Diversity-oriented synthesis of 1-hydroxy-2,4-benzodioates by regioselective [3+3] cyclocondensations of 1,3-bis(silyloxy)-1,3-butadienes with 3-alkoxy- and 3-silyloxy-2-alkoxycarbonyl-2-en-1-ones

Article abstract of DOI:10.1039/b900542k

1-Hydroxy-3,5-dimethyl-2,4-benzodioates (4-hydroxyisophthalates) were prepared by [3+3] cyclocondensation of 1,3-bis(silyloxy)-1,3-butadienes with 3-ethoxycarbonyl-4-trimethylsilyloxy-3-penten-2-one which is synthesized from (symmetrical) ethyl 2-acetylacetoacetate. The [3+3] cyclization of 1,3-bis(silyloxy)-1,3-butadienes with 3-alkoxy-2-alkoxycarbonyl-2-en-1-ones, readily available by reaction of β-ketoesters with trialkyl orthoformiates, provide a convenient and regioselective approach to a great variety of 3-substituted 1-hydroxy-2,4-benzodioates that are not readily available by other methods.

Full text of DOI:10.1039/b900542k

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Diversity-oriented synthesis of 1-hydroxy-2,4-benzodioates by regioselective
[3+3] cyclocondensations of 1,3-bis(silyloxy)-1,3-butadienes with 3-alkoxy-
and 3-silyloxy-2-alkoxycarbonyl-2-en-1-ones†
Mohanad Shkoor,a Abdolmajid Riahi,a,b Olumide Fatunsin,a Ibrar Hussain,a Mirza A. Yawer,a Mathias Lubbe,a
Stefanie Reim,a Helmut Reinke,b Christine Fischerb and Peter Langer*a,b
Received 12th January 2009, Accepted 18th February 2009
First published as an Advance Article on the web 31st March 2009
DOI: 10.1039/b900542k
1-Hydroxy-3,5-dimethyl-2,4-benzodioates (4-hydroxyisophthalates) were prepared by [3+3]
cyclocondensation of 1,3-bis(silyloxy)-1,3-butadienes with 3-ethoxycarbonyl-4-
trimethylsilyloxy-3-penten-2-one which is synthesized from (symmetrical) ethyl 2-acetylacetoacetate.
The [3+3] cyclization of 1,3-bis(silyloxy)-1,3-butadienes with 3-alkoxy-2-alkoxycarbonyl-2-en-1-ones,
readily available by reaction of b-ketoesters with trialkyl orthoformiates, provide a convenient and
regioselective approach to a great variety of 3-substituted 1-hydroxy-2,4-benzodioates that are not
readily available by other methods.
trianions with carboxylic acid derivatives and subsequent in-
Introduction
tramolecular aldol reaction of the polyketides thus formed.4
Functionalized benzene derivatives, such as hydroxylated ben-
zoates, are of considerable interest as lead structures in medicinal
and agricultural chemistry and as synthetic building blocks.1
Other syntheses rely on the [4+2] cycloaddition of electron-
rich dienes (e.g. 2-silyloxy-1,3-butadienes) with electron-poor
alkynes. We have recently reported the synthesis of 4-hydroxy-
Several syntheses of functionalized benzoates rely on electrophilic
and 2,4-dihydroxy-homophthalates by [4+2] cycloaddition of
substitutions, oxidations or transition metal-catalyzed cross-
1,3-bis(trimethylsilyloxy)-1,3-butadienes with dimethyl allene-1,3-
coupling reactions.2 Despite their great utility, these transforma-
tions have several drawbacks. Classical electrophilic substitutions
often proceed with poor o/p-regioselectivity. Oxidations of toluene
to benzoic acid derivatives require harsh reaction conditions,
which can lead to destruction of more complex, polyfunctionalized
substrates. In contrast, transition metal-catalyzed cross-coupling
reactions proceed under mild conditions. Despite much progress
in this area, cross-coupling reactions of sterically encumbered or
functionalized substrates often proceed in low yields or not at
all. In addition, the synthesis of more complex starting materials,
highly functionalized or substituted aryl halides or triflates, can
be a very difficult task.
