Scheme 2. Reagents and conditions: (a) sodium azide, aqueous ammonium chloride solution, methanol, 70 °C, 4 h; (b) 4-pentynoic acid methyl ester, tert-butanol, water, aqueous sodium ascorbate solution, aqueous copper(ii) sulfate solution, room temperature, 24 h; (c) Dess–Martin periodinane, dry CH2Cl2, room temperature, 5 h; (d) aqueous KOH solution, methanol, room temperature, 3–5 h.
