Reaction mechanism for the cyclization of 3-[γ,γ-dimethylallyl] coumaric acid methyl ester in dimethyl sulfoxide (DMSO)

Full text of DOI:10.3390/50300612

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Molecules 2000, 5
Reaction Mechanism for the Cyclization of
3-[γ,γ-Dimethylallyl]Coumaric Acid Methyl Ester in Dimethyl
Sulfoxide (DMSO)
E.J. Borkowski, C.E. Ardanaz, P.C. Rossomando and C.E. Tonn
INTEQUI - CONICET - Facultad de Química, Bioquímica y Farmacia. Universidad Nacional de San
Luis. Chacabuco y Pedernera. San Luis (5700), Argentina
E-mail: edubor@unsl.edu.ar
Introduction
The reagents used to oxidize alcohols to ketones in DMSO are believed to form sulfoxonium inter-
mediate species by electrophilic attack at the DMSO oxygen [1,2]. In this work we studied the reaction
of 3-[γ,γ - dimethylallyl] – coumaric acid methyl ester with 2,4,4,6-tetrabromo-2,5-cyclohexadienone
(TBC).
Experimental
1
The reaction (Scheme 1) was followed using H NMR and GC-EIMS. The products were isolated
using TLC and HPLC. Conductimetric measurements of solutions of TBC in acetonitrile and DMSO
as a function of time were carried out.
OH
O
R
S
R
Br
Br
Br
Br
Br
a) DMSO
R
+
+
+
O
O
OH
b) Acetonitrile
Br Br
Br
2
3
1
3-(2,2-dimethyl-3-methylsulfanyl-
chroman-6-yl)-acrylic acid
methyl ester
3-[4-hidroxy-3-(3-methyl-but-2-enyl)-phenyl]
acrylic acid methyl ester
3-(3-Bromo-2,2-dimethyl-
chroman-6-il)-acrylic acid methyl
ester
O
Scheme 1
R=
O
Results and Discussion
GC-EIMS experiments indicate that tribromophenol (TBF) is formed almost completely during the
first minute of the reaction as long as ester slowly diminishes its concentration. This suggests the exis-
tence of an intermediate species that captures the bromonium ion. If TBF participate in this specie,
significant changes should not be expected when solvent is changed. However, the reaction speed and

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Molecules 2000, 5
the yield of the brominated cycle increase when the reaction is made in DMSO related to acetonitrile.
Therefore, we propose the species (CH3)2S+-O-Br, which would act as a carrier of bromonium to pro-
duce the cyclization. The postulation of this species would also explain the product 3, which was char-
acterized by 1HNMR and GC-EIMS. From this point of view the reaction can be considered as similar
to a reaction of Pummerer [3].
In order to test this hypothesis, the conductance of solutions of 2, 4, 4, 6 - tetrabromo - 2, 5 - cyclo-
hexadienone (TBC) in DMSO and acetonitrile respectively, as a function of time, was measured. Re-
sults indicate that the first step the reaction would involve the following balance:
O
-
O
Br
Br
H C
3
H
C
3
Br
Br
k
1
S
O
S
O
Br
+
+
k
2
H
C
H C
3
3
Br
Br
K
F
D
S
Br
H
C
3
H C
3
k
3
+
S
O
Br
Br
S
O
+
H C
3
H C
3
D
S
B
Acknowledgements: We are thankful to CONICET (PIP 5030) and UNSL (Project 7301) for financial
support. We especially thank Professor L.F.R. Cafferata (U.N. La Plata), Dra. María Virginia Mirífico
(INIFTA) and Dr. Leónides Sereno (U.N. Río Cuarto) for their helpful comments.
References and Notes
1. Corey, E.J.; Kim, C.U. J. Am. Chem. Soc. 1972, 94, 7586.
2. Corey, E.J.; Kim, C.U. Tetrahedron Letters 1973, 919.
3. Lucchi, O. De; Miotti U.; Modena, G. Organic Reactions; J. Wiley & Sons, Inc: 1991; Chapter 3,
p 40.