aromaticity and delocalized electronic structure of the
polycyclic system and confirm features prerequisite for
materials applications of the molecules.
Our synthetic approach to 3 begins from triester 6
(available from 5 in three steps as reported previously8d) that
is easily saponified to give its corresponding triacid 7
(Scheme 1). Intermediate 7 can alternatively be prepared
more directly via dimethylation and hydrolysis of 5, as
shown. Subsequent dehydrative lactonization of 7 with
polyphosphoric acid (PPA) affords a modest, but reproduc-
ible, supply of benzotrifuranone 3 on a gram scale. The
identity of highly symmetric 3 is quickly confirmed by NMR
analysis (DMSO-d6) where it shows a single peak in the 1H
NMR spectrum at δ ) 4.01 ppm for the six chemically
equivalent methylene protons and just four peaks by 13C
NMR analysis (δ ) 30.1 (CH2), 101.9 (O-CdC), 148.8
(O-CdC), and 173.4 (CdO) ppm).9 Further proof is
available from X-ray crystallographic analysis (vide infra).
The compound is isolated as an off-white powder that is
reasonably soluble in organic solvents that include DMSO,
THF, CH2Cl2, and CHCl3. Alternative strategies to synthesize
3 are conceivable (and have been attempted) based on those
reported for simpler 1 in the literature. Scheme 1 emerges
as particularly successful since it reduces contamination by
partially cyclized intermediates (e.g., benzofuranones and
benzodifuranones) that are difficult to separate from or further
convert to the target.10
Figure 1. Benzofurans 2 can be accessed via O-acylation of
benzofuranones 1. Derivatives shown have been prepared as model
compounds in the current study.
Scheme 1
.
Synthesis of Benzotrifuranone 3 and Heteroaromatic
Benzotrifurans 4
A single crystal of 3 could be obtained by slow diffusion
of pentane into its chloroform solution; the X-ray crystal
structure is shown in Figure 2, and experimental and
as the immediate precursor to representative benzotrifuran
derivatives 4. Demonstrated for 3 are rare polar crystal
formation and subtle electronic coupling between the three
lactone rings, properties incommensurate with its simple
structure and high symmetry. Subsequently shown is O-
acylation of 3 using representative electrophiles and efficient
generation of the benzotrifuran core of 4. Spectroscopic
measurements and DFT calculations go on to address the
(6) See, for example:(a) Destrade, C.; Tinh, N. H.; Mamlok, L.; Malthete,
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Gasparoux, H.; Mamlok, L. Liq. Cryst. 1987, 2, 229–233. (c) Nicolas, Y.;
Blanchard, P.; Levillain, E.; Allain, M.; Mercier, N.; Roncali, J. Org. Lett.
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Bendikov, M. Angew. Chem., Int. Ed. 2007, 46, 8814–8818. (f) Lai, W.-
Y.; He, Q.-Y.; Zhu, R.; Chen, Q.-Q.; Huang, W. AdV. Funct. Mater. 2008,
18, 265–276. (g) Talarico, M.; Termine, R.; Garc´ıa-Frutos, E. M.; Omenat,
A.; Serrano, J. L.; Go´mez-Lor, B.; Golemme, A. Chem. Mater. 2008, 20,
6589–6591. (h) Rose, K. G.; Jaber, D. A.; Gondo, C. A.; Hamilton, D. G.
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Figure 2. X-ray crystal structure of 3: (a) ORTEP plot of molecule
A (thermal ellipsoids shown at the 50% probability level and
hydrogen atoms have been removed for clarity); (b) O···CdO
interactions between molecules A and B that define the molecular
packing (distances in Å); (c) helical arrangement of 3 along the b
axis.
(7) (a) Hantzsch, A. Chem. Ber. 1886, 19, 2934–2939. (b) Japp, F. R.;
Meldrum, A. N. J. Chem. Soc., Trans. 1899, 75, 1035–1043. (c) Brown,
B. R.; Somerfield, G. A.; Weitzman, P. D. J. J. Chem. Soc. 1958, 4305–
4308.
(8) (a) Li, H.; Homan, E. A.; Lampkins, A. J.; Ghiviriga, I.; Castellano,
R. K. Org. Lett. 2005, 7, 443–446. (b) Lampkins, A. J.; Abdul-Rahim, O.;
Li, H.; Castellano, R. K. Org. Lett. 2005, 7, 4471–4474. (c) Lampkins,
A. J.; Abdul-Rahim, O.; Castellano, R. K. J. Org. Chem. 2006, 71, 5815–
5818. (d) Lampkins, A. J.; Li, Y.; Al Abbas, A.; Abboud, K. A.; Ghiviriga,
I.; Castellano, R. K. Chem.sEur. J. 2008, 14, 1452–1463.
calculated (at the RB3LYP/6-311+G* level) structural data
are given in Table S6 (Supporting Information). Most
surprisingly, the crystal is noncentrosymmetric and occupies
the orthorhombic space group aba2 (point group mm2); the
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