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2,2-Dimethoxypropane (87 ml, 0.707 mol) and p-toluenesulphonic acid monohydrate (1.33 g, 7 mmol) were added to a stirred solution of the diol 4 (14.39 g, 70 mmol) in dichloromethane (319 ml) at 25° C. The reaction was monitored by TLC (ethyl acetate-hexanes 2:1) until complete (36 h). The reaction mixture was then washed with aqueous NaHCO3 (5%, 2×50 ml), brine (50 ml), dried (MgSO4) and concentrated to form a colourless oil, which crystallised upon standing. The ratio of the desired five-membered ring product 5 to the undesired six-membered ring product 6 was 6.4:3.6. Recrystallisation from heptane gave 5 as colourless needles (8.2 g, 48%). Spectral data for this sample were consistent with those given in the literature3. 1 H NMR (200 MHz, CDCl3) 4.17 (m, 1H), 3.97 (m, 1H), 3.86-3.42 (m, 3H), 3.33 (br, 1H), 1.76 (m, 2H), 1.5 (s, 3H), 1.46 (s, 9H); 13C NMR (50 MHz, CDCl3) 153.9, 93.6, 80.9, 68.2, 58.6, 53.9, 37.7, 27.7, 26.3, 243.

1. washThe reaction mixture was then washed with aqueous NaHCO3 (5%, 2×50 ml), brine (50 ml)
2. dry with materialdried (MgSO4)
3. concentrationconcentrated
4. customto form a colourless oil, which
5. customcrystallised
6. customRecrystallisation from heptane