A practical synthesis of benzocrown ethers under phase-transfer catalysis conditions

Article abstract of DOI:10.1055/s-2002-34853

Convenient and effective procedures for the synthesis of benzo-12-crown-4, benzo-15-crown-5, benzo-18-crown-6, benzo-21-crown-7 and benzo-24-crown-8 under phase-transfer catalysis conditions have been developed.

Full text of DOI:10.1055/s-2002-34853

2266
PAPER
A Practical Synthesis of Benzocrown Ethers under Phase-Transfer Catalysis
Conditions
S
ynthesisof Be
a
nzocrow
n
E
t
ther
s
yana Bogaschenko, Stepan Basok, Catherine Kulygina, Alexander Lyapunov, Nikolay Lukyanenko*
A.V. Bogatsky Physico-Chemical Institute, National Academy of Sciences of Ukraine, 86 Lustdorfskaya doroga, Odessa 65080, Ukraine
Fax +38(48)2652012; E-mail: ngl@farlep.net
Received 24 May 2002; revised 20 July 2002
centrated alkaline solutions (usually 50% aq NaOH) or
Abstract: Convenient and effective procedures for the synthesis of
solid bases (NaOH, KOH, K2CO3) instead of expensive,
and sometimes dangerous, hydrides, amides and organic
benzo-12-crown-4, benzo-15-crown-5, benzo-18-crown-6, benzo-
21-crown-7 and benzo-24-crown-8 under phase-transfer catalysis
alkoxides in strictly anhydrous media, (ii) under PTC con-
ditions, the rates of nucleophilic reactions essentially in-
crease, that should result in an increase of the yield of
macrocyclic compounds; and (iii) the concentration of
ionic reagent in an organic phase or on a phase boundary
cannot appreciably exceed the concentration of phase-
transfer catalyst. Therefore, even at rather high concentra-
tions of reactants, it is possible to create conditions of
pseudo-high dilution by changing the catalyst concentra-
tion.
conditions have been developed.
Key words: benzocrown ethers, macrocycles, phase-transfer catal-
ysis, diols, ring closure
Crown ethers are an interesting class of macrocyclic host
compounds on account of their ability to bind inorganic
and organic cations and some neutral substrates strongly
and selectively.1–4 They also can be widely used as plat-
forms for the design of new macrocyclic host molecules.
As a result, several interesting new classes of molecules
including cryptands,4–6 rotaxanes and catenanes7–9 have
been developed. The modification of crown ethers by in-
troduction of the substituents allows variation of their
complexing properties and enables them to be incorporat-
ed them into polymeric backbones. It facilitates the prep-
aration of new macrocyclic receptors (e. g. lariat crown
ethers10) and polymers with unusual properties.11,12 In this
respect benzocrown ethers have advantages over their al-
iphatic analogs because of the possibility of easy introduc-
tion of the substituents in the benzene ring.
In continuation of our earlier work on the synthesis of
macrocyclic compounds under PTC conditions,16,17 we
herein present a practical synthesis of benzocrown ethers.
Four procedures, commonly used for benzocrown ethers
synthesis,3 were tested: (i) reaction of catechol (1) with
the corresponding oligo(ethylene glycol) ditosylates (pro-
cedure A), (ii) alkylation of bis(o-phenylene)glycol 2 with
oligo(ethylene glycol) ditosylates (procedure B), (iii) re-
action of bis(o-phenylene)glycol ditosylate 3 with oli-
go(ethylene glycol)s (procedure C), and (iv)
intramolecular ring closure of bis(o-phenylene)glycols
4a–d monotosylates, generated in situ (procedure D)
(Scheme 1). Most attractive is the one-step procedure A,
in which the readily available and inexpensive catechol
(1) and oligo(ethylene glycol) ditosylates were used as
starting materials. The procedures B–D require the prepa-
ration of the starting bis(o-phenylene)glycols 2 and 4a–d.
While methodologies for the construction of crown ethers
continue to generate considerable interest in synthetic
chemistry, their synthesis is often inefficient,3 as cycliza-
tion is always accompanied by side process of polycon-
densation. This means that macrocyclization reactions
had to be fast in order to maintain a very low concentra-
tion of reactants. Among the general strategies for ring
closure, the high dilution principle,13 the template
method3 and the cesium effect14 have proved to be most
profitable in that the formation of linear oligo- or poly-
condesation products is suppressed or minimized.
