3.2.3. Acetonide Formation of 1 and 2

A mixture of 1 (1.0 mg), 2,2-dimethoxypropane (2.0 mL), and p-TsOH (0.2 mg) was stirred at room temperature for 0.5 h. Saturated aqueous NaHCO₃ (6.0 mL) was then added, and the reaction mixture was extracted with EtOAc (24 mL × 3). The organic solvents were removed with a high-vacuum pump, and the crude mixture was subjected to preparative HPLC to obtain acetonide product 1a (0.96 mg). Acetonide product 2a (0.91 mg) was obtained from 2 under the same conditions as 1a.

Compound 1a: ¹H NMR (600 MHz, CDCl₃) δ 7.17 (1H, dd, J = 15.1, 10.9 Hz, H-10), 6.74 (1H, dd, J = 15.0, 5.6 Hz, H-11), 6.37 (1H, dd, J = 15.0, 10.9 Hz, H-9), 6.06 (1H, s, H-4), 5.96 (1H, dd, J = 15.0, 7.2 Hz, H-12), 4.59 (1H, m, H-13), 4.35 (1H, s, H-14), 3.88 (3H, m, H-7), 1.94 (3H, s, H-8), 1.38 (3H, s, H-21), 1.34 (3H, s, H-22), 1.30-1.50 (8H, m, H-15/16/17/18), 0.88 (3H, t, J = 6.6 Hz, H-19).

Compound 2a: ¹H NMR (600 MHz, CDCl₃) δ 7.15 (1H, dd, J = 15.0, 11.1 Hz, H-10), 6.43 (1H, dd, J = 15.0, 11.1 Hz, H-11), 6.09 (1H, d, J = 15.0 Hz, H-9), 6.06 (1H, s, H-4), 5.94 (1H, dd, J = 15.0, 7.2 Hz, H-12), 4.09 (1H, m, H-13), 3.88 (3H, s, H-8), 3.70 (1H, m, H-14), 1.95 (3H, s, H-8), 1.26 (3H, s, H-21), 1.25 (3H, s, H-22), 1.30-1.50 (8H, m, H-15/16/17/18), 0.89 (3H, t, J = 6.6 Hz, H-19).