Organic Letters
was examined. As depicted, the reaction furnished the
corresponding products 4l−4n in 64−72% yields. Diynamides
with other electron-rich aromatic groups, including furanyl or
thiophenyl aromatic rings, have also been tested. Products with
a similar polycyclic framework were not observed (results not
shown). These results demonstrated that this gold-catalyzed
reaction provides a highly efficient and practical avenue for the
construction of fused tetracyclic compounds bearing β-
2
1
carboline cores, which can be found in various bioactive
2
2
alkaloids.
As shown in Scheme 5, the obtained furo[2,3-c]-
isoquinolines could be functionalized under a variety of
a
Scheme 5. Synthetic Transformations of 2a
Figure 1. (a) Basic optical properties of furo[2,3-c]isoquinolines 2.
−
4
UV−vis absorption spectra in MeCN (ca. 10 M). (b) Fluorescence
−
4
spectra of MeCN (ca. 10 M) were obtained upon sample excitation
with 350 nm.
catalyzed oxidation of alkynes to form dicarbonyl compounds.
The products obtained could be functionalized and trans-
formed to other potentially bioactive molecules. Moreover,
according to optical property studies, the polycyclic hetero-
cyclic products display pronounced blue luminescence,
showing remarkably high fluorescence quantum yields (up to
75%). Hence, future application in functional materials and
pharmaceutical sciences may benefit from the current method-
ology.
a
Reagents and conditions: (a) NBS, THF, 40 °C, 24 h; (b) m-CPBA,
DCM, RT, 24 h; (c) LPO, TfOH, MeCN, 80 °C, 12 h; (d) PhCHO,
TBHP, TFA, blue LEDs, N , EA, RT, 24 h; (e) HBr, AcOH, reflux, 48
h.
2
conditions. For example, treatment of 2a with NBS could
lead to the formation of 5a, in which the Br group provides a
platform for further transition-metal-catalyzed cross-coupling
reactions. Oxidization of 2a with m-CPBA was achieved under
mild conditions, affording 6a in 60% yield. A metal-free
protocol for Minisci C−H alkylation of 2a using LPO as the
ASSOCIATED CONTENT
sı Supporting Information
■
*
Experimental procedures and characterization data for
23
alkylating reagent afforded 7a in 83% yield. We have also
tested a visible-light-mediated direct cross-coupling between
24
2a and benzaldehyde, furnishing 8a in 76% yield. The 2-
CCDC 1976508 contains the supplementary crystallographic
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
phenylfuro[2,3-c]isoquinolin-7-ol 9a was found to be produced
in 89% yield under acidic conditions.
Interestingly, upon observation of all solutions, the furo[2,3-
c]isoquinolines 2 with striking blue fluorescences could be
observed under illumination with a UV lamp. So the
absorption emission properties of 2 and 4 were examined.
Representative spectra for compounds 2 are shown in Figure
25
Corresponding Author
1
(a, b). Their emission wavelengths in acetonitrile are located
■
in the range of the blue region, and remarkably high
fluorescence quantum yields (Φ ) up to 75% have been
Xueliang Huang − Key Laboratory of Coal to Ethylene Glycol
and Its Related Technology, Center for Excellence in Molecular
Synthesis, Fujian Institute of Research on the Structure of
Matter, Fujian College, Chinese Academy of Sciences, Fuzhou,
FL
observed in acetonitrile solution (Table 2). These optical
properties may suggest a promising perspective of their
potential application in the area of developing new materials
for blue organic light-emitting diodes.
In summary, we have presented an unprecedented gold-
catalyzed oxidative cascade cyclization of readily accessible
diynamides to construct a furopyridinyl core. Through this
reaction, compounds furo[2,3-c]isoquinolines and 6H-furo-
Authors
[
3′,2′:5,6]pyrido[3,4-b]indoles possessing a highly conjugated
Jibing Liu − Key Laboratory of Coal to Ethylene Glycol and Its
Related Technology, Center for Excellence in Molecular
Synthesis, Fujian Institute of Research on the Structure of
Matter, Fujian College, Chinese Academy of Sciences, Fuzhou,
Fujian 350002, China; University of Chinese Academy of
Sciences, Beijing 100049, China
system could be prepared in a straightforward manner. The
salient features are highly efficient on multiple new bond
formations and fused heterocyclic ring construction via a single
operation under mild conditions. Such a intrinsic selectivity is
remarkable, as it overrides the traditional reactivity on gold-
D
Org. Lett. XXXX, XXX, XXX−XXX