On the Effect of Diene Geometry in Intramolecular Allyl Cation Cycloadditions

References

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• Examination of molecular models clearly demonstrates the difficulty of attempting to align the allyl cation and the Z-diene unit so that concerted cycloaddition would be possible. These reactive partners are in fact orthogonal to each other in the most favored conformations.
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• Two minor unidentified isomeric compounds (FW=148) are also indicated by GC/MS (product ratio: 3b:3a:x:x=16:55:05:13, in order of increasing retention time).
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• The major cyclization product shows six olefinic hydrogens in three regions (6.5, 5.8, 5.0 ppm), whereas bicyclic 5 shows five olefinic hydrogens in two regions (5.8, 4.8 ppm). The 13C-NMR data were also dissimilar: bicyclic 5 exhibits four olefinic carbons (157.8, 143.1, 113.1, 104.9 ppm) while the major cyclization product exhibits six olefinic peaks (147.2, 136.5, 135.2, 122.4, 114.6, 112.9 ppm).
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• In addition, dissimilarities in the MS fragmentation patterns of monocyclic trienes 3a,b compared to [5,5]-bicyclic 5 were significant.
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• SODAR is an acronym for Sum Of Double bonds And/or Rings.
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• This trans-rotameric preference in 2a is confirmed by the dominant formation of the trans-[5,7]-hydroazulene from the allyl cation cycloaddition of the Z,E-stereoisomer, confer reference 4.
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• Trans-fusion in [3.3.0] bicyclic ring systems is rare and strongly thermodynamically disfavored.
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• The 1H-NMR data for the tetraene 7 shows nine olefinic hydrogens in four regions (6.65, 6.0, 5.45, 5.0 ppm) and the corresponding 13C-NMR reveals the expected eight olefinic carbons (148.2, 143.1, 133.2, 132.7, 129.6, 117.2, 112.8, 112.4 ppm).
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