Generation and Diels-Alder trapping of 4,5-bis(bromomethylene)-4,5-dihydrothiazole

Article abstract of DOI:10.1002/(SICI)1099-0690(199809)1998:9<2047::AID-EJOC2047>3.0.CO;2-Z

Treatment of 4,5-bis(dlbromomethyl)thiazole (1a) with sodium iodide in DMF led to 4,5-bis(bromomethylene)-4,5-dihydrothiazole (2a). Trapping the latter in situ with dienophiles afforded directly the aromatized cycloadducts. Starting with 5-hydroxynaphthoquinone or its 2- and 3-bromo derivatives 6 gave a mixture of the tetracyclic quinones 10 + 11 from which the 1,6-regioisomer 10 predominates. Using acrylate dienophiles gave 6-substituted benzothiazoles 13 as the major products. The regiochemistry of the cycloadditions between 2a and naphthoquinones 6 agrees with the predictions of the frontier molecular orbital theory. In contrast, the regioselectivity observed from 2a and acrylate dienophiles 12, similar to that previously obtained with 5-bromomethylene-4-methylene-4,5-dihydrothiazole (2b), is opposite to that predicted and could be better accomodated by a competitive Michael addition followed by a cyclization-elimination step.

Full text of DOI:10.1002/(SICI)1099-0690(199809)1998:9<2047::AID-EJOC2047>3.0.CO;2-Z

FULL PAPER
Generation and Diels-Alder Trapping of
,5-Bis(bromomethylene)-4,5-dihydrothiazole
4
a
a
b
a
Karine Jouve , F e´ lix Pautet , Monique Domard , and Houda Fillion*
Laboratoire de Chimie Organique, Facult e´ de Pharmacie, Universit e´ Claude Bernarda,
, Avenue Rockefeller, F-69373 Lyon Cedex 08, France
8
Fax: (internat.) ϩ 33-4/78777158
E-mail: fillion@cismsun.univ-lyon1.fr
Laboratoire de Chimie Physique et Analytique II, Facult e´ de Pharmacie, Universit e´ Claude Bernardb,
8, Avenue Rockefeller, F-69373 Lyon Cedex 08, France
Received March 16, 1998
Keywords: Quinodimethanes / Cycloadditions / Thiazole derivatives / Quinones / Regioselectivity
Treatment of 4,5-bis(dibromomethyl)thiazole (1a) with
sodium iodide in DMF led to 4,5-bis(bromomethylene)-4,5-
dihydrothiazole (2a). Trapping the latter in situ with
dienophiles afforded directly the aromatized cycloadducts.
of the cycloadditions between 2a and naphthoquinones 6
agrees with the predictions of the frontier molecular orbital
theory. In contrast, the regioselectivity observed from 2a and
acrylate dienophiles 12, similar to that previously obtained
Starting with 5-hydroxynaphthoquinone or its 2- and 3- with
5-bromomethylene-4-methylene-4,5-dihydrothiazole
bromo derivatives 6 gave a mixture of the tetracyclic (2b), is opposite to that predicted and could be better
quinones 10
+
11 from which the 1,6-regioisomer 10
accomodated by a competitive Michael addition followed by
a cyclization-elimination step.
predominates. Using acrylate dienophiles gave 6-substituted
benzothiazoles 13 as the major products. The regiochemistry
ortho-Quinodimethanes (o-QDMs) are highly reactive di- ations may be induced by a nucleophilic attack of fluoride
enes in [4ϩ2] cycloaddition reactions and their applications ion on the silicon atom of substrates bearing a silyl group
in this field have proved very useful for the building of poly- and a good leaving group (X ϭ SiR , Y ϭ OAc or
3
cyclic aromatic compounds.[ Their five-membered hetero- R N ), by treatment of a tributylstannylfuryl derivatives
1]
ϩ [9]
3
cyclic analogues have received recent attention due to the (X ϭ Bu Sn, Y ϭ OAc) with boron trifluorideϪdiethyl
3
wide variety of the pentagonal heteroaromatic rings.[
2]
ether
[10]
or by the well-known reductive debromination of
However, their synthetic potential through Diels-Alder re- vicinal bromomethyl derivatives.[ This last method, al-
actions has been little explored. Among the methods em- though older, is still valuable due to the easy availability of
ployed for their generation (Scheme 1), flash vacuum pyro- the o-QDM precursors.
