One-shot photochemical synthesis of 5-(thiophen-3-yl)pyrano[2,3-c]chromen- 2(3H)-ones from 3-propynyloxy-chromenones: A case of an intramolecular Paterno-Buchi reaction

Article abstract of DOI:10.1039/c3pp50396h

5-(Thiophen-3-yl)pyrano[2,3-c]chromen-2(3H)-ones (2), angular tricyclic compounds, were synthesized in significantly high yields through the photoinduced intramolecular coupling of the acetylenic group with the carbonyl centre in 3-(prop-2-ynyloxy)-2-(thi

Full text of DOI:10.1039/c3pp50396h

Photochemical &
Photobiological Sciences
View Article Online
View Journal | View Issue
COMMUNICATION
One-shot photochemical synthesis of
5-(thiophen-3-yl)pyrano[2,3-c]chromen-2(3H)-
ones from 3-propynyloxy-chromenones: a case of
an intramolecular Paterno–Buchi reaction†
Cite this: Photochem. Photobiol. Sci.,
2014, 13, 488
Received 20th November 2013,
Accepted 20th December 2013
Pooja Jindal,a Rimpy Bhatia,a Sadhika Khullar,b Sanjay K. Mandalb and
DOI: 10.1039/c3pp50396h
www.rsc.org/pps
Ramesh C. Kamboj*a
5-(Thiophen-3-yl)pyrano[2,3-c]chromen-2(3H)-ones (2), angular 3-propynyloxy-chromenones which led to the expedient syn-
tricyclic compounds, were synthesized in significantly high yields thesis of 5-(thiophen-3-yl)pyrano[2,3-c]chromen-2(3H)-ones (2),
through the photoinduced intramolecular coupling of the acetyle- the angular pyranochromenones. These synthesized pyrano-
nic group with the carbonyl centre in 3-(prop-2-ynyloxy)-2-(thio- chromenones may mimic the activities (antineoplastic,12 anti-
phen-3-yl)-4H-chromen-4-ones (1). This photoreaction is a case microbial,13 anti-cancer,14 anti-HIV,15 antiproliferative,16 etc.)
of an intramolecular Paterno–Buchi reaction and is unprecedented elicited by many pyranochromenone analogs.
in 3-propynyloxy-chromenones. The structure of
2
has been
The substrates, 3-(prop-2-ynyloxy)-2-(thiophen-3-yl)-4H-
determined by spectroscopic (FTIR, NMR and mass) and single chromen-4-ones (1),17 were synthesized by (a) the conden-
crystal X-ray crystallographic studies.
sation of 2-hydroxyacetophenones with thiophene-3-carbox-
aldehyde in the presence of NaOH/EtOH, (b) reacting the
resulting chalcones with 50% H2O2/OH− under Algar–Flynn–
Oyamada18 reaction conditions, and (c) subsequent alkylation
of the 3-hydroxychromenones with propargyl bromide in the
presence of dry acetone, freshly dried K2CO3 and tetra-n-butyl
ammonium iodide. These chromenones 1(a–c) showed λmax in
the range of 316–320 nm (Fig. 1).
The chromatographic separation of the photolysate
obtained by the photo-irradiation (Scheme 1) of a methanolic
solution of 1 with pyrex filtered UV-light using a 125 W Hg
lamp under a nitrogen atmosphere yielded the 5-(thiophen-
3-yl)pyrano[2,3-c]chromen-2(3H)-ones 219 (yield, mp: 2a, 32%,
76 °C; 2b, 29%, 65 °C and 2c, 31%, 94 °C) as one of the
The photoinduced [2 + 2] cycloaddition of an olefin to a carbo-
nyl center – the Paterno–Buchi reaction1 – is a facile and versa-
tile access for oxetane2 preparation with high regio- and
stereoselectivity.3,4 The ring opening of oxetanes followed by
bond-formation has been found to be very useful in synthetic
chemistry.4,5 Insect pheromones, asteltoxin6 (a potent inhibi-
tor of ATP synthesis) and (+)-Preussin6 (an antifungal agent)
can also be synthesized by Paterno–Buchi reactions followed
by ring opening. The photoinduced [2 + 2] cycloaddition of
acetylene with carbonyl compounds leads to the formation of
a 1,4-biradical which on cyclization gives labile oxetenes.7,8
There are many reports9 on the photoproduction of enones on
irradiation of carbonyl compounds in the presence of acetyl-
enes. In most of these, an unstable oxetene intermediate was
believed to be formed which easily fragmented to give enone
products. In the framework of our research program on the
determination of the photolytic behavior of 3-alkoxy-
chromones10 bearing a propynyloxy group at the 3-position and
its extension, herein, we report our investigations on the
photocoupling through the intramolecular Paterno–Buchi reac-
tion11 of the acetylenic moiety with the carbonyl center in
aDepartment of Chemistry, Kurukshetra University, Kurukshetra-136119, Haryana,
India. E-mail: rckamboj@hotmail.com; Fax: +91-1744-238277; Tel: +91-9466650266
bDepartment of Chemical Sciences, Indian Institute of Science Education and
Research, Mohali, Sector 81, Manauli PO, S.A.S. Nagar, Mohali 140306, Punjab,
India
†CCDC 956397. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3pp50396h
Fig. 1 Electronic spectra of 1.
