Photoaddition of N-acylbenzoxazole-2-thiones to alkenes
Takehiko Nishio
Department of Chemistry, University of Tsukuba, Tsukuba-shi, Ibaraki, 305-8571 Japan
Table 1 Yields of photoproducts (3–14)
N-Acylbenzoxazole-2-thiones 1 undergo photoaddition
with alkenes 2 in a regiospecific manner to yield benz-
oxazole derivatives 3–9 and/or the unexpected, imino-
thietanes 10–14 via spirocyclic aminothietane intermedi-
ates, depending on the nature of the substituents both on
the N-atom of 1 and those present in the alkenes 2.
Thione (1)
Alkene (2)
Product
R
R1
R2
R3
R4
(yield, %)a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1bc
1bd
Me
2a Me
2b Me
2c Me
2d Me
2e Ph
2g Me
Me
Me
Me
Me
H
CN
Me
Me
Ph
H
H
H
Me
3 (59)
4 (61) 10 (tr.)
5 (7) 11 (52)
Me
Me
H
H
H
Me
Me
H
Thiocarbonyl compounds are known to undergo photocyclo-
addition to a variety of alkenes.1 Thioamides also undergo both
inter- and intra-molecular photocycloaddition with alkenes.2
Aminothietanes are believed to be intermediates in these
reactions. However, such products have not been isolated except
for one example,3 probably because the nitrogen lone-pair
electron-assisted cleavage of the C᎐S bond of the thietane ring
facilitates the formation of a zwitterion.2 We are interested in
exploring the effect of the substituents on the nitrogen atom of
thioamides upon [2 ϩ 2] photocycloaddition of thioamides
with alkenes. We recently reported that the photocycloaddition
of N-unsubstituted benzoxazole-2-thiones to alkenes gave 1:1-
adducts of the reactants, 2-alkylated benzoxazoles, and that of
N-alkylbenzoxazole-2-thiones to alkenes gave the ring-opened
products, amide derivatives, which were derived from the add-
ition of water to 1:1-adducts of the reactants.2e We now report
the results of photoreaction of N-acylbenzoxazole-2-thiones 1,
substituted with an electron-withdrawing group on the nitrogen
atom, and alkenes 2 yielding 2-substituted benzoxazoles 3–9
and the unexpected products, iminothietanes 10–14: these are
formed by intramolecular trapping of zwitterion intermediates,
derived from [2 ϩ 2] cycloadducts, spirocyclic aminothietanes,
of 1 and 2.
Irradiation of a benzene solution of N-acetylbenzoxazole-2-
thione 1a and 1,1-disubstituted alkenes such as 2-methylprop-
2-ene 2a and methacrylonitrile 2g in a Pyrex vessel with a
high-pressure mercury lamp under argon gave 2-substituted
benzoxazoles 3 and 7, exclusively, which were 1:1-adducts
of the reactants, in 59 and 63% isolated yields, respectively.
Irradiation of 1a in benzene in the presence of 1,1,2-tri-
substituted alkenes, 2-methylbut-2-ene 2b and 2,5-dimethyl-
hexa-2,4-diene 2d and the 1,1,2,2-tetrasubstituted alkene, 2,3-
dimethylbut-2-ene 2c under the same conditions gave 2-
CH᎐CMe2 6 (28) 12 (31)
᎐
b
Ph
H
H
Me
H
H
—
7 (63)
Me2CH 2b Me
13 (53)
14 (51)
2c Me
2f Ph
2g Me
2g
8 (55)
9 (54)
9 (77)
9 (59)
CN
H
2g
a
Isolated yield. b Recovered 1a (88%). c In MeOH. d In CH3CN.
substituted benzoxazoles 4–6 and the unexpected products,
iminothietanes 10–12. On the other hand, irradiation of 1a in
the presence of 1,2-disubstituted alkenes such as stilbene 2e
resulted in recovery of unchanged 1a. The structures of all new
photoproducts 3–9 and 10–12 were confirmed on the basis of
spectral data and analytical results. The IR spectra of 2-
substituted benzoxazoles 3–7 showed the characteristic absorp-
tion of thioester carbonyls (1675–1700 cmϪ1), while those of
iminothietanes 10–12 showed absorptions characteristic of an
ester group (1755–1760 cmϪ1). The 13C NMR spectra of
2-substituted benzoxazoles 3–7 exhibited the presence of a
thioester carbonyl carbon (δ 192.8–196.6) and an imino
carbon (δ 161.5–170.7) of a benzoxazole ring. Those of
iminothietanes 10–12 exhibited the signals at δ 168.8–173.2 and
at δ 166.9–168.9, which were assigned to ester carbonyl and
imino carbons, respectively. In contrast, irradiation of N-2-
methylpropanoylbenzoxazole-2-thione 1b in benzene in the
presence of alkenes 2 gave 2-substituted benzoxazoles 8–9 or
iminothietanes 13–14, exclusively, depending on the substitu-
tion pattern of the alkenes. Similar results were obtained when
1b was irradiated in methanol and acetonitrile in the presence
of alkene 2g. The structures of the photoproducts were
elucidated on the basis of analytical results, spectral data and
chemical evidence. The photoproduct, 2-(1,3-benzoxazol-2-yl)-
2-cyanopropyl 2-methylpropanethioate 9, was hydrolyzed with
sodium hydroxide in methanol to give 2-(1,3-benzoxazol-2-yl)-
2-cyanopropanethiol 15. The structure of 15 was confirmed
by spectral comparison with an authentic sample which was
independently prepared by the photoaddition of benzoxazole
1c with methacrylonitrile 2g.2e Treatment of iminothietane 13
with sodium methoxide in methanol gave N-(3,3,4-trimethyl-2-
thietanylidene)-o-hydroxyaniline 16. The structure of 16 was
elucidated on the basis of its spectral properties and elemental
analysis.4 The formation of 2-substituted benzoxazoles 3–9
and iminothietanes 10–14 in the photocycloaddition of N-
acylbenzoxazole-2-thiones 1 with alkenes 2 could be explained
in terms of the pathways shown in Scheme 3. The amino-
O
R4
R3
O
N
O
hυ
S
R1
R2
R3
R4
S
R
N
R1
R2
O
2
R
3–9
1a: R = Me
1b: R = Me2CH
O
R
O
R1
N
R2
R3
S
R4
10–14
Scheme 1
J. Chem. Soc., Perkin Trans. 1, 1998
1007
Nishio, Takehiko
