Photochemistry of chromones: Photoreorganisation of 3-alkoxy-2-thienyl-4-oxo-4H-1-benzopyrans

Article abstract of DOI:10.1039/a901090d

Photoirradiation of a methanolic solution of 3-alkoxy-2-thienyl-4-oxo-4H-1-benzopyrans with Pyrex filtered UV light leads to cyclised and cyclodehydrogenated angular products involving both thiophene and alkoxy groups. The reaction is initiated through H-abstraction. The product distribution depends upon the substituents on the thiophene ring.

Full text of DOI:10.1039/a901090d

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Photochemistry of chromones: photoreorganisation of
3-alkoxy-2-thienyl-4-oxo-4H-1-benzopyrans
Satish C. Gupta,* Somesh Sharma, Ashok Saini and Som N. Dhawan
Department of Chemistry, Kurukshetra University, Kurukshetra-136119, India
Received (in Cambridge, UK) 9th February 1999, Accepted 25th June 1999
Photoirradiation of a methanolic solution of 3-alkoxy-2-thienyl-4-oxo-4H-1-benzopyrans with Pyrex filtered UV
light leads to cyclised and cyclodehydrogenated angular products involving both thiophene and alkoxy groups. The
reaction is initiated through H-abstraction. The product distribution depends upon the substituents on the thiophene
ring.
Enones bearing alkoxy or alkyl groups at C-3, on irradiation
with UV light lead to the formation of a 1,4-biradical through
abstraction of a H-atom from the alkoxy/alkyl moiety by the
excited carbonyl group.1 The photoproducts formed depend
upon the substituents at C-2. The photolysis of 2-phenyl-2 and
2-furyl-3-alkoxy-4-oxo-4H-1-benzopyrans3 produced angular
tetracyclics, 2-styryl derivatives4 produced linear tricyclics
whereas 2-methyl-3-alkoxy derivatives provided both oxetanols
and desmethoxy chromones.5 Here we report the results of our
investigations on the phototransformations of some 3-alkoxy-
4-oxo-4H-1-benzopyrans bearing thiophene at C-2.
In this research we were prompted by two considerations:
(a) how a thiophene moiety [stabilisation energy (SE) 32 kcal]
affects the product formation/distribution compared with a
furan (SE 16–18 kcal) or phenyl moiety (SE 36 kcal) at C-2 and
(b) whether phototranspositions could become available in
these photoreactions—a route common in the photolysis of
2-aryl/alkyl thiophenes.6
The required thienylbenzopyrans 1 and 2 were synthesised
commencing with the condensation of (5-chloro-2-hydroxy-
phenyl)ethanone with a thiophene-2-carbaldehyde (Scheme 1).
The structures of the benzopyrans 1 and 2 and their
precursors were found to be consistent with their spectral
parameters (vide Experimental section). The 1H NMR spectral
assignments were confirmed through decoupling experiments:
in 1a decoupling of signals at δH 7.90 (3Ј-H) converted the dd
at δH 7.15 (4Ј-H) into a d (J 5 Hz); similarly the dd at δH 7.90
changed to a singlet (w1/2 1.5 Hz) on irradiation of the dd at
δH 7.15.
The photolysis of 1c in methanol solution (0.001 M) with
Scheme 1 Reactions and conditions: i, NaOH, EtOH; ii, H2O2 (30%),
Pyrex filtered UV light from a 450 W Hg lamp exhibited the
KOH; iii, K2CO3, Me2CO, DMF, RЈX (RЈ = CH3, C2H5, C6H5CH2;
complete disappearance (TLC) of 1c in 60 min. A chromato-
graphic separation produced 7c and 8c (Scheme 2).
X = I, Cl).
That tetracyclic compound 7c was only a photoreorganised
product of 1c is evident from its mass spectrum, m/z 368/70
located at m/z 212 and 154 (RDA). The thienyl protons reson-
ated at δH 7.57 (d, J 4.5 Hz, 2-H) and 6.82 (d, J 4.5 Hz, 3-H).
