Phototransformation of 3-alkoxychromenones: Regioselective photocyclisation and dealkoxylation

Article abstract of DOI:10.1039/c5pp00318k

The phototransformation of some 2-(3-methoxyphenyl)-4H-chromen-4-ones bearing a propynyloxy moiety at the 3-position has been described. On photolysis with pyrex-filtered UV light from a Hg lamp (125 W), these chromenones produced a major amount of 5-ethy

Full text of DOI:10.1039/c5pp00318k

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Phototransformation of 3-alkoxychromenones:
regioselective photocyclisation and dealkoxylation†
Cite this: DOI: 10.1039/c5pp00318k
Radhika Khanna, Aarti Dalal, Ramesh Kumar and Ramesh C. Kamboj*
The phototransformation of some 2-(3-methoxyphenyl)-4H-chromen-4-ones bearing a propynyloxy
moiety at the 3-position has been described. On photolysis with pyrex-filtered UV light from a Hg lamp
(125 W), these chromenones produced a major amount of 5-ethynyl-2-methoxy-6-oxa-benzo[5,6-c]-
xanthen-7-ones consisting of an exotic tetracyclic scaffold. These photoproducts have been envisioned
to be produced through regioselective ring closure at the 6’-position of the 2-(3’-methoxy)phenyl moiety
of the initially formed 1,4-biradical via a γ-H abstraction mechanism. No product whatsoever was
observed through ring closure at the 2’-position. This behaviour has been found to be in accordance with
the directive influence observed in free radical aromatic substitutions. This regioselective photocyclisation
is further supported by calculations made from 3D structures (MM2 program). In addition, during the
irradiation of these substrates, 2-(3-methoxyphenyl)-4H-chromen-4-ones were also realised through
dealkoxylation. The structures of the substrates and photoproduct(s) have been determined by their
spectroscopic (IR, NMR, mass spectrometry) studies.
Received 25th August 2015,
Accepted 1st October 2015
DOI: 10.1039/c5pp00318k
www.rsc.org/pps
degenerative impairments including Parkinson’s disease, Alz-
heimer’s disease, Down’s syndrome, HIV-associated dementia
Introduction
Photochemical intramolecular H abstractions in carbonyl com- and schizophrenia.23–29 In view of these and our continuous
pounds have found several synthetic applications1–11 including effort in developing the photochemistry of such molecules, in
the synthesis of numerous exotic carbocyclic and heterocyclic the present paper we wish to report the results of our investi-
scaffolds that are otherwise very difficult to obtain under gations on the photolysis of some 4H-chromen-4-ones bearing
thermal conditions. The exceptional behaviour of the propar- the propynyloxy moiety at the 3-position and the 3-methoxy-
gyl 1,4-biradical in forming photoproducts via unsaturated carb- phenyl moiety at the 2-position, with the motive to determine
enes12,13 has been investigated by Agosta and Margaretha,14 the regioselective photocyclisation and other reactions, as
and has encouraged us to study the photochemistry of chrome- elicited earlier in other similar molecules.15–17,30–32
nones containing the propargyl group.15–17 In these studies, it
was observed that the propynyloxy group behaved in many
ways, such as as a suitable candidate for H abstraction
followed by ring closure, in tandem electrocyclisation and
Results and discussion
rearrangement, and in intramolecular Paterno–Buchi cyclisa-
tion. In most of these reactions, xanthenones are the main
photoproducts. Xanthenones are secondary metabolites
occurring in higher plants, fungi and lichens18 and are utilized
as leuco-dyes, pH-sensitive fluorescent materials for tracking
biomolecules and in laser technologies.19–22 The reactive
chromene core in the xanthenone moiety is responsible for a
wide range of biological activities ranging from anti-cancer,
anti-viral, diuretic, spasmolytic, anti-coagulant, and anti-ana-
phylactic to cognitive enhancement for the treatment of neuro-
The chromenones, 2-(3-methoxyphenyl)-3-(prop-2-ynyloxy)-4H-
chromen-4-ones 5, consisting of an acetylenic pendant, were
synthesized by (i) condensing acetophenones 1 with 3-methoxy-
benzaldehyde (2) in the presence of NaOH/EtOH, (ii) followed
by the cyclisation of chalcones 3 to 3-hydroxychromenones
4 under Algar–Flynn–Oyamada33,34 reaction conditions, and
(iii) subsequent alkylation of the latter with propargyl bromide
in the presence of dry acetone, freshly dried K2CO3 and tetra-
n-butyl ammonium iodide (Scheme 1). The structures of
compounds 3, 4 and 5 30 were confirmed by their spectroscopic
data (vide experimental).