An alternative approach is based on a building block strat-
egy. Functionalized phenols and benzoates have been prepared,
for example, by base-mediated cyclization reactions of acti-
vated (symmetrical) 1,3,5-tricarbonyl compounds, e.g. diethyl
acetone-1,3-dicarboxylate, with enolizable 1,3-dicarbonyl deriva-
tives, ynones or enones.3 Harris and coworkers reported the
(biomimetic) synthesis of densely functionalized phenols by
condensation of 1,3-dicarbonyl dianions or 1,3,5-tricarbonyl
dicarboxylate.5 4,5-Diaryl-1,2,3-benzenetricarboxylates have been
prepared by [4+2] cycloaddition of 4-hydroxycyclopent-2-en-1-
one-2-carboxylates, precursors of highly reactive fulvenones, with
dimethyl acetylenedicarboxylate.6
In 1980, Chan and Brownbridge reported7 the first example of a
new approach to salicylates based on the formal [3+3] cyclization
of 1,3-bis(silyloxy)-1,3-butadienes, electroneutral 1,3-dicarbonyl
dianion equivalents,8 with dielectrophilic 3-silyloxy-2-en-1-ones.
In recent years, this strategy has been applied to the synthesis
of various functionalized arenes.9 However, the scope is limited
to 3-silyloxy-2-en-1-ones derived from symmetrical 1,3-diketones.
Starting with these substrates, salicylates containing the same
substituents located at carbon atoms C3 and C5 can be prepared.
Although a few exceptions have been reported,7 cyclizations of
3-silyloxy-2-en-1-ones derived from unsymmetrical 1,3-dicarbonyl
compounds often proceed with low regioselectivity. Following an
explanation first suggested by Chan and Brownbridge,7 this is a
result of the fact that 3-silyloxy-2-en-1-ones can undergo a TiCl4-
mediated isomerization (migration of the TMS-group from one
oxygen atom to the other).
In their early work, Chan and Brownbridge mentioned7 that
the cyclocondensation of 1-methoxy-1,3-bis(trimethylsilyloxy)-
1,3-butadiene with 1-methoxybut-1-en-3-one proceeds with good
regioselectivity. Based on this observation, we recently started a
program directed towards the development of new cyclizations
of acceptor-substituted 3-alkoxy-2-en-1-ones. These cyclizations
proceed with very good regioselectivity, which might be explained
by the assumption that the substrates do not undergo the above
mentioned TiCl4-mediated isomerization process. Our strategy
aInstitut fu¨r Chemie, Universita¨t Rostock, Albert-Einstein-Str. 3a, 18059,
Rostock, Germany. E-mail: peter.langer@uni-rostock.de; Fax: 00 49 381
4986412; Tel: 00 49 381 4986410
bLeibniz-Institut fu¨r Katalyse e. V. an der Universita¨t Rostock, Albert-
Einstein-Str. 29a, 18059, Rostock, Germany
† Electronic supplementary information (ESI) available: Compound char-
acterization, copies of NMR spectra and details of the X-ray struc-
ture analyses. CCDC reference numbers 718444 and 718445. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b900542k
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takes advantage of the fact that the required starting materials
are readily available by reaction of acceptor-substituted ketones
with triethyl or trimethyl orthoformiate.
1-Hydroxy-2,4-benzodioates (4-hydroxyisophthalates) are im-
portant core structures of pharmacologically important natural
products and also represent versatile building blocks in organic
synthesis.10 The most widely applied synthetic approach to such
molecules relies on the oxidation of appropriate benzene deriva-
tives and on the functionalization of phenol or benzoic acid
derivatives.10–14 Recently, we have reported the synthesis of 1-
hydroxy-3,5-dimethyl-2,4-benzodioates by TiCl4-mediated [3+3]
cyclization of 1,3-bis(silyloxy)-1,3-butadienes with 3-silyloxy-2-
en-1-ones derived from (symmetrical) ethyl 2-acetylacetoacetate.15
Herein, we report full details of this reaction and a significant
extension of its scope. In addition, we report, for the first time, the
[3+3] cyclocondensation of 1,3-bis(silyloxy)-1,3-butadienes with
readily available 3-alkoxy-2-alkoxycarbonyl-2-en-1-ones. These
reactions rely on the general concept outlined above and provide
a convenient and regioselective approach to a great variety of
functionalized 1-hydroxy-2,4-benzodioates that are not readily
available by other methods.