488 | Photochem. Photobiol. Sci., 2014, 13, 488–491
This journal is © The Royal Society of Chemistry and Owner Societies 2014

View Article Online
Photochemical & Photobiological Sciences
Communication
Scheme 1 Conversion of 1 to 2 via the photoirradiation process.
Table 1 1H NMR shifts of photoproducts
H’s
2a
2b
2c
C1-H
C3-H
6.33(s)
4.68(s)
6.26(s)
4.73(s)
6.69(s)
4.67(s)
C7-H
C8-H
7.39–7.37(d)
7.45–7.42(dd)
—
2.46(s)
7.71(d)
8.22–8.21(dd)
7.54–7.52(dd)
7.78–7.77(dd)
7.32–7.30(d)
—
2.54(s)
—
7.77(d)
8.24–8.18(dd)
7.49–7.45(dd)
8.05–7.97(dd)
7.47–7.45(d)
7.52–7.49(dd)
—
—
7.78(d)
8.12–8.05(dd)
7.66–7.62(dd)
8.20–8.19(dd)
C8-CH3
C9-CH3
C10-H
C2′-H
C4′-H
C5′-H
Fig. 2 2D-COSY spectrum of 2a.
products. The formation of these pyranochromenones (yield:
2a, 31%; 2b, 29% and 2c, 30%) from 1 also occurs in benzene.
Moreover, no chromenone-sensitized addition of methanol to
the acetylenic moiety in 3-(prop-2-ynyloxy)-4H-chromen-4-ones
was observed.
The FTIR spectrum of 2a showed a strong absorption band
at 1643 cm−1 that may be assigned to the CvO of the pyrone
moiety. In the NMR spectrum (Table 1) of 2a, a doublet at δ
7.71 (J = 1.64 Hz) for H-10 was observed. For the other two
benzenoid protons, resonances between δ 7.45 and 7.42 (dd,
Jo = 8.4 Hz and Jm = 1.64 Hz, H-8) and between δ 7.39 and 7.37
(d, Jo = 8.4 Hz, H-7) were observed. The three thienyl protons
gave one signal as dd between δ 8.22 and 8.21 (J2′,5′ = 3.0 Hz
and J2′,4′ = 1.2 Hz, H-2′), another between δ 7.78 and 7.77 (J5′,4′
= 5.2 Hz and J5′,2′ = 1.2 Hz, H-5′) and third one between δ 7.54
Fig. 3 Structure of 2a determined by single crystal X-ray analysis.
The structure of 2a was further corroborated (Fig. 3) by its
and 7.52 (J4′,5′ = 5.2 Hz and J4′,2′ = 1.2 Hz, H-4′). This shows single X-ray structure analysis.20 The compound crystallizes in
that during photo-irradiation the thienyl ring remained intact. the monoclinic P21/n space group. In addition to FTIR spectro-
The H-1 proton absorption appeared as a singlet at δ 6.33. The scopy, the presence of the carbonyl group is also confirmed by
–O-CH2-protons (C3-H’s) exhibited a singlet at δ 4.68 and the the CvO distance of 1.230(4) Å in 2a. The O6–C4 and O6–C5
methyl group (9-CH3) protons showed their signal as a singlet bonds (1.371(4) Å and 1.369(4) Å, respectively) are shorter than
at δ 2.46. These assignments in the NMR spectrum of 2a are the O4–C3 bond (1.422(4) Å) but are similar to the O4–C13
consistent with the structure of the pyranochromenone 2a bond (1.366(4) Å). This indicates that C3 is a sp3 hybridized
obtained as a photoproduct from 1a.
