3.1.15. (3*S*,4*S*)-5,8-dihydroxy-4-methoxy-1-oxo-3-pentylisochromane-7-carbaldehyde (**12**)

To a solution of diMOM aldehyde derivative **12a** (10.0 mg, 25.2 μmol) in THF (1.9 mL) was added 6 M aqueous HCl (0.63 mL) at 0 ◦C. After stirring for 4 h at room temperature, the reaction was quenched by adding saturated aqueous NaHCO3 at 0 ◦C. The mixture was extracted with EtOAc (×3) and the combined organic layers were washed with brine, dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified with PTLC (EtOAc:*n*-hexane = 2:3) to give benzaldehyde derivative **12** (6.0 mg, 77%) as a pale yellow solid. m.p. 170 ◦C (dec); 1H-NMR (400 MHz, CDCl3) δ 11.33 (1H, s), 10.47 (1H, s), 7.70 (1H, d, *J* = 1.5 Hz), 6.62 (1H, br-s), 4.75 (1H, d, *J* = 2.2 Hz), 4.49 (1H, ddd, *J* = 2.2, 5.6, 8.0 Hz), 3.43 (3H, s), 2.03 (1H, s), 1.88 (1H, m), 1.61 (1H, m), 1.48 (1H, m), 1.42–1.30 (4H, overlapped), 0.92 (3H, t, *J* = 6.8 Hz); 13C-NMR (125 MHz, CDCl3) δ 189.0, 168.8, 158.9, 146.0, 131.3, 124.9, 121.5, 110.2, 82.2, 69.2, 57.9, 31.9, 30.3, 25.1, 22.8, 14.3.; IR (KBr) 3444, 3169, 2953, 2940, 2920, 1676, 1455, 1395, 1299 cm<sup>−</sup>1; HRMS (ESI) *m*/*z* (M + H)<sup>+</sup> calculated for (C16H21O6) <sup>+</sup> 309.1338, found 309.1342.