Compounds 2,18,19 4a,20,21 4b,22,23 and 4d23 were obtained
earlier by various, often multistep and time consuming
methods in moderate yields. To the best of our knowledge
the asymmetric phenylene glycol 4c, is not described ear-
lier.
In our opinion, good alternative to traditional methods are
the synthesis of macrocyclic compounds under phase-
transfer catalysis (PTC) conditions. Our hypothesis is
based on the following: (i) the reactions of anions gener-
ated by deprotonation of alcohols and phenols represent a
widely exploited part of PTC application for formation
new carbon–oxygen bonds.15 Such anions are obtained in
liquid-liquid or solid-liquid two-phase systems using con-
We have now developed a convenient and general ap-
proach to symmetric, as well as, asymmetric bis(ethyle-
neoxy) derivatives 4a–d of catechol (1). Compounds 4b
and 4d were produced in 82 and 74% yields, respectively,
by the reaction of catechol (1) with diethylene or triethyl-
ene glycol chlorohydrins in DMF in the presence of potas-
sium carbonate. The asymmetric glycols 4a and 4c were
synthesized in two stages via the intermediate compound
6 in 44 and 47% overall yield, respectively. The 1,2-bis(2-
hydroxyethoxy)benzene (2) was obtained in 90% yield,
by the reaction of catechol (1) with ethylene chlorohydrin
in an aqueous solution of sodium hydroxide (Scheme 2).
Synthesis 2002, No. 15, Print: 29 10 2002.
Art Id.1437-210X,E;2002,0,15,2266,2270,ftx,en;Z08302SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881

PAPER
Synthesis of Benzocrown Ethers
2267
n
As a preliminary to the synthesis of a series of ben-
zocrown ethers, we have analyzed the influence of exper-
imental conditions on the yield of benzo-15-crown-5 (5b),
prepared by the reaction of catechol (1) with ditosylate or
dihalides of triethylene glycol (Scheme 3, Table 1).
OH
TosO
O
OTos
OH
n = 2 - 6
A
1
2
3
OH
TosO
n
O
O
O
O
O
OH
OH
X
X
O
O
3
O
OTos
O
aq NaOH / MePh / (n-C4H9)4NI
n = 0 - 4
B
OH
O
O
O
O
a)
1
5b
n
OTos
HO
n
Scheme 3 Reaction conditions: reflux for 16 h; reactants concentra-
O
O
tion: 0.05 mol/L
5a n = 1
5b n = 2
5c n = 3
5d n = 4
5e n = 5
O
OH
n = 0 - 4
C
The gas chromatographic analysis of the reaction mixture
has shown noticeable increase of the content of crown
ether 5b in toluene during 15–16 hours. Further enhance-
ment of a reaction time up to 30 hours results in an in-
OTos
O
m
O
O
OH
appreciable augmentation (3–5%) of
a
product
TosCl
concentration, most probably because of thermal decom-
position of catalyst.15 We determined also, that the best re-
sults are reached at 0.05 mol/L concentration of reactants
and at a volume ratio of 1:6 of aqueous and organic phases
(Table 1).
OH
D
O
k
4a m = 1, k = 0; 4b m = k = 1;
4c m = 2, k = 1; 4d m = k = 2
Scheme 1 Reaction conditions: a) 50% aq NaOH, toluene, 25 mol%
TBAI, reflux, 16 h
Table 1 Synthesis of Benzo-15-crown-5 (5b)
Entry
Catalyst
(mol %)
NaOH
(%)
X
Yielda
(%)
OH
O
1
2
0
5
50
50
50
50
50
50
50
50
10
25
50
50
50
50
OTos
OTos
OTos
OTos
OTos
OTos
OTos
OTos
OTos
OTos
OTos
Cl
0
21
42
52
56
48
45
43
35
50
58
6
OH
aq NaOH
Cl
O
3
15
25
50
75
100
125
25
25
25
25
25
25
OH
2, 90%
4
m
m
O
O
5
O
O
O
OH
OH
OH
OH
Cl
6
DMF, K2CO3
OH
m
7
1
O
8
4b m = 1, 82%
4d m = 2, 74%
Cl
OH
9
O
DMF, K2CO3
10
11
12
13
14
O
O
OH
m
O
Cl
OH
OH
m
O
Br
52
24
DMF, K2CO3
4c m = 2, 91%
I
O
O
a Yield of isolated purified product.