The photolysis of 1a and 1b produced 7a, 8a and 7b, 8b (yield
<30%) respectively, although the reaction was much slower as
compared to that of 1c. The only photoproducts obtained from
2a and 2b were 9a and 9b and not even a trace (<5%) of products
(Mϩ ); two other fragments at m/z 214 and 154 lent further
᝽
support to the structure 7c as they could become available
through the retro-Diels–Alder (RDA) mode of cleavage.
1
The acceptability of H NMR assignments were confirmed as
1
follows: (a) irradiation of the signal at δH 6.46 (d, 2-H) con-
verted the dd at δH 5.17 (3-H) to a d (J3a,3 3 Hz), (b) decoupling
of the d at δH 5.05 (11b-H) simplified the multiplet at δH 3.45
(3a-H) to a dd (J3a,4 10.5 Hz, J3a,3 3 Hz), (c) signals at δH 3.45
were reduced to a dd (J3a,11b 8 Hz, J3a,3 3 Hz) on irradiation of
the absorption at δH 4.78 (4-H).
corresponding to 8 seemed to be formed (TLC, H NMR of
crude photolysate). The exclusive formation of 9a and 9b was
confirmed by recording the UV spectrum of 2b (2.0 × 10Ϫ4 M in
MeOH) at different times of irradiations (0, 1, 2, 3, 4 min) when
absorption at λ 361 nm started diminishing and the changes in
the spectrum involved an isosbestic point at λ 260 nm.3
The second photoproduct 8c had in its mass spectrum the
molecular ion peak at m/z 366/68 pointing to the loss of 2 mass
units in its formation from 1c. The other prominent ions were
Regarding the C/D ring fusion in 7a, 7b, 7c, 9a and 9b, it is
concluded from the use of Dreiding models that the five mem-
bered ring D and pyran ring C must be cis fused and this is
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Table 1
Compound
δ3-H
7a
7b
7c
∆δH
9a
9b
∆δH
5.55
5.55
5.17
0.38
5.10
4.78
0.32
Scheme 3
1c and 2b was carried out in the presence of I2 or O2, only 8c
(R = H) and 9c (R = CH3) were produced.
Even the solvent polarity (C6H6, MeOH) did not have any
significant influence on the product distribution (7, 9:8). This
suggests that the formation of 7 is intramolecular and probably
occurs through a 1,5-sigmatropic migration, in enol 14, formed
initially by the abstraction of H from 3-alkoxy group by the
Scheme 2
excited C᎐O group of the pyrone 1, 2 (Scheme 4).
᎐
Fig. 1
borne out by the fact7 that J3a,11b is 8 Hz (φ-3a, 11b 27.32Њ).
Similarly the relative stereochemical disposition of 3a-H and
4-H is cis-axial (φ ~ 0Њ), thus placing the bulkier group, CH3 or
C6H5, in 7b, 7c and 9b in a pseudo-equatorial conformation.
Such a contention gets further support from the observation
that in 7c and 9b, 3-H appeared at higher field3 than the same
proton in 7a, 7b and 9a by ~0.35 ppm (Table 1). The only
explanation for such an observation can be that it is only in the
pseudo-equatorial position that the phenyl ring at 4-C would
shield the proton 3-H (I, Fig. 1). MMP calculations show that
in no other conformation such as in II (φ3a,4 60Њ), would 3-H
experience shielding.8
Scheme 4
Regarding the effect of substitution at C-2 of the pyrone ring
in 1 or 2 on product distribution/formation, an examination of
Table 2 shows that in the case of furyl or methylfuryl or methyl-
thienyl chromones 10, 11 and 2b only the photocyclised prod-
ucts (A) are obtained. In others (1c and 12 carrying phenyl or
thiophene rings), both photocyclised (A) and photocyclo-
dehydrogenated products (B) are formed. For such dichotomy,
the only assignable reason could be the difference in electron
density in the ring moiety at C-2. An examination of the results
in the Table 2 reveals that as we go down the table, the electron
density in the ring at C-2 decreases and the formation of
cyclodehydrogenated products (B) become predominant. It is
possible that, as shown in Scheme 4, the transfer of H to the
The compounds 8a, 8b and 8c are formed independently9
from 1a, 1b and 1c and not via 7a, 7b and 7c. This is confirmed
from the photoirradiation of 7c when only polymeric products
were obtained and no product corresponding to 8 was realised.
