Conversions of thiirene 1-oxides to α-iminothioketones or oxazoles through probable intermediates sulfilimines

Article abstract of DOI:10.1246/cl.2002.314

Three thiirene 1-oxides 1, which possess bulky alkyl substituents at the 2- and 3-positions, afforded α-iminothioketones in 59-69% yields on successive treatment with trifluoroacetic anhydride and p-toluenesulfonamide, whereas 1 furnished oxazoles in 65-8

Full text of DOI:10.1246/cl.2002.314

314
Chemistry Letters 2002
Conversions of Thiirene 1-Oxides to ꢀ-Iminothioketones or Oxazoles
through Probable Intermediates Sulfilimines
Yutaka Ono, Yoshiaki Sugihara, Akihiko Ishii, and Juzo Nakayamaꢀ
Department of Chemistry, Faculty of Science, Saitama University, Saitama, Saitama 338-8570
(Received November 8, 2001; CL-011121)
Three thiirene 1-oxides 1, which possess bulky alkyl
substituents at the 2- and 3-positions, afforded ꢀ-iminothio-
ketones in 59–69% yields on successive treatment with
trifluoroacetic anhydride and p-toluenesulfonamide, whereas 1
furnished oxazoles in 65–89% yields on the same treatment with
trifluoroacetic anhydride and p-toluamide. A mechanism invol-
ving ring-expansion of the initial intermediates, sulfilimines, is
proposed for these reactions.
Thiirene 1-oxides, the smallest cyclic and unsaturated
sulfoxide, are of much interest in syntheses, structures, and
reactions, but their chemistry is still in the development stage.1{3
Recently, we devised a practical synthesis of thiirene 1-oxides 1a-
c that possess bulky alkyl substituents at the 2- and 3-positions.4
We have now examined conversions of 1 to the corresponding
sulfilimines. This work led us to the finding that the resulting
sulfilimines undergo a very quick ring-expansion to produce 1,2-
thiazetes.
Thiirene 1-oxides are known to isomerize to ꢀ-oxothioke-
tones 4 through 1,2-oxathietes 8 when heated.4 However, this
would not be true for the formation of 4 in the present case
because, interestingly, treatment of 1a or 1b with trifluoroacetic
anhydride alone inCH2Cl2 produced anearly quantitative yield of
ꢀ-oxothioketone 4a or 4b, respectively. Therefore, another
mechanism probably involving 5 as intermediates seems to be
operative for the formation of 4.
3,4-Di-t-butylthiophene 1-oxide and other sulfoxides are
satisfactorily converted to the corresponding sulfilimines through
intermediates such as 2 by treatment with trifluoroacetic
anhydride and then with p-toluenesulfonamide.5;6 Thus, thiirene
1-oxides 1 were treated with these reagents with expectation of
obtaining the corresponding sulfilimines 6, whose synthesis was
not hitherto reported. Unexpectedly, however, successive treat-
ment of thiirene 1-oxide 1a with trifluoroacetic anhydride and p-
toluenesulfonamide in CH2Cl27 produced ꢀ-iminothioketone 3a8
and ꢀ-oxothioketone 4a4 in 59 and 11% yields, respectively, with
recovery of 1a in 9% yield. Similar treatment of 1b gave 3b9 and
4b4 in 66 and 7% yields, respectively, with recovery of 1b in 10%
yield. Expected sulfilimines 6 were not formed even in trace
amounts. The formation of 3 can best be explained by assuming i)
the initial formation of the sulfilimines 6 through 5, ii) ring-
expansion of 6 to 1,2-thiazetes 7, and iii) ring-opening of 7 to 3.
Previously, we proposed that the thiazete 7b tautomerizes to the
ring-opened form 3b exclusively.9
As expected from the mechanisms discussed above,
successive treatment of unsymmetrically substituted thiirene 1-
oxide 1c with trifluoroacetic anhydride and p-toluenesulfonamide
produced 1 : 1 mixtures of 3c and 3c08 and of 4c and 4c04 in 69 and
10% yields, respectively, with recovery of 1c in 10% yield.
When p-toluamide was used in place of p-toluenesulfona-
mide, the reactions provided more unexpected results. Thus,
treatment of 1a and 1b with trifluoroacetic anhydride and then
with p-toluamide in CH2Cl2 produced oxazoles 13a8 and 13b8 in
89 and 65% yields, respectively, in addition to elemental sulfur.
Also in these cases, 1,2-thiazetes 10 would be formed initially via
