124. The [2 + 2] photocycloaddition of cyclopentenone with methyl vinyl ether can produce four products (differing in regiochemistry and stereochemistry). Calculate the energies of the four structures. Use MMX to estimate the vicinal coupling constants of the C-H at the carbon bearing the OCH3 group. Compare your results with the calculated and experimental values in the cited reference.
[Griesbeck, A. G.; Stadtmüller, S.; Busse, H.; Bringmann, G.; Buddrus, J. Chem. Ber. 1992, 125, 933.]
176. A recent article describes the total synthesis of (±)-bilobalide. Shown to the right are two conformations of a compound that is to be subjected to intramolecular [2+2] photocycloaddition. Compute the energies of these conformations and compare the energy difference with that cited for structures 18 and 19 in the article. [Note: for convenience, enantiomeric structures were used in these illustrations.] Similarly, compute the difference in energy between conformations 20 and 21 in the article. Do you agree or disagree with the authors that these differences are related to the excellent stereoselectivity in the first case but not in the second?
[Crimmins, M. T.; Jung, D. K.; Gray, J. L. J. Am. Chem. Soc. 1993, 115, 3146.]
219. Intramolecular [4+4] photo-cycloaddition of the bis-pyridone to the right gives a remarkable product whose X-ray crystal structure reveals that the cyclohexane ring is in a boat conformation. Do MMX calculations on the simplified model compound (to the far right) in its various possible conformations (twist, half-chair) in order to determine which conformation is preferred. Compare your results with those in the article.
[Sieburth, S. M.; Ravindran, K. Tetrahedron Lett. 1994, 35, 3861; Sieburth, S. M.; Lin, C.- H. J. Org. Chem. 1994, 59, 3597.]
251. Irradiation with ultraviolet light sets up an equilibrium between the "Dewar" naphthalene and -paracyclophane derivatives shown to the right. Each of these is strained and each of these exists in two stable conformations (corresponding to different arrangements of the (CH2)5 chain). The authors of the article obtained a 95/5 ratio of conformations of the aromatic compound, but were unable to assign structures to the major and minor components. Do MMX calculations on these two compounds (in their two conformations); see if you can help in the assignment of structure; try removing the two methyl groups and see what effect this has.
[van Es, D. S.; de Kanter, F. J. J.; de Wolf, W. H.; Bickelhaupt, F. Angew. Chem., Int. Ed. Engl. 1995, 34, 2553.]
254. Double [2+2] photodimerization of 2,7- divinylphenanthrene produces a mixture of products: the exo/exo (shown to the right) along with the exo/endo and endo/endo. Do MMX calculations on all three; in particular note the amount by which the phenanthrene rings are splayed out. Compare your results with those in the reference.
[Nakamura, Y.; Tsuihiji, T.; Mita, T.; Minowa, T.; Tobita, S.; Shizuka, H.; Nishimura, J. J. Am. Chem. Soc. 1996, 118, 1006.]
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