Further confirmation of the structure 7c was obtained by its
DDQ oxidation to 8c (Scheme 3). Also when the photolysis of
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Table 2
Compounds
A
B
Ratio of A:Ba
1:03
1:03
1:0
1:0.2
1:42
a Ratios calculated from the isolated yield.
ring junction (at 11-b) is assisted and this becomes faster
than cleavage of 3a-H leading to 8. However in the case of
3-methoxy/ethoxy derivatives, the trend remains the same
except that the ratio of A:B reaches 1:1 in 1a and 1b indicat-
ing the effect of the relative stability of the alkoxyl radical at
C-3.
(suspended in water) and iodine vapour was used as the visual-
ising agent. Silica gel (60–120 mesh) was used for column
chromatography. The columns were packed and left overnight,
before being used for fractionations. Light petroleum refers to
the fraction with distillation range 60–80 ЊC.
Phototranspositions,6 a reaction prevalent in alkyl/aryl thio-
phenes through ring opening and isomerisation frequently
occur, but in the present study all our attempts to observe
such a mode of reaction in the photolysis of 1 and 2
proved unsuccessful. Even in the photolysis of 2-(2Ј-thienyl)-
4-oxo-4H-1-benzopyrans 13, no such reaction could be
effected and the compound was recovered unchanged quanti-
tatively.
In summary, the formation and distribution of cyclised and
cyclodehydrogenated products from the photoirradiation of 3-
alkoxy-2-arylchromones depend upon the electron density in
aryl ring at C-2. Hydrogen abstraction from the 3-alkoxy group
by the carbonyl of the chromones is preferred to photo-
transpositions in the 3-alkoxy-2-thienylchromones.
1-(5Ј-Chloro-2Ј-hydroxyphenyl)-3-(2Љ-thienyl)propenone 3
A solution of 1-(5-chloro-2-hydroxyphenyl)ethanone (6.8 g,
0.04 mol) and 2-thiophenecarbaldehyde (4.58 g, 0.04 mol) in
methanol at 0 ЊC was stirred with powdered NaOH (8 g, 0.04
mol) for 5 h and was then poured onto ice–HCl to give 3 (7.58 g,
75%) mp 110–112 ЊC (lit.,10 114 ЊC); νmax/cmϪ1 3400(OH),
1635(CO); δH(90 MHz; CDCl3) 12.75 (1H, br s, 2Ј-OH), 7.82
(1H, d, J 2 Hz, 6Ј-H), 7.55–7.27 (4H, m, 2-, 3-, 4Ј- and 5Љ-H),
7.30–7.00 (2H, m, 3Љ- and 4Љ-H), 6.95 (1H, d, J 9 Hz, 3Ј-H).
1-(5Ј-Chloro-2Ј-hydroxyphenyl)-3-(5Љ-methyl-2Љ-thienyl)-
propenone 4. (81%), Mp 138 ЊC; (Found: C, 60.1; H, 3.8.
C14H11ClO2S requires C, 60.4; H, 4.0%); νmax/cmϪ1 3400(OH),
1635(CO); δH(90 MHz; CDCl3) 12.85 (1H, br s, 2Ј-OH), 7.80
(1H, d, J 15 Hz, 3-H), 7.72 (1H, d, J 2 Hz, 6Ј-H), 7.25 (1H, dd,
J 2, 9 Hz, 4Ј-H), 7.18 (1H, d, J 4 Hz, 3Љ-H), 7.11 (1H, d, J 15 Hz,
2-H), 6.90 (1H, d, J 9 Hz, 3Ј-H), 6.79 (1H, d, J 4 Hz, 4Љ-H), 2.50
(3H, s, 5Љ-Me).