Results and discussion
The triethylamine-mediated reaction of ethyl 2-acetylacetoacetate
(2) with trimethylchlorosilane afforded the 2-ethoxycarbonyl-3-
silyloxy-2-en-1-one 3 (Scheme 1).
Scheme 2 Possible mechanism of the formation of 4a.
Scheme 1 Synthesis of silyl enol ether 3.
Scheme 3 Synthesis of 4a–o.
Table 1 Synthesis of 4a–o
The TiCl4-mediated formal [3+3] cyclization of 3 with 1,3-
bis(silyloxy)-1,3-butadiene 1a, prepared from methyl acetoacetate
in two steps,7 afforded the novel functionalized 1-hydroxy-3,5-
dimethyl-2,4-benzodioate 4a (Scheme 2). The best yield was
obtained when (a) the reaction was carried out in a highly
concentrated solution (2 mL per 1.0 mmol of 3), (b) the reagents
were added in a stoichiometric ratio and (c) hydrochloric acid
(10%) was employed for the aqueous work-up.
The formation of 4a can be explained by reaction of 3 with TiCl4
to give intermediate A. The attack of the terminal carbon atom
of 1a onto A afforded intermediate B. The elimination of TMS-
siloxane generated intermediate C and subsequent cyclization gave
intermediate D (Scheme 2). The elimination of titanium hydroxide
(before or during the aqueous work-up) and aromatization
resulted in the formation of product 4a. Due to the symmetrical
structure of A, the attack of 1a on either terminal allylic carbon
atom would result in the formation of the same product (4a).
1,3-Bis(silyloxy)-1,3-butadienes 1a–o were prepared in analogy
to the procedure previously reported.7 The TiCl4 mediated for-
mal [3+3] cyclization of 3 with 1,3-bis(silyloxy)-1,3-butadienes
1a–m afforded the 1-hydroxy-2,4-benzodioates 4a–m (Scheme 3,
Table 1). The 2-acetyl- and 2-benzoyl-1-hydroxy-4-benzoates 4n
and 4o were prepared from dienes 1n and 1o which are derived
from acetylacetone and benzoylacetone, respectively. The yields
1,4
R1
R2
% (4)a
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
H
Me
Et
OMe
OEt
OEt
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OEt
OEt
OEt
Me
30
48
49
35
40
41
38
37
34
65
48
42
51
38
36
nHex
nNon
nDec
4-MeC6H4
4-ClC6H4
4-(MeO)C6H4
OMe
OPh
O(2-MeC6H4)
O(3-MeC6H4)
H
H
Ph
a Yields of isolated products.
of products 4b–m, containing a substituent located at carbon
C6, were higher (albeit not in all cases significantly) than the
yield of 4a. The best yield was obtained for product 4j con-
taining a methoxy group located at carbon C6. The syntheses of
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Table 2 Synthesis of 6a–e
5,6
R
R1
R2
% (6)a
a
b
c
d
e
Me
Me
Me
Et
Me
Et
nPr
Ph
Me
Me
Me
Et
82
84
83
81
80
Et
CH2Cl
Et
a Yields of isolated products.
2-hydroxy-2,4-benzodioates 4a–o bear, as discussed above, no issue
of regioselectivity.
The regioselective synthesis of 1-hydroxy-2,4-benzodioates de-
rived fromunsymmetrical substrates was next studied. Our strategy
is based, as outlined in the introduction, on the employment
of 3-alkoxy- rather than 3-silyloxy-2-en-1-ones as dielectrophilic
building blocks. 2-Alkoxycarbonyl-3-methoxy-2-en-1-ones 6a–e
were prepared, following a known procedure,16 by reaction of b-
ketoesters 5a–e with trimethyl orthoformiate and acetic anhydride
(Scheme 4, Table 2). The synthesis of 6a has been previously
reported.16 All products, including the chlorinated enone 6e, were
prepared in good yields.
Scheme 5 Possible mechanism of the formation of 7a.
Scheme 4 Synthesis of 6a–e; (i) 5a–e (1.0 equiv.), HC(OR)3 (1.2 equiv.),
Ac2O, reflux, 2 h.
