Photoreaction of o-acylstyrenes: Formation of benzo-fused 8-oxabicyclo[3.2.1]octanes

Article abstract of DOI:10.1246/cl.2000.1386

Irradiation of o-acylstyrenes affords a pair of stereoisomers of photodimerized benzo-fused oxygen-bridged compounds, 8-oxabicyclo[3.2.1]octane derivatives.

Full text of DOI:10.1246/cl.2000.1386

1386
Chemistry Letters 2000
Photoreaction of o-Acylstyrenes: Formation of Benzo-Fused
8-Oxabicyclo[3.2.1]octanes
Kazuaki Oda,* Rikiji Nakagami, Naozumi Nishizono, and Minoru Machida*
Faculty of Pharmaceutical Sciences, Health Sciences University of Hokkaido, Ishikari-Tobetsu, Hokkaido 061-0293
(Received September 7, 2000; CL-000841)
Irradiation of o-acylstyrenes affords a pair of stereoisomers
of photodimerized benzo-fused oxygen-bridged compounds, 8-
oxabicyclo[3.2.1]octane derivatives.
In the context of extensive investigation in the photochem-
istry of carbonyl compounds and styrenes, the photochemistry
of o-acylstyrenes, which consist of a carbonyl group and a
styrene moiety in conjugation system, also is a subject of con-
tinuing interest, because it is expected that some novel photore-
actions ensue. Indeed, it is reported that on irradiation of o-
formylstyrene (1), a novel photoreaction occurred, and conse-
quently benzo-fused oxygen-bridged compound, 6,8-dioxa-
bicyclooctane derivative (2) was obtained accompanied by a
small amount of isocoumarin (3) (scheme 1).1 During the
course of our systematic study of the photochemistry of the
arenecarbothioamide–alkene system,2–6 we also observed the
formation of oxygen-bridged compound 8-oxabicylooctane as a
small amount of by-product in the photoreaction of
benzenecarbothioamide with o-acylstyrenes (Scheme 1).
methide (8) and an oxatricyclotriene intermediate (9) as shown
in Scheme 3. The ketene methide intermediate 8, which is gen-
erated through initial 1,5-hydrogen shift from formyl hydrogen
to a vinyl terminus,9 undergoes [4 + 2] addition with a carbonyl
in 1 to give isocoumarin 3.10 The other intermediate oxatricyclo-
triene 9, which arises from an initial [π2 + π4 ] rearrange-
a
s
ment,1,11 reacts intermolecularly with a carbonyl group in 1 to
lead dioxabicyclooctane 2.
The generation of ketene methide 8 is possible only for o-
formylstyrene. In fact, in the cases of 5a and 5b, no iso-
coumarin was obtained, and only oxygen-bridged compounds
were obtained. When an acyl group is an acetyl and a benzoyl,
oxatricyclotriene intermediate 10 was exclusively generated,
and subsequently underwent addition with a vinyl moiety in a
second substrate 5 to give 8-oxabicyclo[3.2.1]octanes 6, 7,
while 6,8-dioxabicyclo[3.2.1]octane derivative was not isolated
(Scheme 3). Probably, this selective addition of vinyl moiety is
due to a steric factor of an acyl group, since in photoreaction of
1, 2 and 4 were formed almost equal amounts.
In order to investigate the pathway of formation of oxygen-
bridged compounds, photoreaction of o-acylstyrenes was car-
ried out in benzene using a 1 kW high-pressure mercury lamp
through a Pyrex filter under a nitrogen atmosphere. Irradiation
of 1 for 5 h gave 6,8-dioxabicyclo[3.2.1]octane derivative 2,
isocoumarin derivative 3, and 8-oxabicyclo-[3.2.1]octane deriv-
ative (4),7 in 22, 16, and 19% yields, respectively (Scheme 2).
Interestingly, this product distribution was different from that of
Kessar and Mankotia, in which compound 2 is obtained in 60%
yield accompanied by a small amount of 3.1 Similarly, photo-
reaction of o-acylstyrenes (5a: R = CH3, 5b: R = Ph) was exam-
ined. 8-Oxabicyclo[3.2.1]octanes (6, 7) were effectively
obtained as a pair of stereoisomers (Scheme 2). Structural
assignments for 6 and 7 were made on the basis of spectral data
