*3.1. Chemistry*

Melting points were obtained on a *KSPI* melting-point meter (A. KRÜSS Optronic, Hamburg, Germany) and were uncorrected. IR spectra were recorded on a Bruker Tensor 27 instrument (Bruker Optik GmbH, Ettlingen, Germany). NMR spectra were recorded on a Bruker 500 MHz spectrometer (Bruker BioSpin, Rheinstetten, Germany). Chemical shifts are reported in ppm downfield from TMS. UV–vis spectra were recorded on a Varian BioChem 100 spectrophotometer. Mass spectra were recorded on a Thermo Scientific ISQ LT instrument (Thermo Fisher Scientific Inc., Waltham, MA, USA). All reagents were commercially available and used without further purification. Elemental analysis, nuclear magnetic resonance data and copies of 13C-NMR spectra are included in the Supplementary Material.

3.1.1. 1-(2-Hydroxy-3,5-diiodophenyl)-1-oxoethan-2-yl *N*,*N*-diethylamino-1-carbodithioate (**2**)

To a solution of 2-bromo-1-(2-hydroxy-3,5-diiodophenyl)ethan-1-one (**1**, 1.4 g, 3 mmol) in acetone (10 mL), a solution of sodium *N*,*N*-diethyldithiocarbamate trihydrate (0.68 g, 3 mmol) in acetone/water (10 mL, 1:1 *v*/*v*) was added. The resulting mixture was refluxed for 10 min, cooled to room temperature and poured into water (100 mL) with vigorous stirring. The precipitate thus formed was vacuum-filtered and recrystallized from ethanol, yielding 1.3 g (81%) of yellow crystals; M.p. = 162–163 ◦C. IR (ATR, cm<sup>−</sup>1) 1699, 1499, 1425, 1245, 1174, 821, 621. 1H NMR (CDCl3) δ 12.71 (s, 1H), 8.30 (d, *J* = 1.6 Hz, 1H), 8.25 (d, *J* = 1.7 Hz, 1H), 4.86 (s, 2H), 4.03 (q, *J* = 6.9 Hz, 2H), 3.83 (q, *J* = 6.9 Hz, 2H), 1.38 (t, *J* = 6.9 Hz, 3H), 1.30 (t, *J* = 6.9 Hz, 3H). 13C NMR (CDCl3) δ 198.1, 192.8, 160.6, 152.9, 138.7, 120.6, 88.2, 80.6, 50.5, 47.3, 43.5, 12.6, 11.5. UV–vis (λmax, nm) 373. MS (EI) (*m*/*z*): 534.8 (M+, 37%) for C13H15I2NO2S2.

3.1.2. General Procedure for 6,8-Diiodo-2-(4-methylphenyl)-4-oxochroman-3-yl *N*,*N*-diethyldithiocarbamate (**4a**)

To a solution of 1-(3,5-diiodo-2-hydroxyphenyl)-1-oxoethan-2-yl *N*,*N*-diethyldithiocarbamate (**2**) (0.268 g, 0.5 mmol) in a mixture of CHCl3/MeOH (12 mL, 1:1 *v*/*v*) aminal **3a** (0.13 g, 0.5 mmol) was added and the reaction mixture was heated under reflux for 4 h. After cooling, the solid material was filtered off and purified by recrystallization from ethanol to give **4a** (0.23 g, 72%) as colourless crystals. IR (ATR, cm<sup>−</sup>1) 2738, 1698, 1419, 1255, 1203, 963, 811, 506, 485, 430. 1H NMR (CDCl3, selected data for the major isomer) δ 8.26 (d, *J* = 1.7 Hz, 1H), 8.13 (d, *J* = 1.7 Hz, 1H), 7.38 (d, *J* = 7.7 Hz, 2H), 7.16 (d, *J* = 7.7 Hz, 2H), 6.01 (d, *J* = 6.2 Hz, 1H), 5.75 (d, *J* = 6.2 Hz, 1H), 3.99 (m, 2H), 3.68 (m, 2H), 2.35 (s, 3H), 1.25 (t, *J* = 6.9 Hz, 6H). 13C NMR (CDCl3, selected data for the major isomer) δ 191.6, 186.2, 158.8, 152.6, 138.7, 136.3, 132.8, 129.3, 127.2, 122.9, 87.6, 84.7, 83.0, 57.8, 50.5, 47.3, 21.2, 12.6, 11.4. UV–vis (λmax, nm) 314. MS (EI) *m*/*z*: 636.8 (M+, 17%) for C21H21I2NO2S2.
