Microwave-induced Selective Alkoxylation of 1,4-Naphthoquinones

Article abstract of DOI:10.1039/a601810f

A new efficient alkoxylation of 1,4-naphthoquinones at the active quinonoid position is reported using alkanols and an alkenol in the presence of cerium chloride and iodine under microwave irradiation.

Full text of DOI:10.1039/a601810f

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2
4 J. CHEM. RESEARCH (S), 1997
J. Chem. Research (S),
Microwave-induced Selective Alkoxylation of
,4-Naphthoquinones†
1
997, 24–25†
1
Mazaahir Kidwai,* Parven Kumar and Seema Kohli
Department of Chemistry, University of Delhi, Delhi-110007, India
A new efficient alkoxylation of 1,4-naphthoquinones at the active quinonoid position is reported using alkanols and an
alkenol in the presence of cerium chloride and iodine under microwave irradiation.
Alkoxy-1,4-naphthoquinones occur in nature and some are
complexation, with the metal salt facilitating an electrophilic
attack on the olefinic double bond. In our case, alkoxylation
does not proceed in the absence of either iodine or cerium
chloride, implying that the initial step is the electrophilic
attack of iodine on the quinonoid double bond to form an
iodonium intermediate which is then attacked by the alkoxy
1
,2
biologically important. Recently 2,3-disubstituted ana-
logues of 1,4-naphthoquinones have shown potent antiplate-
3
let, antiallergic and antiinflammatory activities. Alkoxylation
of 1,4-quinones has been reported using alkanols in the
4
,5
6
6
presence of metal/metal salts, Cu-bronze/I , silica gel and
2
6
7
Raney Ni. Khanna et al. have reported the use of transition
metal salts in the presence of iodine for the alkoxylation of
1
,4-napthoquinones.
Keeping in mind the current interest in lanthanide salts
and the importance of microwaves in organic synthesis, we
report CeCl /I as a new reagent for the selective alkoxylation
3
2
of 1,4-naphthoquinones at the active quinonoid position. In
order to show the general use of the reagent we used substi-
tuted 1,4-naphthoquinones as starting materials and different
alkanols and an alkenol as alkoxylating agents. All the reac-
tions were carried out with conventional heating as well as
microwave irradiation. A comparative study (method A and
method B, Table 1) showed that the use of microwaves sub-
stantially reduces the reaction time and appreciably increases
the yields. All the products (6–13) were characterised and
compared with authentic samples (TLC, mp and IR).
Table 1 Comparison of reaction times and yields obtained using method A and method B
a
Reaction time [Yield (%)]
Reactant
Compound
Method
1
2
3
4
5
7
6
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
4 [70]
[82]
4.5 [68]
[70]
5.5 [65]
[70]
6 [60]
[65]
7 [54]
[60]
7.5 [50]
[57]
8 [55]
[60]
3 [75]
[85]
4 [70]
[78]
5.5 [62]
[70]
6 [60]
[63]
6.5 [57]
[60]
7 [55]
[60[]
7 [60]
[65]
2 [78]
[90]
3.5 [75]
[83]
5.5 [65]
[78]
5.5 [60]
[67]
6 [58]
[65]
6.5 [55]
[60]
7 [68]
[75]
—
—
—
—
7
7
—
—
—
—
6
8
—
—
—
—
7
9
—
—
—
—
1
6
1
7
1
2
1
1
1
1
0
1
2
3
—
—
—
—
—
—
—
–
—
—
—
—
—
—
—
—
—
—
7.0 [60]
[62]
8.0 [57]
[60]
a
Reaction time for method A is in hours and all method B reactions were carried out for 5 min.
Alkoxylation of 1,4-naphthoquinones in the presence of
anion from the corresponding alkanol to give the alkoxylated
CeCl /I follows an ionic pathway as the reaction is not
1,4-napthoquinone.
3
2
quenched by the addition of a radical quencher like hydro-
On alkoxylation, 2-chloro-5-hydroxy-1,4-naphthoquinones
(4) and 5-hydroxy-1,4-naphthoquinones (5) both afforded
2-alkoxy-5-hydroxy-1,4-naphthoquinones (13) in comparable
yields, which indicates that the ease of elimination of H and
Cl are almost identical under the above reaction condi-
tions.
8
quinone. Georgoulis et al. have showed that the metal salt
aids the abstraction of the iodide ion due to the reversible
attack of the alkoxy ion on the double bond and no organo-
metallic intermediates are involved during this process. It has
9
–10
also been reported
that molecular iodine is activated by
*
†
To receive any correspondence.
Experimental
This is a Short Paper as defined in the Instructions for Authors,
General Procedure.—Method A (thermal). In a 100 ml conical
flask, a mixture of cerium chloride (1.0 mmol), iodine (0.1 mmol)
and a solution of 1,4-naphthoquinone (1.0 mmol) in alkanol/alke-
Section 5.0 [see J. Chem. Research (S), 1997, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).

View Article Online
J. CHEM. RESEARCH (S), 1997 25
We are grateful to the University Grant Commission, New
Delhi, for providing financial assistance.
Received, 14th March 1996; Accepted, 11th October 1996
Paper E/6/01810F
References
1
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3
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4
(
4
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6
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mixture of cerium chloride (1.0 mmol), iodine (0.1 mmol) and a
solution of 1,4-napthoquinone (1.0 mmol) in alkanol/alkenol (50
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6
9.
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1
1
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