and used as received, except for pyrrole, which was distilled and
stored in the freezer. Silica gel 60 (0.04–0.063 mm particle
size, 230–400 mesh, Merck) was employed for flash chromatography.
Silica gel 60 preparative thin-layer chromatographic plates (20 cm
× 20 cm × 0.5 mm, Merck) were used for final purification
of all compounds.