O
OH
O
OH
Cl
OH
aq NaOH
O
OH
Crown ether 5b is not formed in the absence of catalyst
tetrabutylammonium iodide (TBAI). Isolated yields of 5b
increase when the catalyst concentration rise up to 50
mol%, and then slowly decrease (Table 1, entries 1–8).
OH
6, 52%
4a, 85%
Scheme 2
Synthesis 2002, No. 15, 2266–2270 ISSN 0039-7881 © Thieme Stuttgart · New York

2268
T. Bogaschenko et al.
PAPER
Most likely, in the presence of a large amount of catalyst, Let us remark, however, that similar phenomena have
the concentration of phenolic anions essentially increases, been observed also in the synthesis of aliphatic crown
which enhances the contribution of side reactions like ethers on changing the relative length of reacting glycols
polycondensation. In a concentration range of 25–50 and ditosylates.25,26 The procedures B and D yield practi-
mol% of the catalyst, the yield of 5b raised insignificant- cally identical results for the synthesis of benzocrown
ly. Maximum yield of crown ether 5b was achieved by ap- ethers 5b–d.
plication of a strong solution of sodium hydroxide (entries
In conclusion, the PTC conditions are very convenient
9–11) and of triethylene glycol tosylate as alkylating
and effective for the preparation of benzocrown ethers.
agent (entries 11–14).
We believe that the described approach may serve as a
Taking into consideration the obtained results, syntheses practical alternative to currently available methods for the
of benzocrown ethers 5a–e were performed in two-phase synthesis of different types of macrocycles.
system – 50% aqueous solution of sodium hydroxide/tol-
uene, in the presence of 25 mol% of TBAI and at a 1:6
1
Mps were determined in open capillaries and are uncorrected. H
volume ratio of aqueous and organic phases, at reflux tem-
perature for 16 hours. In all cases, the concentrations of
reactants were 0.05 mol/L (Scheme 1). The results sum-
marized in Table 2 demonstrate the universality of appli-
cation of PTC conditions for the preparation of
benzocrown ethers 5a–e.
NMR spectra were recorded on Bruker AM-250 and Bruker DR-
500 spectrometers at 250 and 500 MHz, respectively. Mass spectra
were obtained at 70 eV on a MX 1321 spectrometer. Oligo(ethylene
glycol) ditosylates and ditosylate 3 were prepared according to lit-
erature procedures.27 The structures of the all known products were
confirmed by comparison of their mps, analytical and spectral data
(MS, 1H NMR) with those reported in the literature.
Table 2 Preparation of Benzocrown Ethers 5a–e
2-[2-(2-Hydroxyethoxy)phenoxy]-1-ethanol (2)
A solution of NaOH (9.60 g, 240 mmol) in H2O (500 mL) was add-
ed in portions to a stirred mixture of catechol (1; 11.01 g, 100 mmol)
and ethylene chlorohydrin (19.32 g, 240 mmol) in H2O (200 mL)
under N2 taking care that the temperature of the reaction mixture did
not exceed 30 °C. When the addition was completed, the tempera-
ture was raised to 95 °C, and the mixture was stirred for further 1.5
h. After cooling of the mixture to 40 °C, H2SO4 (1 mL) was added.
The resulting mixture was stirred at 15 °C for 4 h and 2 was sepa-
rated by filtration, dried, and purified by recrystallization from
EtOH to yield 2 as white crystals (17.80 g, 90%); mp 89–91 °C
(Lit.18 mp 93–94 °C).
Entry Compounda,b Yield (%)c
Procedure Procedure Procedure Procedure
A
B
C
D
1
2
3
4
5
5a
5b
5c
5d
5e
18
58
77
22
15
13
60
69
56
77
5
7
12
8
61
68
68
6
MS (EI+): m/z = 198 (M+).
d
5
–
a Satisfactory microanalysis obtained: C 0.33; H 0.28.
b All products were >98% pure (GC).
c Yield of isolated, purified product.
d Not determined.