In order to provide further confirmation of these H NMR
carbon whereas C5 and C4 are sp2 hybridized.
1
The formation of 2 from 1 can be envisioned (Scheme 2) to
assignments to the different protons in 2a, its 2D-COSY (Fig. 2) occur through the intramolecular Paterno–Buchi reaction,
was recorded. As can be seen in Fig. 2, H-1 is directly interact- involving the initial C–O bond formation to give a 1,4-biradical
ing with H-10 and the latter is also correlating to H-8. The H-7 A which cyclizes to an oxetene B. This oxetene thus formed
and H-8 as expected are close and thus showing their inter- seems to be highly strained and thus easily opens up to
action with each other. In the case of thienyl protons, the furnish 2. Earlier also, the formation of such oxetenes7–9,21 has
interactions of H-2′ with H-4′ and H-5′ and between H-5′ and been demonstrated, which on subsequent cleavage gave
H-4′ are observed.
enones as observed here.
This journal is © The Royal Society of Chemistry and Owner Societies 2014
Photochem. Photobiol. Sci., 2014, 13, 488–491 | 489

View Article Online
Communication
Photochemical & Photobiological Sciences
3 M. Demuth and G. Mikhail, New Developments in the field
of Photochemical Syntheses, Synthesis, 1989, 145–162.
4 J. A. Porco and S. L. Schreiber, in Comprehensive Organic
Synthesis, ed. B. M. Trost, I. Fleming and L. A. Paquette,
Pergamon, New York, 1991, vol. 5, p. 151.
5 (a) T. Bach, The Paternò-Büchi Reaction of Heteroatom-sub-
stituted Alkenes as a Stereoselective entry to Polyfunctional
Cyclic and Acyclic Molecules, Liebigs Ann., 1997, 1627–
1634; (b) T. Bach, Stereoselective Intermolecular [2+2]-
Photocycloaddition Reactions and their Application in
Synthesis, Synthesis, 1998, 683–703; (c) T. R. Hoyl,
W. S. Richardson and A. Short, Oxetane-based Synthesis of
( )-Sarracenin, J. Org. Chem., 1989, 54, 688–693; (d) M. Abe,
E. Torii and M. Nojima, Lewis acid Promoted Ring-opening
Reactions of 2-Methoxy-2-siloxyoxetane in the presence of
Allylsilane: Notable Lewis acid Effect on the Ring-opening
Regioselectivity of the oxetane, Main Group Metal Chem.,
2000, 23, 651–654.
6 T. Bach and H. Brummerhop, Unprecedented Facial
Diastereoselectivity in the Paternò–Büchi Reaction of a
Chiral Dihydropyrrole-A Short Total Synthesis of
(+)-Preussin, Angew. Chem., Int. Ed., 1998, 37, 3400–3402.
7 (a) J. A. Barltrop and B. Hesp, Organic photochemistry. Part
V. The Illumination of Some Quinones in the presence of
Conjugated Dienes and other Olefinic Systems, J. Chem.
Soc. (C), 1967, 1625–1635; (b) S. Farid, W. Kothe and
G. Pfundt, Competitive Photoadditions of Acetylenes to the
C = C and C = O Bonds of p-Quinones, Tetrahedron Lett.,
1968, 4147–4150; (c) S. P. Pappas and N. A. Portnoy, Substi-
tuent Effects on the Photoaddition of Diphenylacetylene to
1,4-Naphthoquinones, J. Org. Chem., 1968, 33, 2200–2203.
8 (a) L. E. Friedrich and J. D. Bower, Detection of an Oxetene
Intermediate in the Photoreaction of Benzaldehyde with
2-Butyne, J. Am. Chem. Soc., 1973, 95, 6869–6870;
(b) L. E. Friedrich and P. Y.-S. Lam, Syntheses and Reac-
tions of 3-Phenyloxete and the Parent Unsubstituted Oxete,
J. Org. Chem., 1981, 46, 306–311; (c) L. E. Friedrich and
G. B. Schuster, 2,3,4,4-Tetramethyloxetene. Kinetics of
Rearrangement compared to Cyclobutene-butadiene Con-
versions, J. Am. Chem. Soc., 1971, 93, 4602–4604.