Experimental
Mps were determined on a sulfuric acid bath and are un-
corrected. The UV spectra were recorded on a U-2000 Hitachi
UV-spectrophotometer. The IR spectra (Nujol) were scanned
6-Chloro-3-hydroxy-2-(2Ј-thienyl)-4-oxo-4H-1-benzopyran 5
1
on a Perkin-Elmer IR-842 spectrophotometer. The H NMR
spectra (CDCl3) were recorded on a Perkin-Elmer R-32 (90
MHz) spectrometer. The chemical shifts are expressed on the
δ scale with SiMe4 as internal standard, with J values given in
Hz. Mass spectra were recorded at 70 eV using a VG micromass
7070F instrument. TLC plates were coated with silica gel-G
To a stirred solution of compound 3 (3 g, 0.015 mol) in meth-
anol (30 cm3)–aq. NaOH (10 cm3, 20%) cooled to 0 ЊC was
added 30% H2O2 (5 cm3) dropwise. The mixture was then
poured onto ice–HCl to give 5 (2.58 g, 86%), mp 200–202 ЊC
(lit.,10 mp 202 ЊC); νmax/cmϪ1 3260(OH), 1630(CO); δH(90 MHz;
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CDCl3) 11.0 (1H, br s, 3-OH), 8.20 (1H, d, J 2 Hz, 5-H), 8.00
(1H, dd, J 1, 4 Hz, 3Ј-H), 7.70 (1H, dd, J 2, 9 Hz, 7-H), 7.60
(1H, dd, J 1, 5 Hz, 5Ј-H), 7.30 (1H, d, J 9 Hz, 8-H), 7.20 (1H,
dd, J 4, 5 Hz, 4Ј-H).
magnesium-dried methanol (80 cm3) and the solution was
deoxygenated by bubbling in dry nitrogen gas for 10 min.
Thereafter the solution was irradiated in an immersion appar-
atus with a 450 W high-pressure mercury arc (Hanovia) sur-
rounded by a Pyrex water-cooled jacket. After 1 h, removal of
the solvent under reduced pressure yielded a dark red gummy
product, which was chromatographed over silica gel. The
column was eluted with light petroleum (300 cm3), light
petroleum–benzene (100 cm3, 1:1) to elute the starting com-
pound 1c (10 mg) followed by a white solid 7c (100 mg, 25%),
mp 190–192 ЊC (Found: C, 65.5; H, 3.5. C20H13ClO3S requires
C, 65.2; H, 3.5%); νmax/cmϪ1 1640(CO); m/z 368/370 (Mϩ); δH(90
MHz; CDCl3) 8.20 (1H, d, J 2 Hz, 7-H), 7.55 (1H, dd, J 2, 9 Hz,
9-H), 7.45–7.20 (6H, m, 10–H, 4-Ph), 6.46 (1H, d, J 5 Hz, 2-H),
5.17 (1H, dd, J 3, 5 Hz, 3-H), 5.05 (1H, d, J 8 Hz, 11b-H), 4.78
(1H, d, J 10.5 Hz, 4-H), 3.55–3.35 (1H, m, 3a-H).
6-Chloro-3-hydroxy-2-(5Ј-methyl-2Ј-thienyl)-4-oxo-4H-1-
benzopyran 6. (81%), Mp 215 ЊC (Found: C, 57.2; H, 3.0.
C14H9ClO3S requires C, 57.5; H, 3.1%); νmax/cmϪ1 3260(OH),
1632(CO); δH(90 MHz; CDCl3) 11.50 (1H, br s, 3-OH), 8.05
(1H, d, J 2 Hz, 5-H), 7.65 (1H, d, J 4 Hz, 3Ј-H), 7.45 (1H, dd,
J 2, 9 Hz, 7-H), 7.28 (1H, d, J 9 Hz, 8-H), 7.10 (1H, d, J 4 Hz,
4Ј-H), 2.50 (3H, s, 5Ј-Me).
