Figure 2.

Synthesis of BD750: a mixture of compounds 1 (5.5 g, 33.3 mmol) and 2 (5.0 g, 29.4 mmol) in toluene (70 mL) with a catalylic amount of acetic acid (0.1 mL) was refluxed for 5 h. The reaction was checked by TLC (Merck precoated 60F₂₅₄ plates), and spots were detected by viewing under a UV light, colourizing with charring after dipping in 5% sulfuric acid and ethanol solution. After completion of the reaction, the solvent was evaporated under reduced pressure. BD750 (3.6 g, 45% yield) was recrystalized from ethanol as a yellow amorphous powder.