Chemistry of L-ascorbic acid. Part 3.1 Photoreduction of quinones
with 5,6-O-isopropylidene-L-ascorbic acid†
Mukund G. Kulkarni* and Sandesh D. Kate
Department of Chemistry, University of Pune, Pune 411007, India.
E-mail: mgkul@chem.unipune.ernet.in; Fax ϩ91 020 565 1728
Received (in Cambridge, UK) 26th June 2000, Accepted 31st October 2000
First published as an Advance Article on the web 21st November 2000
Upon irradiation with UV light, instead of undergoing the
Paternò–Büchi reaction, 5,6-O-isopropylidene-L-ascorbic
acid reduced quinones quite efficiently and rapidly to the
corresponding hydroquinones.
at all of 2-hydroxy-1,4-naphthoquinone by 2 in the dark was
observed. This would imply that the photo-excited state of
2-hydroxy-1,4-naphthoquinone is reduced by 2.
It was essential to check the possible involvement of acetone
in the above reaction, either as a sensitizer or as a redox reagent,
before exploring the scope of this reaction. The simplest way
to check this could be to carry out the reaction in a photo-
chemically inert solvent like dichloromethane or acetonitrile.
However the -ascorbic acid derivative 2 is insoluble in these
solvents. But it was gratifying to observe that 2 had sufficiently
good solubility in 1,2-dimethoxyethane (DME). Accordingly, a
solution (0.1 molar solution) of an equimolar mixture of 5,6-O-
isopropylidene--ascorbic acid 2 and 2-hydroxy-1,4-naphtho-
quinone in DME was irradiated with UV light (125 W lamp)
using a pyrex filter at 25 ЊC in an immersion well photo-reactor.
Under these conditions as well, reduction of 2-hydroxy-1,4-
naphthoquinone to 1,2,4-trihydroxynaphthalene was over in
20 h. No difference in the outcome of the photoreaction, as
compared to the reaction in acetone, was observed. This con-
firmed that acetone is not playing any other role, except that
of a solvent, in the photochemical reaction.
The biological and pharmacological activity as well as thera-
peutic potential of -ascorbic acid and its derivatives have been
studied extensively.2 However, despite the fact that -ascorbic
acid possesses several interesting functional groups, its organic
chemistry and synthetic potential has not been explored to any
significant extent. The complex chemical properties2–4 of this
‘mysterious small molecule’ may have hindered developments
in this regard.
Recently it has been shown1 that the derivatives of -ascorbic
acid participate in a Paternò–Büchi reaction to furnish chiral
oxetanes. In continuation of this work, it was of interest to
study such an addition of -ascorbic acid to quinones as they
are known to participate5 in the Paternò–Büchi reaction,
affording the corresponding oxetanes. However -ascorbic acid
is known to reduce different systems,6 including inorganic
compounds,7 under photolytic conditions. -Ascorbic acid is
also reported8 to reduce quinones in a ground state reaction,
albeit in a sluggish manner. Therefore, it was essential to check
whether, under photolytic conditions, -ascorbic acid would
simply reduce the quinones to the corresponding hydro-
quinones or would undergo a Paternò–Büchi reaction to give
the respective oxetanes. Results pertaining to the above investi-
gation are described in the present communication.
In the initial phase of the work, a solution of 5,6-O-iso-
propylidene-2,3-di-O-methyl--ascorbic acid 1 and 2-hydroxy-
1,4-naphthoquinone in acetone (0.1 molar solution) was
irradiated with UV light (125 W lamp) using a pyrex filter at
25 ЊC in an immersion well photo-reactor. No appreciable
reaction was observed even after prolonged period (>250 h)
of irradiation. However, when an equimolar mixture of 5,6-O-
isopropylidene--ascorbic acid9 2 and 2-hydroxy-1,4-naphth-
In order to explore the generality of this observation, a series
of quinones were irradiated with -ascorbic acid derivative 2 in
DME under the above reaction conditions. From the results
obtained (Table 1) it is clear that, barring a few exceptions, most
of the quinones undergo photo-reduction. Reduction of these
quinones in the dark, under identical temperature and concen-
tration conditions, was studied simultaneously. In general,
naphthoquinones did not undergo reduction while benzo-
quinones were found to undergo reduction in the dark, though
the rate was found to be quite slow (Table 1). Furthermore, the
photoreduction of naphthoquinones and anthroquinones was
found to be much slower than that of the benzoquinones. When
the photoreduction of 2-hydroxy-1,4-naphthoquinone was
carried out using 400 W UV lamp,11 under otherwise identical
reaction conditions, the reaction was complete in considerably
reduced time (<5 h). Similarly, in the case of unsubstituted
benzoquinone, the reduction was complete in a significantly
shorter time (<30 min).
A study of the effect of the concentration and the temper-
ature on the rate of the reduction was undertaken to confirm
that the observed enhancement in the rate of the reductions of
quinones is really a photochemical effect and not possibly due
to the heating by the UV lamp. For this purpose the reduction
of benzoquinone with the -ascorbic acid derivative 2, at
different concentrations and temperatures, was conducted.
Simultaneously, the effect of concentration and temperature on
the dark reaction was studied as well. From the results of this
study (Table 2) it is clear that the rate of the reduction, in the
photochemical reaction, is independent of the concentration of
the reacting species and the temperature. This confirms that the
observed reduction of quinones with -ascorbic acid derivative
2 is a photochemical reaction.
oquinone was irradiated in an identical manner, within
20 h, complete disappearance (TLC) of both the starting
materials and concomitant formation of a single product was
observed.10a,b From the spectral data of the product, it was clear
that -ascorbic acid derivative 2 has effected reduction of 2-
hydroxy-1,4-naphthoquinone to 1,2,4-trihydroxynaphthalene,
rather than undergoing a Paternò–Büchi reaction. No reduction
Under photolytic conditions, using wavelength >400 nm,
quinones have been reported12 to furnish addition products
with electron rich olefins. Attempted reaction of 5,6-O-
† The IUPAC name for ascorbic acid is 5-(1,2-dimethylethyl)-3,4-
dihydroxy-2,5-dihydrofuran-2-one.
4242
J. Chem. Soc., Perkin Trans. 1, 2000, 4242–4244
This journal is © The Royal Society of Chemistry 2000
DOI: 10.1039/b005120i