Electrocyclic Reactions in Organic Synthesis
An often studied electrocyclic reaction is the conrotatory thermal ring-opening of benzocyclobutane. The reaction product is a very unstable ortho-quinodimethane but this molecule can be trapped in an endo addition with a strong dienophile such as maleic anhydride to the Diels-Alder adduct. The chemical yield for the ring opening of the benzocyclobutane depicted in scheme 2 is found to depend on the nature of the substituent R. With a reaction solvent such as toluene and a reaction temperature of 110°C, the yield increases going from methyl to isobutylmethyl to trimethylsilylmethyl. The increased reaction rate for the trimethylsilyl compound can be explained by silicon hyperconjugation as the βC-Si bond weakens the cyclobutane C-C bond by donating electrons.
An biomimetic electrocyclic cascade reaction was discovered in relation to the isolation and synthesis of certain endiandric acids :
Asymmetric electrocyclic reactions are an emerging field in contemporary organic synthesis. The most commonly studied reactions in this field are the 4π Staudinger β-lactam synthesis and the 4π Nazarov reaction; asymmetric catalysis of both reactions have been controlled by use of a chiral auxiliary, and the Nazarov reaction has been performed catalytically using chiral Lewis acids, Brønsted acids and chiral amines.
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