and elemental analyses.8
As for a pathway in the photoreaction of o-formylstyrene,
Kessar and Mankotia1 proposed two pathways via a ketene
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
1387
In conclusion, photoreaction of o-acylstyrenes seems to
proceed through two fold photoreactions such as rearrangement
to oxatricyclotriene followed by addition of a second substrate,
i.e., photodimerization. Such an unusual photodimerization is a
first example and the new photodimerization reaction may be
used as a tool for the synthesis of novel benzo-fused oxygen-
bridged compounds. The synthetic application of the novel
photodimerization is underway.
128.0 (d), 129.1 (d), 131.9 (d), 133.0 (s), 133.1 (d), 134.6
(s), 136.5 (s), 140.2 (s), 192.9 (d); MS m/z: 264 (M+);
Anal. Calcd for C19H16O2: C, 81.79; H, 6.10%. Found: C,
82.02; H, 6.18%.
8
The stereochemistry of 6a and 7a was assigned on the
basis of their 1H-NMR spectra by considering the
anisotropic shielding effects between the two phenyl rings.
Compd 6a: Colorless plates; mp 141–143 °C; IR (Nujol)
1
1670 cm–1, H-NMR (400 MHz, CDCl3) δ 1.28 (3H, s),
This work was supported in part by Grant-in-Aid for High
Technology Research Programs from the Ministry of
Education, Science, Sports and Culture in Japan.
2.20–2.38 (2H, m), 2.56 (3H, s), 3.46 (2H, dd, J= 4.4, 16.1
Hz), 3.98 (1H, m) 4.94 (1H, m). 7.09–7.61 (8H, m); 13C-
NMR (100 MHz, CDCl3) δ 20.3 (q), 30.3 (q), 37.3 (t),
40.5 (t), 52.5 (d), 73.6 (d), 84.5 (s), 124.0 (d), 125.9 (d),
126.3 (d), 126.7 (d), 127.6 (d), 129.2 (d), 129.7 (d), 131.3
(d), 137.1 (s), 139.1 (s), 143.7 (s), 144.9 (s), 202.7 (s); MS
m/z: 292 (M+); Anal. Calcd for C20H20O2: C, 82.16; H,
6.90%. Found: C, 82.11; H, 6.98%. Compd 7a: Colorless
References and Notes
1
S. V. Kessar and A. K. S. Mankotia, J. Chem. Soc., Chem.
Commun., 1993, 1828. Their preparative photoreaction was
carried out with nitrogen-purged benzene solution using a
450 W medium-pressure mercury lamp in a Pyrex immer-
sion well.
1
oil; IR (Nujol) 1670 cm–1; H-NMR (400 MHz, CDCl3) δ
1.54 (3H, s), 1.86 (1H, m), 2.61 (3H, s), 2.69 (1H, d,
J=16.1 Hz), 2.80 (1H, m), 3.50 (1H, dd, J= 4.9, 16.1 Hz),
4.06 (1H, t, J=10.3 Hz), 4.70 (1H, m), 6.07 (1H, d, J=7.8
Hz), 6.35 (1H, d, J= 7.3 Hz), 6.88–7.49 (6H, m); 13C-NMR
(100 MHz, CDCl3) δ 20.3 (q), 30.3 (q), 37.3 (t), 40.5 (t),
52.5 (d), 73.6 (d), 84.5 (s), 124.0 (d), 125.9 (d), 126.3 (d),
126.7 (d), 127.6 (d), 129.2 (d), 129.7 (d), 131.3 (d), 137.1
(s), 139 (s), 143.7 (s), 144.9 (s), 202.7 (s); MS m/z
292(M+); Anal. Calcd for C20H20O2: C, 82.16; H, 6.90%.
Found: C, 82.01; H, 6.77%.
2
3
4
5
6
7
K. Oda and M. Machida, J. Chem. Soc., Chem. Commun.,
1994, 1477.
K. Oda, H. Tsujita, K. Ohno, and M. Machida, J. Chem.
Soc., Perkin Trans. 1, 1995, 2931.
K. Oda, M. Sakai, and M. Machida, Chem. Pharm. Bull.,
45, 584 (1997).
K. Oda, H. Tsujita, M. Sakai, and M. Machida, Chem.
Pharm. Bull., 46, 1522 (1998).
K. Oda, R. Nakagami, N. Nishizono, and M. Machida,
Chem. Commun., 1999, 2371.
9
S. V. Kessar, A. K. S. Mankotia, J. C. Scaiano, M. Barra, J.
Gebicki, and K. Huben, J. Am. Chem. Soc., 118, 4361
(1996).
Compd 4: Colorless plates; mp 117–118.5 °C; IR (Nujol)
1
1670 cm–1, H-NMR (400 MHz, CDCl3) δ 2.04 (2H, m),
2.62 (1H, m), 2.68 (1H, d, J= 16.6 Hz), 3.46 (1H, dd, J=
4.4, 16.1 Hz), 4.94 (2H, m), 6.03 (1H, d, J= 7.3 Hz), 6.54
(1H, d, J= 7.8 Hz), 6.70–7.78 (6H, m), 10.37 (1H, s); 13C-
NMR (100 MHz, CDCl3) δ 34.8 (t), 37.1 (t), 48.7 (d), 74.9
(d), 80.5 (d), 125.0 (d), 125.9 (d), 126.7 (d), 127.2 (d),
10 S. V. Kessar, P. Singh, R. Vohra, N. P. Kaur, and D.
Venugopal, J. Org. Chem., 57, 6716 (1992).
11 J. C. Scaiano, M. V. Encinas, and M. V. George, J. Chem.
Soc., Perkin Trans. 2, 1980, 724.