These chromenones 5 (absorption maxima, λmax, between
350–357 nm in MeOH) were tailored and designed with the
objectives; (i) to illustrate regioselectivity, because these have a
propensity for cyclisation at two possible sites, i.e. the 2′- and
Department of Chemistry, Kurukshetra University, Kurukshetra-136119, Haryana,
India. E-mail: rckamboj@hotmail; Fax: +91-1744-238277; Tel: +91-9466650266
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5pp00318k
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Scheme 1 Synthesis of 2-(3-methoxyphenyl)-3-(prop-2-ynyloxy)-4H-chromen-4-ones 5.
6′-positions of the methoxyphenyl moiety, and (ii) to observe Irradiation of 5(a–d) in methanol with pyrex-filtered light from
the effect of the propynyloxy group on photoproduct formation a 125 W Hg UV lamp under an N2 atmosphere furnished two
in comparison to other functionalities.31
new compounds 6(a–d) and 7(a–d) in each case, as indicated
by the thin layer chromatographic examination of their
reaction mixtures (Scheme 2). The removal of solvent under
reduced pressure and the chromatographic separation of
photolysate with increasing proportions of ethyl acetate in
ethyl acetate–petroleum ether yielded the xanthenones
6 (35–45%) and the dealkoxylated chromenones 7 (20–24%)
as the photoproducts, approximately in the ratio 2 : 1,
respectively. The product composition remained the same
when benzene was used as the solvent instead of methanol.
The structures of these photoproducts were obtained by their
spectroscopic studies. In the NMR spectrum of product 6, a
doublet was observed at δ 8.3–7.9 ppm (Jm = 2.4 Hz) for proton
Scheme 2 Conversion of 5 to 6 via photochemical transformation.
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doublet at around δ 6.4 ppm (Jm = 2.4 Hz). The rest of the
protons, i.e. H1, H3 and H4, were observed at δ 7.5–7.4 ppm (d,
Jm = 1.6 Hz), 7.0 ppm (dd, Jo = 7.6 Hz and Jm = 1.6 Hz) and
7.48 ppm (d, Jo = 7.6 Hz) respectively. This splitting and
coupling pattern of H1, H3 and H4 is in agreement with the
proposed structure, i.e. 6(a–d), not with the structures 8(a–d).
The structure of the dealkoxylated product formed (7) was
confirmed by comparing its NMR spectrum with the substrate
(5), where the former showed the presence of singlet at 6.8 ppm
for H3 and the disappearance of doublets at δ 5.0 and 2.3 ppm
(vide experimental) present in the 1H NMR spectrum of 5.
This structure of 6 was further confirmed by 2D COSY of
6b, where it was clearly observed that H8 was interacting with
H10, which was also correlating with H11, and the acetylenic
Fig. 1 Possible product via cyclisation at 2’-position.
H8. Proton H11 resonated between δ 7.4 and 7.5 ppm (d, Jo =
8.8 Hz) and proton H10, as in 6b–6d, was observed between 7.7
and 7.6 ppm (dd, Jo = 8.8 Hz and Jm = 2.4 Hz). The acetylenic
proton gave a signal at nearly δ 2.4 ppm (d, Jm = 2.4 Hz), which
showed allylic coupling with proton H5, which resonated as a
Scheme 3 Suggested mechanism for the formation of products (6 & 7) from 2-(3-methoxyphenyl)-3-(prop-2-ynyloxy)-4H-chromen-4-ones 5 by
photolysis.
