8
8 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
Lewis Acid-catalysed Facile Elimination of the Diazo
Group in 3-Diazochromanones. Novel Conversion of
1998, 88±89�
Chromanones into Chromones{
Pranab Mandal and Ramanathapuram V. Venkateswaran*
Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta-
700 032, India
3
-Diazochromanones undergo rapid elimination of the diazo group in presence of BF ÁEt O to furnish chromones.
3
2
The ready accessibility of a-diazocarbonyl compounds and
the versatility of their transformations have made them one
of the most useful functional groups in organic synthesis;
was obtained in very good yield. This suggests a route to
various 3-alkoxy substituted chromanones.
When this diazo elimination reaction was extended to the
alicyclic situation, as in 2-diazotetralin-1-one, it furnished a-
naphthol in over 70% yield as a result of elimination and
consequent aromatisation.
1
the wide variety of a-diazocarbonyl compounds which have
been brought within the scope of such transformations attest
to the immense potential of this functional group. However,
extension of such varied transformations to carbocyclic
diazoketones has been rather limited and there has been no
report of any transformation involving the 3-diazo-4-
chromanone system. As an extension of our continuing syn-
The metal-catalysed decomposition of a diazoketone con-
taining an a-hydrogen has been reported to lead to an1
alkene, depending on the nature of the ligand on the metal.
In keeping with this, decomposition of 3-diazochromanone
2a with a catalytic amount of dirhodium tetraacetate in
benzene furnished chromone 3a in more than 70% yield.
In summary, we describe an interesting Lewis acid-
catalysed elimination of the diazo group in 3-diazo-
chromanones as a novel route to chromones. This should
serve as a useful complementary procedure to other existing
methods for the transformation of chromanones into
2
thetic studies involving chromone precursors, we have also
started investigations into the possible utilisation of 3-diazo-
chromanones and herein report the facile elimination of
the diazo group in the presence of BF ÁEt O to furnish chro-
3
2
mones (Scheme 1). The 3-diazochromanones were prepared
from the corresponding chromanones via a formylation and
deformylative diazo transfer reaction and were obtained in
overall yields of 55±60%. Treatment of these diazo-
chromanones (2a±e) with a catalytic amount of BF ÁEt O in
3
chromones.
3
2
methylene chloride at ambient temperature for 1±1.5 h furn-
ished the chromones 3a±e, respectively, in excellent yields,
providing a new procedure for the conversion of chroma-
Experimental
General Procedure as Illustrated for 3a.ÐTo a stirred slurry of
sodium hydride (1.44 g) in diethyl ether (40 ml) under nitrogen was
added a drop of ethanol followed by a solution of chromanone (1a)
3
nones into chromones (Table 1). The structures of the
(
2.96 g) in diethyl ether (5 ml). The reaction mixture was cooled in
products were established through detailed spectral analysis
and comparison with authentic samples. When methanol
was added to the reaction medium, 2a aorded a mixture of
chromone 3a and 3-methoxychromanone (4a, Scheme 1), in
varying proportions, and when methanol alone was used as
the solvent, it gave 4a exclusively in more than 85% yield.
Similarly, in presence of allyl alcohol, the allyl derivative 4b
an ice-bath and ethyl formate (4.44 g) was added dropwise, and the
mixture stirred overnight. The reaction mixture was then poured
into ice-cold water, the diethyl ether layer separated and the
aqueous layer acidi®ed with cold dil. HCl and extracted with diethyl
ether. The ethereal extracts were combined, washed with brine,
2 4
dried (Na SO ) and concentrated to aorded the 3-formyl-
chromanone (2.95 g, 83%) which was used directly in the next step.
The above formylchromanone was dissolved in methylene
chloride (20 ml) and triethylamine (3.75 g) was added. The reaction
mixture was cooled in an ice-bath and a solution of toluene-p-
sulfonyl azide (7.3 g) in methylene chloride (5 ml) was added drop-
wise. The cold reaction mixture was stirred for 3 h and subsequently
overnight at room temp. Aqueous potassium hydroxide (10%,
O
R1
2
0 ml) was then added and the solution stirred for 30 min. The
R2
ii
O
R3
O
O
O
O
layers produced were separated and the aqueous layer extracted
with diethyl ether. The combined organic extracts were washed with
aqueous potassium hydroxide (5%) and water, and then dried
3
R1
R1
N2
R3
i
(
Na
the oily residue over neutral alumina (benzene±light petroleum, bp
0±60 8C, 2:3) furnished the 3-diazochromanone 2a (2.1 g, 72%)
2 4
SO ). Removal of the solvent followed by chromatography of
R2
R3 R2
iii
O
4
OR
�
1
as a yellow solid, mp 52±55 8C; �max/cm 2100, 1670; d
CDCl , 60 MHz) 5.22 (s, 2 H), 7.0±7.52 (m, 3 H), 7.96 (d, 1 H,
J 8 Hz).
H
(ppm)
1
2
(
3
–3a R1 = R2 = R = H
b R = Me, R = R = H
3
1
O
1
2
3
Compound 2a (1 g) was taken in methylene chloride (5 ml) and a
drop of boron tri¯uoride etherate was added by syringe. The reac-
tion mixture was stirred for 1.5 h at room temp. and decomposed
1
3
2
4a R = Me
b R = CH2–CH CH2
c R = R = H, R = Me
1
2
3
d R = H, R = R = Me
1
3
2
e R = R = H, R = OMe
Scheme 1 Reagents: i, (a) NaH/HCO
ii, BF .Et O/CH Cl ; iii, (a) BF .Et O/MeOH (b) BF
CH �OH
=CH�CH
3
with aqueous NaHCO . The organic layer was separated and the
aqueous layer extracted with diethyl ether. The combined organic
extracts were washed with water, dried and the solvent distilled o,
to furnish chromone 3a (0.79 g, 95%) in satisfactorily pure form,
identical with an authentic sample.
2
Et/Et
2
O (b) Et
.Et O/
3 3
N/TsN ;
3
2
2
2
3
2
3
2
2
2
Table 1 lists the physical data and yields of the diazochroman-
ones 2a±e and the products 3a±e.
�
�
[
To receive any correspondence..
This is a Short Paper as de®ned in the Instructions of Authors
J. Chem. Research (S), 1997 Issue 1, p. xi], there is therefore no
corresponding material in J. Chem. Research (M).
Formation of 3-Methoxychromanone (4a) from 3-Diazochromanone
(2a).ÐThe above reaction was carried out using methanol as
solvent in place of methylene chloride. After 1.5 h the reaction
3
mixture was decomposed with aqueous NaHCO and extracted with
Mandal, Pranab
