Typical experimental procedure for dimerisation of glycals
A precooled solution of acetyl perchlorate (18.4 ml, 0.1 M in dichloromethane) was added dropwise to a stirring solution of the glycal (3.7 mmol) in dichloromethane (18.4 ml) under dry conditions, at -78 °C. Stirring at this temperature was continued for 20 h before quenching with an equivalent volume of saturated NHCO3 solution and warming to room temp. The organic phase was extracted and the aqueous layer washed with dichloromethane (3x20 ml). The combined organic extracts were dried (MgSO4) and solvent removed in vacuo followed by flash column chromatography on silica gel to afford the separated anomeric products.
Data for the axial dimer of tri-O-acetyl-D-glucal
Rf = 0.08 (30% ethyl acetate in light petroleum). delta-H (400 MHz, CDCl3) 2.02 (3H, s, OAc), 2.04 (3H, s, OAc), 2.07 (6H, s, 2xOAc), 2.11 (3H, s, OAc), 2.14 (3H, s, OAc), 2.53 (1H, ddd, J=8.6, 11.2, 7.2, 3.2), 3.95-3.97 (1H, m), 3.97-4.08 (3H, m), 4.19 (1H, dd, J=8.0, 12.4), 4.22-4.30 (1H, dd), 5.00-5.04 (1H, m), 5.06 (1H, apparent t, J=9.7), 5.51 (1H, dd, J=9.7, 11.2), 5.76-5.88 (2H, m, CH=CH), 6.35 (1H, d, J= 3.2, anomeric H). delta-C (100 MHz, CDCl3) 20.62, 20.69, 20.72, 20.97, 46.20, 61.78, 62.37, 64.42, 68.97, 69.33, 69.50, 70.90, 90.42, 124.59, 130.80, 168.28, 169.79, 170.15, 170.21, 170.60, 170.89. vmax 2990, 2964, 2910, 1748 (C=O), 1434, 1367, 1223, 1077 cm-1. (EI) 111, 153, 202, 213, 244, 249, 305, 364, 425, 485 (M-Ac)+. C22H29O12 requires [(M-OAc)+] = 485.1659. Found [(M-OAc)+] = 485.1659. Calcd for C24H32O14: C, 52.94; H, 5.92. Found: C, 52.74; H, 5.88%.
