Search for the pseudopericyclic nature of [1,7]-sigmatropic rearrangements
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The stereochemistry of pericyclic [1,7]-sigmatropic migrations of hydrogen atoms in hydrocarbons is known to be antarafaical. However, qualitative theories suggest that a [1,7]-hydrogen migration would also be allowed via a planar pathway. A computational survey of several 5-hydroxy-2,4-diene systems, calculated at the B3LYP/6-31G (d,p) level of theory, suggested that bicyclo[3.3.0]octa-1-ene-2,8-dicarbaldehyde (1) had a suitable geometry to permit an in-plane arrangement of the eight central atoms at the transition state . The activation energies for the hydrogen migration were found to be 2.56 kcal/mol, 0.02 kcal/mol and -2.82 kcal/mol when calculated at the MP2, B3LYP and CCSD (T) levels of theory respectively. These low barriers and also a planar transition state, suggested a pseudopericyclic mechanism for the [1,7]-sigmatropic hydrogen rearrangement in 1. It was then proposed to design a synthesis for 1 and test experimentally, the above prediction that [1,7]-sigmatropic rearrangements of hydrogen are pseudopericyclic in nature. Two retrosynthetic schemes were suggested, with the aim of constructing a bicyclic framework with the right number of carbons at the desired oxidation states. The first scheme involved the formation of the bicyclic ring through the Weiss condensation. However, the reduction reactions to get the seven central atoms to the proper oxidation states proved to be unsuccessful. The second scheme involved the synthesis of 1 by an epoxide ring opening reaction followed by a cyclopentannulation as the key steps. The experimental investigation of this scheme resulted in the development of a synthetic method for 1, with ethyl 2-oxo-cyclopentanoate as the starting material. Preliminary NMR studies suggest that the [1,7]-pseudopericyclic rearrangement is possible in 1.