Highly enantioselective epoxidation of unfunctionalized olefins catalyzed by a novel recyclable chiral poly-salen-Mn(III) complex

Article abstract of DOI:10.1039/b201294b

A new soluble recyclable poly-salen-Mn(III) complex has been synthesized and found to give high enantioselectivities (up to 97% ee) for the asymmetric epoxidation of unfunctionalized olefins.

Full text of DOI:10.1039/b201294b

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Highly enantioselective epoxidation of unfunctionalized olefins
catalyzed by a novel recyclable chiral poly-salen–Mn(III) complex
Yuming Song, Xiaoquan Yao, Huilin Chen, Guizhi Pan, Xinquan Hu and Zhuo Zheng*
1
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023,
P. R. China
Received (in Cambridge, UK) 4th February 2002, Accepted 15th February 2002
First published as an Advance Article on the web 6th March 2002
A new soluble recyclable poly-salen–Mn() complex has been synthesized and found to give high enantioselectivities
up to 97% ee) for the asymmetric epoxidation of unfunctionalized olefins.
(
Introduction
aldehydes and (R,R)-cyclohexane-1,2-diamine have been
reported. Unlike the modified catalysts mentioned above,
these two poly-salen–Mn() complexes can be regarded as
polymers of “monomer” salen–Mn() catalyst, which, how-
ever, showed somewhat reduce enantioselectivities relative to
Jacobsen’s catalyst (90% ee for 6-cyano-2,2-dimethylchromene
Great progress has been achieved in the area of asymmetric
catalysis over the last decade. Salen–Mn() complex catalyzed
asymmetric epoxidation of unfunctionalized olefins has proved
to be a most useful reaction, and the most used Salen–Mn()
complex (Jacobsen’s catalyst, Fig. 1) exhibits excellent activities
1
0c). However, both catalysts could be easily recovered and
reused. Considering that Jacobsen’s catalyst has two hindered
tert-butyl groups at the 5,5Ј positions, which are considered
very important in introducing high ee during enantioselective
epoxidation, while our earlier polymers have only –CH – or
2
–
CH –O–CH – groups at the same position, we sought to
2 2
introduce two geminal methyl groups on the linking carbon
atom, with the expectation of achieving the same enantio-
selectivities as Jacobsen’s catalyst.
Fig. 1 Jacobsen’s catalyst.
Results and discussion
1
and enantioselectivities for libraries of substrates. However,
Synthesis of the polymer catalyst
salen–Mn() complexes undergo decomposition under the
reaction conditions and cannot be recovered and re-used, there-
fore many groups have directed their research to modifications
of typical salen–Mn() complexes, such as polymer-supported
The bisphenol 3 can be prepared in 50% yield by the con-
densation of 2-tert-butylphenol 2 and acetone catalyzed by
8
H S–HCl at ambient temperature. No ether linkage product
2
2
systems; non-covalent immobilization on zeolites, clays, or
was found in the condensation. The bis-salicylaldehyde 4 was
readily prepared in 40% yield via a modified Duff reaction, and
the corresponding polymeric ligand 5 with an average molec-
ular weight up to 19893 (GPC, Mn) was synthesised through
the condensation of 4 and (R,R)-cyclohexane-1,2-diamine via a
3
siloxane membranes; grafted catalyst systems onto silica or
4
5
MCM-41, and FBS (fluorous biphase systems). Although
these modified catalysts showed very good separation features,
their catalytic efficiencies seldom reached that of the original
salen–Mn() complex. Recent work reported by Seebach’s
9
modified procedure for synthesis of salen ligands. The poly-
6
group, however makes a major contribution in this respect.
salen–Mn() catalyst 1, which is very soluble in dichloro-
methane, slightly soluble in ether and insoluble in hexane, was
prepared according to the same procedure as our previous
In our previous work involving the enantioselective epoxid-
7
ation of unfunctionalized olefins, two soluble poly-salen–
7
Mn() complexes derived from two substituted bis-salicyl-
polymer catalysts (Scheme 1).
Scheme 1 Synthetic route for the novel poly-salen–Mn() complex.