2-{2-[2-(2-Hydroxyethoxy)ethoxy]phenoxy}-1-ethanol (4a)
A solution of NaOH (7.20 g, 180 mmol) in H2O (38 mL) was added
in portions to a stirred mixture of 6 (29.70 g, 150 mmol), ethylene
chlorohydrin (14.49 g, 180 mmol) and H2O (30 mL) under N2 tak-
ing care that the temperature did not exceed 30 °C. The temperature
was raised to 95 °C within 2 h and the mixture was stirred for further
1.5 h. After cooling down to r.t., the aqueous layer was extracted
with CHCl3 (3 100 mL), and the organic solutions were combined
The procedure A yields good results, as compared with lit-
erature ones,24 for the synthesis of benzo-12-crown-4
(5a), benzo-15-crown-5 (5b) and benzo-18-crown-6 (5c). and dried (Na2SO4). The solvent was removed under reduced pres-
sure, and the residue purified by distillation under vacuum to afford
However, benzo-21-crown-7 (5d) and benzo-24-crown-8
4a (31.5 g, 85%) as a colorless liquid; bp 166–170 °C/1.33 mbar.
(5e) were obtained in moderate yields under these condi-
1H NMR (500 MHz, CDCl3): = 3.71 (m, 2 H), 3.73 (m, 2 H), 3.88
tions. The synthesis of crown ethers in single-phase sys-
(m, 2 H), 3.93 (m, 2 H), 4.06 (m, 2 H), 4.13 (m, 2 H), 4.32 (br s, 1
tems, usually suppose that the reason for the higher yield
of the smaller crown ethers in comparison to the large
H), 4.80 (br s, 1 H), 6.92 (m, 4 H).
MS (EI+): m/z = 242 (M+).
ones is due to the fact that cavity size of smaller macrocy-
cles better matches the ionic radius of the cation (Na+).14
Anal. Calcd for C12H18O5: C, 59.49; H, 7.49. Found: C, 59.32; H,
7.27.
This surmise is inconsistent with our results on a good
yield synthesis of 5d and 5e, according to procedures B or
C. Probably, under PTC conditions, the template effect of
cation influences the result of intramolecular ring closure
much smaller than in single-phase systems.
2-(2-{2-[2-(2-Hydroxyethoxy)ethoxy]phenoxy}ethoxy)-1-etha-
nol (4b)
A solution of diethylene glycol chlorohydrin (209 g, 1.68 mol) and
catechol (1; 66.6 g, 600 mmol) in anhyd DMF (690 mL), containing
K2CO3 (265.5 g, 1.9 mol) was stirred at 95–100 °C for 10 h under
N2. After cooling down to r.t., the mixture was filtered and concen-
trated to dryness under reduced pressure. The residue was poured
into H2O (400 mL), and extracted with CHCl3 (4 200 mL). The
combined organic phases were dried (Na2SO4), and the solvent was
Procedure B, in all examined cases gave benzocrown
ethers in good yield. Therefore, the poor yields of 5a–e,
according to the alternate procedure C, which, essentially,
is unsymmetric to the procedure B, is rather surprising.
We do not have any clear explanations now on this fact.
Synthesis 2002, No. 15, 2266–2270 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Synthesis of Benzocrown Ethers
2269
evaporated. The residue was distilled under vacuum; yield: 140.7 g
(82%); colorless liquid; bp 210–212 °C/1.33 mbar.
1H NMR (500 MHz, CDCl3): = 2.98 (br s, 2 H), 3.66 (m, 4 H),
3.73 (m, 4 H), 3.88 (m, 4 H), 4.14 (m, 4 H), 6.89 (m, 4 H).
ture was vigorous stirred at reflux for 16 h. The organic layer was
separated and washed with H2O (3 200 mL), brine (100 mL), and
dried (MgSO4). The solvent was removed under vacuum, and the
residue was extracted with boiling hexane (3 150 mL). After cool-
ing, the pure products were isolated as colorless crystals (5a–c) or
oils (5d, 5e).
MS (EI+): m/z = 286 (M+).
Anal. Calcd for C14H22O6: C, 58.73; H, 7.74. Found: C, 58.82; H,
7.86
Procedure D: A solution of p-toluenesulfonyl chloride (50 mmol)
and the appropriate glycol 4a–d (50 mmol) in toluene (300 mL) was
added dropwise within 2.5 h to a vigorously stirred mixture of
Bu4NI (1.25 mmol), toluene (300 mL) and 50% aq NaOH (100 mL)
maintained at 70–75 °C. The mixture was stirred for a further 13 h
at this temperature. The organic layer was separated, filtered,
washed with H2O (3 200 mL), brine (100 mL), and dried
(MgSO4). The pure compounds 5a–d were obtained as describe
above.
