The legacy of Hans Fischer - revision of one of the largest libraries of pyrroles and porphyrins

• Hans Fischer’s collection of compounds represents one of the largest chemical libraries of pyrroles and porphyrins. • The structures of the compounds are now being re-evaluated using state-of-the-art analytical tools. • Most of the historical compounds analyzed so far are still intact and exhibit the same structures as originally described by Hans Fischer and his co-workers. • The collection will be fully digitized, presented, and made accessible. The chemist and Nobel Prize winner Hans Fischer (1881-1945) pioneered the analysis and synthesis of pyrroles and pyrrole-derived pigments such as heme and chlorophyll. Numerous pyrroles, dipyrroles, tetrapyrroles, and porphyrins were synthesized for the first time under his guidance. The structural analysis of these novel compounds was based on the limited tools that were available at the beginning of the last century, such as elemental analysis, melting point determination, chemical degradation, and a handheld optical spectroscope. Many of the compounds synthesized and analyzed by Fischer and his co-workers were kept in ampoules and preserved as a great historical collection. Only in 2022 was this chemical library reopened to be characterized using modern, state-of-the-art analytical methods. More specifically, NMR, GC-MS, FT-IR, and UV/Vis experiments were carried out - mainly by students of the Technical University of Munich (TUM) - to confirm or refute the structures proposed by Hans Fischer and his co-workers. To date, most of these compounds have been found to remain intact and to match the structures that were originally assigned to them. Here, we present a first set of exemplary data on four historical samples: 2-chloro-1-(3,5-dimethyl-1H-pyrrole-2-yl)ethanone, ( Z )-2-(3,5-dimethyl-1H-pyrrol-2-yl)-2-oxoacetaldehyde oxime, ethyl-5-(chloromethyl)-4-ethyl-3-methyl-1H-pyrrole-2-carboxylate, and ( Z )-2-((3,5-dimethyl-2H-pyrrol-2-ylidene)methyl)-3,5-dimethyl-1H-pyrrole (monohydrobromide).