402 J. CHEM. RESEARCH (S), 1997
J. Chem. Research (S),
1997, 402–403†
Direct Synthesis of trans-1,4-Diacetoxycyclohexa-2,5-diene
by Electrochemical Reduction of r-1,t-4-Diacetoxy-
t-2,c-3-dibromocyclohex-5-ene†
a
b
b
¨
Latif Kelebekli, Umit Demir and Yunus Kara*
aEducation Faculty of Agrı, 04200 Agrı, Turkey
bDepartment of Chemistry, Atatu¨rk University, 25240 Erzurum, Turkey
Electrochemical reduction of r-1,t-4-diacetoxy-t-2,c-3-dibromocyclohex-5-ene 1 gives only trans-1,4-diacetoxycyclohexa-
2,5-diene 2 in good yield while the commonly used Zn reduction gives a product mixture containing 2 and acetoxybenzene
3 derived from acetoxy elimination.
Cyclohexadienediols have been identified as intermediates in
OAc
OAc
the biological breakdown of arene oxides by Burice et al.1
These compounds are also useful intermediates for the pre-
paration of natural aromatic compounds, quinone deriva-
tives,2 conduritols3 and cyclitols.4 The direct and easy electro-
chemical synthesis of trans-1,4-diacetoxycyclohexa-2,5-diene
2 leading to conduritol and inositols will facilitate the investi-
gation of biological activities of this compound acting as an
Zn/solvent
+
OAc
AcO
Br
Br
2
2
3
OAc
Electrochemistry
Hg, DMF, 2e–
OAc
AcO
1
inhibitor of -glycosidases and exhibiting an inhibitory effect
D
on the growth of tumour cells.5
Scheme
For the synthesis of cyclohexadienediols, a method based
on 1,2-nucleophilic addition of various alkyl metal reagents
such as RMgX,6 RLi7 and R3Al8 to p-benzoquinones has been
used. These type of reactions result in poor yields and dia-
stereoisomeric mixtures.9 Another method would be the
direct reduction of p-benzoquinone with metal hydrides (e.g.,
LiAlH4, NaBH4); however, these reagents give mainly hydro-
quinone and a small amount of 1,4-dihydroxycyclohexadiene
either with little or no stereoselectivity.10 Therefore, in order
to block the formation of hydroquinone one of the double
bonds must be protected by bromination before reducing
with metal hydrides. Then, there are two possible ways to
eliminate the bromines: (a) dehalogenation with conven-
tional reducing reagents, or (b) electrochemical reduction.
The dehalogenation of vicinal dehalides with zinc11 has been
performed for the synthesis of new double bonds which can
be reduced to yield the conduritols having the desired confor-
mation.12 In addition to zinc, magnesium, iodine and electro-
chemical reduction can be used.13 Electroreduction of
1,2-dibromides as a preparative method offers an attractive
alternative to chemical procedures due to the potential mild-
ness of the reaction conditions. Consequently, there has been
a need for a simple, efficient, and a stereospecific procedure
for the preparation of cyclohexadienediols and compounds of
related structure.
In this study, we report the ready synthesis of trans-1,4-di-
acetoxycyclohexa-2,5-diene 2, which we first synthesized by
the electroreduction of r-1,t-4-diacetoxy-t-2,c-3-dibromo-
cyclohex-5-ene 112 and to make comparisons between the Zn
and electrochemical reductions.
Reductions performed with Zn in different solvents gave
mixtures of two compounds, acetoxybenzene 3 and the
desired diene product 2, depending on the reaction condi-
tions. The ratio of 2 to 3 in the reaction mixture was found to
be dependent on the nature of the solvent used and the
temperature. While compound 3 was the only product in
DMSO at 90 °C, a mixture containing compounds 2 and 3 was
obtained in diethyl ether and MeCO2H solutions at 45 °C for
the reductions with Zn. In the latter reaction conditions,
compound 2 was the predominant product (2:3 = 3:1).
Cyclic voltammetry was performed on 1 using a mercury
electrode in order to determine the reduction potential of 1.
Only a broad and irreversible peak was observed at about
ꢀ1.5 V (SCE) at room temperature at sweep rates up to 100
mV sꢀ1. Cyclic voltammograms showed the same peak shape
and cathodic peak potential as in the literature.14 The reduc-
tions were carried out in a divided cell on a stirred mercury
electrode using DMF as solvent and 0.1 mol dmꢀ3 LiClO4 as
supporting electrolyte. A constant potential electrolysis (cpe)
at ꢀ1.7 V of 1 gave exclusively compound 2 in high yields.
It has been reported that b-haloethers and esters on treat-
ment with Zn undergo alkoxy-halo-elimination reaction11,15
or acetoxy elimination resulting from removal of one acetoxy
group by Lewis acid (ZnBr2) formed in the reduction stage
after formation of 2 by debromination. In the case of acetoxy-
halo-elimination, benzene or 1-acetoxy-2-bromocyclohexa-
3,5-diene would be obtained, but these compounds were
never observed. The formation of compound 3 can be
explained by removal of one acetoxy group by ZnBr2 yielding
a cation and then an aromatization process.16
In contrast to Zn reductions, we obtained quantitatively
only compound 2 by the electrochemical reduction of 1 on Hg
electrodes in DMF. Similar results have been reported for
the electrochemical reduction of vicinal dibromides.14,17,18
Two possible mechanisms are proposed for this reduction in
these studies: first carbanion formation followed either by
proton abstraction resulting in a monobromo compound or
elimination of the second bromide giving compound 2. The
second mechanism is the concerted elimination yielding only
compound 2. In both aprotic and protic conditions, since we
never observed either a monobromo compound or an
aromatic compound, the mechanism of electrochemical
debromination can therefore be considered as concerted.
In conclusion, a short and practical synthesis of 2 has been
described. We have shown that electrochemical reduction of
1 for the synthesis of 2 has some advantages over Zn reduc-
tion. These are selectivity, mild conditions of the reaction,
simplicity of the procedure, and good yields of the product.
Experimental
Cyclic voltammetric determinations were performed using a
Potentioscan Wenking POS 73 potentiostat, YEW 3022 A4 X-Y
recorder. NMR spectra were recorded on a Varian-Gemini 2000
spectrometer at 200 MHz for 1H NMR and 50 MHz for 13C NMR.
The IR spectra were recorded on a Mattson 1000 FTIR spectro-
*To receive any correspondence (e-mail: yunus%tratauni@vm.
ege.edu.tr).
†This is a Short Paper as defined in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1997, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).