Scheme 2 Mechanistic rationale of formation of 2 from 1.
Conclusions
We have developed a highly efficient synthesis of 5-(thiophen-
3-yl)pyrano[2,3-c]chromen-2(3H)-ones (2), the angular tricyclic
compounds in one shot, in appreciable yield from the photo-
reaction of 3-(prop-2-ynyloxy)-2-(thiophen-3-yl)-4H-chromen-4-
ones (1). We believe this method should provide a practical
entry for synthesizing these types of angular pyranochrome-
nones in environment-benign conditions. Further details of
this reaction, the application to other aromatic systems and
the exploration of biological activities of these angular pyrano-
chromenones are ongoing in our laboratory and will be pre-
sented in due course.
Acknowledgements
The work has been supported by the grants as JRF to P.J. from
the University Grant Commission (UGC), New Delhi. S.K.
would like to thank CSIR, India for a research fellowship. X-ray
facility at IISER, Mohali, is gratefully acknowledged.
Notes and references
1 (a) E. Paternó and G. Chieffi, Synthesis in Organic Chem-
istry using Light. Note II. Compounds of Unsaturated
Hydrocarbons with Aldehydes and Ketones, Gazz. Chim.
Ital., 1909, 39, 341–361; (b) G. Büchi, C. G. Ihman and
E. S. Lipinsky, Light-catalyzed Organic Reactions. I. The
Reaction of Carbonyl Compounds with 2-Methyl-2-butene
in the presence of Ultraviolet Light, J. Am. Chem. Soc., 1954,
76, 4327–4331; (c) D. R. Arnold, R. L. Hinman and
A. H. Glick, Chemical Properties of the Carbonyl n,π* State.
The Photochemical Preparation of Oxetenes, Tetrahedron
Lett., 1964, 5, 1425–1430; (d) N. C. Yang, M. Nussim,
M. J. Jorgenson and S. Murov, Photochemical Reactions of
Carbonyl Compounds in Solution: The Paterno-Buchi reac-
tion, Tetrahedron Lett., 1964, 5, 3657–3664.
9 (a) G. Buchi, J. T. Kofron, E. Koller and D. Rosenthal, Light
Catalysed Organic Reactions. V. The Addition of Aromatic
Carbonyl Compounds to a Disubstituted Acetylene, J. Am.
Chem. Soc., 1956, 78, 876–877; (b) D. Bryce-Smith,
A. Gilbert and M. G. Johnson, Photoadditions of Dipheny-
lacetylene, Cyclo-octa-1,3-diene, and Cyclo-octene to
Anthraquinone, Tetrahedron Lett., 1968, 9, 2863–2866;
(c) S. P. Pappas and B. C. Pappas, Photoaddition of Alkynes
to Methoxy-p-benzoquinone, Tetrahedron Lett., 1967, 1597–
1600; (d) D. Bryce-Smith, G. I. Fray and A. Gilbert, A Photo-
adduct of p-Benzoquinones and Diphenylacetylene, Tetra-
hedron Lett., 1964, 2137–2139; (e) H. E. Zimmerman and
L. Craft, The Photochemical Reaction of Benzoquinone
with Tolan, Tetrahedron Lett., 1964, 2131–2136;
(f) H. J. T. Bos, H. T. Van Der Bend, J. S. M. Boleij,
C. J. A. Everaars and H. Polman, Chemistry of Acetylenic
2 J. F. Leibman and A. Greenberg, A Survey of Strained
Organic Molecules, Chem. Rev., 1976, 76, 311–365.
490 | Photochem. Photobiol. Sci., 2014, 13, 488–491
This journal is © The Royal Society of Chemistry and Owner Societies 2014

View Article Online
Photochemical & Photobiological Sciences
Communication
Ethers 95: Photochemical Additions of Acetylenes II Reac-
tions of Ketones with 1-Alkynyl Ethers, Recl. Trav. Chim.
Pays-Bas, 1972, 91, 65–74; (g) H. J. T. Bos and J. S. M. Boleij,
Chemistry of Acetylenic Ethers 93. Photochemical
Additions of Acetylenes (I) Reactions of Ketones with
1-Methylthio-1-propyne, Recl. Trav. Chim. Pays-Bas, 1969,
88, 465–473; (h) H. Polman, J. S. M. Boleij and H. J. T. Bos,
Chemistry of Acetylenic Ethers 98: Photochemical Addition
of Benzophenone and 4,4′-Dimethyl-benzophenone to
1-Methoxy-1-propyne Alkylidenecycloheptatrienes, Recl.
