Scheme 3. Synthesis of Dixanthone 12 via the Intermediate
Imino-Xanthone 11
Figure 4. Hydrogen-bond stabilization of intermediates in (a) acid-
catalyzed cyclization of dinitrile 10 and (b) acid hydrolysis of the
di-imininium precursor to imino-xanthone 11.
Cyclization of the resorcinol-derived dinitrile 10 required
more forcing conditions than those described above, and after
the product was treated with dilute sodium hydroxide at room
temperature, the imino-xanthone 11 was obtained (Scheme
3) rather than the expected di-imine. Analysis of the
intermediate iminium triflate by FAB-MS indicated that the
hydrolysis had occurred during cyclization rather than on
contact with the aqueous base.
13. Initial studies involving nickel-catalyzed coupling of
4-bromo-2-fluorobenzonitrile, in the presence of [Ni(PPh3)4]
and zinc powder,10 yielded complex mixtures of products
that proved very difficult to separate, but a clean, high-
yielding synthesis of 13 was eventually developed using [Ni-
(1,5-cyclooctadiene)2] as a stoichiometric coupling agent
(Scheme 4).11
The structure of 11 was established by single-crystal X-ray
analysis.9 The unit cell contains two independent molecules
with identical regiochemistry, the relative positions of the
imine and carbonyl groups being unambiguously determined
from both the location of the imino hydrogen atom and the
lengths of the CdN and CdO bonds (Figure 3). The CdN
double bond is bent significantly away from one of its
adjacent hydrogens as a consequence of repulsion between
the aromatic and imino hydrogen atoms [C(2)-C(1)-N(1)
) 127°]. The molecules form hydrogen-bonded tapes that
π-stack with their centrosymmetrically related counterparts.
The dixanthone 12 was obtained by hydrolysis of 11 with
50% sulfuric acid at high temperature (Scheme 3). The
regioselectivity of cyclization onto the resorcinol residue
(1,2,3,4- rather than 1,3,4,6-substitution) may result from
stabilization of the Wheland intermediate by intramolecular
hydrogen bonding [Figure 4a] and a similar explanation, in
terms of hydrogen-bond stabilization of the intermediate,
could account for the enhanced susceptibility of the endo-
imino group toward hydrolysis [Figure 4b].
Scheme 4. Synthesis of Monomer 13
Polycondensation of 13 with 1,5-dihydroxynaphthalene
and with 4,4′-dihydroxydiphenyl gave rather insoluble,
crystalline polymers of very low molecular weight, but the
polyethernitrile 14, from condensation of 13 with 4,4′-
hexafluoroisopropylidene-diphenol (Scheme 5), proved to be
To extend this chemistry to macromolecular systems, we
next developed a route to the new biphenyl-based monomer
(7) Booth, B. L.; Noori, G. F. M. J. Chem. Soc., Perkin Trans. 1 1980,
2894-2900.
(8) Crystal data for 7: C42H12O4, M ) 364.34, monoclinic, space group
P21/n, a ) 7.4211(12), b ) 4.7907(5), c ) 23.106(2) Å, â ) 95.728 (7)°,
V ) 817.4(2) Å3, T ) 296 K, Z ) 2, Dc ) 1.480 g cm-3, µ(Cu KR) ) 8.27
cm-1, F(000) ) 376; 1284 independent reflections, R1 ) 0.060, wR2
)
0.149 for 887 independent observed reflections [2θ e 128°, Fo > 4σ(Fo)].
Crystal dimensions 0.03 × 0.05 × 0.50 mm3. Data were measured on a
Siemens P4/RA diffractometer with graphite-monochromated Cu KR
radiation using ω-scans. The structure was solved by direct methods, and
non-hydrogen atoms were refined anisotropically using F2 data and the
SHELXTL program package version 5.03.
(9) Crystal data for 11: C20H11NO3, M ) 313.30, triclinic, space group
P1h, a ) 7.2454(9), b ) 13.7838(12), c ) 14.205(2) Å, R ) 87.375(5)°, â
) 85.609(8)°, γ ) 83.879(7)°, V ) 1407.2(3) Å3, T ) 293 K, Z ) 4, Dc
) 1.479 g cm-3, µ(Cu KR) ) 8.22 cm-1, F(000) ) 648; 4182 independent
reflections, R1 ) 0.071, wR2 ) 0.156 for 2466 independent observed
reflections [2θ e 120°, Fo > 4σ(Fo)]. Crystal dimensions 0.40 × 0.06 ×
0.03 mm3. Data collection and structure solution as for compound 7.
(10) Colon, I.; Kelsey, D. J. Org. Chem. 1986, 51, 2627-2637.
(11) Semmelhack, M. F.; Helquist, P. M.; Jones, L. D. J. Am. Chem.
Soc. 1971, 93, 5908-5910.
Figure 5. Crystal packing in dixanthone 7.
Org. Lett., Vol. 3, No. 15, 2001
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