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proton was correlating with H5. The interactions of H3 with Although the mechanism of this reaction is not established yet,
H1 and H4 were also observed. the formation of 6 (path 1) and 7 (path 2) from 5 can be en-
Study of the structures of the xanthenones 6(a–d) obtained visioned (Scheme 3) to occur through the initial formation of a
here showed that these have been produced through regio- 1,4-biradical via a γ-H abstraction mechanism,30 followed by
selective cyclisation at the 6′-position of the 2-(3′-methoxyphenyl) ring closure by coupling of the –O–ĊH– radical with the 6′-posi-
moiety. No photoproduct similar to 8 (Fig. 1) was realised tion of the 2-(3′-methoxyphenyl) moiety regioselectively. Photo-
through regioselective cyclisation at the 2′-position, which has a product 7 has been conceived to be formed through the
fair probability of occurrence, as observed earlier, with any formation of oxetane 7B, followed by its fragmentation (con-
alkoxy group30 other than the propargyloxy group. Also no certed or stepwise), as observed earlier36 in the photochemical
dihydro product, i.e. 5-ethynyl-4a,5-dihydro-4-methoxyiso- cycloreversion of oxetane dyads. Further, the higher reactivity of
chromeno[4,3-b]chromen-7(2H)-one, similar to 4 in ref. 31 was oxetane 7B formed in situ toward fragmentation may probably
obtained.
be related to its folded conformations (Fig. 2), and also to the
1
The H NMR spectral analysis of the photolysate also indi- presence of the electron-donating methoxyphenyl chromophore
cated the formation of a trace amount of pyranochromenones and the electron-withdrawing carbonyl group in the chrome-
9 (singlet at δ 6.8 ppm and doublet at δ 5.3 ppm), with no for- none moiety. The formation of 7 could also be through the
mation of xanthenones 10, similar to those obtained from the heterolytic mechanism (Scheme 4), involving the formation of
photolytic studies15,17,35 on 3-(prop-2nyloxy)-2-(thiophen-3-yl)- alkoxide plus a chromenone cation (7D), which would be like a
4H-chromen-4-ones. However, these products, being in trace 2-oxocyclohexadienyl carbene (7E), being part of the pyrrilium
amounts, could not be isolated in spite of our best efforts. ring system.37 A ring oxygen in the 3-position would certainly
Even the change of solvent from methanol to benzene had no support its formation, as would a carbonyl oxygen next to it.
effect on the product formation and isolation.
Fig. 2 MM2 energy-minimized 3D structure of tentative oxetane 7B – a
folded conformation (a representative case).
Scheme 4 Alternate plausible mechanism for the formation of 7.
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CDCl3 on a 300 MHz/400 MHz Bruker spectrometer using TMS
as an internal standard. Mass spectra were recorded on a
LC-MS Spectrometer Model Q-ToF Micro Waters source in ES+
mode. The columns for purification were packed with Silica
gel 100–200 mesh in petroleum ether/ethyl acetate (9 : 1) and
left overnight before use. The elution was carried out with
increasing proportions of ethyl acetate in the petroleum ether–
ethyl acetate mixture. The yields reported are based on the
amount of isolated photoproduct and are calculated by exclud-
ing the recovered substrates. The solvents used in the study for
purification and other purposes have been recovered.
Synthesis of 1-(2-hydroxyphenyl)-3-(3-methoxyphenyl)prop-2-
en-1-ones (3a–3d). To a well-stirred suspension of powdered
NaOH (0.1 mol) in EtOH at 0 °C were added substituted
2-hydroxyacetophenone (0.05 mol) and 3-methoxybenzalde-
hyde (0.055 mol). The reaction mixture, which became deep
red in color after 3 h, was stirred for 7–8 h. Thereafter, it was
poured over ice and was neutralized with dil. HCl to obtain
acrylophenone, which was crystallized from EtOH to give
yellow needles of 3.
