General procedure for the synthesis of 2,3-O-isopropylidene-D-erythroketonolactone from 2,2-dimethoxypropane and (3R,4R)-3,4-dihydroxydihydrofuran-2(3H)-one: Na2CO3 (42.4 g, 0.400 mol) was added batchwise to aqueous (500) solution of D-isoscorbic acid (35.2 g, 0.200 mol) at 0 °C. mL) solution. Subsequently, H2O2 (31.3% w/w, 44.0 mL, 0.450 mol) was added slowly and dropwise over 30 min. The reaction solution was stirred at 0 °C for 30 min, then warmed up to 40 °C and continued stirring for 1 h. Decolorized charcoal (Norit A, 8.0 g) was added to break down the excess peroxide and stirred until the starch-iodide test was negative (about 30 min). The reaction mixture was filtered through Celite and washed with water. The filtrate was acidified to pH 1 with 6 M HCl and subsequently concentrated under reduced pressure. To the residue was added acetone (175 mL) and MgSO4 (50 g), stirred and then 2,2-dimethoxypropane (350 mL, 2.85 mol) and TsOH-H2O (420 mg, 2.21 mmol) were added sequentially. The reaction mixture was stirred at room temperature for 16 h. Concentrated aqueous NH4OH (20 mL) was added and stirring was continued for 10 min. The reaction mixture was diluted with Et2O (500 mL) and filtered. The filter cake was washed with Et2O (300 mL), the organic phases were combined and concentrated under reduced pressure. The residue was dissolved in Et2O, dried by adding MgSO4 (10 g), filtered through Celite and the filtrate concentrated under reduced pressure. Purification by recrystallization (Et2O/30-40 °C petroleum ether) afforded the target product 2,3-O-isopropylidene-D-erythroketonolactone (13.5 g, 43%, >99:1 dr) as a light yellow solid. [α]25D = -113 (c 1.0 in H2O); literature value [α]25D = -120 (c 1.0 in H2O); melting point 55-58 °C; literature value melting point 64-65 °C; 1H NMR (400 MHz, CDCl3) δ 1.41 (3H, s, MeCMe), 1.49 (3H, s, MeCMe), 4.39- 4.50 (2H, m, C(5)H2), 4.76 (1H, d, J=5.7 Hz, C(3)H), 4.89 (1H, dd, J=5.7,3.8 Hz, C(4)H).