8
70
J. Chem. Soc., Perkin Trans. 1, 2002, 870–873
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b201294b

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a
b
Table 1 The poly-salen–Mn() complex catalysed enantioselective epoxidation with NaClO–4-PPNO and MCPBA–NMO
c
d
e
Entry
Substrate
Product
T /ЊC
Oxidant
t/h
Yield (%)
Ee (%)
Configuration
1
2
3
4
5
6
7
8
9
0
1
2
3
6
7
0
Ϫ22
Ϫ78
0
NaClO–4-PPNO
MCPBA–NMO
MCPBA–NMO
NaClO–4-PPNO
MCPBA–NMO
NaClO–4-PPNO
MCPBA–NMO
NaClO–4-PPNO
MCPBA–NMO
NaClO–4-PPNO
MCPBA–NMO
NaClO–4-PPNO
MCPBA–NMO
2.5
0.5
1
3
0.5
2
0.5
5.5
0.5
6
0.5
4
86
83
ND
37
51
36
88
89
96
97
95
97
96
96
94
95
R
R
R
6
7
g
6
7
f
8
9
87
87
81
73
90
92
96
89
86
83
1S, 2R
1S, 2R
3R, 4R
3R, 4R
f
8
9
0
0
0
0
0
0
0
0
10a
10a
10b
10b
10c
10c
10d
10d
11a
11a
11b
11b
11c
11c
11d
11d
g
NDg
ND
1
1
1
1
3R, 4R
3R, 4R
3R, 4R
3R, 4R
0
0.5
a
The reactions were carried out in 2 ml of CH Cl with 1.0 mmol of olefin substrate. The mole ratio of substrate–NaClO–4-PPNO–catalytic unit was
: 2 : 0.2 : 0.04. TLC was used to detect the total conversion. The reaction was carried out in 8 ml of CH Cl with 1.0 mmol of olefin substrate. The
2 2
c d
2
2
b
1
mole ratio of substrate–MCPBA–NMO–catalytic unit was 1 : 2 : 5 : 0.04. TLC was used to detect the total conversion. Isolated yield. The ee’s for
the epoxides were determined by GC using a chiral capillary column (cyclodex-β,2,3,6-methylated, 30 m × 0.25 mm id; carrier, H for 2,2-
2
e
10
dimethylchromene oxides and N for styrene oxide and (Z)-β-methylstyrene oxides). Absolute configurations were compared with the literature.
2
f
g
Overall yield of (Z)-β-methylstyrene oxides and the ee of cis products. ND, not determined.
Poly-salen–Mn(III) complex 1 catalyzed epoxidation of
unfunctionalized olefins
Table
2
Recycling enantioselective epoxidation a of 2,2-dimethyl-
chromene 10a using NaClO–4-PPNO as the oxidant
b
The epoxidation process was performed following typical
Entry
Cycle
t/h
Isolated yield (%)
Ee (%)
Jacobsen’s procedures using either NaClO–4-PPNO or
10
MCPBA–NMO (Scheme 2). The results are summarized in
1
2
3
4
5
Fresh
1.5
1.5
1.5
2
81
77
75
70
63
96
95
94
96
95
1
2
3
4
2
a
The reactions were carried out in 8 ml of CH Cl with 4.0 mmol of
2
2
1
0a. The mole ratio of 10a–NaClO–4-PPNO–catalytic unit was 1 : 2 :
.2 : 0.04. The aqueous NaClO was added dropwise. After the total
0
coversion (as detected with TLC), the aqueous layer and most of the
solvent were removed. The catalyst was recovered by precipitation from
b
the mixture using hexanes and drying in vacuo. The ee’s for the
epoxides were determined by GC using a chiral capillary column
(
cyclodex-β,2,3,6-methylated, 30 m × 0.25 mm id; carrier, H ).
2
showed much lower activity and enantioselectivity for styrene
oxide under these conditions (entry 3, Table 1). It is possible
that the linear polymer catalyst is in a rigid and contracted form
at Ϫ78 ЊC. Indeed some direct evidence was that solid catalyst
granules could be observed during the reaction. Therefore, it
was impossible to re-use the poly-salen–Mn() catalyst 1 with
the MCPBA–NMO system.
Scheme 2 Enantioselective epoxidation catalyzed by poly-salen–
Mn() catalyst 1.