Fig. 3 MM2 energy-minimized 3D structure of compound 5.
This photocyclisation is in accordance38 with the
directive influence observed in free radical aromatic substi-
tution (o- and p-directing). In this case, regioselectivity for
ring formation exclusively at the 6′-position (p- to methoxy
and o- to the chromenyl group) with no ring formation at the
2′-position has been observed, which can explained by the
steric hindrance experienced at the 2′-position from the
methoxy and chromenyl groups tethered to the phenyl moiety
at the 3′- and 1′-positions respectively. This regioselectivity for
photocyclisation was further substantiated by the proximity
effect, as revealed by the calculations made from 3D structures
3a – Orange solid in 77% yield, mp 91–94 °C; νmax (cm−1):
3405 (–OH), 1645 (CvO); 1H NMR [CDCl3, 400 MHz, δ (ppm)]:
12.67 (1H, s, OH), 7.90 (1H, s, H6′), 7.86 (1H, d, J3,2 = 15.2 Hz,
H3), 7.51 (1H, d, J2,3 = 15.2 Hz, H2), 7.38 (1H, t, Jo = 8.0 Hz, Jo =
7.2 Hz, H5″), 7.28 (1H, dd, Jo = 8.0 Hz, Jm = 2.0 Hz, H6″), 7.17
(1H, d, Jm = 2.0 Hz, H2″), 7.01 (1H, td, Jo = 8.0 Hz, Jm = 2.0 Hz,
H4″), 6.91 (1H, s, H3′), 3.88 (3H, s, OCH3), 2.39 ppm (3H, s,
CH3).
3b – Orange solid in 78% yield, mp 88–92 °C; νmax (cm−1):
3415 (–OH), 1641 (CvO); 1H NMR [CDCl3, 400 MHz, δ (ppm)]:
12.76 (1H, s, OH), 7.96 (1H, d, Jm = 2.8 Hz, H6′), 7.84 (1H, d,
J3,2 = 15.2 Hz, H3), 7.56 (1H, d, J2,3 = 15.2 Hz, H2), 7.52 (1H, d,
Jo = 6.8 Hz, Jm = 2.8 Hz, H4′), 7.32 (1H, dd, Jo = 8.0 Hz, H5″),
7.11 (1H, d, Jm = 2.4 Hz, H2″), 7.04 (2H, m, H4″, 6″), 6.91 (1H,
d, Jo = 6.8 Hz, H6′), 3.90 ppm (3H, s, OCH3).
(MM2 programme) for the substrates (5). The –O–C̲H2– and
the 6′-position (the two clipping atoms for ring formation)
have been observed in close proximity; the contact averaged at
3.263 Å, which is quite a bit shorter than the close contact
(average 4.924 Å) between –O–C̲H2– and the 2′-position of the
methoxyphenyl ring at the 2-position (Fig. 3).
3c – Orange solid in 75% yield, mp 78–81 °C; νmax (cm−1):
3445 (–OH), 1643 (CvO); 1H NMR [CDCl3, 400 MHz, δ (ppm)]:
12.79 (1H, s, OH), 7.93 (1H, dd, Jo = 6.8 Hz, Jm = 2.8 Hz H6′),
7.86 (1H, d, J3,2 = 15.6 Hz, H3), 7.65 (1H, d, J2,3 = 15.6 Hz, H2),
7.52 (1H, td, Jo = 6.8 Hz, Jm = 1.6 Hz, H4′), 7.37 (1H, td, Jo =
6.8 Hz, Jm = 2.8 Hz, H5′), 7.29 (1H, dd, Jo = 8.0 Hz, H5″),
Conclusion
It may be concluded that 2-(3-methoxyphenyl)-3-(prop-2-
ynyloxy)-4-H-chromen-4-ones yield tetracyclic product(s)
5-ethynyl-2-methoxy-6-oxa-benzo[5,6-c]xanthen-7-ones
on
7.17 (1H, d, Jm = 1.6 Hz, H2″), 7.04 (1H, td, Jo = 8.0 Hz, Jm
=
phototransformation, through γ-H abstraction followed by
regioselective cyclisation at 6-position, which has been found
to be controlled by the steric influence. Further, these angular
tetracyclics, the benzoxanthenones, have been produced by an
environmentally benign green method without using any toxic
reagents. The solvents used in the study for purification and
other purposes have been recovered substantially and reused.
