Rapid Communication
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2009, 62, 407–412
Structural Studies on α-Pyrone Cycloadducts. Manifestation
of the Early Stages of CO Extrusion by retro
2
Hetero-Diels–Alder Reaction
A
A
A
Jesse Roth-Barton, Yit Wooi Goh, Asimo Karnezis,
A,B
and Jonathan M. White
A
School of Chemistry and Bio-21 Institute, University of Melbourne, Vic. 3010, Australia.
Corresponding author. Email: whitejm@unimelb.edu.au
B
Structures of the α-pyrone (pyran-2-one) cycloadducts 4–8 show deviations of some bond distances from their normal
values, consistent with manifestation of the early stages of the retro hetero-Diels–Alder decarboxylation reaction in the
ground state structures. Thus the bridgehead C–O(CO) and C–CO(O) bonds are lengthened and the bridging C–O bond is
shortened. The degree of lengthening of the C–O(CO) and C–CO(O) bonds is similar, whereas in the calculated transition
state structure there is significant asymmetry in the extent of C–CO(O) and C–O(CO) bond breaking.
Manuscript received: 9 January 2009.
Final version: 20 March 2009.
Introduction
character during the progress of the reaction, may be shortened
compared with standard values.
α-Pyrone (pyran-2-one) derivatives 1 are valuable synthetic
building blocks that react readily with a variety of dienophiles to
give cycloadducts 2, which aside from the typical retro-Diels–
Alder pathway (Scheme 1, step 1) that can result in endo–exo
isomerization, they can also undergo decarboxylation to afford
CO2 and an alternative diene product 3.[
The decarboxylation reaction is particularly facile when
the dienophile is an alkyne, as the resulting product is aro-
matic. Many examples of highly unsaturated cycloadducts that
undergo facile decarboxylation have been reported. If the
dienophile is an alkene then the driving force for decarboxy-
lation is decreased and the intermediate cycloadduct 2 can often
be isolated and characterized. For example, the reaction of
X-Ray data were obtained at low temperature to minimize
the unwanted effects of thermal libration, and thermal ellipsoid
plots for 4–6 are presented in Fig. 2 (see Fig. 3 for structures).
Selected structural parameters for 4–6 are presented in Table 1,
which also includes structural data for the cycloadducts 7 and
8, which were extracted from the Cambridge Crystallographic
database (CCDC).[
1]
6]
In order to obtain ‘standard’ bond distances for comparison,
the non-reactive saturated adduct 9 was prepared by hydrogena-
tion of 6, and the structures 10–12 (Fig. 3), which contain a
similar carboxylate bridge but also cannot undergo the retro
[2]
Diels–Alder reaction, were extracted from the CCDC.
From the data summarized in Table 1,[
7–11]
it is clear that
α-pyrone with maleic anhydride gives the isolatable cycloadduct
◦
4
, which requires heating at 160 C to bring about decarboxy-
both the bridgehead C–CO(O) (bond a), and C–O(CO) (bond b)
bonds are significantly lengthened in those adducts 4–8 that can
[3]
lation to give cis-1,2-dihydrophthalic anhydride (Scheme 2).
As part of our studies on the structural effects that occur
in cycloadducts that have a propensity towards the retro Diels–
undergo thermally allowed loss of CO compared with the cor-
responding bonds in the ‘saturated analogues’ 9–12, which do
2
[
4]
Alder reaction, we were interested in establishing whether the
decarboxylation reaction, which is formally a retro hetero-Diels–
Alder reaction, would manifest in the ground state structures of
α-pyrone cycloadducts. According to the structure-correlation
not readily lose CO . In addition, the bridging O–CO (bond c)
2
is shortened (but to a lesser degree) in 4–8 than in 9–12. These
structural effects are consistent with the manifestation of the
early stages of decarboxylation in the α-pyrone cycloadducts
4–8. Thus bonds a and b, which break during this reaction, are
lengthened, whereas bond c, which changes from a C–O single
bond to a C=O double bond during the reaction, is shortened.
The degree of lengthening of each of bond a and b compared
with standard values, is similar; does this imply that these
cycloadducts would decarboxylate by a symmetrical transition
state? Our previous experience from structures of cycloadducts
thatcanundergotheretroDiels–Alderreactionwouldsuggestso.
For example, in the cyclohexadiene–benzoquinone cycloadduct
13, bonds a and b are lengthened to the same degree in the
ground state, and the transition state for the retro Diels–Alder
[
5]
principle, structural changes that occur along the reaction
coordinate for a particular reaction can manifest in the ground
state structures as deviations of bond distances and angles from
their ‘normal’distances along the reaction coordinate. This prin-
ciple applies provided that the ground state structure of the
molecule is similar to the transition state for the reaction, as this
allows those orbitals whose interaction facilitates the reaction to
mix in the ground state. The largest effects are on those bonds
that are formally broken during the course of the reaction. Thus
in the cycloadducts 2 (Fig. 1) bonds a and b may be longer than
standard values, whereas bond c, which develops double-bond
©
CSIRO 2009
10.1071/CH09018
0004-9425/09/050407