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5999
References
1. For excellent accounts on the stereoselectivity in [2+2]
photocycloaddition reactions, see: (a) Crimmins, M. T.
Chem. Rev. 1988, 1453–1473; (b) Fleming, S. A.; Brad-
ford, C. L.; Gao, J. J. In Organic Photochemistry; Rama-
murthy, V.; Schanze, K. S., Eds.; Marcel Dekker: New
York, 1997; Vol. 1, Chapter 6, pp. 187–243.
2. Mesmaeker, A. D.; Veenstra, S. J.; Ernst, B. Tetrahedron
Lett. 1988, 29, 459–462.
3. Ihara, M.; Suzuki, T.; Katogi, M.; Taniguchi, N.; Fuku-
moto, K. J. Chem. Soc., Chem. Commun. 1991, 646–647.
4. Ferrier, R. J.; Middleton, S. Chem. Rev. 1993, 93, 2779–
2831.
5. (a) Shing, T. K. M.; Fung, W.-C.; Wong, C.-H. J. Chem.
Soc., Chem. Commun. 1994, 449–450; (b) Patra, R.; Bar,
N. C.; Roy, A.; Achari, B.; Ghoshal, N.; Mandal, S. B.
Tetrahedron 1996, 34, 11265–11272.
6. For [2+2] photocycloaddition in pyranosugars, see: (a)
Tenaglia, A.; Barille, D. Synlett 1995, 776–778; (b)
Gomez, A. M.; Mantecon, S.; Velazquez, S.; Valverde, S.;
Herczegh, P.; Lopez, J. C. Synlett 1998, 1402–1404; (c)
Holt, D. J.; Barker, W. D.; Jenkins, P. R.; Ghosh, S.;
Russell, D. R.; Fawcett, J. Synlett 1999, 1003–1005.
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3473–3476.
Scheme 3.
8. All new compounds reported here were duly character-
biradical 16 as the later experiences severe 1,3-diaxial
interactions involving the benzyloxy group and the
alkyl chains bearing the radicals. Such 1,3-diaxial inter-
actions between alkoxy and alkyl groups have been
found to determine the stereochemistry of the product
in 5-exo-trig radical cyclisations.16 Ring closure of the
1,4-biradical 17 then leads to the product 12. In light of
this observation, it is apparent that photocycloaddition
of the diene 4 in the presence of Cu(I) catalyst does not
proceed through the complex 8. Instead, it probably
proceeds through photoexcitation of the complex 18.
Sequential bond formation,17 similar to the sensitised
process, preferentially leads to the radical intermediate
19 which finally collapses to the product. Thus, the
unusual stereochemical outcome in both the Cu(I)-
catalysed and -sensitised photocycloadditions arises
from the steric effects of the substituents in the sugar
template.
1
ised on the basis of spectral (IR, H and 13C NMR) and
C, H microanalytical data. Spectral data for selected
compounds. Compound 5b: 1H NMR (300 MHz,
CDCl3): l 1.26 (3H, s, Me), 1.36 (3H, s, Me), 1.59 (3H,
s, Me), 1.76–2.15 (6H, m), 2.46 (1H, m), 4.47–4.57 (4H,
m, PhCH2, C4-H and C7-H), 5.99 (1H, d, J=3.9 Hz,
C5-H), 7.24–7.4 (5H, m, ArH); 13C NMR (75 MHz,
CDCl3): l 13.1 (CH2), 26.9 (Me), 27.0 (Me), 27.8 (Me),
34.3 (CH2), 38.1 (C), 44.6 (CH2), 46.6 (CH), 67.0 (CH2),
80.6 (CH), 87.4 (CH), 97.4 (C), 106.5 (CH), 112.5 (C),
127.4 (CH), 127.9 (CH), 128.2 (CH), 128.4 (CH), 128.8
(CH), 138.3 (C). Compound 6b: 1H NMR (300 MHz,
CDCl3): l 1.28 (3H, s, Me), 1.36 (3H, s, Me), 1.54 (3H,
s, Me), 1.79–2.1 (6H, m), 2.48 (1H, m), 4.15 (1H, d,
J=6.3 Hz, C7-H), 4.27 (1H, d, J=3.6 Hz, C4-H), 5.89
(1H, d, J=3.6 Hz, C5-H); 13C NMR (75 MHz, CDCl3): l
13.3 (CH2), 26.7 (Me), 26.9 (Me), 28.0 (Me), 29.7 (C),
34.2 (CH2), 46.7 (CH), 48.8 (CH2), 81.3 (CH), 88.2 (CH),
1
92.0 (C), 105.8 (CH), 112.4 (C). Compound 12: H NMR
(300 MHz, CDCl3): l 1.25 (3H, t, J=7.2 Hz, Me), 1.39
(3H, s, Me), 1.61 (3H, s), 2.21–2.50 (3H, m), 2.77–2.95
(3H, m), 3.09–3.20 (1H, m), 4.13 (2H, q, J=7.2 Hz), 4.49
(1H, d, J=11 Hz, PhCH), 4.50 (1H, d, J=4.8 Hz, C7-H),
4.61 (1H, d, J=11 Hz, PhCH), 4.62 (1H, d, J=3.9 Hz,
C4-H), 5.98 (1H, d, J=3.9 Hz, C5-H), 7.14–7.36 (5H, m);
13C NMR (75 MHz, CDCl3): l 14.6 (Me), 21.0 (CH2),
27.2 (Me), 27.4 (Me), 38.6 (CH2), 38.7 (CH), 42.3 (CH),
45.3 (CH), 60.8 (CH2), 67.5 (CH2), 81.1 (CH), 86.0 (CH),
98.7 (C), 106.4 (CH), 113.1 (C), 127.8 (CH), 127.9 (CH),
128.7 (CH), 138.8 (C), 175.9 (CO).
In conclusion, we have demonstrated that thermody-
namically less stable cis-syn-cis linearly arrayed tricyclic
structures, which are not usually obtained in cycloaddi-
tion reactions, can be synthesised in [2+2] photocy-
cloaddition reactions through the correct choice of
substrate and reaction parameters.
Acknowledgements
9. Gregg, P. J.; Samuelsson, B. Synthesis 1979, 469–470.
10. Salomon, R. G.; Coughlin, D. J.; Ghosh, S.; Zagorski,
M. G. J. Am. Chem. Soc. 1982, 104, 998–1007.
We are grateful to Dr. Dipankar Datta of the Depart-
ment of Inorganic Chemistry for the AM1 calculations.
Financial support from CSIR, New Delhi through
Grant No. 01(1625)/EMR-II is gratefully acknowl-
edged. SPB thanks CSIR for a fellowship.
11. Crystal data for 5b: C20H26O4, M=330.41, monoclinic,
space group P21 (no. 4), a=11.9951(5), b=5.8347(3),
3
,
,
c=12.6220(6) A, i=97.325(2)°, V=876.18(7) A , Z=2,