2.4, H6″), 7.00 (1H, td, Jo = 8.0 Hz, Jm = 2.4 and 1.6 Hz, H4″),
6.92 (1H, dd, Jo = 6.8 Hz, Jm = 2.8 Hz, H3′), 3.84 ppm (3H,
s, OCH3).
3d – Orange solid in 71% yield, mp 79–83 °C; νmax (cm−1):
3405 (–OH), 1645 (CvO); 1H NMR [CDCl3, 400 MHz, δ (ppm)]:
12.76 (1H, s, OH), 7.93 (1H,d, Jm = 2.8 Hz, H6′), 7.88 (1H, d, J3,2
= 15.2 Hz, H3), 7.56 (1H, d, J2,3 = 15.2 Hz, H2), 7.50 (1H, dd,
Jo = 8.4 Hz, Jm = 2.8 Hz, H4′), 7.41 (1H, dd, Jo = 8.0 Hz, H5″),
7.11 (1H, d, Jm = 2.0 Hz, H2″), 7.04 (2H, m, H4″, 6″), 6.90 (1H,
d, Jo = 8.4 Hz, H3′), 3.90 (3H, s, OCH3), 2.53 ppm (3H, s,
5′-CH3).
Experimental
General
Melting points recorded are uncorrected and taken in open
Synthesis of 3-hydroxy-2(3-methoxyphenyl)-4H-chromen-4-
capillaries. IR spectra were recorded on a Perkin-Elmer IR ones (4a–4d). To a suspension of chalcone (0.01mol) in
1
spectrophotometer. H and 13C NMR spectra were recorded in methanol was added 10.0 ml of 20% aq. KOH, and this
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mixture was cooled to 0 °C. To this dark red well-stirred 141.6, 135.7, 127.9, 127.0, 125.3, 123.5, 122.0, 121.7, 119.6,
solution was added H2O2 (30%) dropwise until the color 118.4, 116.8, 116.2, 78.3, 76.4, 58.1, 55.0.
changed to yellow, and the stirring was continued for 1 h. The
5c – White solid in 74% yield, mp 110–112 °C; λmax
reaction mixture was neutralized with ice–HCl to give light (MeOH): 353 nm; νmax (cm−1): 2132 (CuC), 1625 (CvO);
yellow precipitates. The solid was filtered, dried and crystal- 1H NMR [CDCl3, 400 MHz, δ (ppm)]: 8.28 (1H, dd, Jo = 8.0 Hz,
lized (CHCl3–MeOH) to give yellow crystals of 3-hydroxy- Jm = 1.6 Hz, H5), 7.73 (1H, td, Jo = 8.0 Hz, Jm = 1.6 Hz, H6), 7.72
chromenones 4.
(1H, dd, Jo = 8.4 Hz, Jm = 1.6 Hz, H6′) 7.70 (1H, td, Jo = 8.0 Hz,
4a – cream solid in 72% yield, mp 112–116 °C; νmax (cm−1): Jm = 1.6 Hz, H7), 7.61 (1H, dd, Jo = 8.8 Hz, H5′), 7.45 (2H, m,
3202 (–OH), 1610 (CvO); 1H NMR [CDCl3, δ (ppm), 400 MHz]: H2′ and 8), 7.08 (1H, td, Jo = 8.0 Hz, Jm = 1.6 Hz, H4′), 5.00 (2H,
8.20 (1H, s, H5), 7.87 (1H, dd, Jo = 7.2 Hz, H5′), 6.96 (4H, m, d, J1″,3″ = 2.4 Hz, H1″), 3.90 (3H, s, OCH3), 2.36 (1H, t, J3″,1″
=
H8, 2′, 4′ and 6′), 3.80 (3H, s, OCH3), 2.53 (3H, s, CH3).