3-Alkoxy-6-chloro-2-(2Ј-thienyl)-4-oxo-4H-1-benzopyran 1 and
3-alkoxy-6-chloro-2-(5Ј-methyl-2Ј-thienyl)-4-oxo-4H-1-benzo-
pyran 2
Further elution of the column with benzene–ethyl acetate
(19:1) furnished a second white solid compound 8c (20 mg,
5%), mp 232 ЊC (Found: C, 65.3; H, 3.2. C20H11ClO3S requires
C, 65.6; H, 3.0%); νmax/cmϪ1 1640 (CO); m/z 366/368 (Mϩ);
δH(90 MHz; CDCl3) 8.23 (1H, d, J 2 Hz, 7-H), 7.57 (1H, d, J 4.5
Hz, 2-H), 7.53 (1H, dd, J 2, 9 Hz, 9-H), 7.48–7.37 (6H, m, 10-H,
4-Ph), 6.82 (1H, d, J 4.5 Hz, 3-H), 6.53 (1H, s, 4-H).
A suspension of compound 5 or 6 (2.8 g, 0.01 mol), alkyl hal-
ides (0.01 mol CH3I/C2H5I/C6H5CH2Cl), KI (1 g) and freshly
ignited K2CO3 (5 g) in dry acetone (30 cm3) was refluxed for 5 h.
Filtration, evaporation and percolation of the residue through
a column of silica gel gave compound 1a, 1b and 1c and 2a
and 2b.
3-Benzyloxy-6-chloro-2-(2Ј-thienyl)-4-oxo-4H-1-benzopyran
1c. (70%), Mp 138 ЊC (Found: C, 65.4; H, 3.3. C20H13ClO3S
requires C, 65.2; H, 3.5%); λmax (MeOH)/nm 346 and 260.5
(ε/dm3 molϪ1 cmϪ1 49000 and 34000); νmax/cmϪ1 1630 (CO);
δH(90 MHz; CDCl3) 8.20 (1H, d, J 2 Hz, 5-H), 7.90 (1H, dd, J 1,
4 Hz, 3Ј-H), 7.60–7.20 (8H, m, 7- and 8-H, 5Ј-H, 3-OCH2C6H5),
7.10 (1H, dd, J 4, 5 Hz, 4Ј-H), 5.25 (2H, s, 3-OCH2C6H5).
Irradiation of benzopyran 1c in benzene. A solution of com-
pound 1c (400 mg, 0.001 mol) in thiophene free dry benzene (80
cm3) was photolysed under the above conditions. The reaction
mixture, on chromatography, gave 7c in 25% and 8c in 5% yield
respectively.
Irradiation of compound 1c in the presence of O2. An oxy-
genated solution of 1c (300 mg, 0.0008 mol) in dry benzene
(80 cm3), after the usual work up and chromatography provided
8c (100 mg) in 33% yield characterized by TLC, mp and mmp.
6-Chloro-3-ethoxy-2-(2Ј-thienyl)-4-oxo-4H-1-benzopyran 1b.
(68%), Mp 135 ЊC (Found: C, 58.9; H, 3.8. C15H11ClO3S
requires C, 58.8; H, 3.6%); λmax (MeOH)/nm 343.5 and 260.5
(ε/dm3 molϪ1 cmϪ1 34000 and 21000); νmax/cmϪ1 1630 (CO);
δH(90 MHz; CDCl3) 8.19 (1H, d, J 2 Hz, 5-H), 7.90 (1H, dd, J 1,
4 Hz, 3Ј-H), 7.60–7.40 (2H, m, 5Ј-H, 7-H), 7.35 (1H, d, J 9 Hz,
8-H), 7.20 (1H, dd, J 3, 5 Hz, 4Ј-H), 4.30 (2H, q, J 8 Hz,
3-OCH2CH3), 1.50 (3H, t, J 8 Hz, 3-OCH2CH3).