The TiCl4-mediated cyclization of 6a with 1a afforded
the 1-hydroxy-2,4-benzodioate 7a with excellent regioselectivity
(Scheme 5). The best yield was (again) obtained when the reaction
was carried out in a highly concentrated solution (2 mL per 1 mmol
of 6a). The yield decreased when the reaction mixture was more
dilute (30 mL per 1.0 mmol of 6a). The starting materials 6a,
1a and TiCl4 were employed in a 1.0 : 1.1 : 1.1 ratio. The yield
decreased when the stoichiometric ratio was 1.0 : 1.0 : 2.0, 1.0 : 2.0 :
1.0 or 1.0 : 2.0 : 2.0. The temperature had to slowly warm from
-78 to 20 ◦C. No product could be isolated when the reaction was
carried out at 0 ◦C. The regioselectivity can be explained by steric
and electronic effects.
Scheme 6 Synthesis of 7a–ae.
The structures of 1-hydroxy-2,4-benzodioates 7a and 7i were
easily confirmed based on the large coupling constants (1H NMR)
of the neighbouring aromatic hydrogen atoms. The elucidation of
the structure of all other derivatives was considerably more difficult
and had to rely on 2D NMR experiments (NOESY, HMBC). The
structures of 7b and 7k were independently confirmed by X-ray
crystal structure analyses (Fig. 1 and 2).† In the structure of 7k
the OH group is involved both in an intra- and an intermolecular
hydrogen bond. So the distance between the symmetry equivalent
atoms O1 is only 277 pm.
The formal [3+3] cyclization of 2-alkoxycarbonyl-3-alkoxy-2-
en-1-ones 6a–d with 1,3-bis(silyloxy)-1,3-butadienes 1a, 1c–h, 1j,
and 1p–t afforded the 1-hydroxy-2,4-benzodioates 7a–ae in 43–
60% yield (Scheme 6, Table 3).
The substituents R1, located next to the carbonyl group of 6a–d,
have no significant influence on the yields. 1,3-Bis(silyloxy)-1,3-
butadienes 1c–e and 1p–s contain an alkyl group located at carbon
atom C4 of the diene. Products 7a,i have been prepared from the
unsubstituted dienes 1a,t. The yields of products 7b–h, derived
from 6a and from 1c–e and 1p–s, are slightly higher than the
yields of 7a,i. There are no significant variations in the yields
of the products containing the same substituent R3 but different
substituents R1. Thus, the substituent introduced by the enone has
no significant influence on the yield (except for 6e which failed to
give the desired product 7af).
Conclusions
In conclusion, we have reported a convenient and regioselective
synthesis of a great variety of 1-hydroxy-2,4-benzodioates (4-
hydroxyisophthalates) that is, to the best of our knowledge, the
first formal [3+3] cyclization of 1,3-bis(silyloxy)-1,3-butadienes
with 3-silyloxy- and 3-alkoxy-2-alkoxycarbonyl-2-en-1-ones. The
reactions are carried out easily and the starting materials are read-
ily available. The highly functionalized and sterically encumbered
benzene derivatives prepared are not readily available by other
methods.
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Table 3 Synthesis of 7a–ae
1
6
7
R1
R2
R3
H
R4
Yield (%) a
a
p
c
q
d
r
s
e
t
g
h
j
p
c
q
d
r
s
e
f
g
p
c
q
d
r
s
e
f
p
c
a
a
a
a
a
a
a
a
a
a
a
a
a
b
b
b
b
b
b
b
b
b
c
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
Et
Et
Et
Et
Et
Et
Et
Et
Me
Me Me
Me Et
Me
Me
Et
43
57
50
52
55
51
48
52
48
53
48
52
52
50
52
52
51
50
51
49
51
51
53
51
45
46
45
50
55
58
60
0
Me nBu
Me nHex
Me nHept
Me nOct
Me nNon
Me
Me
Me
Me
Me
(CH2)2OMe
Me
Me
Me
Me
Et
Me
Me
Me
Me
Me
Me
Me
Me
Et
Me
Me
Me
Me
Me
Me
Me
Et
Me
H
Me 4-MeC6H4
Me 4-ClC6H4
Me OMe
Me Me
Me Et
Me nBu
Me nHex
Me nHept
Me nOct
Me nNon
Me nDec
Me 4-MeC6H4
Me Me
Me Et
Me nBu
Me nHex
Me nHept
Me nOct
Me nNon
Me nDec
Fig. 2 Ortep plot of 7k.