2.4 Hz, H3″); 13C NMR [CDCl3, δ (ppm)]: 174.9, 159.4, 155.3,
4b – cream solid in 76% yield, mp 108–110 °C; νmax (cm−1): 133.6, 132.1, 129.9, 129.4, 125.8, 124.8, 124.0, 121.3, 119.3,
3210 (–OH), 1605 (CvO); 1H NMR [CDCl3, 400 MHz, δ (ppm)]: 118.0, 116.8, 114.2, 78.6, 76.1, 59.2, 55.0.
8.02 (1H, d, Jm = 1.6 Hz, H5), 7.88 (1H, dd, Jo = 8.4 Hz, Jm = 1.6
5d – White solid in 73% yield, mp 91–94 °C; λmax (MeOH):
Hz, H7), 7.74 (1H, dd, Jo = 6.8 Hz, Jm = 1.6 Hz, H6′), 7.56 (1H, 357 nm; νmax (cm−1): 2128 (CuC), 1622 (CvO); 1H NMR
d, Jo = 8.4 Hz, H5′), 7.44 (2H, m, H2′ and 8), 7.10 (1H, td, Jo = [CDCl3, 400 MHz, δ (ppm)]: 8.27 (1H, d, Jm = 2.4 Hz, H5), 7.79
8.4 Hz, Jm = 1.6 Hz, H4′), 3.90 (3H, s, OCH3).
(1H, dd, Jo = 8.4 Hz, Jm = 2.4 Hz, H7), 7.54 (1H, dd, Jo = 8.1 Hz,
4c – cream solid in 71% yield, mp 101–103 °C; νmax (cm−1): H5′), 7.32 (3H, m, H2′, 6′ and 8), 7.01 (1H, dd, Jo = 8.0 Hz, Jm
=
3205 (–OH), 1615 (CvO); 1H NMR [CDCl3, 400 MHz, δ (ppm)]: 1.6 Hz, H4′), 4.99 (2H, d, J1″,3″ = 2.4 Hz, H1″), 3.90 (3H, s,
8.27 (1H, dd, Jo = 8.0 Hz, Jm = 1.6 Hz, H5), 7.73 (1H, td, Jo = OCH3), 2.43 (3H, s, CH3), 2.33 (1H, t, J3″,1″ = 2.4 Hz, H3″);
6.8 Hz, Jm = 1.6 Hz, H6), 7.70 (1H, td, Jo = 6.8 Hz, Jm = 1.6 Hz, 13C NMR [CDCl3, δ (ppm)]: 169.7, 159.1, 158.1, 138.1, 133.6,
H7), 7.68 (1H, d, Jm = 1.6 Hz, H2′), 7.56 (1H, dd, Jo = 8.0 Hz, 131.1, 129.9, 126.1, 124.0, 122.8, 121.1, 118.9, 116.4, 115.8,
H5′), 7.45 (1H, dd, Jo = 8.8 Hz, Jm = 2.8 Hz, H8), 7.42 (1H, dd, 114.6, 78.6, 76.6, 59.6 55.8, 20.4.