Recycling of the polymer catalyst 1
Re-use of the recovered poly-salen–Mn() catalyst 1 was per-
formed with the NaClO–4-PPNO system employing 2,2-
dimethylchromene (10a) as the substrate and the results of five
cycling experiments are listed in Table 2. Despite the possible
loss of catalyst during the recovery and re-use process, the
cycling experiments were successfully carried out up to four
times. It proved very important to add the pre-cooled aqueous
NaClO dropwise, otherwise there was 2–4% drop in ee if
aqueous NaClO was added in one portion. All reactions were
complete within 2 hours. The data in Table 2 show that the
recovered catalyst maintains its enantioselectivity but there is a
small loss in the yield of the epoxide in each cycle, suggesting
that some catalyst decomposed.
Table 1. In the NaClO–4-PPNO system, the poly-salen–Mn()
catalyst 1 exhibited excellent catalysis under typical conditions.
Almost the same enantioselectivities were obtained as those
using Jacobsen’s catalyst for (Z)-β-methylstyrene 8 and substi-
tuted 2,2-dimethylchromenes 10 as epoxidation substrates
(
88–96% ee; for the typical Jacobsen’s catalyst, 88–98% ee could
be obtained). The catalyst could be recovered as an oil layer by
precipitation from the reaction mixture with hexanes and used
for the next reaction without further purification. In the
MCPBA–NMO system, excellent ee’s could also be obtained at
0
ЊC for (Z)-β-methylstyrene 8 and substituted 2,2-dimethyl-
chromenes 10 as epoxidation substrates (89–97% ee). The best
result for styrene oxide was obtained with 51% ee at Ϫ22 ЊC
(
entry 3, Table 1), which is much better than that from the
Conclusions
NaClO–4-PPNO system (37% ee, entry 1, Table 1). However,
obvious decomposition of the poly-salen–Mn() catalyst 1 in
the MCPBA–NMO system was observed, the decomposed
species from the catalyst being detected by TLC. Lowering the
reaction temperature to Ϫ78 ЊC to slow down the potential
decomposition of the catalyst was helpful, but the catalyst
In conclusion, we have prepared a novel tertiary carbon-linked
poly-salen–Mn() catalyst, which exhibits activities and
enantioselectivities as good as the Jacobsen’s homogeneous
catalyst, and this catalyst can be easily recovered and reused
several times without loss of enantioselectivity.
11
J. Chem. Soc., Perkin Trans. 1, 2002, 870–873
871

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Experimental
of EtOH–H
2
O (5 : 1) were placed in a 500 ml three-necked
round-bottomed flask equipped with a reflux condenser and an
addition funnel. The mixture was stirred and heated to 80 ЊC
under Ar, after which a solution of 5.0 g of 4 (12.6 mmol) in
General
Melting points were measured on a Yazawa micro melting point
apparatus (uncorrected). Optical rotations were measured on a
100 ml of ethanol and 10 ml of CH Cl was added dropwise
2 2
Horiba SEPA-200 highly sensitive polarimeter. [α] values are
over 45 min through the funnel, and the funnel was washed with
50 ml of ethanol. The resulting yellow suspension was refluxed
for 6 h. Water (100 ml) was added to dilute the mixture, which
was then cooled to room temperature. The solid was collected
by suction filtration, the wet cake was carefully washed with
20 ml portions of ethanol until the filtrate was colorless. The
solid was dissolved in 50 ml of CH Cl and washed with brine
D
Ϫ1
2
Ϫ1
1
given in units of 10 deg cm g . The H NMR spectra were
recorded on a Bruker DRX 400 system with TMS as an internal
standard. M and M /M were measured on a PL-GPC210
n
w
n
instrument. Enantiomeric excesses (%ee) were determined by
GC (HP 4890 series) analysis.
2
2
Materials
(50 ml × 3). The organic layer was dried over anhydrous
Na SO and the solvent was removed under reduced pressure.
The resulting yellow powder was washed with 50 ml portions of
n-hexane until the hexane washings were colorless. The solid
2
-tert-Butylphenol, MCPBA and 4-PPNO (4-phenylpyridine
2
4
N-oxide) were obtained from Acros and (Z)-β-methylstyrene
was obtained from TCI. Substituted 2,2-dimethylchrom-
enes and (R,R)-cyclohexane-1,2-diyldiammonium mono-(ϩ)-
tartrate salt were prepared according to the reported method.
