3.1.28. (3*S*,4*S*)-5,8-dihydroxy-7-iodo-4-methoxy-3-pentylisochroman-1-one (**20**)

To a solution of **3** (12.6 mg, 38.8 μmol) in DMF (0.35 mL) was added the solution of *N*-iodosuccinimide (17.5 mg, 77.6 μmol) in DMF (50 μL) at room temperature. After stirring for 3 h at room temperature, the reaction was quenched by adding saturated aqueous NaHCO3 at 0 ◦C. The mixture was extracted with CH2Cl2 (×3) and the combined organic layers were washed with brine, dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was pathed through SiO2 plug and the resultant mixture of monoMOM iodo derivative **20a** was used for the next reaction without further purification. To a solution of crude mixture of **20a** in MeOH (0.83 mL) was added 6 M aqueous HCl (0.30 mL) at 0 ◦C. After stirring for 5 h at 40 ◦C, 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 = 1:9) to give iodo derivative **20** (4.0 mg, 87%) as a pale-yellow oil. m.p. 109–110 ◦C; 1H-NMR (500 MHz, CDCl3) δ 11.44 (1H, s), 7.57 (1H, s), 6.11 (1H, br-s), 4.51 (1H, ddd, *J* = 2.8, 5.4, 8.5 Hz), 3.40 (3H, s), 1.94 (1H, m), 1.85 (1H, m), 1.75–1.50 (4H, overlapped), 1.45 (1H, m), 1.40–1.30 (4H, overlapped), 0.91 (3H, t, *J* = 7.0 Hz); 13C-NMR (125 MHz, CDCl3) δ 168.3, 155.3, 146.3, 133.8, 122.6, 107.1, 85.5, 81.5, 69.8, 56.9, 31.5, 29.7, 24.8, 22.5, 14.0; IR (KBr) 3293, 2977, 298, 2857, 1674, 1427, 1197 cm<sup>−</sup>1; HRMS (ESI) *m*/*z* (M + Na)<sup>+</sup> calculated for (C15H19O5Ina)<sup>+</sup> 429.0175, found 429.0174.

### *3.2. Bactericidal Assay*

Methicillin-susceptible *Staphylococcus aureus* (MSSA) ATCC25923 and methicillin-resistant *Staphylococcus aureus* (MRSA) ATCC 33,591 were aerobically incubated at 37 ◦C in Luria–Bertani medium (LB, Nippon Becton Dickinson Company, Tokyo, Japan). *Porphyromonas gingivalis* W83 was anaerobically incubated at 37 ◦C in Gifu anaerobic medium (GAM, Nissui, Tokyo, Japan). Each culture (20 μL) prepared to an optical density of 1.5 at 600 nm were appropriately incubated with various concentrations of synthesized compounds in 200 μL of culture medium at 37 ◦C for 24 h in 96-well plate (Thermo scientific, MA, USA). Compounds were dissolved in DMSO (Wako, Osaka, Japan). The degree of turbidity in the broth culture was measured at absorbance 600 nm using microplate reader (Thermo scientific, MA, USA).