2-[2-(2-{2-[2-(2-Hydroxyethoxy)ethoxy]ethoxy}phenoxy)-
ethoxy]-1-ethanol (4c)
A mixture of 6 (89.10 g, 450 mmol), triethylene glycol chlorohydrin
(84.30 g, 500 mmol) and K2CO3 (93.15 g, 675 mmol) in anhyd
DMF (600 mL) was stirred at 95–100 °C for 10 h under N2. After
cooling down to r.t., the mixture was filtered, and concentrated to
dryness under reduced pressure. The residue was poured into H2O
(400 mL), and extracted with CHCl3 (4 200 mL). The combined
organic phases were dried (Na2SO4), and the solvent was evaporat-
ed. The residue was distilled in vacuum to afford 4c; yield: 135.1 g
(91%); colorless liquid; bp 220–222 °C/0.23 mbar.
Analytical and spectral data (MS, 1H NMR) of benzocrown ethers
5a–d were in good agreement with the data reported.24,28
References
1H NMR (500 MHz, CDCl3): = 3.34 (br s, 2 H), 3.59 (m, 2 H),
3.69 (m, 4 H), 3.72 (m, 2 H), 3.76 (m, 4 H), 3.90 (m, 4 H), 4.17 (m,
4 H), 6.91 (m, 4 H).
(1) Pedersen, C. J. Angew. Chem. Int. Ed. 1988, 27, 1021.
(2) Vögtle, F.; Weber, E. In The Chemistry of Ethers, Crown
Ethers, Hydroxyl Groups and Their Sulphur Analogues;
Patai, S., Ed.; Wiley: Chichester, 1980, 59–156.
(3) Laidler, D. A.; Stoddart, J. F. In The Chemistry of Ethers,
Crown Ethers, Hydroxyl Groups and Their Sulphur
Analogues; Patai, S., Ed.; Wiley: Chichester, 1980, 1–58.
(4) Viout, P.; Lehn, J.-M. Macrocyclic Chemistry; VCH: New
York, 1993.
MS (EI+): m/z = 330 (M+).
Anal. Calcd for C16H26O7: C, 58.17; H, 7.93. Found: C, 57.92; H,
7.83
2-{2-[2-(2-{2-[2-(2-Hydroxyethoxy)ethoxy]ethoxy}phenoxy)-
ethoxy]ethoxy}-1-ethanol(4d)
(5) Vögtle, F. Supramolecular Chemistry; Wiley: New York,
1991.
(6) Lehn, J.-M. Supramolecular Chemistry; VCH: New York,
1995.
Following the method described for compound 4b, the title com-
pound was obtained from the catechol (1; 66.6 g, 600 mmol) and tri-
ethylene glycol chlorohydrin (222.0 g, 1.32 mol); yield: 165.5 g
(74%); colorless liquid; bp 220–225 °C/0.13 mbar.
(7) Fyfe, M. C. T.; Stoddart, J. F. Adv. Supramol. Chem. 1999,
5, 1.
(8) Cantrill, S. J.; Pease, A. R.; Stoddart, J. F. J. Chem. Soc.,
Dalton Trans. 2000, 3715.
(9) Amabilino, D. B.; Stoddart, J. F. Chem. Rev. 1995, 95, 2725.
(10) Gokel, G. W. Chem. Soc. Rev. 1992, 21, 39.
(11) Gibson, H. W. Macromol. Chem. Phys. 2000, 201, 815.
(12) Kakuchi, T.; Watanabe, T.; Matsunami, S.; Kamimura, H.
Polymer 1997, 38, 1233.
(13) Knops, P.; Sendhoff, N.; Mekelburger, H.-B.; Vögtle, F.
Top. Curr. Chem. 1992, 161, 3.
(14) Ostrowicki, A.; Koepp, E.; Vögtle, F. Top. Curr. Chem.
1992, 161, 39.
(15) Dehmlow, E. V.; Dehmlow, S. S. Phase Transfer Catalysis;
VCH: Weinheim, 1993.
(16) Bogatsky, A. V.; Lukyanenko, N. G.; Basok, S. S.;
Ostrovskaya, L. K. Synthesis 1984, 138.
