A new photochemical ring expansion of 1,2-benzisothiazole 1,1-dioxides

Article abstract of DOI:10.1016/S0040-4039(01)00992-3

A new facile photoconversion of 1,2-benzisothiazole 1,1-dioxides into 1,3-2H-benzothiazine 1,1-dioxides upon 254 nm irradiation in methanol or acetonitrile is reported.

Full text of DOI:10.1016/S0040-4039(01)00992-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 5651–5653
A new photochemical ring expansion of 1,2-benzisothiazole
1,1-dioxides
Ibrahim Elghamry* and Dietrich Do¨pp
Division of Organic Chemistry, Gerhard-Mercator-Universita¨t Duisburg, D-47048 Duisburg, Germany
Received 22 May 2001; revised 10 June 2001; accepted 11 June 2001
Abstract—A new facile photoconversion of 1,2-benzisothiazole 1,1-dioxides into 1,3-2H-benzothiazine 1,1-dioxides upon 254 nm
irradiation in methanol or acetonitrile is reported. © 2001 Elsevier Science Ltd. All rights reserved.
Considerable attention has been focused on the photo-
chemistry of five-membered heterocyclic ring systems.1 By
far the greatest area of activity in this field has been a
study of the photoisomerization of the heterocyclic
nucleus. The literature on the photoprocesses observed
with isothiazoles has recently been reviewed.1,2 The
photoisomerization of various 1,2-benzisothiazoles1a has
also been studied in our laboratory,3–6 as well as by
others7–9 and these studies have dealt with the synthetic
usefulness as well as the reaction mode of this ring system.
nitrogen does occur.4 We wished to clarify whether a
similar oxygen shift could also be possible by introducing
at the N-methylene group an acceptor (Acc) substituent
instead of a donor as in compounds 2. Substituents
CH2-Acc would indeed have a migratory aptitude differ-
ent from that of CH2-donor groups if the rearrangement
followed a polar mechanism.5
Thus, sultams 2 were prepared by treating starting
materials 1 with primary alkyl bromides Br-CH2-R2 (see
Table1)inthepresenceofsodiumhydrideand18-crown-6
(Scheme 1). When the N-alkylated sultams 2 were
subjected to 254 nm photolysis10 in acetonitrile or
methanol for the periods listed in Table 1, a single
photoproduct was obtained in all cases (except from
sultam 2a) which was demonstrated to be the 2-substi-
tuted-1,3-2H-benzothiazine 1,1-dioxides 3b–e. Sultam
The photoconversion of 1 (R1 being methyl or phenyl)
to the isomeric 2,3-dihydro-2-hydroxy-1,2-benzisothia-
zole 1-oxides by a formal oxygen shift from sulfur to
nitrogen has been firmly established.3–5 The hydrogen
atom at nitrogen in 1 was replaced by CH2OR (R=CH3,
i-Pr) and still the formal oxygen shift from sulfur to
Table 1. Photoconversion of sultams 2 to 1,3-benzothiazines 3
2, 3, 4
R1
R2
Period of irradiation (h)
Yield of 3 (%)a
Chemical shiftsc l (ppm)
H-2
C-2
a
CH3
Ph
2
6
2
1
1.5
7
45b
60
81
86
84
58
5.50
5.31
5.27
5.32
5.85d
72.6
71.5
62.0
70.3
74.1d
b
c
d
e
CH3
CH3
CH3
Ph
CO2CH3
CN
COCH3
Ph
a Based on the consumed starting material.
b Additionally, 20% of compound 4a have been formed.
c In CDCl3 unless stated otherwise.
d In DMSO-d6.
Keywords: photorearrangement; ring expansion; 1,2-benzisothiazoles; 1,3-benzothiazines.
* Corresponding author. Fax: +49-203379 4192; e-mail: elghamry@hotmail.com
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00992-3

5652
I. Elghamry, D. Do¨pp / Tetrahedron Letters 42 (2001) 5651–5653
Scheme 1.
2a led to the formal oxygen shift3–5 photoproduct 4a
besides 3a (Scheme 1). The a priori alternative structure
5 could be ruled out on the basis of the spectral data11
obtained for 4a.
material upon standing at room temperature in protic
solvents or exposure to acid. Therefore, it is not sur-
prising that, while compound 4a could be isolated (as
an oil), after a short irradiation time (2 h) no 4a was
detected in the photolysis mixture after 6 h. When a
sample of neat 4a had been left standing for 2 weeks at
room temperature, the flask only contained crystalline
2a.
A rationale which explains the photoisomerization of 2
to 3 is outlined in Scheme 2.
Photoexcited 2 undergoes SꢀN bond homolysis3–9 to
give biradical 6. A subsequent 1,2-hydrogen shift from
carbon to nitrogen produces biradical 7 followed by
cyclization to afford the final ring-expansion photo-
Furthermore, it has been reported3,4 earlier that com-
pounds having structure 4 could revert back to starting
Scheme 2.

