52nd International Chemistry Olympiad, Istanbul, TURKEY
Preparatory problems: THEORETICAL
Problem 11. Benzoporphyrin The name "porphyrin" derives from the Greek word porphyra, meaning purple. Porphyrins are a group of macrocycle organic compounds, composed of four modified pyrrole subunits. They have a total of 26 π-electrons, 18 of which form a planar porphyrin ring structure. They are often described as aromatic. Metal complexes derived from porphyrins occur naturally. One of the best-known families of porphyrin complexes is heme, the pigment in red blood cells. A benzoporphyrin is a porphyrin with a benzene ring fused to pyrrole unit(s). 11.1. Benzoporphyrins can be prepared starting from a masked pyrrole derivative E. The synthesis of E starts with a reaction of cis-1,2-dichloroethene and thiophenol to give A. Oxidation of A yields B having phenylsulfonyl units. The cis-product B is then converted to its trans isomer C when treated with a catalytic amount of Br2 under UV light. The Diels–Alder reaction between C and 1,3-cyclohexadiene under thermal conditions gives the product D, which is converted to a pyrrole carboxylic acid ester when reacted with ethyl isocyanoacetate. Ester then is treated with TFA to give the pyrrole derivative E. Draw the structures of compounds A–E including stereochemistry when necessary.
11.2. Porphyrins can easily be prepared via a cyclization reaction of pyrrole derivatives with aldehydes. Draw the structure of aldehyde F and determine the oxidation state of zinc in compound H.
73
