Images were recorded by a Falcon III direct electron detector at full 4k resolution resulting in a pixel size of 0.373 nm/pixel. The defocus value was set to 4.9 µm. The reaction mixture was heated for 4 h at 100 °C, and then cooled to room temperature. The crystals of product PB-1 were filtered off, washed twice with ethanol, and dried in vacuo.