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Goh et al.
SCHEME 1
in the lengths of bonds a and b in the ground state in accord
with previous observations on the cyclohexadiene cycloadduct
8.4 Thus the exo cycloadducts 21 and endo cycloadduct 22 were
prepared by combining 2-methoxyfuran with maleic anhydride
and N-methylmaleimide, respectively. Crystals of suitable
quality for X-ray analysis were obtained at -20 °C. Selected
structural data for 21 and 22 are presented in Table 6, in addition
to the previously determined cycloadduct 23,4 which is included
here for comparison.
1 and 2. This appeared to be counter to the predictions of the
structure correlation principle. A possible explanation is that
the presence of the electronegative oxygen of the furan moiety
decreases the “natural” C-C bond distances (a and b) to the
bridgehead carbons, suggesting that perhaps a direct comparison
between these bond distances with the corresponding distances
in the cyclopentadiene cycloadducts is not justified. Thus it was
deemed more appropriate to determine the degree of lengthening
of the C-C bond distances in the furan cycloadducts compared
with their corresponding saturated analogues, which cannot
undergo the rDA reaction, and compare this to the degree of
lengthening of the C-C bonds in the cyclopentadiene cycload-
ducts with their saturated analogues. After reduction of the furan
cycloadducts 12-17 by hydrogenation the only suitable crystals
for X-ray analysis were obtained for the saturated exo furan
maleic anhydride cycloadduct 20, which was compared with
the exo cycloadduct 15. Selected data for compounds 2, 18,
15, and 20 are presented in Table 5.
Surprisingly, there was found to be very little difference in
the bond distances a and b in both 21 and 22, which contrasts
with the result obtained for 23 where the difference is much
more significant. An explanation for this “anomalous” result is
provided by examination of the conformation of the OCH3
substituent as defined by the CH3-O-C3-C2 dihedral angle.
In compound 23 the CH3-O-C3-C2 dihedral angle is 69.1-
(2)°, which allows for effective overlap of a nonbonded orbital
on the oxygen with the C3-C2 σ* orbital (Figure 2); it is this
interaction that was proposed to cause the extra lengthening of
bond a compared to bond b.4 However, in compounds 21 and
22 the CH3-O-C3-C2 dihedral angles are 177.9° and 180.0°,
respectively, which places the OCH3 group antiperiplanar to
the C3-C2 bond, resulting in overlap of a nonbonded orbital
on the oxygen with the bridging C-O σ* orbital (Figure 2)
rather than with the C3-C2 σ* orbital. Consistent with this is
significant asymmetry in the two C-O bond distances involving
the bridging oxygen (Table 6). The C3-O3 distance is
significantly longer than the C6-O3 distance, which is an
example of the structural anomeric effect that has been described
previously for other oxygen-containing systems.15
From these data it is quite clear that the “natural” C-C bond
distances to the bridgehead carbons as provided by the saturated
furan cycloadduct 20 are significantly shorter than the similar
C-C bonds in the saturated cyclopentadiene adduct 18, thus
demonstrating that the electronegative oxygen does indeed exert
a marked “bond-shortening” effect. This effect can be more
generally demonstrated from a search of the Cambridge Crystal-
lographic database.13 Molecules containing the C-C-O frag-
ment gave a mean C-C distance of 1.531 Å while similar
C-C-C fragments had a mean C-C distance of 1.547 Å.
Importantly, these data show that the degree of bond lengthening
observed in the furan cycloadduct 15 relative to the saturated
analogue 20, which is 0.042 Å, is significantly greater than the
degree of bond lengthening observed in the cyclopentadiene
cycloadduct 2, which is 0.023 Å. Thus, according to the structure
correlation principle,5 this is consistent with the greater reactivity
of the furan cycloadducts toward the rDA reaction.
Interesting, upon standing at room temperature, cycloaducts
21 and 22 both underwent decomposition giving the aromatic
products 24 and 25, respectively, by initial cleavage of the
C3-O bond followed by aromatization by elimination of H2O.
The initial step of this rearrangement no doubt involves the same
interaction causing lengthening of the C3-O bond distance
(Scheme 1).
Conclusions
Relationships between structure and reactivity are demon-
strated by comparing the trimethysilylcyclopentadiene-maleic
anhydride cycloadduct 9 with the silicon-free analogue 1, and
by comparison of the endo and exo isomers of furan cycload-
ducts 13 and 16. The C-C bonds, which break in the retro-
Diels-Alder (rDA) reaction for these structures, are slightly
longer in the more reactive derivatives. This effect is small,
thus demonstrating that small increases in C-C bond distance
have a large effect on the rDA reactivity. However, care must
be taken when comparing C-C bond distances in unlike
systems, for example, the bond distances for the C-C bonds
The structural effects arising in the cycloadducts of 2-meth-
oxyfuran with maleic anhydride and N-methylmaleimide were
investigated, as previous reports suggest that cycloadditions
involving 2-methoxyfuran as dienophile proceed in a stepwise
fashion.14 We therefore expected there to be marked differences
(13) Allen, F. H.; Bellard, S.; Brice, M. D.; Cartwright, B. A.; Doubleday,
A.; Higgs, H.; Hummelink, T.; Hummelink-Peters, T.; Kennard, O.;
Motherwell, W. D. S.; Rogers, J. R.; Watson, D. G. Acta Crystallogr. 1979,
B35, 2331.
(14) Bridson, J. N.; Bennett, S. M.; Butler, G. J. Chem. Soc., Chem.
Commun. 1980, 413.
(15) Kirby, A. J. The Anomeric Effect and Related Stereoelectronic
Effects at Oxygen; Springer-Verlag: New York, 1983.
154 J. Org. Chem., Vol. 73, No. 1, 2008