Trav. Chim. Pays-Bas, 1972, 91, 1088–1094; (i) T. Miyamoto,
Y. Shigemitsu and Y. Odaira, Photoaddition of Carboxylate
APEX2.
6-Methyl-3-(prop-2-ynyloxy)-2-(thiophen-3-yl)-4H-
chromen-4-one, 1a: white solid; mp 112–114 °C; yield 77.8;
νmax (cm−1): 2129 (CuC), 1612 (CvO); H NMR (400 MHz:
1
CDCl3): δ = 8.41 (1H, dd, J2′,4′ = 3.0 Hz, J2′,5′ = 1.2 Hz, H-2′),
8.03 (1H, d, Jm = 2.4 Hz, H-5), 7.90 (1H, dd, J5′,4′ = 5.1 Hz,
J5′,2′ = 1.2 Hz, H-5′), 7.50 (1H, dd, Jo = 9.0 Hz, Jm = 2.4 Hz,
H-7), 7.46–7.43 (2H, m, H-8, H-4′), 5.09 (2H, d, J1″,3″ = 2.4
Hz, H-1″), 2.48 (3H, s, C6-CH3), 2.41 (1H, t, J3″,1″ = 2.4 Hz,
H-3″); 13C NMR (100 MHz, CDCl3): δ = 174.59 (C-4), 153.22,
152.81, 137.45, 134.79, 134.71, 132.06, 129.46, 127.37,
125.57, 124.95, 123.70, 117.64, 78.74, 76.08, 59.09, 20.90
(C6-CH3); mass (m/z, +Q1): 297.1 (M+, 100%).
Esters to Diphenylacetylene, J. Chem. Soc. D, 1969, 1410; 18 (a) T. Oyamada, A New General Method for the Synthesis of
( j) W. Templeton, Photochemical Cyclization of Diphenyl-
acetylene to Phenanthrene, J. Chem. Soc. (D), 1970, 1412.
10 R. C. Kamboj, R. Arora, D. Kumar and G. Sharma, Photo-
Flavonol Derivatives, J. Chem. Soc. Jpn., 1934, 55, 1256;
(b) J. Algar and J. P. Flynn, New Synthesis of Flavonols,
Proc. R. Irish Acad., 1934, 42B, 1.
reorganization of 3-Alkoxy-6-chloro-2-(thiophen-3-yl)-4H- 19 A deoxygenated 1.0 mM methanolic solution of 1 taken in
chromen-4-ones: Regioselective Cyclization via γ-Hydrogen
Abstraction, J. Photochem. Photobiol., A, 2011, 220, 124–133.
11 R. A. Valiulin, D. G. Dressen, J. R. Riggs, F. M. Habte and
A. G. Kutateladze, Effect of Intramolecular Paternò−Büchi
Reaction on the Thermodynamics and Kinetics of nearly
degenerate [3,3]-Sigmatropic Shift in Fluxional Polycycles,
J. Org. Chem., 2009, 74, 3484–3490.
12 L. Kováčiková, R. Gašparová, A. Boháč, M. Ďurana and
M. Lácová, Synthesis of 3-Phenyl-2H,5H-pyrano[3,2-c]-
chromen-2-one Derivatives and their Antineoplastic
Activity, ARKIVOC, XI, 2010, 188–203.
a Pyrex glass vessel was purged with nitrogen for 30 min
and then irradiated under nitrogen with light from a 125 W
Hg vapor lamp for 45 min. The removal of solvent under
reduced pressure yielded a gummy mass that was chro-
matographed over a column of silica gel. The column was
eluted with increasing proportion of benzene in
a
benzene–petroleum ether mixture to obtain 5-(thiophen-3-
yl)-pyrano[2,3-c]chromen-2(3H)-ones, 2 (2a; 32%, 2b; 29%,
2c; 31% yields). 9-Methyl-5-(thiophen-3-yl)pyrano[2,3-c]-
chromen-2(3H)-one, 2a: greenish white solid; mp 76 °C;
νmax (cm−1):1643 (CvO); 1H NMR (400 MHz, CDCl3): δ =
8.22–8.21 (1H, J2′,5′ = 3.0 Hz and J2′,4′ = 1.2 Hz, H-2′),
7.78–7.77 (1H, J5′,4′ = 5.2 Hz and J5′,2′ = 1.2 Hz, H-5′), 7.71
(1H, d, J = 1.64 Hz, H-10), 7.54–7.52 (1H, J4′,5′ = 5.2 Hz and
13 C. B. Sangani, D. C. Mungra, M. P. Patel and R. G. Patel,
Synthesis and Antimicrobial Screening of Pyrano[3,2-c]-
chromene Derivatives of 1H-Pyrazoles, Cent. Eur. J. Chem.,
2011, 9, 635–647.
