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 [5]-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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