11]
lysis (FVP) of furanocyclohexene, suitable esters or chloro-
methyl derivatives needs usually very harsh conditions[
3]
Scheme 1
(temperatures may vary from 600 to 900°C) while chelo-
tropic extrusion of sulfur dioxide from five-membered het-
eroaromatic fused 3-sulfolenes[
a versatile alternative using temperatures between 170 and
20°C. Moreover, furano-, thieno- and pyrrolo-fused sul-
tines are described as precursors for non-classical o-
4][5]
has been shown to be
2
[
6]
QDMs while pyrrolopyranones are considered as stable
cyclic analogues of pyrrole-2,3-quinodimethane. The latter
undergo Diels-Alder reactions with alkynes to give indoles
after loss of carbon dioxide.[ Tautomerism of imino de-
rivatives bearing a methyl group in an ortho position offers
another method for the generation of five-membered het-
7]
[
8]
erocyclic o-QDMs. Trapping such intermediates with di-
enophiles by an intramolecular [4ϩ2] cycloaddition has
In continuation of our investigations directed towards the
constituted an efficient strategy towards the total synthesis ability of thiazole derivatives to generate o-quinodimeth-
of indole alkaloids. 1,4-Exocyclic eliminations are also larg- anes by heating with sodium iodide,[ we decided to evalu-
ely employed to generate o-QDMs since they are carried ate the suitability of 4,5-bis(dibromomethyl)thiazole (1a)[
out under very mild conditions. Thus, o-QDMs are pre- as a precursor for 4,5-bis(bromomethylene)-4,5-dihydro-
pared from various precursors and reagents. Indeed, elimin- thiazole (2a). Trapping the latter with acrylate dienophiles
12]
13]
Eur. J. Org. Chem. 1998, 2047Ϫ2050

WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
1434Ϫ193X/98/0909Ϫ2047 $ 17.50ϩ.50/0
2047

K. Jouve, F. Pautet, M. Domard, H. Fillion
FULL PAPER
Scheme 2
Scheme 3
would be of interest due to the directly aromatized adducts
that it may afford. Moreover, a comparison of the regioch-
emistry of its [4ϩ2] cycloadditions with that of 5-bromo-
methylene-4-methylene-4,5-dihydrothiazole (2b) previously
prepared and trapped with unsymmetrical dienophiles[
would be helpful to elucidate the regiocontrol of such reac-
tions.
12]
Results and Discussion
Subsequent generation of o-QDM 2a was performed ac-
cording to the procedure described for 2b by treating the
tetrabrominated precursor 1a with sodium iodide in DMF was observed in the cycloaddition of 2a and 3-bromojug-
Scheme 2). Then, 2a was trapped in situ with a set of di- lone (6c) comparatively to 6a and 6b.
(
enophiles. Starting with the symmetrical dienophiles 3, 4
Trapping o-QDM 2a with acrylate dienophiles 12
˚
and 5 and following method A, we obtained the aromatized (method B, molecular sieves 4 A) afforded a mixture of the
cycloadducts 7 and 9 in higher amounts than from 2b. In aromatized adducts 13 and 14 (Scheme 4, Table 1). In the
contrast, compound 8 was isolated in a very poor yield absence of molecular sieves (method A), the reaction be-
(Scheme 3 and Table 1). Trapping 2a with juglone (6a) and tween 2a and 12b afforded the mixture of 13b and 14b in
its 2- or 3-bromo derivatives 6b and 6c afforded a mixture the same ratio (70:30) but with a lower yield (30%). The
of the regioisomeric tetracyclic quinones 10 and 11. Com- structural assignment for the regioisomers 13 and 14 was
1
parison of these results with those obtained from 2b (Table made by comparison of their H-NMR spectral data with
1
) indicates that compounds 10 and 11 are isolated in a those of samples obtained by aromatization of 15 and 16.