sulfilimines 9. Thiazetes 10 then undergo a ring-opening to give
11. An electrocyclization of 11 would produce thiiranes 12.10
Finally, sulfur extrusion of 12 furnishes hetroaromatic oxazoles
13.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
315
references cited therein.
S. Sato, S.-Z. Zhang, and N. Furukawa, Heteroat. Chem., 12,
444 (2001).
Thiirene 1-oxide 1a was dissolved in CH2Cl2 containing
trifluoroacetic anhydride (2.3 equiv) at À78 ꢁC. After the
solution had been stirred for 5 min, p-toluenesulfonamide (2.0
equiv) was added. The mixture was stirred for 1 h at À78 ꢁC
and then warmed to room temperature. The reaction was
quenched by addition of aq. NaHCO3.
6
7
8
Satisfactory elemental analyses were obtained for all new
compounds. The following are relevant spectroscopic data
including mp or bp. 3a: mp 55–56 ꢁC; purple blocks; 1H NMR
(CDCl3) ꢁ 1.24 (9H, s), 1.57 (9H, s), 2.42 (3H, s), 7.29 (2H, d,
J ¼ 8:3 Hz), 7.74 (2H, d, J ¼ 8:3 Hz); 13C NMR (CDCl3) ꢁ
21.6, 30.2, 31.8, 42.0, 52.6, 127.2, 129.4, 137.3, 143.7, 194.4,
266.8. A 1 : 1 mixture of 3c and 3c0: mp 98–100 ꢁC; purple
needles; 1H NMR (CDCl3) ꢁ 1.23 (9H, s), 1.56 (9H, s), 1.60–
2.56 (30H, m), 2.42 (6H, s), 7.28 (2H, d, J ¼ 8:6 Hz), 7.29
(2H, d, J ¼ 8:0 Hz), 7.727 (2H, d, J ¼ 8:6 Hz), 7.733 (2H, d,
J ¼ 8:0 Hz); 13C NMR (CDCl3) ꢁ 21.6, 27.0, 30.3, 31.6, 31.8,
36.1, 36.2, 36.31, 36.44, 41.9, 42.1, 43.5, 52.6, 55.3, 124.2,
127.1, 129.4, 137.4, 137.5, 143.6, 194.0, 266.1, 266.4. 13a: bp
130–135 ꢁC/5 mmHg (bulb-to-bulb distillation); colorless oil;
1H NMR (CDCl3) ꢁ 1.42 (9H, s), 1.46 (9H, s), 2.37 (3H, s),
7.20(2H d, J ¼ 7:9 Hz), 7.85 (2H, d, J ¼ 7:9 Hz); 13C NMR
(CDCl3) ꢁ 21.4, 30.9, 31.7, 32.2, 33.0, 125.6, 125.8, 129.2,
139.2, 143.0, 152.4, 155.6; MS m=z 271 (Mþ). 13b: mp 196–
197 ꢁC; colorless crystals; 1H NMR (CDCl3) ꢁ 1.78 (12H, m),
2.09 (6H, m), 2.14 (6H, m), 2.17 (6H, m), 2.36 (3H, s), 7.19
(2H, d, J ¼ 8:4 Hz), 7.84 (2H, d, J ¼ 8:4 Hz); 13C NMR
(CDCl3) ꢁ 21.4, 28.6, 29.0, 34.6, 35.7, 36.7, 36.8, 41.9, 42.9,
125.6, 125.8, 129.1, 139.1, 143.9, 153.5, 155.8; MS m=z 427
(Mþ). A 1 : 1 mixture of 13c and 13c0: mp 105–106 ꢁC;
colorless crystals; 1H NMR (CDCl3) ꢁ 1.43 (9H, s), 1.47 (9H,
s), 1.78 (12H, m), 2.08 (12H, m), 2.12 (6H, m), 2.16 (6H, m),
2.37 (6H, s), 7.20(4H, d, J ¼ 8:3 Hz), 7.84 (4H, d,
J ¼ 8:3 Hz); 13C NMR (CDCl3) ꢁ 21.4, 28.6, 28.9, 31.2,
32.0, 32.3, 33.2, 34.5, 35.5, 36.7, 36.8, 41.7, 42.6, 125.6,
125.7, 125.8, 129.2, 139.1, 139.2, 143.2, 143.6, 153.0, 153.1,
155.6, 155.8; MS m=z 349 (Mþ). 14: mp 85–89 ꢁC; colorless
solid; 1H NMR (CDCl3) ꢁ1.77–1.83 (12H, m), 2.11 (6H, br s),
2.25 (6H, d, J ¼ 2:7 Hz), 2.31 (6H, d, J ¼ 2:7 Hz), 7.30–7.39
(3H, m), 7.89 (2H, dd, J ¼ 8:3, 1.6 Hz); 13C NMR (CDCl3) ꢁ
29.2, 29.3, 36.4, 36.9, 37.1, 39.7, 43.5, 45.0, 125.9, 128.6,
128.7, 134.5, 146.1, 158.4, 159.2; MS m=z 349 (Mþ).
J. Nakayama, N. Masui, Y. Sugihara, and A. Ishii, Bull. Chem.
Soc. Jpn., 71, 1181 (1998).
As expected from the mechanism, 1c produced a 1 : 1
mixture of 13c and 13c0 in 81% yield by the same treatment.
The use of thiobenzamide, instead of p-toluamide, in the
reaction of 1b produced thiazole 14 in 50% yield probably by the
same mechanism as that for the formation of 13.
The reaction reported above would be important not only as a
new synthesis of oxazoles (thiazole) but also as a synthesis of
oxazoles that possess two bulky alkyl substituents at vicinal
positions.11
It would be thus concluded that sulfilimines 6 and 9 are more
susceptible to ring-opening than the corresponding thiirene 1-
oxides, and quickly isomerize to 1,2-thiazetes from which final
products are derived.12
This work was supported a Grant-in-Aid for Scientific
Research in Priority Areas (No. 13029016) from the Ministry of
Education, Science, Sports, Culture and Technology.
References and Notes
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3
4
11 F. W. Hartner, Jr., ‘‘Comprehensive Heterocyclic Chemistry
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1
12 Attempts to detect sulfilimines or 1,2-thiazetes by H NMR
were all unsuccessful.