separated from the reaction media by a permanent mag-
net without the need for filtration or centrifugation or a
tedious workup procedure.5 While recent studies primarily
focused on their potential synthetic applications,6 utiliza-
tion of the derived MNPs to function as an external
stimulus in molecular switching systems has never been
reported. Here, we describe a one-pot, microwave-assisted,
pseudo multicomponent synthesis of two new pyranocou-
marins to investigate their ring-opening properties using
UV light as controllable parameters. The feasibility of
accelerating the reverse ring-closing reaction with the
imidazoline-functionalized MNPs was also explored.
Scheme 1 shows the one-pot synthesis of pyranocou-
marins 1 and 2. An acetic anhydride-mediated, pseudo
three (or four) component condensation of 4-chloro-
2-oxo-2H-chromene-3-carbaldehyde (3)7 or 4-chloro-7-
dimethylamino-2-oxo-2H-chromene-3-carbaldehyde (5)8
with 4-methylquinoline (4) in acetic anhydride under
microwave irradiation conditions (300 W) at 130 °C gives 1
and 2, respectively. The molecular structures of 1 and 2
pyranocoumarin skeleton, which generally requires several
synthetic steps to accomplish.9
With the availability of 1 and 2, their photochemical
properties were then investigated. Both compounds were
Figure 1. ORTEP crystal structures of 1 and 2.
Scheme 1
Scheme 2
1
were elucidated by H and 13C NMR spectroscopy and
X-ray crystallography(Figure1), clearlyrevealing aquino-
line substituted pyranocoumarin skeleton. Scheme 2 de-
picts the proposed mechanism for the formation of 2. It
begins with the reaction of 4-methylquinoline with acetic
anhydride to yield 1-acetyl-4-methylene-1,4-dihydroquin-
oline (6). The coupling of 6 with aldehyde 5 gives inter-
mediate 7, which subsequently loses a molecule of Ac2O
to afford alcohol 8. 8 then reacts with acetic anhydride
again to give the 4-methylene-1,4-dihydroquinoline 9. The
coupling of 9 with a second molecule of 5 generates the
zwitterion 10. Final cyclization of 10 following by dehydra-
tion furnishes the final product 2. Essentially, this one-pot,
microwave-promoted, Ac2O-mediated multicomponent
reaction provides a quick access to the bulky substituted
found to be sensitive to light. For instance, the yellow
solution of 2 turned red within seconds upon UV irradia-
tion (306 nm). Figure 2 shows the UVÀvis absorbance
changes of 2 in chloroform prior to and after irradiation.
With the increase of exposure time, a new absorption band
with the peak wavelength at ∼504 nm gradually emerged,
along with the appearance of two isosbestic points at 362
and 429 nm. Although the proposed photogenerated
products 11À14 (Scheme 3) were not stable enough to be
isolated and characterized, their formation was partially
1
supported by the H NMR spectra of 2 in deuterated
chloroform after irradiation, which exhibited a time-
dependent downfield shift of Ha and Hb absorption
peaks (see Scheme 3 for H-atom labeling and Figure S4,
Supporting Information (SI)). The discernible downfield
shift of Ha (from 6.982 to 7.186 ppm) and Hb (from 7.344
to 7.570 ppm) after irradiation presumably resulted from
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Org. Lett., Vol. 14, No. 5, 2012
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