systems.6 We have successfully extended this classic photo-
chemical reaction to the preparation of anthracene bisimides
and anthracene-based polyimides and polyesters.7 Our ap-
proach employs bis(o-methylbenzophenone) derivatives, such
as 2,5-diaroyl-p-xylenes, to create the framework for the
polycylic aromatic backbone via two tandem enolization and
trapping sequences. The structure of these diketones is critical
since it dictates the geometry of the resulting oligoacene
backbone. Once these diketones are obtained, a library of
molecules with a specific aromatic backbone can be readily
accessed by varying the structure of the dienophile.
Our prior efforts have focused on diketones which afford
linear aromatic systems. Recently, we have extended this
chemistry to the synthesis of angular arenes, including
sterically congested phenanthrenes, 1, and the first example
of a benzo[e]pyrene bisimide, 2.8 Steric crowding within the
o-xylylenol can inhibit Diels-Alder trapping by hindering
its access to dienophile. In addition, sterically congested
o-xylylenols are well-known to undergo cyclization to the
corresponding benzocyclobutanols.9 Herein, we report the
results of this study.
unsuccessful due to a competing polymerization of the
acrylate.11 Trapping with dimethyl acetylenedicarboxylate
required a large excess of dienophile (10 equiv) and only
produced bisadduct 4d in 30% yield. This is due to the fact
that 4d is itself a good dienophile and can be consumed by
reaction with o-xylylenols. Serendipitously, the highest yield,
86%, was obtained with N-octylmaleimide. Conversion of
bisadducts into the corresponding phenanthrenes, 1a-c, was
achieved in good overall yield using standard acid-catalyzed
dehydration followed by aromatization with DDQ.
While o-xylylenols produced from 3 are sterically crowded,
the reasonable yields obtained by cycloaddition with N-
phenylmaleimide and N-octylmaleimide suggested that trap-
ping of sterically congested o-xylylenols is possible with
compact, reactive dienophiles. To explore this, we turned
our attention to the trapping of o-xylylenols produced from
1,4-dibenzoyl-9,10-dihydroanthracene, 11. Successful trap-
ping of these o-xylylenols would afford access into a new
class of more complex aromatic bisimides based upon benzo-
[e]pyrene.
Diketone 11 was prepared from commercially available
leucoquinazarin 6 as outlined in Scheme 2. Treatment of 6
The synthesis of phenanthrenes, 1, is outlined in Scheme
1. 3,6-Dibenzoyl-o-xylene, 3, was prepared in 80% overall
yield from the reaction of phenylmagnesium bromide with
3,6-dicyano-o-xylene.10
Scheme 2
Scheme 1
with excess PCl5, followed by refluxing the resulting
chlorinated intermediate in 1-pentanol, produced 1,4-dichlo-
(8) Typical benzo[e]pyrene derivatives do not include bisimide func-
tionality. For examples, see: (a) Pothuluri, J. V.; Evans, F. E.; Heinze, T.
M.; Cerniglia, C. E. Appl. Microbiol. Biotechnol. 1996, 45, 677, (b) Lee,
H.; Shyamasundar, N.; Harvey, R. G. J. Org. Chem. 1981, 46, 2889, (c)
Lehr, R. E.; Tayler, C. W.; Kumar, S.; Mah, H. D.; Jerina, D. M. J. Org.
Chem. 1978, 43, 3462, (d) Wood, A. W.; Levin, W.; Thakker, D. R.; Yagi,
H.; Chang, R. L.; Ryan, D. E.; Thomas, P. E.; Dansette, P. M.; Whittaker,
N.; Turujman, S.; Lehr, R. E.; Kumar, S.; Jerina, D. M.; Conney, A. H. J.
Biol. Chem. 1979, 254, 4408.
(9) (a) Sammes, P. G. Tetrahedron 1976, 32, 405, (b) Itoh, Y.; Kano,
G.; Nakamura, M. J. Org. Chem. 1998, 63, 5643.
(10) Obtained from the Spectrum Group, Ltd., and prepared following a
modification of the methods previously reported by: (a) Lai, Y.; Hui-Tin
Yap, A. J. Chem. Soc., Perkin Trans. 2 1993, 1373. and (b) Doyle, M. P.;
Siegfried, B.; Dellaria, J. F. J. Org. Chem. 1977, 42, 2426.
Photolysis of an N2-degassed benzene solution of 3 and a
slight stoichiometric excess (2.2 equiv) of either dimethyl
fumarate, N-phenyl- or N-octylmaleimide produced the
corresponding bisadducts 4a-c in good to moderate yields.
Attempts to use methylacrylate as the dienophile proved
(6) (a) Yang, N. C.; Rivas, C. J, J. Am. Chem. Soc. 1961, 83, 2213, For
an excellent review of this subject, see: (b) Sammes, P. G. Tetrahedron
1976, 32, 405.
(7) (a) Ilhan, F.; Tyson, D. S.; Meador, M. A. Chem. Mater. 2004, 16,
2978, (b) Meador, M. A. B.; Meador, M. A.; Williams, L. L.; Scheiman,
D. A. Macromolecules 1996, 29, 8983.
578
Org. Lett., Vol. 8, No. 4, 2006