Highly facile and stereoselective intramolecular [2+2]photocycloadditions of bis(alkenoyl)ketenedithioacetals

Article abstract of DOI:10.1039/b200395n

The conformational change induced by the introduction of a ketenedithioacetal moiety at C-4 of 1,7-substituted-1,6-heptadiene-3,5-diones results in favorable spatial relationships between the alkenoyl groups to effect efficient intramolecular cycloadditio

Full text of DOI:10.1039/b200395n

Highly facile and stereoselective intramolecular [2+2]photocycloadditions
of bis(alkenoyl)ketenedithioacetals
Bubbly K. Joseph,b Babu Verghese,c C. Sudarsanakumar,*b S. Deepa,a Dhanya Viswam,a Prakash
Chandrana and C. V. Asokan*a
a School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India 686 560.
E-mail: asokancv@yahoo.com; Tel: 91 481 598015
b School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, India 686 560.
E-mail: csudarsanakumar@netscape.net
c
Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, Chennai 600036.
E-mail: babu@rsic.iitm.ernet.in
Received (in Cambridge, UK) 10th January 2002, Accepted 19th February 2002
First published as an Advance Article on the web 7th March 2002
The conformational change induced by the introduction of a
ketenedithioacetal moiety at C-4 of 1,7-substituted-1,6-hep-
tadiene-3,5-diones results in favorable spatial relationships
between the alkenoyl groups to effect efficient intra-
molecular cycloadditions: irradiation of bis(alkenoyl)-
ketenedithioacetals in solution leads to facile and ster-
eospecific intramolecular [2 + 2] photocycloadditions
resulting in the formation of substituted bicyclo[3.2.0]hep-
tane-2,4-diones, the observed conformational rigidity of
which is attributed to the push–pull character of the
ketenedithioacetal group.
developing partial negative charges on oxygen atom and partial
positive charges on sulfur atoms as indicated in 2A. This is
supported by the observed bond lengths in the crystal structure
of 2a and also by the fact that both the carbonyl groups as well
as the carbon and sulfur atoms of the ketenedithioacetal moiety
lie on the same plane.
Over the years, the photochemistry of cinnamic acid derivatives
has attracted interest owing to their applications as photo-
sensitive groups in polymer materials, molecular assemblies
and UV filters.1 While trans-cinnamic acid and cinnamates
photodimerize readily in the crystalline state, in solution they
undergo mostly cis–trans isomerisation. Though intramolecular
[2+2] cycloadditions of cinnamate groups linked by spacers2
have been reported, the double bonds are far apart when fewer
numbers of atoms separate them, unless they are brought close
to each other due to the geometrical constraints in the substrate.
Recently, in an effort to transfer the topochemical control of the
crystalline state to solution, appropriately substituted [2,2]
paracyclophane derivatives have been shown to undergo
efficient stereospecific intramolecular [2 + 2] addition of the
cinnamate groups.3 Banwell et al. have elegantly used the
change in the geometrical relations induced by the introduction
of a cyclopropyl group to a syn-cyclopropylidene dimer to bring
two p-benzoquinone moieties close enough to effect intra-
molecular [2 + 2] cycloaddition.4 In this communication, we
show that the push–pull character of polarized ketenedithioace-
tal group can bring about favorable spatial relationships
between the double bonds of bis(cinnamoyl)ketenedithioacetals
to effect efficient and stereospecific intramolecular photo-
chemical [2 + 2] cycloadditions in solution.
Scheme 1
The bis(alkenoyl)ketenedithioacetals 2a–g were readily
available by the Claisen–Schmidt condensation of 3-(1,3-di-
thiolane-2-yliden)pentane-2,4-dione 1 with aromatic aldehydes
(Scheme 1). The crystal structure of bis(cinnamoyl)ketenedi-
thioacetal (Fig. 1) shows that the cinnamoyl groups are aligned
parallel and close to each other. The carbon atoms a to the
carbonyl groups are separated by just 2.9 Å. The crystal studies
on 1,6-heptadiene-3,5-diones, having an unsubstituted methyl-
ene group show that they have a linear structure as should be
expected due to the keto–enol tautomerisation.5 Thus the
conformational change induced by the simple introduction of
the cyclic ketenedithioacetal group brings the double bonds
spatially close enough for an effective intramolecular photo-
chemical [2 + 2] cycloaddition. The parallel alignment of the
cinnamoyl group in bis(cinnamoyl)ketenedithioacetal appar-
ently results from the push–pull nature of the ketenedithioacetal
group. The delocalization of the electrons would result in
Fig. 1 ORTEP drawing of 4-(1,3-dithiolan-2-yliden)-1,7-diphenyl-1,6-hep-
tadiene-3,5-dione 2a.