J4′,2′ = 1.2 Hz, H-4′), 7.45–7.42 (1H, dd, Jo = 8.4 Hz, Jm
=
14 P. K. Paliwal, S. R. Jetti and S. Jain, Green Approach
towards the Facile Synthesis of Dihydropyrano(c)chromene
and Pyrano[2,3-d]-pyrimidine Derivatives and their Biologi-
cal Evaluation, Med. Chem. Res., 2013, 22, 2984–2990.
15 D. C. Karia, H. K. Pandya, N. K. Godvani and A. Shah, Syn-
thesis, Characterization and anti-HIV Activity of 4-Hydroxy-
1.64 Hz, H-8), 7.39–7.37 (1H, d, Jo = 8.4 Hz, H-7), 6.33 (1H,
s, H-1), 4.68 (2H, s, H-3), 2.46 (3H, s, 9-CH3); 13C NMR
(100 MHz, CDCl3): δ = 24.79, 62.94, 107.62, 117.05, 117.46,
119.64, 122.84, 126.16, 126.75, 127.42, 129.34, 132.33,
134.87, 145.48, 150.11, 154.59, 162.07; mass (m/z, +Q1):
297.1 (M+, 100%).
3-(5-methyl-1-phenyl-1H-pyrazol-3-yl)pyrano[3,2-c]chromene- 20 Crystal data for 2a: C17H12O3S, M = 296.33, monoclinic,
2,5-dione, Elixir Org. Chem., 2012, 47C, 8797–8799.
16 C. P. Dell and C. W. Smith, Antiproliferative Derivatives of
4H-naphtho[1,2-b]pyran and Process for their Preparation,
Eur. Pat, 537949, 1993; C. P. Dell and C. W. Smith, Chem.
Abstr., 1993, 119, 139102d.
17 Solvents were dried and purified by conventional methods
prior to use. 1H and 13C NMR spectroscopic data were
recorded on a Bruker 400 MHz spectrometer in CDCl3 solu-
tion using trimethylsilane as an internal standard. The
space group P21/c, a = 5.0173(5) Å, b = 14.9931(14) Å, c =
18.012(2) Å, β = 93.897(7)°, V = 1351.8(2) Å3, Z = 4, Dc
=
1.456 Mg cm−3, μ(Mo-Kα) = 0.246 mm−1, T = 296 K, 8163
reflections collected. Refinement of 1461 reflections (191
parameters) with I > 2σ(I) converged at a final R1 = 0.0526,
wR2 = 0.1390, gof = 1.055. Crystallographic data (excluding
structure factors) for the structure in this paper have been
deposited with the Cambridge Crystallographic Data
Centre as supplementary publication no. CCDC 956397.
infrared (IR) spectra were scanned in KBr pellets on a 21 (a) E. Bosch, S. M. Hubig and J. K. Kochi, Paterno−Büchi
MB3000 FT-IR with HORIZON MB™ FTIR software from
ABB Bomen. The mass spectral data were obtained on
WATERS, Q-TOF MICROMASS (LC-MS) spectrometer. The
X-ray structure of 2a was collected on a Bruker Kappa APEX
II diffractometer equipped with CCDC detector and sealed-
tube monochromated MoKα radiation using the program
Coupling of (Diaryl)acetylenes and Quinone via Photo-
induced Electron Transfer, J. Am. Chem. Soc., 1998, 120,
386–395; (b) E. Bosch, S. M. Hubig, S. V. Lindeman and
J. K. Kochi, Photoinduced Coupling of Acetylenes and
Quinone in the Solid State as Preorganized Donor−Accep-
tor Pairs, J. Org. Chem., 1998, 63, 592–601.
This journal is © The Royal Society of Chemistry and Owner Societies 2014
Photochem. Photobiol. Sci., 2014, 13, 488–491 | 491