similar range of yield (66Ϫ75% on one hand and 60Ϫ73% Indeed, for the 6-substituted benzothiazoles 13, the protons
on the other hand) but using 2a the regioselectivity ob- 2-H and 7-H are deshielded while the signals of 4-H are
served is lower than that obtained from 2b. In contrast with shifted to higher fields (Table 2). Thus, with these unsym-
our precedent results,[ no inversion of the regiochemistry metrical dienophiles the orientation of the cycloadditions is
12]
Table 1. Trapping of o-QDMs 2 with dienophiles
o-QDM
Dienophile
Method
Adducts
Yield (%)[a]
Ratio of regioisomers
2a
2b
2a
2b
2a
2b
2a
2b
2a
2b
2a
2b
2a
2b
2a
2b
2a
2b
3
A
A
A
A
A
A
B
7
73[
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
12]
3
7
56
4
8
8
4
8
30
95
5
9
[
[
[
[
[
[
12]
12]
12]
12]
12]
12]
12]
5
9
70
75
70
66
60
71
73
57
57
50
75
51
6a
6a
6b
6b
6c
6c
12a
12a
12b
12b
12c
12c
10؉11
10ϩ11
10؉11
10ϩ11
10؉11
10ϩ11
13a؉14a
15aϩ16a
13b؉14b
15bϩ16b
13c؉14c
15cϩ14c
10/11 ϭ 75:25
10/11 ϭ 70:30
10/11 ϭ 79:21
10/11 ϭ 92:8
10/11 ϭ 70:30
10/11 ϭ 08:92
13a/14a ϭ 67:33
15a/16a ϭ 95:5
13b/14b ϭ 71:29
15b/16b ϭ 87:13
13c/14c ϭ 68:32
15c/14c ϭ 81:19
B
B
64[
[
a]
Yields are calculated from isolated pure products.
048
2
Eur. J. Org. Chem. 1998, 2047Ϫ2050

Generation and Diels-Alder Trapping of 4,5-Bis(bromomethylene)-4,5-dihydrothiazole
FULL PAPER
Scheme 4
Scheme 5
proposed for the trapping of benzofuran-2,3-quinodimeth-
ane with methyl vinyl ketone.[
1
Table 2. H-NMR data (δ ppm, CDCl
3
, 200 Mhz) of 2-H, 4-H and
16]
7
-H for benzothiazoles 13 and 14
In summary this work describes the generation of 4,5-
bis(bromomethylene)-4,5-dihydrothiazole (2a) by a re-
ductive debromination of 4,5-bis(dibromomethyl)thiazole
(1a) with sodium iodide. Diels-Alder trapping of 2a with
unsymmetrical dienophiles is found regioselective although
the regioselectivity is lower than that observed with
Compound
2-H
4-H
7-H
13a
14a
13b
14b
13c
14c
9.15
9.08
9.09
9.01
9.17
9.10
8.15
8.38
8.15
8.76
8.18
8.71
8.26
8.01
8.64
8.15
8.62
8.08
5
-bromomethylene-4-methylene-4,5-dihydrothiazole (2b).
Starting with juglone (6a) or its 2- and 3-bromo derivatives
b and 6c, respectively, the reactions afford a mixture of the
6
tetracyclic quinones 10 and 11 from which the 1,6-re-
gioisomer 10 predominates. The regiochemistry of the [4ϩ2]
cycloadditions agree with the predictions of the frontier
molecular orbital theory. Trapping 2a with the acrylate de-
rivatives 12 gives the directly aromatized 6-substituted
benzothiazoles 13 as the major regioisomers. This orien-
tation, similar to that observed with 2b, is in contradiction
with the calculations of the frontier orbital coefficients. It
could be better explained by a competition between a
Michael addition and the Diels-Alder reaction.
similar to that observed from 2b although with a lower re-
gioselectivity.