Jo = 6.8 Hz, Jm = 1.6 Hz, H6′), 7.08 (1H, td, Jo = 8.0 Hz, Jm
1.6 Hz, H4′), 3.90 (3H, s, OCH3).
=
Photolysis of 2-(3-methoxyphenyl)-3-(prop-2-ynyloxy)-4H-
chromen-4-ones (5a–5d). A deoxygenated 1.0 mM methanolic
4d – cream solid in 69% yield, mp 110–113 °C; νmax (cm−1): solution of chromenone 5 contained in a pyrex glass vessel
3232 (–OH), 1605 (CvO); 1H NMR [CDCl3, 400 MHz, δ (ppm)]: was purged with nitrogen for 30 min and then irradiated
7.96 (1H, d Jm = 2.0 Hz, H5), 7.78 (1H, dd, Jo = 8.4 Hz, Jm
=
under nitrogen with light from a 125 W Hg vapor lamp for
2.0 Hz, H7), 7.74 (1H, dd, Jo = 7.6 Hz, Jm = 1.6 Hz, H6′), 7.64 45 min. The removal of solvent under reduced pressure yielded
(1H, dd, Jo = 7.6 Hz, and 8.4 Hz, H5′), 7.51 (2H, m, H2′ and 8), a gummy mass that was chromatographed over a column of
7.03 (1H, d, Jo = 8.4 Hz, Jm = 1.6 Hz, H4′), 3.90 (3H, s, OCH3), silica gel. The column was eluted with increasing proportions
2.54 (3H, s, CH3).
of ethyl acetate in an ethyl acetate–petroleum ether mixture,
Synthesis of 2-(3-methoxyphenyl)-3-(prop-2-ynyloxy)-4H- yielding photoproducts 6 and 7.
chromen-4-ones (5a–5d). To a suspension of compound 4
9-Chloro-5-ethynyl-2-methoxy-10-methyl-6-oxa-benzo[5,6-c]-
(0.001mol) and freshly dried K2CO3 (0.005mol) in dry acetone xanthen-7-one 6a. White solid in 37% yield, mp 202–204 °C;
was added propargyl bromide (0.001 mol) and tetra-n-butyl- νmax (cm−1): 2128 (CuC), 1628 (CvO); 1H NMR [CDCl3,
ammonium iodide (0.050 g). The reaction mixture was refluxed 400 MHz, δ (ppm)]: 8.27 (1H, s, H8), 7.48 (2H, m, H1 and 11),
for 4 h and the color of the reaction mixture changed from 7.36 (1H, d, Jo = 8.0 Hz, H4), 7.08 (1H, dd, Jo = 8.0 Hz, Jm
reddish-orange to white. Filtration, evaporation of solvent and 2.8 Hz, H3), 6.39 (1H, d, J5,2′ = 2.4 Hz, H5), 3.94 (3H, s, OCH3),
crystallization of the residue (MeOH) gave 5.
2.52 (3H, s, CH3), 2.39 (1H, d, J2′,5 = 2.4 Hz, H2′); 13C NMR
=
5a – White solid in 72% yield, mp 101–104 °C; λmax [CDCl3, δ (ppm)]: 160.5, 154.3, 153.0, 142.7, 142.6, 131.5,
(MeOH): 351 nm; νmax (cm−1): 2124 (CuC), 1621 (CvO); 130.1, 126.4, 125.7, 123.3, 120.4, 119.7, 117.4, 114.4, 113.9,
1H NMR [CDCl3, 400 MHz, δ (ppm)]: 8.20 (1H, s, H5), 7.70 (1H, 107.4, 77.3, 61.9, 55.9, 20.8; Mass (m/z): 352.0 (M+, 100%).
