 10 g of fucose, always in quantitative yield from the corresponding isopropylidene protected sugar (over 2 seps). It should however be noted that this protection can also be carried out utilising other Lewis acids, such as perchloric acid (70 % aqueous) [more exothermic, faster reaction] and hydrobromic acid (in acetic acid) [slower reaction] to give alpha-sugars only (see other references). Those methods usually work very well (e.g. galactose, literature yields were achieved), but are incompatible with the crude sugar employed in this procedure.
In the nmr data the anomeric proton is referred to as H-1.
beta-isomer: dH(300 MHz; CDCl3) 5.65 (1 H, d, J 8.5, H-1), 5.29 (1 H, dd, J 10.5 and 8.5, H-2), 5.24 (1 H, dd, J 3.5 and 1.0, H-4), 5.04 (1 H, dd, J 10.5 and 3.5, H-3), 3.93 (1 H, qd, J 6.5 and 1.0, H-5), 2.16 (3 H, s, CH3CO), 2.09 (3 H, s, CH3CO), 2.01 (3 H, s, CH3CO), 1.96 (3 H, s, CH3CO), 1.20 (3 H, d, J 6.5, CH3);
alpha-isomer: dH(300 MHz; CDCl3) 6.30 (1 H, d, J 2.5, H-1), 5.32-5.28 (3 H, m, H-2, H-3 and H-4), 4.24 (1 H, q, J 6.5, H-5), 2.15 (3 H, s, CH3CO), 2.11 (3 H, s, CH3CO), 1.98 (3 H, s, CH3CO), 1.97 (3 H, s, CH3CO), 1.12 (3 H, d, J 6.5, CH3)
B. Iselin, T. Reichstein, Helv. Chim. Acta, 1944, 27, 1200
(a) A. P. Kozikowski, J. Lee, J. Org. Chem., 1990, 55, 863 (perchloric acid); (b) B. K. Shull, B. K., Z. Wu, M. Koreeda, J. Carbohydr. Chem., 1996, 15, 8, 955 (hydrobromic acid)
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