Other materials were laboratory grade species from local
was dried in vacuo to afford 4.4 g of polymer ligand 4. The yield
1
12
was 74%. H NMR (CDCl ): δ 1.33 (s, 18H), 1.45 (s, 6H), 1.32–
3
2
.00 (m, 8H), 3.31 (m, 2H), 6.90 (d, J = 2.0 Hz, 2H), 7.60 (d,
13
J = 2.0 Hz, 2H), 8.25 (s, 2H), 13.75 (s, 2H); C NMR: δ 24.26,
suppliers. All solvents were purified by standard procedures.
2
9.55, 30.97, 33.28, 34.83, 41.63, 72.23, 117.80, 127.26, 128.43,
25
2
,2Ј-Di-tert-butyl-4,4Ј-(propane-2,2-diyl)diphenol 3
136.23, 139.41, 158.10, 165.66; [α] = 156.34 (c 1.014, CH Cl );
D
2 2
GPC: M = 134640, M = 122711, M = 19893, M = 429682,
p
w
n
z
Dry gaseous H S was bubbled through 77.4 g of 2 (0.52 mol)
and 10.9 g of acetone (0.19 mol) in 100 ml petroleum ether at
2
M /M = 6.169.
w
n
2
0 ЊC until the mixture was saturated, after which gaseous HCl
Poly-salen–Mn(III) catalyst 1
MnCl ؒ4H O (1.50 g, 7.58 mmol) and 20 ml of methanol were
was continuously bubbled through for three days at 22–23 ЊC.
The mixture was filtered, the isolated solid was washed with
water (50 ml × 4) and petroleum ether (50 ml × 4) and dried
in vacuo as a first portion of the product. The filtrate was
washed with saturated aqueous NaHCO (100 ml × 3) and
brine (100 ml × 3), and dried over Na SO . After removal of the
solvent from the filtrate, 2-tert-butylphenol was collected by
vacuum distillation. The residue was recrystallized to afford
a second portion of the product powder. A combined yield
of 32 g of pure 3 was obtained. The yield was 50% based on
acetone. Mp 114–116 ЊC; H NMR (CDCl ): δ 1.35 (s, 18H),
2
2
placed in a 100 ml three-necked round-bottomed flask equipped
with a reflux condenser and an addition funnel. The mixture
was heated to reflux until the solid was completely dissolved,
after which a solution of 1.20 g of 5 (2.53 mmol) in 25 ml of
toluene was added dropwise. When the addition was complete,
air was bubbled through for 2 h, and the resulting black mixture
was refluxed for further 4 h then cooled to room temperature.
The solution was washed with 50 ml of brine and the organic
3
2
4
1
layer was dried over anhydrous Na
removed under reduced pressure. The solid obtained was dis-
solved in 30 ml of CH Cl and washed with brine (30 ml × 3),
the organic layer was dried over anhydrous Na SO , the solvent
SO . The solvent was
3
2 4
1.62 (s, 6H), 6.52 (d, J = 4.2 Hz, 2H), 6.91 (dd, J = 2.7 Hz, J =
1 2
13
8
3
.2 Hz, 2H), 7.12 (d, J = 2.3 Hz, 2H); C NMR δ 29.64, 31.19,
4.62, 42.09, 115.84, 125.02, 125.67, 134.97, 142.84, 151.70.
2
2
2
4
was removed under reduced pressure, and the solid was dried
in vacuo to afford 1.49 g of the polymer catalyst 1 as a black
powder. The yield was 84%.
5
,5Ј-Di-tert-butyl-6,6Ј-dihydroxy-3,3Ј-(propane-2,2-diyl)-
dibenzaldehyde 4
4.0 g of 3 (0.10 mol), 56.0 g of hexamethylenetetramine
0.40 mol) and 100 ml of glacial acetic acid were placed in a
00 ml three-necked round-bottomed flask equipped with a
3
(
5
10
General procedures for enantioselective epoxidation
Method A. A solution of 1.0 mmol of olefin in 2.0 ml of
mechanical stirrer. The mixture was heated slowly to 130 ЊC
under Ar, during which it became turbid then viscous, with the
subsequent formation of a large quantity of solid material.