Irradiation of compound 1c in the presence of I2. Irradiation
of 1c (50 mg) in thiophene free dry benzene (30 cm3) in the
presence of I2 (10 mg) under the above conditions, produced
only 8c (20 mg) after the usual chromatographic work up.
DDQ treatment of compound 7c. A mixture of 7c (50 mg, 0.13
mol) and DDQ (30 mg, 0.13 mol) in xylene (20 cm3) was
refluxed for 20 h. Evaporation of solvent and percolation of the
residue through a silica gel column eluted 8c (35 mg), character-
ized by mmp.
6-Chloro-3-methoxy-2-(2Ј-thienyl)-4-oxo-4H-1-benzopyran
1a. (88%), Mp 135–136 ЊC (Found: C, 57.3; H, 3.3. C14H9ClO3S
requires C, 57.5; H, 3.1%); λmax (MeOH)/nm 343 and 260.5
(ε/dm3 molϪ1 cmϪ1 33000 and 23000); νmax/cmϪ1 1628 (CO);
δH(90 MHz; CDCl3) 8.25 (1H, d, J 2 Hz, 5-H), 7.90 (1H, dd, J 1,
4 Hz, 3Ј-H), 7.68–7.58 (2H, m, 5Ј-H, 7-H), 7.45 (1H, d, J 9 Hz,
8-H), 7.15–7.05 (1H, dd, J 3.5 Hz, 4Ј-H), 4.05 (3H, s, 3-OMe).
Irradiation of 6-chloro-3-ethoxy-2-(2Ј-thienyl)-4-oxo-4H-1-
benzopyran 1b
A solution of compound 1b (400 mg, 0.0013 mol) in methanol
(80 ml) was photolysed under similar conditions to those
employed for 1c. The reaction mixture on chromatographic
separation with pet. ether–benzene (1:1) and benzene–ethyl
acetate (19:1) provided the starting compound 1b (100 mg), mp
135 ЊC (mmp); followed by two tetracyclic compounds 7b (50
mg, 16%), mp 195–196 ЊC (Found: C, 59.0; H, 3.8. C15H11ClO3S
requires C, 58.8; H, 3.6%); νmax/cmϪ1 1640 (CO); m/z 306/308
(Mϩ); δH(90 MHz; CDCl3) 8.10 (1H, d, J 2 Hz, 7-H), 7.50 (1H,
dd, J 2, 9 Hz, 9-H), 7.35 (1H, d, J 9 Hz, 10-H), 6.35 (1H, d, J 5
Hz, 2-H), 5.55 (1H, dd, J 3, 5 Hz, 3-H), 4.92 (1H, d, J 8 Hz,
11b-H), 4.05–3.75 (1H, q of d, J 6 Hz, 4-H), 3.15–2.95 (1H, m,
3a-H), 1.45 (3H, d, J 6 Hz, 4-Me) and 8b (40 mg, 14%), mp
230 ЊC (Found: C, 60.0; H, 3.0. C15H9ClO3S requires C, 59.2; H,
3.0%); νmax/cmϪ1 1640 (CO); m/z 304/306 (Mϩ); δH(90 MHz;
CDCl3) 8.20 (1H, d, J 2 Hz, 7-H), 7.57 (1H, J 4.5 Hz, 2-H), 7.53
(1H, dd, J 2, 9 Hz, 9-H), 7.37 (1H, d, J 9 Hz, 10-H), 6.80 (1H, d,
J 4.5 Hz, 3-H), 5.55 (1H, q, J 7 Hz, 4-H), 1.65 (3H, d, J 7 Hz,
4-Me).
6-Chloro-3-methoxy-2-(5Ј-methyl-2Ј-thienyl)-4-oxo-4H-1-
benzopyran 2a. (80%), Mp 130 ЊC (Found: C, 58.9; H, 3.9.