To understand the regiochemistry observed, we calcu-
lated by the semiempirical PM3 method,[ the energies of
HOMO and LUMO for the most stable (Z,Z) configuration
of 2a. Comparison of these values (HOMO ϭ Ϫ9.151 eV
and LUMO ϭ Ϫ1.235 eV) with those of buta-1,3-diene
14]
(HOMO ϭ Ϫ9.502 eV and LUMO ϭ 0.282 eV) and 1-
methoxybuta-1,3-diene (HOMO Ϫ8.854 eV and
ϭ
LUMO ϭ 0.351 eV) indicates that the behaviour of 2a
towards dienophiles would be intermediate between that of
a neutral and an electron-rich diene. Then, calculations of
HOMO orbital coefficients at the ends of (Z,Z)-2a show
that it is a polarized diene as well as 2b (Scheme 2).
Experimental Section
General: Melting points were determined in open capillary tubes
1
with a Büchi 510 apparatus. Ϫ H-NMR spectra were recorded at
2
00 and 300 MHz with Bruker AM 200 or AM 300 spectrometers.
Having in mind that the larger LUMO orbital coefficients
Chemical shifts are given as δ values (int. standard: TMS). Elemen-
for naphthoquinones 6 are located at C-2 independently of
tal analyses were performed at the Centre de Microanalyses du
the presence or the position of the bromine atom,[ we CNRS at Solaize. The energies and coefficients of the frontier mo-
12]
observed that the regiochemistry of the [4ϩ2] cycload- lecular orbitals were calculated using MOPAC of the SYBYL pro-
[17]
ditions between 2a and 6 is in good agreement with the gram. Ϫ 2-Bromo-5-hydroxynaphthoquinone (6b) and 3-bromo-
-hydroxynaphthoquinone (6c)[ were prepared according to pro-
18]
5
molecular frontier orbital theory since the major re-
gioisomer is, in each case, compound 10.
cedures described in the respective references.
In the trapping of o-QDMs 2 with acrylate dienophiles,
General Procedure for the Generation and Trapping of 4,5-
the observed regiochemistry is opposite to that predicted by Bis(bromomethylene)-4,5-dihydrothiazole (2a). Ϫ Method A: A
solution of the tetrabromo derivative 1a (0.25 g, 0.6 mmol) in DMF
the calculations. Indeed, the larger LUMO orbital coef-
ficients, previously calculated for dienophiles 12,[12] are lo-
(2 ml) was added over 10 min to a stirred and heated (70°C) mix-
ture of NaI (0.36 g, 2.4 mmol) and the appropriate dienophile (3
mmol) in DMF (4 ml). Then, the reaction mixture was heated for
cated at the unsubstituted carbon atom. A similar failure is
reported for the Diels-Alder reaction between acrylic acid
and vinylacrylic acid.[ To explain this opposite orien-
tation, we can envisage the possibility for a portion of the
major regioisomers 13 or 15 to result from a competitive
3
0 min. After cooling, 50 ml of water was added to the brown
solution and the latter was decolorated with a 10% aqueous solu-
tion of NaHSO . The solution was extracted with 2 ϫ 30 ml of
EtOAc and the organic phase dried with MgSO . Evaporation of
15]
3
4
initial Michael addition followed by a cyclization-elimi- the solvent left a residue which was purified by column chromatog-
nation step (Scheme 5). An analogous mechanism has been raphy on silica gel using EtOAc/hexane (5:5) as the eluent or by
Eur. J. Org. Chem. 1998, 2047Ϫ2050
2049

K. Jouve, F. Pautet, M. Domard, H. Fillion
FULL PAPER
4
138Ϫ4144. Ϫ C. H. Chou, W. S. Trahanovsky J. Org. Chem.,
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ture of the tetrabromo derivative 1a (0. 25 g, 0.6 mmol) in DMF
1
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(2 ml) and the corresponding acrylate derivative 12 (20 mmol) in 3
ml of DMF was added over 10 min to a stirred, heated (70°C)
solution of NaI (0.6 g, 4 mmol) and highly activated molecular
[
[
4]
5]
˚
sieves 4 A (0.6 g) in DMF (3 ml). Stirring and heating were con-
tinued for 30 min. After elimination of molecular sieves and the
same work-up as above, the residue was purified by column chro-
matography on silica gel using EtOAc/petroleum ether (4:6) as
the eluent.