When 3 was completely consumed (as monitored by TLC), the
CH Cl , 0.0225 g of catalyst (0.04 mmol, based on single
2
2
catalytic unit) and 0.0342 g of 4-PPNO (0.20 mmol) was
cooled to 0 ЊC and stirred for 30 min. Pre-cooled buffered
bleach (3.7 ml, 2.0 mmol, 0.55 M, pH = 11.3, 0 ЊC) was added to
the solution and the mixture was stirred at 0 ЊC until total
conversion of the substrate olefin was achieved, as monitored
by TLC. The phases were separated and the aqueous layer was
extracted with CH Cl (2.0 ml × 3). The combined organic
mixture was cooled to 75 ЊC. 33% H SO (100 ml) was then
2
4
added; and the stirred mixture was heated to reflux for an hour
and cooled to room temperature. EtOAc (100 ml) was added to
dissolve the solid, the organic layer was separated and the
aqueous layer was extracted with EtOAc (50 ml × 3). The
organic layers were combined and washed with saturated aque-
2
2
layers were washed with brine (2.0 ml × 3) and dried over
anhydrous Na SO . The epoxide was separated by flash column
2
4
ous NaHCO until neutral. The organic layer was dried over
anhydrous Na SO . The solvent was removed under reduced
3
chromatography.
2
4
pressure, and the residue was purified on a 100 g silica gel
column, eluent ethyl acetate–petroleum ether (1 : 60). The crude
product was recrystallized from EtOH to afford 16.0 g of the
Method B. A solution of 1.0 mmol of olefin, 0.0225 g of
catalyst (0.04 mmol, based on single catalytic unit) and 0.675 g
of NMO (N-methylmorpholine N-oxide) (5.0 mmol) in 8 ml
CH Cl , was cooled to the appropriate temperature for 30 min,
bis-salicylaldehyde 4 as yellow powder. The yield was 40%. Mp
2
2
1
1
7
2
1
20–122 ЊC. H NMR (CDCl ): δ 1.34 (s, 18H), 1.70 (s, 6H),
3
after which pre-cooled solid MCPBA (0.345 g, 2.0 mmol) was
added in portions over 5 minutes. When the reaction was com-
plete (as monitored by TLC), NaOH (10 ml, 1.0 M) was added,
the phases were separated and the aqueous layer was extracted
with CH Cl (5.0 ml × 3). The combined organic layers were
.31 (dd, J = 2.4 Hz, J = 8.8 Hz, 4H), 9.85 (s, 2H), 11.72 (s,
H); C NMR δ 29.17, 30.79, 34.91, 42.09, 119.90, 128.83,
33.43, 137.90, 140.57, 159.42, 197.16.
1
2
13
2
2
Polymer ligand
washed with brine (10 ml × 2) and dried over anhydrous
Na SO . The pure product was obtained by flash column
(
R,R)-Cyclohexane-1,2-diammonium mono-(ϩ)-tartrate salt
2
4
(
3.4 g, 12.9 mmol), 3.6 g of K CO (26.0 mmol) and 200 ml
chromatography.
2
3
8
72
J. Chem. Soc., Perkin Trans. 1, 2002, 870–873

View Article Online
Modified Jacobsen’s procedure for the recycling of 2,2-dimethyl-
chromene
in Catalytic Asymmetric Synthesis, ed. I. Ojima, VCH Publishers,
New York, 2000, 2nd edn., p. 287.
2
(a) L. Canali, E. Cowan, H. Deleuze, C. L. Gibson and
D. C. Sherrington, Chem. Commun., 1998, 2561; (b) T. S. Reger
and K. D. Janda, J. Am. Chem. Soc., 2000, 122, 6929; (c) C. E. Song,
E. J. Roh, B. M. Yu, D. Y. Chi, S. C. Kim and K. J. Lee,
Chem. Commun., 2000, 615; (d ) L. Canali, E. Cowan, H. Deleuze,
G. L. Gibson and D. C. Sherrington, J. Chem. Soc., Perkin Trans. 1,
The procedure is similar to that described above except that 10a
(
(
0.64 g, 4.0 mmol) is employed and pre-cooled buffered bleach
14.6 ml, 8.0 mmol, 0.55 M, pH = 11.3, 0 ЊC) was added drop-
wise. After removal of most of the solvent, hexane (25 ml) was
added to the stirred mixture, and the hexane layer was decanted
off. The residue was washed with hexane (5 ml × 3), and dried
in vacuo to recover the catalyst. Further purification of the
hexane layers afforded the olefin oxide.
2
000, 2055.