C15H11ClO3S requires C, 58.8; H, 3.6%); λmax (MeOH)/nm 357.5
and 263.5 (ε/dm3 molϪ1 cmϪ1 24600 and 13100); νmax/cmϪ1 1638
(CO); δH(90 MHz; CDCl3) 8.15 (1H, d, J 2 Hz, 5-H), 7.65 (1H,
d, J 4 Hz, 3Ј-H), 7.45 (1H, dd, J 2, 9 Hz, 7-H), 7.35 (1H, d, J 9
Hz, 8-H), 6.78 (1H, d, J 4 Hz, 4Ј-H), 3.95 (3H, s, 3-OMe), 2.50
(3H, s, 5Ј-Me).
3-Benzyloxy-6-chloro-2-(5Ј-methyl-2Ј-thienyl)-4-oxo-4H-1-
benzopyran 2b. (80%), Mp 143–144 ЊC (Found: C, 65.7; H, 3.7.
C21H15ClO3S requires, C, 66.0; H, 3.9%); λmax(MeOH)/nm 361
and 263.5 (ε/dm3 molϪ1 cmϪ1 28900 and 14700); νmax/cmϪ1 1638
(CO); δH(90 MHz; CDCl3) 8.10 (1H, d, J 2 Hz, 5-H), 7.65 (1H,
d, J 4 Hz, 3Ј-H), 7.60–7.20 (7H, m, 7- and 8-H, 3-OCH2C6H5),
6.78 (1H, d, J 4 Hz, 4Ј-H), 5.20 (2H, s, 3-OCH2C6H5), 2.40 (3H,
s, 5Ј-Me).
Irradiation of 3-benzyloxy-6-chloro-2-(2Ј-thienyl)-4-oxo-4H-1-
benzopyran 1c
The chromone 1b (30 mg) was photolysed in dry benzene
(80 cm3) in the presence of nitrogen to afford 7b and 8b in 10%
and 20% yields respectively.
The benzopyran 1c (400 mg, 0.001 mol) was dissolved in
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Irradiation of 6-chloro-3-methoxy-2-(2Ј-thienyl)-4-oxo-4H-1-
benzopyran 1a in methanol
after 30 min at room temperature was precipitated with H2O,
filtered and crystallised (EtOH–CHCl3) to obtain 13 (900 mg,
90%), mp 170 ЊC (Found: C, 59.3; H, 2.9. C13H7ClO2S requires
C, 59.5; H, 2.7%); νmax/cmϪ1 1635(CO); δH(90 MHz; CDCl3)
8.15 (1H, d, J 2 Hz, 5-H), 7.80–7.35 (4H, m, 3Ј-H, 5Ј-H, 7-H
and 8-H), 7.30–7.10 (1H, m, 4Ј-H), 6.60 (1H, s, 3-H).
A solution of compound 1a (300 mg, 0.001 mol) in methanol
(80 cm3) was photolysed under similar conditions to those
employed above and was chromatographed to give the starting
compound 1a (70 mg), mp 135–136 ЊC (mmp and co-TLC) and
7a (30 mg, 14%), mp 190–192 ЊC; m/z 292/294 (Found: C, 57.2;
H, 3.0. C14H9ClO3S requires C, 57.5; H, 3.1%); νmax/cmϪ1 1638
(CO); δH(90 MHz; CDCl3) 8.15 (1H, d, J 2 Hz, 7-H), 7.60 (1H,
dd, J 2, 9 Hz, 9-H), 7.35 (1H, d, J 9 Hz, 10-H), 6.30 (1H, d, J 5
Hz, 2-H), 5.55 (1H, dd, J 3, 5 Hz, 3-H), 4.85 (1H, d, J 8 Hz,
11b-H), 4.20–4.05 (1H, dd, J 4, 10 Hz, 4a-H), 3.85 (1H, d, J 10
Hz, 4b-H), 3.75–3.20 (1H, m, 3a-H) and 8a (20 mg, 10%), mp
228–229 ЊC (Found: C, 58.0; H, 2.5. C14H7ClO3S requires C,
57.9; H, 2.4%); νmax/cmϪ1 1640 (CO); m/z 290/292 (Mϩ); δH(90
MHz; CDCl3) 8.20 (1H, d, J 2 Hz, 7-H), 7.60 (1H, d, J 2 Hz,
2-H), 7.53 (1H, dd, J 2, 9 Hz, 9-H), 7.37 (1H, d, J 9 Hz, 10-H),
6.90 (1H, d, J 4.5 Hz, 3-H), 5.30 (2H, s, 4-H).