General procedure for the synthesis of 1-hydroxy-2,4-benzodioates
4a–o
s
t
u
v
w
x
y
z
To a CH2Cl2 solution (2 mL per 1.0 mmol of 3) of 3 (1.0 equiv.)
was added 1 (1.0 equiv.) and, subsequently, TiCl4 (1.0 equiv.) at
-78 ◦C. The temperature of the solution was allowed to warm
to 20 ◦C during 14 h with stirring. To the solution was added
hydrochloric acid (10%, 20 mL) and the organic and the aqueous
layer were separated. The latter was extracted with CH2Cl2 (3 ¥
20 mL). The combined organic layers were dried (Na2SO4), filtered
and the filtrate was concentrated in vacuo. The residue was
purified by chromatography (silica gel, n-heptane–EtOAc) to give
product 4.
nPr
nPr
nPr
nPr
nPr
c
c
c
c
c
aa nPr
ab nPr
ac nPr
ad Ph
ae Ph
c
c
d
d
e
Et
Et
Me
Et
H
af
CH2Cl Et
Me
a Yields of isolated products.
1-Ethyl 3-methyl 4-hydroxy-2,6-dimethylisophthalate (4a)
Starting with 3 (0.489 g, 2.0 mmol) and 1a (0.521 g, 2.0 mmol), 4a
was isolated after chromatography (silica gel, heptanes–EtOAc)
1
as an orange oil (0.149 g, 30%). H NMR (CDCl3, 250 MHz):
3
d = 1.36 (t, J = 7.1 Hz, 3H, OCH2CH3), 2.25 (s, 3H, CH3),
2.44 (s, 3H, CH3), 3.94 (s, 3H, OCH3), 4.36 (q, 3J = 7.1 Hz, 2H,
OCH2CH3), 6.69 (s, 1H, CHAr), 11.23 (s, 1H, OH). 13C NMR
(CDCl3, 75 MHz): d = 14.2 (OCH2CH3), 20.0, 20.2 (CH3), 52.2
(OCH3), 61.1 (OCH2CH3), 110.4 (CAr), 116.9 (CHAr), 128.6, 137.8,
141.8 (CAr), 162.6 (COH), 169.7 (CO), 171.7 (CO). IR (neat, cm-1):
nmax = 3421 (br, w), 2983 (m), 2956 (m), 1725 (s), 1666 (s), 1606 (m),
1579 (m), 1444 (s), 1360 (s), 1324 (s), 1259 (s), 1232 (s), 1185 (s),
1115 (s), 1053 (m), 1036 (m). MS (GC/MS, 70 eV): m/z (%) = 252
(M+, 27), 220 (100), 207 (28), 175 (61). Anal.: calcd. for C13H16O5
(252.26): C, 61.90; H, 6.39. Found: C, 61.69; H, 6.51.
Fig. 1 Ortep plot of 7b (only one of the symmetry independent molecules
shown).
Typical experimental procedure for the synthesis of 7a–ae
Experimental section
To a CH2Cl2 solution (2 mL per 1 mmol of 6a–e) of 6a–e was
added 1 (1.1 mmol) and, subsequently, TiCl4 (1.1 mmol) at -78 ◦C.
The temperature of the solution was allowed to warm to 20 ◦C
during 14 h with stirring. To the solution was added hydrochloric
acid (10%, 20 mL) and the organic and the aqueous layer were
separated. The latter was extracted with CH2Cl2 (3 ¥ 20 mL).
The combined organic layers were dried (Na2SO4), filtered and
the filtrate was concentrated in vacuo. The residue was purified by
chromatography (silica gel, heptanes–EtOAc) to give 7a–ae.
General comments
All solvents were dried by standard methods and all reactions
were carried out under an inert atmosphere. For 1H and 13C
NMR spectra, the indicated deuterated solvents were used. Mass
spectrometric data (MS) were obtained by electron ionization
(EI, 70 eV), chemical ionization (CI, isobutane) or electrospray
ionization (ESI). For preparative scale chromatography, silica gel
60 (0.063–0.200 mm, 70–230 mesh) was used.
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Dimethyl 4-hydroxy-2-methylisophthalate (7a)
derivatives, see: M. Yamaguchi, K. Hasebe and T. Minabi, Tetrahedron
Lett., 1986, 27, 2401; (e) for the cyclization of a 1,3-dicarbonyl dianion
with 3-(N,N-dimethylamino)-2-ethylacroleine, see: D. H. R. Barton, G.