3 (a) S. Ogunwumi and T. Bein, Chem. Commun., 1997, 901; (b) K. B.
M. Jannsen, I. Laquiere, W. Dehaen, R. F. Parton, I. F. J.
Vankelecom and P. A. Jacobs, Tetrahedron: Asymmetry, 1997, 8,
3
481.
4
(a) M. J. Sabater, A. Corma, A. Domenech, V. Fornes and
H. Garcia, Chem. Commun., 1997, 1285; (b) D. Pini, A. Mandoli,
S. Orlandi and P. Salvadori, Tetrahedron: Asymmetry, 1999, 10,
Determination of enantiomeric excesses
The following are the retention times for racemic products.
3
883; (c) G.-J. Kim and J.-H. Shin, Tetrahedron Lett., 1999, 40,
827; (d ) X.-G. Zhou, X.-Q. Yu, J.-S. Huang, S.-G. Li, I.-S. Li and
(
1
a) Chiral capillary GC column: (Cyclodex-β,2,3,6-methylated,
5 m × 0.25 mm id), carrier = N , β-methylstyrene oxides,
6
2
C.-M. Che, Chem. Commun., 1999, 1789; (e) P. Piaggo, C. Langham,
P. McMorn, D. Bethell, P. C. Bulman-Page, F. E. Hancock, C. Sly
and G. Hutchings, J. Chem. Soc., Perkin Trans. 2, 2000, 143;
( f ) P. Piaggo, P. McMorn, D. Murphy, D. Bethell, P. C. B. Page,
F. I. Hancock, K. O. J. Sly and G. J. Hutchings, J. Chem. Soc.,
Perkin Trans. 2, 2000, 2008.
T = 110 ЊC, t = 5.04 min, t = 5.23 min, t = 5.78 min, t =
1
2
3
4
6
.48 min. (b) Chiral capillary column (Cyclodex-β,2,3,6-
methylated, 30 m × 0.25 mm id), carrier = N , styrene oxides,
2
T = 100 ЊC, t = 16.25 min, t = 16.83 min. (c) Chiral capillary
R
S
column (Cyclodex-β,2,3,6-methylated, 30 m × 0.25 mm id):
carrier = H , 2,2-dimethylchromene oxides, T = 120 ЊC, t
5
(a) G. Pozzi, F. Cinato, F. Montanari and S. Quici, Chem. Commun.,
=
2
SS
1
998, 877; (b) G. Pozzi, M. Cavazzini, F. Cinato, F. Montanari
1
3.63 min, tRR = 14.61 min; 6-bromo-2,2-dimethylchromene
oxides, T = 165 ЊC, t = 9.87 min, t = 10.28 min; 6-cyano-2,2-
and S. Quici, Eur. J. Org. Chem., 1999, 1947; (c) M. Cavazzini,
A. Manfredi, F. Montanari, S. Quici and G. Pozzi, Chem. Commun.,
2000, 2171.
1
2
dimethylchromene oxides, T = 175 ЊC, tSS = 13.39 min, tRR
=
1
3.99 min; 6-nitro-2,2-dimethylchromene oxides, T = 175 ЊC,
6 H. Sellner, J. K. Karjalainen and D. Seebach, Chem. Eur. J., 2001, 7,
2
873.
tSS = 21.16 min, tRR = 22.18 min.
7
X. Yao, H. Chen, W. Lü, G. Pan, X. Hu and Z. Zheng, Tetrahadron
Lett., 2000, 41, 10267.
A. J. Dietzler, US Pat. 2917550/1959.
8
9
Acknowledgements
J. F. Larrow and E. N. Jacobsen, J. Org. Chem., 1994, 59, 1939.
We are grateful to S. Liao for valuable discussions. This work
was supported by the National Natural Science Foundation of
China (29933050).
10 (a) E. N. Jacobsen, W. Zhang, A. R. Muci, J. R. Ecker and L. Deng,
J. Am. Chem. Soc., 1991, 113, 7063; (b) M. Palucki, P. J. Pospisil,
W. Zhang and E. N. Jacobsen, J. Am. Chem. Soc., 1994, 116, 9333.
1
1 D. L. Hughes, G. B. Smith, J. Liu, G. C. Dezeny, C. H. Senanayake,
R. D. Larsen, T. R. Verhoeven and P. J. Reider, J. Org. Chem., 1997,
6
2, 2222.
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873