17. Do the complete conformational analysis of the perhydrophenanthrenes (six isomers).
[Hönig, H.; Allinger, N. L. J. Org. Chem. 1985, 50, 4630.]
18. Do the complete
conformational analysis of the perhydroanthracenes (five isomers).
[Allinger, N. L.; Wuesthoff, M. T. J. Org. Chem. 1971, 36, 2051.]
19. Do the complete
conformational analysis of the perhydrophenalenes (four isomers).
[Dillen, J. L. M. J. Org. Chem. 1984, 49, 3800.]
22. Do the complete conformational analysis of the various bicyclo[3.n.1]alkanes. For n = 3, calculate the energies of the three reasonable conformations: chair-chair, chair-twist, and twist-twist; comment on the strain factors which affect the relative energies. Similarly, calculate the energies of the chair and twist conformations for the molecule with n = 1 and the one with n = 2.
[Camps, P.; Castane, J.; Feliz, M.; Jaime, C.; Minguillon, C. Chem. Ber. 1989, 122, 1313.]
23. Do the conformational analysis of the bicyclo[2.2.n]alkanes. Calculate the geometries and energies for n = 1, 2, 3, etc. How large does n have to be for the six-membered ring to exist in an unstrained chair conformation?
24. Do the conformational
analysis of the doubly 1,4-bridged cyclohexanes. Calculate the geometries
and energies for n = 1, 2, 3, etc. When n = 2, the molecule is called twistane
- how large must n be before the central ring can exist in an unstrained
48. Consider the case of hexakis(bromomethyl)benzene, whose structure is shown to the right. There are eight distinctly different conformations for this compound (see Figure 4 in the cited reference for a shorthand notation). Compute the relative energies of these eight conformations; compare your values with those in the article; assign the variation in conformational energy to the responsible factors.
[Golan, N. Z. O.; Biali, S. E. J. Org. Chem. 1991, 56, 2444.]
59. Calculate the torsion angles between the various aromatic rings in bimesityl (1), diphenylbimesityl (2), and dimesitylbimesityl (also known as quatermesityl) (3). Compare your answers with those from the X-ray crystal structures given in Table I of the reference.
[Fischer, E.; Hess, H.; Lorenz, T.; Musso, H.; Rossnagel, I. Chem. Ber. 1991, 124 , 783.]
138. The "ufolanes" are structures having one five- and two six-membered rings mutually fused. Unlike the related perhydrophenalenes for which there are only four distinct stereoisomers (Problem 20), there are six distinct isomers (three of which have been synthesized) for the ufolanes. Compute the MMX energies and structures of all six isomers; comment on the sources of the strain which are present in the higher-energy isomers.
[Boldt, P.; Arensmann, E.; Blenkle, M.; Kersten, H.; Tendler,
H.; Trog, R.-S.;
Jones, P. G.; Döring, D. Chem. Ber. 1992, 125, 1147.]
143. For each of the dimethyl-substituted cis-decalins shown to the right, there are three distinct stereoisomers (both methyls alpha, both beta, or one alpha and one beta). Each of these, of course, is conformationally mobile. Compute the MMX energies and the distance between methyl groups of both conformations of each the six stereoisomers [for two of the isomers, the inverting conformations are identical]. Compare yours results with those in Table I of the reference. Discuss the factor(s) responsible for the position of conformational equilibrium in each compound.
[Beeson, C.; Dix, T. A. J. Org. Chem.1992, 57, 4387.]
159. There is a report that suggests that some 1,6-disubstituted-(Z,Z)-1,6- cyclodecadienes might exist preferentially in boat-like rather than chair-like conformations. Do MM calculations on the tub, boat, and chair conformations of the parent compound (R = H) and of its 1,6- dimethyl derivative. Determine if there is a change in relative stabilities of these conformations between the two molecules.
[Hamon, D. P. G.; Krippner, G. Y. J. Chem. Soc., Chem. Commun.1992, 1507.]
217. Like hindered biphenyls, the dibenzocyclooctatetraene molecule (to the right) is chiral with a significant twist angle between the benzene rings; the barrier to racemization is 30 kcal/mol. Do MMX calculations on this compound in both its twisted and all-planar conformations. Then, test the ability of related molecules to be twisted and/or planar by doing calculations on: the two possible dihydro derivatives and the one tetrahydro derivative; also on those compounds with three or two or one CH2 group(s) bridging the ortho positions of a biphenyl nucleus. Discuss the factors that favor twist vs. coplanarity in these structures.
[Burger, U.; Lottaz, P.-A.; Millasson, P.; Bernadinelli, G. Helv. Chim. Acta1994, 77, 851.]
247. trans-anti-trans-Pehrydrophenanthrene (1) is an unstrained molecule with three chair cyclohexane rings. It was recently alleged that introduction of an alpha-methyl at position a and a cyclopentane fused beta at positions c and d (see structure 2) causes ring C (the substituted ring) to adopt a twist conformation. Do MMX calculations on 2 in both a chair and twist conformation and decide which is more stable; see what happens to the relative stability of chair and twist if either the methyl is removed or the cyclopentane is removed. Compare and contrast your results with those in the cited article.
[Dasgupta, S.; Tang, Y.; Moldowan, J. M.; Carlson, R. M. K.;
Goddard, W. A., III J. Am. Chem. Soc.1995, 117, 6532.]
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