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Vitamin B12 and the genesis of a new theory of chemistry.

Thursday, December 20th, 2012

I have written earlier about dihydrocostunolide, and how in 1963 Corey missed spotting the electronic origins of a key step in its synthesis.[1]. A nice juxtaposition to this failed opportunity relates to Woodward’s project at around the same time to synthesize vitamin B12. The step in the synthesis that caused him to ponder is shown below.

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References

  1. E.J. Corey, and A.G. Hortmann, "The Total Synthesis of Dihydrocostunolide", Journal of the American Chemical Society, vol. 87, pp. 5736-5742, 1965. https://doi.org/10.1021/ja00952a037

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Posted in Interesting chemistry, pericyclic, reaction mechanism | No Comments »

A Disrotatory 4n+2 electron anti-aromatic Möbius transition state for a thermal electrocyclic reaction.

Thursday, April 2nd, 2009

Mauksch and Tsogoeva have recently published an article illustrating how a thermal electrocyclic reaction can proceed with distoratory ring closure, whilst simultaneously also exhibiting 4n electron Möbius-aromatic character[1]. Why is this remarkable? Because the simple Woodward-Hoffmann rules state that a disrotatory thermal electrocyclic reaction should proceed via a Hückel-aromatic 4n+2 electron transition state. Famously, Woodward and Hoffmann stated there were no exceptions to this rule. Yet here we apparently have one! So what is the more fundamental? The disrotatory character, or the 4n/Möbius character in the example above? Mauksch and Tsogoeva are in no doubt; it is the former that gives, and the latter is correct.

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References

  1. M. Mauksch, and S. Tsogoeva, "A Preferred Disrotatory 4<i>n</i> Electron Möbius Aromatic Transition State for a Thermal Electrocyclic Reaction", Angewandte Chemie International Edition, vol. 48, pp. 2959-2963, 2009. https://doi.org/10.1002/anie.200806009

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