Photolysis of 6-chloro-2-(2Ј-thienyl)-4-oxo-4H-1-benzopyran
13 in benzene. A benzene solution of 13 (130 mg, 0.005 mol, 80
cm3) was photolysed (2 h) under similar conditions to those
employed for 1c in benzene. The photolysate on work up gave
a solid identified as the starting chromone 13 (TLC, mmp).
Acknowledgements
Generous grants from DAE, Bombay and DST, New Delhi,
India are gratefully acknowledged.
The chromone 1a was photolysed in dry benzene (75 ml) in
nitrogen to afford 7a and 8a in 8% and 20% yields respectively.
References
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Irradiation of 6-chloro-3-methoxy-2-(5Ј-methyl-2Ј-thienyl)-4-
oxo-4H-1-benzopyran 2a in dry benzene
The photolysis of a benzene solution of 2a (400 mg, 0.0013
mol, 80 cm3, 1 h) and chromatographic separation under simi-
lar conditions to those employed earlier provided 2a in a small
amount followed by 9a (20%), mp 198 ЊC (Found: C, 60.0; H,
3.7. C15H11ClO3S requires C, 58.8; H, 3.6%); νmax/cmϪ1 1640
(CO); δH(90 MHz; CDCl3) 8.10 (1H, d, J 2 Hz, 7-H), 7.50 (1H,
dd, J 2, 9 Hz, 9-H), 7.35 (1H, d, J 9 Hz, 10-H), 5.10 (1H, d, J 3
Hz, 3-H), 4.90 (1H, d, J 8 Hz, 11b-H), 4.10–3.90 (3H, m, 3a-,
4a- and 4b-H), 1.90 (3H, s, 2-Me). Irradiation of 2a in MeOH
also provided 9a.
Irradiation of 3-benzyloxy-6-chloro-2-(5Ј-methyl-2Ј-thienyl)-
4-oxo-4H-1-benzopyran 2b in benzene. This gave 9b (20%), mp
215 ЊC (Found: C, 66.0; H, 4.0. C21H15ClO3S requires C, 66.0;
H, 3.9%); νmax/cmϪ1 1638 (CO); δH(90 MHz; CDCl3) 8.20 (1H,
d, J 2 Hz, 7-H), 7.50 (1H, dd, J 2, 9 Hz, 9-H), 7.45–7.20 (6H, m,
10-H, 4-Ph), 4.78 (1H, d, J 3 Hz, 3-H), 5.05 (1H, d, J 8 Hz, 11b-
H), 4.75 (1H, d, J 11 Hz, 4-H), 3.50–3.30 (1H, m, 3a-H), 1.90
(3H, s, 2-Me).
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6-Chloro-2-(2Ј-thienyl)-4-oxo-4H-1-benzopyran 13
10 J. Tirouflet and P. L. Cheng, Bull. Soc. Chim. Fr., 1963, 2252.
A
solution of 1-(5Ј-chloro-2Ј-hydroxyphenyl-3-(2Љ-thienyl)-
propenone 3 (1 g, 0.004 mol) in 5 cm3 DMSO was refluxed in the
presence of a crystal of I2 for 10 min. The dark red solution
Paper 9/01090D
J. Chem. Soc., Perkin Trans. 1, 1999, 2391–2395
2395