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Irradiation of a solution of the bis(cinnamoyl)ketenedithio-
acetal 2a in benzene (2.5 3 1023 M) with Pyrex filtered light for
one hour gave bicyclo[3,2,0]heptane-2,4-dione 3a as a yellow
crystalline solid (mp. 170–173 °C) as the only product in 50%
isolated yield, while rest of the starting material could be
recovered unchanged (Scheme 1).
internal photochemical [2 + 2] cycloaddition. Studies on other
possible transformations induced by the conformational rigidity
of polarized ketenedithioacetals as well as the likely applica-
tions of the highly functionalized bicycloheptanediones 3 are in
progress.
Irradiation of a more dilute solution (2.75 3 1025 M) of 2a
has resulted in complete conversion into 3a in less than five min
(Fig. 2).
Notes and references
† Crystal data for 2a: Yellow prismatic crystal, C22H18O2S2, monoclinic
space group P21/c, a = 8.998(4), b = 16.579(6), c = 13.372(6) Å,
b = 106.50(5)°, V = 1913(1) Å3, Z = 4, Dc = 1.314 g cm23, m = (Cu-Ka)
2
= 2.63 cm21. The structure was solved by direct method and refined on ¡F¡
by full-matrix least-squares refinement using the program SHELX-97.7 The
final R and Rw were 0.051 and 0.139, respectively, using 3267 unique
reflections with I > 2s(I). GoF = 1.05. CCDC 169427.
Crystal data for 3a: Yellow needle like crystal, C22H18O2S2, or-
thorhombic space group Pna21, a
= 31.600(7), b = 5.630(1), c =
10.199(6) Å, V = 1814(1) Å3, Z = 4, Dc = 1.385 g cm23, m = (Cu-Ka)
= 2.76 cm21. The structure was solved by direct method and refined on
2
¡F¡ by full-matrix least-squares refinement using the program SHELX-97.7
The final R and Rw were 0.0479 and 0.1263, respectively, using 1501
unique reflections with I > 2s(I). GoF = 1.143. CCDC 169428. See http://
www.rsc.org/suppdata/cc/b2/b200395n/ for crystallographic files in .cif
format.
Fig. 2 Electronic absorption spectra of 2a (2.75 3 1025 M) in methanol
before (a) and after irradiation for 5 minutes at > 300 nm (b).
1 (a) B. Konnig, B. Knieriem and De Meijere, A. Chem. Ber., 1993, 1643;
(b) M. Shirai, T. Orikata and M. Tanaka, J. Polym. Sci. Polym. Chem. Ed.,
1985, 23, 463; (c) Z. Saavion and D. L. Wernick, J. Org. Chem., 1993, 58,
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Chem. Soc., 1993, 115, 10390.
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S. Akabori, T. Kumagai, Y. Habata and S. Sato, J. Chem. Soc., Perkin
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Bouas-Laurent, Leibigs Ann., 1996, 1231; (b) H. Greiving, H. Hopf, P. G.
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4 M. G. Banwell, D. C. R. Hockless and J. M. Walter, Chem. Commun.,
1996, 1469.
The structure of the product 3a was determined on the basis
of spectral and analytical data.6 Finally the stereochemical
features were established by X-ray structural analysis (Fig. 3).†
Similarly, the other 1,7-disubstituted-4-(1,3-dithiolan-2-yli-
den)-1,6-heptadiene-3,5-diones 2b–g also gave the respective
bicyclo[3,2,0]heptane-2,4-diones 3b–g in 40–70% overall
yields (Scheme 1). The structures of the products 3b and 3c
were confirmed with the help of spectral and analytical data.
In conclusion, we have developed a very facile method for
exerting topochemical control over cycloaddition reactions of
the cinnamoyl groups by the incorporation of a ketenedithioace-
tal moiety. It was found that the push–pull nature of the
ketenedithioacetal functionality organizes the cinnamoyl
groups parallel and close to each other, thereby effecting a facile
5 (a) H. H. Tonnensen, J. Karlsen and A. Mostad, Acta Chem. Scand., 1982,
36, 475; (b) H. H. Tonnesen, J. Karlsen, A. Mostad, U. Pedersen, P. B.
Rasmussen and S. O. Lawesson, Acta Chem. Scand., 1983, 37, 179; (c) A.
Mostad, U. Pedersen, P. B. Rasmussen and S. O. Lawesson, Acta Chem.
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Rasmussen and S. O. Lawesson, Acta Chem. Scand., 1986, 40, 420.
6 3-(1,3-Dithiolan-2-yliden)-6,7-diphenyl bicyclo[3,2,0]heptane-2,4-dione
3a. Obtained as a yellow crystalline solid by the irradiation of 400 mL of
2.5 3 1023 M solution of 2a for 1 h . Yield (20 mg, 50%), mp 170–173
°C. 1H NMR (300 MHz, CDCl3) d 3.5 (4H, s, SCH2), 3.6 (2H, d,
cyclobutane), 4.1 (2H, d, cyclobutane), 6.9–7.3 (10H, m, aromatic) ppm.
13C NMR (300 MHz, CDCl3) d 29.1, 37.7, 46.8, 47.2, 47.8, 48.6, 109.2,
127.8, 138.9, 200.9 ppm. IR nmax/cm21 1625, 1440, 1285, 1225. FAB
mass 379 (M+ + 1, 95%), 199 (90%), 165 (24%), 155 (50%), 149 (30%),
138 (58%), 57 (100%), Anal. Calcd. for C22H18O2S2 C, 69.81; H, 4.79;
Found C, 69.72; H, 4.86%.
Fig. 3 ORTEP drawing of 3-(1,3-dithiolan-2-yliden)-6,7-diphenylbicyclo-
[3.2.0]heptane-2,4-dione 3a.
7 G. M. Sheldrick, 1997. SHELX-97. University of Göttingen, Germany.
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