1
417Ϫ1439.
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6
4
2
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16: To a solution of the corresponding dihydro adduct (0.6 mmol)
in diphenyl ether (2 ml) was added 0.15 equiv. of 10% Pd/C. The
suspension was stirred and heated at 220°C until the dihydro ad-
duct disappeared (monitored by TLC). After cooling, the reaction
mixture was filtered through Celite 545 (Merck). The filtrate was
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[
[
6]
7]
8]
Thiazolo[4,5-e]benzofuran-5,7-dione (8): Compound 8 was pre-
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Ϫ1
3
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[
D
6
]DMSO): δ ϭ 9.81 (s, 1 H, 2-H), 9.04 (s, 1 H, 8-H), 8.74 (s, 1
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H
3
3
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[
9]
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2
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Ϫ1
1
[10]
[11]
cm (CN). Ϫ H NMR (200 MHz, CDCl
3
): δ ϭ 9.15 (s, 1 H, 2-
H), 8.26 (d, 1 H, J ϭ 1.5 Hz, 7-H), 8.15 (d, 1 H, J ϭ 8 Hz, 4-H),
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1
7
.70 (dd, 1 H, J ϭ 8 and 1.5 Hz, 5-H). Ϫ 14a: H NMR (200 MHz,
CDCl ): 9.08 (s, 1 H, 2-H), 8.38 (d, 1 H, J ϭ 1.5 Hz, 4-H), 8.01
d, 1 H, J ϭ 8 Hz, 7-H), 7.60 (dd, 1 H, J ϭ 8 and 1.5 Hz, 6-H). Ϫ
S, 0.33 H O (166.1): calcd. C 57.84, H 2.83, N 16.86 S
9.30; found C 57.63, 2.45, 16.79, 19.66.
5
429Ϫ5430. Ϫ D. J. Chadwick, A. Plant, Tetrahedron Lett.
3
1
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C
1
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H
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N
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2
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5- and 6-Ethoxycarbonylbenzothiazole (13b and 14b): Com-
pounds 13b and 14b were both obtained as a white solid mixture
from o-QDM 2a and dienophile 12b (method B) or by dehydrogen-
Ϫ1
ation of 15b and 16b in 51% yield. Ϫ IR (KBr): ν˜ ϭ 1700 cm
1
(CO). Ϫ H NMR (200 MHz, CDCl
3
): 13b: δ ϭ 9.09 (s, 1 H, 2-
H), 8.64 (s, 1 H, 7-H), 8.15 (m, 2 H, 4-H and 5-H), 4.38 (q, 2 H,
J ϭ 6.2 Hz, CH ), 1.37 (t, 3 H, J ϭ 6.2 Hz, CH ); 14b: δ ϭ 9.01
s, 1 H, 2H), 8.76 (d, 1 H, J ϭ 1.3 Hz, 4-H), 8.15 (m, 2 H, 7-H
and 6-H), 4.38 (q, 2 H, J ϭ 6.2 Hz, CH ), 1.37 (t, 3 H, J ϭ 6.2
Hz, CH ). Ϫ C S, 0.33 H O (213.2): calcd. C 57.84, H 2.83,
N 16.86 S 19.30; found C 57.63, 2.45, 16.79, 19.66.
2
3
(
5
1, 9595Ϫ9602.
2
[12]
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8
H
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