*2.1. Synthesis*

Hydroquinones **1**–**4** were prepared following a literature procedure for the parent compound [37], which can be followed in Figure 2. Initially, 2,5-dimethoxybenzoic acid was reacted with oxalyl chloride in dry toluene to afford the acid chloride. Immediate reaction with the respective aniline derivative under basic conditions resulted in the preparation of dimethoxybenzamide compounds **9**–**12** in yields of between 38–69%. Subsequently, the compounds were reacted with boron tribromide at 0 ◦C to yield the hydroquinone series. A number of methods were attempted to successfully oxidise the hydroquinones to their quinone forms. Initially, reactions of the hydroquinone compounds with FeCl3 and cerium ammonium nitrate respectively did not lead to the formation of product. This may be due to the instability of the final product in methanol, which caused conversion back to the hydroquinone species**,** and separation problems as a consequence of additional products. Incomplete conversion to the quinone product in solution led to the formation of a purple compound, identified as a sandwich quinhydrone-like complex which contains one unit each of hydroquinone and quinone [38].

**Figure 2.** Synthetic procedure for the hydroquinone series and subsequent oxidation to the analogous quinone species.

Conversion was successfully achieved by adding silver oxide to a suspension of the hydroquinone derivatives in dichloromethane [39]. Only the quinone compound enters the solution, meaning the formation of quinhydrone can be avoided. The quinone series was successfully isolated with yields ranging from 52–83%. All new compounds were characterized by 1H-NMR and 13C {1H}-NMR. The dimethoxybenzamide and hydroquinone compounds were characterized using Electrospray Ionization (ESI) mass spectrometry, and the quinones using Atmospheric-Pressure Chemical Ionization (APCI) mass spectrometry. Full details and characterization are provided in the Supplementary Materials.

Crystals of Compound **3** were grown by slow evaporation of an acetonitrile solution of the receptor and the structure was elucidated by single crystal X-ray diffraction (CCDC 1919631). The structure (Figure 3a) shows the receptor adopting the expected conformation in the solid state with an intramolecular hydrogen bond between hydroquinone oxygen O17 and amide oxygen O10 (2.52(4) Å). Crystals of Compound **4** were grown in a similar fashion from a saturated acetonitrile solution and the structure was elucidated by single crystal X-ray diffraction (CCDC 1919630). In this case, the compound crystallized as the acetonitrile solvate with the acetonitrile bound via a hydrogen bond to the hydroquinone oxygen (O2 ... N2 2.818(9) Å) (Figure 3b). There was an additional intramolecular hydrogen bond between amide N1 and hydroquinone O2 (N1 ... O2 2.645(8) Å) stabilizing the solvate in an alternate conformation to that adopted by Compound **3** in the solid state.

**Figure 3.** X-ray crystal structures of free receptors **3** (**a**) and **4** (**b**).