dd, Jo = 8.4 Hz, Jm = 1.6 Hz H6′), 7.45 (1H, dd, Jo = 8.4 Hz, H5′),
7.07 (2H, m, H2′ and 4′), 6.98 (1H, s, H8), 4.98 (2H, d, J1″,3″
9-Chloro-5-ethynyl-2-methoxy-6-oxa-benzo[5,6-c]xanthen-
=
7-one 6b. White solid in 34% yield, mp 180–185 °C; νmax
1
2.4 Hz, H1″), 3.89 (3H, s, OCH3), 2.52 (3H, s, CH3), 2.37 (1H, t, (cm−1): 2125 (CuC), 1621 (CvO); H NMR [CDCl3, 400 MHz,
J3″,1″ = 2.4 Hz, H3″); 13C NMR [CDCl3, δ (ppm)]: 160.5, 154.3, δ (ppm)]: 8.30 (1H, d, Jm = 2.4 Hz, H8), 7.64 (1H, dd, Jo
153.0, 142.7, 142.6, 131.5, 130.1, 126.4, 125.7, 123.3, 120.4, 8.8 Hz, Jm = 2.4 Hz, H10), 7.55 (1H, d, Jo = 8.8 Hz, H11), 7.51
119.7, 117.4, 114.4, 113.9, 107.4, 77.3, 61.9, 55.9, 20.8. (1H, d, Jm = 1.6 Hz, H1), 7.49 (1H, d, Jo = 7.6 Hz, H4), 7.09 (1H,
=
5b – White solid in 85% yield, mp 98–102 °C; λmax (MeOH): dd, Jo = 7.6 Hz, Jm = 1.6 Hz, H3), 6.40 (1H, d, J5,2′ = 2.4 Hz, H5),
343 nm; νmax (cm−1): 2128 (CuC), 1615 (CvO); 1H NMR 3.94 (3H, s, OCH3), 2.40 (1H, d, J2′,5 = 2.4 Hz, H2′); 13C NMR
[CDCl3, 400 MHz, δ (ppm)]: 8.08 (1H, d, Jm = 2.8 Hz, H5), 7.70 [CDCl3, δ (ppm)]: 170.8, 154.4, 153.1, 147.4, 135.1, 133.6,
(2H, m, H7, 5′), 7.62 (1H, dd, Jo = 8.8 Hz, H6′), 7.32 (2H, m, H2′ 130.7, 130.2, 125.5, 124.4, 120.5, 119.7, 114.6, 114.0, 79.6, 74.4,
and 8), 7.10 (1H, td, Jo = 8.8 Hz, Jm = 1.6 Hz, H4′), 4.98 (2H, d, 61.9, 56.0, 22.7; Mass (m/z): 338.9 (M+, 100%).
J1″,3″ = 2.4 Hz, H1″), 3.90 (3H, s, OCH3), 2.35 (1H, t, J3″,1″
=
5-Ethynyl-2-methoxy-6-oxa-benzo[5,6-c]xanthen-7-one 6c.
2.4 Hz, H3″); 13C NMR [CDCl3, δ (ppm)]: 173.2, 158.4, 157.1, White solid in 41% yield, mp 192–194 °C; νmax (cm−1): 2115
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(CuC), 1628 (CvO); H NMR [CDCl3, 400 MHz, δ (ppm)]: 7.93
(1H, dd, Jo = 6.8 Hz, Jm = 2.8 Hz, H8), 7.65 (1H, td, Jo = 6.8 Hz,
Acknowledgements
Jm = 2.8 Hz, H10), 7.52 (1H, dd, Jo = 6.8 Hz, Jm = 1.8 Hz, H11), Radhika Khanna is highly thankful to the Department of
7.48 (1H, d, Jm = 2.0 Hz, H1), 7.37 (1H, d, Jo = 7.6 Hz, H4), 7.17 Science and Technology, New Delhi (INSPIRE Fellowship) for
(1H, m, H9), 7.04 (1H, dd, Jo = 7.6 Hz, Jm = 2.0 Hz, H3), 6.41 (1H, generous financial support of this work.
d, J5,2′ = 2.4 Hz, H5), 3.90 (3H, s, OCH3), 2.35 (1H, d, J2′,5
=
2.4 Hz, H2′); 13C NMR [CDCl3, δ (ppm)]: 169.3, 159.5, 157.3,
136.1, 135.3, 135.1, 130.9, 130.4, 128.4, 127.6, 124.2, 121.5, 116.4,
113.2, 110.7, 84.7, 76.4, 65.2, 55.9; Mass (m/z): 304.0 (M+, 100%).
5-Ethynyl-2-methoxy-9-methyl-6-oxa-benzo[5,6-c]xanthen-7-
one 6d. White solid in 46% yield, mp 201–203 °C; νmax
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