I. Elghamry, D. Do¨pp / Tetrahedron Letters 42 (2001) 5651–5653
5653
product 3. It should be emphasized that the rearranged
Chem. 1987, 99, 142–143; Angew. Chem., Int. Ed. Engl.
1987, 26, 146–147.
4. Do¨pp, D.; Lauterfeld, P.; Schneider, M.; Schneider, D.;
Seidel, U. Phosphorus Sulfur Silicon Relat. Elem. 1994,
95–96, 481–482.
5. Do¨pp, D.; Lauterfeld, P.; Schneider, M.; Schneider, D.;
Henkel, G.; Issac, Y.; Elghamry, I. Synthesis 2001, 1223–
1227.
biradical
7
profits from benzylic (R2=Ph) or
captodative12 (R2=CO2Me, CN, COMe) stabilization
and hence there is a driving force for its formation. The
photorearrangement of 3a to the sulfine hydroxamic
acid derivative 4a has been discussed earlier3–5 in other
cases.
The assignment of the photo-ring-expansion products 3
6. Elghamry, I.; Do¨pp, D.; Henkel, G. Synthesis 2001,
1228–1235.
7. Kamigata, N.; Iizuka, H.; Kobayashi, M. Bull. Chem.
Soc. Jpn. 1986, 59, 1601–1602.
8. (a) Sindler-Kulyk, M.; Neckers, D. C. Tetrahedron Lett.
1981, 22, 525–528; (b) Hoffman R. W.; Sieber, W.; Guhn,
G. Chem. Ber. 1965, 98, 3470–3478.
9. (a) Kamigata, N.; Hashimoto, S.; Kobayashi, M.;
Nakanishi, H.; Bull. Chem. Soc. Jpn. 1985, 58, 3131–
3136; (b) Kamigata, N.; Hashimoto, S.; Fujie, S.;
Kobayashi, M. J. Chem. Soc. Chem. Commun. 1983,
765–766.
1
was based on H/13C NMR spectra as well as on the
1
mass spectra.13 Thus, in the H NMR spectra of com-
pounds
3
the one-proton singlet was observed
at d 5.27–5.85 ppm assigned to H-2, and the 13C NMR
spectra revealed signals at d 62.0–74.1 ppm assigned to
the asymmetric carbon atoms C-2. Furthermore, in the
mass spectrum of 3a the major fragment at m/z=104 is
the equivalent of C6H5CNH+. The 2-substituted 1,3-
benzothiazines tend to lose a nitrile fragment as a
characteristic fragment in the mass spectra.14
It is worth mentioning that all the UV spectra of the
sultams 2a–e typically showing structured absorptions
in the 270–250 nm range with log m values below 3 are
dominated mainly by the benzoid ring as a chromo-
phore.
10. Samples of sultams 2 (0.87 mmol) in methanol (or aceto-
nitrile) (100 ml) were irradiated for the periods listed
using a quartz immersion well in connection with a
Hanau TNN 15 low-pressure mercury lamp (15 W input)
with continuous argon purging. After concentration the
residue was chromatographed on silica-gel layers (1 mm)
using ethylacetate/n-hexane (1:1). Rf values refer to ana-
lytical TLC.
11. 2-Benzyloxy-2,3-dihydro-3,3-dimethyl-1,2-benzisothiazole
1-oxide (4a): Rf=0.13 colorless viscous liquid; IR (KBr) w
1144 (SO); 1H NMR (300 MHz, CDCl3) l 1.59 (s, 3H,
CH3), 1.67 (s, 3H, CH3), AB (lA=4.18, lB=4.24, ꢀJABꢀ=
18 Hz, CH2), 7.48 (m, 9H, aryl-H); 13C NMR (75 MHz,
CDCl3) l 21.1 (CH3), 23.1 (CH3), 68.2 (C-4), 75.7 (CH2),
117.6, 126.6, 128.1, 128.4, 129.2, 130.3, 132.1, 138.4,
142.6, 143.8; MS (70 eV) m/z (%) 287 (M+, 3), 272 (28),
166 (4), 149 (11), 91 (100). Anal. calcd for C16H17NO2S
(287.4): C, 66.87; H, 5.96; N, 4.87; S, 11.16. Found: C,
66.78; H, 6.01; N, 4.82; S, 11.11%. Compound 3a: C,
66.71; H, 5.82; N, 4.63; S, 10.91%.
In summary, a new and efficient one-step photoconver-
sion of 2,3-dihydro-1,3-benzisothiazoles 1,1-dioxides to
1,3-benzothiazine 1,1-dioxides has been described. The
yield of the reaction coupled with the easy conversion
starting from readily available precursors makes this
method a most expeditious and efficient one among the
previously reported methods14–16 for constructing simi-
lar ring systems.
Acknowledgements
I. Elghamry is deeply indebted to Alexander-von-Hum-
boldt-Foundation for the award of a research fellow-
ship during the period from Dec. 1999 to June 2001 at
Duisburg University (Germany). Financial support by
Fonds der Chemischen Industrie is gratefully
acknowledged.
12. (a) Viele, H. G.; Janousek, Z.; Mere´nyi, R.; Stella, L.
Acc. Chem. Res. 1985, 18, 148–154; (b) Sustmann, R.;
Korth, H.-G. Adv. Phys. Org. Chem. 1990, 26, 131–178.
13. 3,4-Dihydro-4,4-dimethyl-2-phenyl-1,3-2H-benzothiazine
1,1-dioxide (3a): Rf=0.23 colorless crystals (from benz-
ene); mp 207–208°C; IR (KBr) w 3330 (broad, NH),
1
1279, 1141 (SO2); H NMR (500 MHz, CDCl3) l 1.56 (s,
References
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129.7, 132.3, 137.5, 145.7; MS (70 eV) m/z (%) 287 (M+,
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14. Sindler-Kulyk, S.; Neckers, D. C. J. Org. Chem. 1983, 48,
1275–1281.
15. Zinnes, H.; Comes, R. A.; Shavel, J. J. Org. Chem. 1964,
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1997; pp. 57–110 and references cited therein.
3. Do¨pp, D.; Kru¨ger, C.; Lauterfeld, P.; Raabe, E. Angew.
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