Dressaire, B. J. Willis, A. G. M. Barrett and M. Pfeffer, J. Chem. Soc.,
Perkin Trans. 1, 1982, 665.
4 (a) For a review, see: T. M. Harris and C. M. Harris, Tetrahedron, 1977,
33, 2159; (b) see also: T. P. Murray and T. M. Harris, J. Am. Chem. Soc.,
1972, 94, 8253; (c) C. M. Harris, J. S. Roberson and T. M. Harris, J. Am.
Chem. Soc., 1976, 98, 5380; (d) T. M. Harris and J. V. Hay, J. Am. Chem.
Soc., 1977, 99, 1631; (e) J. S. Hubbard and T. M. Harris, Tetrahedron
Lett., 1978, 47, 4601; (f) R. M. Sandifer, A. K. Bhattacharya and T. M.
Harris, J. Org. Chem., 1981, 46, 2260; (g) S. G. Gilbreath, C. M. Harris
and T. M. Harris, J. Am. Chem. Soc., 1988, 110, 6172.
Starting with 6a (0.237 g, 1.5 mmol) and 1a (0.429 g, 1.7 mmol), 7a
was isolated after chromatography (silica gel, heptanes–EtOAc)
◦
1
as a yellowish solid (0.144 g, 43%), mp. = 88–90 C. H NMR
(250 MHz, CDCl3): d = 2.63 (s, 3H, PhCH3), 3.79 (s, 3H, OCH3),
3
3.91 (s, 3H, OCH3), 6.78 (d, J = 8.7 Hz, 1H, CHAr), 7.76 (d,
3J = 8.9 Hz, 1H, CHAr), 10.98 (s, 1H, OH). 13C NMR (CDCl3,
75 MHz): d = 20.0 (CH3), 52.0, 52.5 (OCH3), 114.5 (CCOOCH3),
115.2 (CHAr), 123.9 (CCOOCH3), 135.8 (CHAr), 143.6 (CAr), 163.9
(COH), 168.0, 171.7 (CO). IR (KBr, cm-1): nmax = 3339 (w), 2989
(w), 2959 (w), 2924 (w), 2853 (w), 1715 (m), 1688 (m), 1651 (m),
1583 (m), 1537 (m), 1430 (m), 1386 (m), 1321 (m), 1243 (m), 1195
(s), 1151 (s), 1050 (m), 1018 (m), 960 (m), 944 (m), 858 (m), 797
(s), 754 (m), 707 (s), 652 (m), 560 (m). GC-MS (EI, 70 eV): m/z
(%) = 224 ([M]+, 31), 193 (30), 192 (100), 161 (56), 160 (26), 149
(13), 133 (12), 132 (12), 105 (10), 77 (15), 51 (11). HRMS (EI):
Calcd. for C11H12O5 ([M]+): 224.06792. Found: 224.067341.
5 I. Hussain, M. A. Yawer, B. Appel, M. Sher, A. Mahal, A. Villinger
and P. Langer, Tetrahedron, 2008, 64, 8003.
6 M. Sher, H. Reinke and P. Langer, Tetrahedron, 2007, 63, 4080.
7 T.-H. Chan and P. Brownbridge, J. Am. Chem. Soc., 1980, 102, 3534.
8 For a review of 1,3-bis(silyloxy)-1,3-butadienes, see: P. Langer, Synthe-
sis 2002, 441.
9 H. Feist, P. Langer, Synthesis 2007, 327.
10 (a) S. Cabiddu, C. Fattuoni, C. Floris, G. Gelli, S. Melis and F. Sotgiu,
Tetrahedron, 1990, 46, 861; (b) R. Fusco and F. Sannicolo, J. Org.
Chem., 1981, 46, 83; (c) J. T. Pinhey and P. T. Xuan, Aust. J. Chem.,
1988, 41, 69; (d) W. A. Bonner and J. I. De Graw, Tetrahedron, 1962,
18, 1295.
Acknowledgements
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Parker, D. M. Spero and K. A. Koziski, J. Org. Chem., 1987, 52, 183;
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Financial support from the State of Mecklenburg-Vorpommern
(scholarship for M. S.) is gratefully acknowledged.
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2186 | Org. Biomol. Chem., 2009, 7, 2182–2186
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