1H NMR (500 MHz, CDCl3): = 2.82 (br s, 2 H), 3.59 (m, 4 H),
3.67 (m, 4 H), 3.70 (m, 4 H), 3.74 (m, 4 H), 3.87 (m, 4 H), 4.17 (m,
4 H), 6.90 (m, 4 H).
MS (EI+): m/z = 374 (M+).
Anal. Calcd for C18H30O8: C, 57.74; H, 8.08. Found: C, 57.82; H,
7.93
2-[2-(2-Hydroxyethoxy)ethoxy]phenol (6)
A mixture of catechol (1; 66.6 g, 600 mmol), diethylene glycol
chlorohydrin (104.5 g, 840 mmol), and K2CO3 (130.5 g, 950 mmol)
in anhyd DMF (700 mL) was stirred at 95–100 °C for 10 h. After
cooling, it was filtered and concentrated to dryness under reduced
pressure. The residue was poured into H2O (400 mL), and the aque-
ous layer was extracted with CHCl3 (4 200 mL). The combined
organic phases were dried (Na2SO4), and the solvent was evaporat-
ed. The residue was purified by distillation under vacuum to afford
6 (61.7 g, 52%); colorless liquid; bp 145–150 °C/1.33 mbar.
(17) Lukyanenko, N. G.; Basok, S. S.; Filonova, L. K. J. Chem.
Soc., Perkin Trans. 1 1988, 3141.
(18) Hodgkinson, L. C.; Sutherland, I. O. J. Chem. Soc., Perkin
Trans. 1 1979, 1908.
(19) Topal, G.; Demire, N.; Togrul, M.; Turgut, Y.; Hosgoren, H.
J. Heterocycl. Chem. 2001, 38, 281.
1H NMR (500 MHz, CDCl3): = 3.71 (m, 2 H), 3.78 (m, 2 H), 3.88
(m, 2 H),3.93 (m, 2 H), 4.06 (m, 2 H), 4.13 (m, 2 H), 4.32 (br s, 1
H), 4.80 (br s, 1 H), 6.92 (m, 4 H).
MS (EI+): m/z = 198 (M+).
(20) Weber, E. Chem. Ber. 1985, 118, 4439.
(21) Draye, M.; Favre-Reguillon, A.; Chomel, R.; Faure, R.;
Guy, A.; Foos, J.; Lemaire, M. Bull. Soc. Chim. Fr. 1996,
133, 183.
(22) Kyba, E. P.; Helheson, R. C.; Madan, K.; Gokel, G. W.;
Tarnowski, T. L.; Cram, D. G. J. Am. Chem. Soc. 1977, 99,
2564.
Anal. Calcd for C10H14O4: C, 60.59; H, 7.12. Found: C, 60.42; H,
6.96.
Benzocrown Ethers 5a–e; General Procedures
Procedures A–C: To a stirred solution of catechol (1; 50 mmol) or
glycol 2 (50 mmol) or the appropriate oligo(ethyleneglycol) (50
mmol) and Bu4NI in toluene (300 mL) was added 50% aq NaOH
(100 mL) at 50–60 °C. The mixture was stirred at this temperature
for further 30 min, whereupon a solution of corresponding ditosyl-
ate (50 mmol) in toluene (300 mL) was added. The resulting mix-
(23) Czech, B.; Czech, A.; Bartch, R. A. J. Heterocycl. Chem.
1984, 21, 341.
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T. Bogaschenko et al.
PAPER
(26) Ouchi, M.; Inoue, Y.; Kanzaki, T.; Hakushi, T. J. Org.
(24) Gokel, G. W.; Korzeniowski, S. H. Macrocyclic Polyether
Syntheses; Springer: Berlin, 1982.
Chem. 1984, 49, 1408.
(25) Gibson, H. W.; Bheda, M. C.; Engen, P.; Shen, Y. X.; Sze,
H.; Zhang, H.; Gibson, M. D.; Delaviz, Y.; Lee, S.-H.; Liu,
S.; Wang, L.; Nagvekar, D.; Rancourt, J.; Taylor, L. T. J.
Org. Chem. 1994, 59, 2186.
(27) Ouchi, M.; Inoue, Y.; Lu, Y.; Nagamune, S.; Nakamura, S.;
Wada, K.; Hakushi, T. Bull. Chem. Soc. Jpn. 1990, 63, 1260.
(28) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 7017.
Synthesis 2002, No. 15, 2266–2270 ISSN 0039-7881 © Thieme Stuttgart · New York