Natural Products
95. Kasallene is
an optically active brominated allene which is isolate from a red alga.
Shown to the right is the acetonide derivative of the diol (OH groups at
C9 and C10 in the natural product). The lH-NMR
vicinal coupling constants were used to establish the stereochemistry at
C9 and C10. Use MM to calculate the energy and all
of the coupling constants expected for the 9alpha,10alpha
and of the 9beta,10beta derivatives. Compare your results
with both the calculate and experimental results in the cited reference.
Indicate whether or not you agree with the authors' assignment of structure.
[Öztunç, A.; Imre, S.; Wagner, H.; Norte, M.; Fernández,
J. J. González, R. Tetrahedron Lett. 1991, 32,
4376.]
99. The natural product
3-carene is alleged to have a planar (or nearly planar) six-membered ring,
rather than the (expected?) boat. Do MM calculations on the boat and planar
forms to see which, in fact, is more stable; compare your dihedral angles
with those in the cited article. Then, in order to understand why
the six-membered ring is flattened, do MM calculations on: the non-methylated
compound; on all of the possible mono-methyl compounds (methyl at
C3 or at C7, either endo or exo); and on the various
di-methyl compounds.
[Tkachev, A. V.; Denisov, A. Y. J. Chem. Soc., Mendel Commun.
1991, 98.]
152. In a total
synthesis of the diterpene 3beta-hydroxykemp-7(8)- en-6-one (the
structure to the right with a double bond between C7 and C8),
it was discovered that the unconjugated isomer (double bond at C8-C9
and Hbeta at C7) does not isomerize to it. Do MMX calculations
on these two compounds and on isomers with the double bond exocyclic (between
C8 and the methyl carbon) and with H7 alpha
as well as beta. Compare your answers with those in Table I of the
reference; discuss those factors that lead to greater stability of the
unconjugated isomer.
[Paquette, L. A.; Sauer, D. R.; Cleary, D. G.; Kinsella, M.
A.; Blackwell, C. M.; Anderson, L. G. J. Am. Chem. Soc. 1992,
114, 7375; for a recent AM1 calculation that tends to refute the
MMX results, see Taber, D. J. Tetrahedron Lett. 1993, 34,
1883.]
192. 18-Oxo-3-virgene
is a diterpene that was recently isolated from tobacco plants. It is one
of the four possible stereoisomers (at the two undefined stereo centers)
of the structure shown to the right. Another of these stereoisomers was
synthesized and then transformed into the natural product. Calculate the
energies of all four structures and compare your results with those in
the reference; comment on the factors responsible for the variation of
energy with structure.
[Wang, X., Paquette, L. A. Tetrahedron Lett. 1993,
34, 4579; Paquette, L. A.; Wang, X. J. Org. Chem. 1994,
59, 2052.]
224. It was recently
reported that diene 1a isomerizes in acid to its isomer 2a
and that the latter does not isomerize to 3a; both of these
facts run counter to expectations based on alkyl stabilization of alkenes;
they are also in contrast to the behavior of natural product cis-alpha-himachalene
(2b) which does isomerize to 3b (and maybe 1b). Use
MMX calculations to see if an explanation for this behavior can be derived.
[Gorman, D. B.; Gassman, P. G. J. Org. Chem., 1995,
60, 977.]
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