Asymmetric synthesis of C2-symmetric 5,6-bis(benzyloxy)-cyclohexa-1,3-diene and a tricarbonyliron complex

Article abstract of DOI:10.1039/b205526k

The C₂-symmetric 5,6-bis(benzyloxy)cyclohexa-1,3-diene and the corresponding tricarbonyliron complex have been synthesized in enantiomerically pure form. Reaction of the complex with trialkylaluminium gave a mono-alkylated product accompanied by racemization.

Full text of DOI:10.1039/b205526k

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Asymmetric synthesis of C2-symmetric 5,6-bis(benzyloxy)-
cyclohexa-1,3-diene and a tricarbonyliron complex
Akiko Watanabe, Takao Kamahori, Mariko Aso and Hiroshi Suemune*
1
Graduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka 812-8582, Japan
Received (in Cambridge, UK) 10th June 2002, Accepted 17th September 2002
First published as an Advance Article on the web 18th October 2002
The C2-symmetric 5,6-bis(benzyloxy)cyclohexa-1,3-diene and the corresponding tricarbonyliron complex have been
synthesized in enantiomerically pure form. Reaction of the complex with trialkylaluminium gave a mono-alkylated
product accompanied by racemization.
Table 1 Synthesis of tricarbonyliron–diene complex 6
Introduction
In the course of our studies on the development of asymmetric
reactions using a metal catalyst with novel diene compounds
as chiral ligands,1 we describe the asymmetric synthesis of
C2-symmetric 5,6-bis(benzyloxy)cyclohexa-1,3-diene 5 and its
tricarbonyliron complex 6. Previously, we reported the enantio-
selective synthesis of cyclohexa-3,5-diene-1,2-diol 3, starting
fom optically active cyclohex-4-ene-1,2-diol 1 prepared by
kinetic resolution using an enzymatic process. In this work, we
designed the corresponding benzyl ether 5 as a key compound
for the construction of the tricarbonyliron complex 6.
Entry
Solvent
Fe2(CO)9/eq.
Time/h
Yield (%)
1
2
3
4
5
6
7
2
2
2
3
2
2
2
1
4
6
2
4
4
4
42
58
42
64
THF
a
Et2O
(i-Pr)2O
dioxane
–
25
7b
a The ( )-5 was recovered. b Reaction gave a complex mixture.
Results and discussion
A preliminary synthetic approach to 5 by direct O-benzylation
of cyclohexa-3,5-diene-1,2-diol 3 gave a complex mixture. Next,
O-benzylation of C2-symmetric (1R,2R,4R,5R)-4,5-dibromo-
cyclohexane-1,2,-diol 2 under usual conditions gave the desired
product 4 in 98% yield. Reaction of 4 with DBU in benzene
at 50 ЊC gave the corresponding diene2 5 in 49% yield accom-
panied by an aromatized product, (phenylmethoxy)benzene,
in 8% yield. The structure of 5 was confirmed by spectro-
Scheme 2
summarized in Table 1 and (Scheme 2). It was found that the
reaction was highly affected by the solvent used. Among the
solvents such as Et2O, (i-Pr)2O, dioxane and tetrahydrofuran
(THF), only THF gave reasonable results affording the desired
product ( )-6 in 42–64% yield (entries 1–4 in Table 1), although
the reproducibility of this reaction was somewhat poor. On the
other hand, reaction in Et2O did not afford ( )-6 at all.
1
scopic analyses. In the H-NMR spectrum, C1–H and C2–H
were observed at δ 4.44 (2H, s), and the olefinic protons (C(3–
6)–H) at δ 5.92–6.00 (4H). In the 13C-NMR spectrum, eight
carbon signals were observed for the C2-symmetric structure
(Scheme 1).
1
In the H-NMR spectrum of ( )-6, olefinic protons were
observed at δ 5.4–5.5 (C2,3–H, 2H), δ 3.10 (C4–H, 1H) and
δ 3.01 (C1–H, 1H). A pair of benzylic methylene protons was
observed at δ 4.53 (C6–exo–OCH2, 2H, s) and another pair at
δ 4.50 and 4.38 (1H each, endo-C6–OCH2, d, J = 11.6 Hz). The
protons at the C5,6-positions were also observed at δ 3.84 and
3.53, respectively.3a Reaction of trans-diol ( )-3 under the same
conditions in THF gave a complex mixture, and the isolable
product was a reduced derivative ( )-7 (8% yield). Compound
( )-7 was converted into the corresponding benzoate ( )-8
in 80% yield. The structures of 7 and 8 were determined by
spectroscopic analyses. The reaction of prochiral cis-diol 9 with
Fe2(CO)9 gave a complex 10 in 49% yield, which was not
stable for purification by silica-gel column chromatography.3b
The crude complex 10 was converted into the corresponding
dibenzoate 11 in 94% yield, which was sufficiently stable for
purification (Scheme 3). The stereochemical structure of com-
plex 11 was determined by X-ray crystallographic analysis as
shown in Fig. 1 and Table 2. It was interesting that the relative
stereochemistry between the iron and the bis(benzoyloxy)
group was endo, which suggested that chelation control between
Scheme 1
Next, syntheses of tricarbonyliron complexes of cyclohexa-
1,3-diene derivatives3 were studied by using ( )-3, ( )-5 and
cis-cyclohexa-3,5-diene-1,2-diol 9. At first, complex form-
ation of ( )-5 with Fe2(CO)9 was investigated.4 The results are
DOI: 10.1039/b205526k
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This journal is © The Royal Society of Chemistry 2002
2539

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Table 2 Crystal and diffraction parameters of 11
Table 3 Complexation of (R, R)-5
11
Empirical formula
Mr
Crystal dimensions/mm
Data collection temp.
Crystal system
Lattice parameters:
a, b, c/Å
C23H16O7Fe1
460.22
0.50 × 0.30 × 0.20
Ϫ150 ЊC
Reaction duration/h
% Ee of (R,R)-6
Orthorhombic
2
4
6
9
95
89
80
44
11.2695(7), 17.0311(8), 10.3072(6)
90, 90, 90
1978.3
α, β, γ/Њ
V/Å3
Space group
P212121
4
1.545
C2-symmetry. The optical purity of complex 6 was determined
by high performance liquid chromatography with a CHIRAL-
PAK AD column detected using UV radiation at 260 nm.6 To
our surprise, prolonging the reaction decreased the ee of the
complex as shown in Table 3. On the other hand, the ee of the
isolated complex 6 (95% ee) was not decreased by the 5 h reac-
tion with Fe2(CO)9. This racemization during the formation of
the complex might be caused by some species with weak Lewis
acidity derived from Fe2(CO)9.
Z value
Dcalc/g/cm3
µ(Mo-Kα)/cmϪ1
No. of observations
No. of variables
R, Rw
8.06
2583(I > Ϫ10.0σ(I))
346
0.047, 0.070
2360(I > 2.0σ(I)
0.028
No. of reflections of calc. R1
R1
Solvent of crystallization
EtOH
Synthetic approach to cyclohexadiene derivatives
In order to study the synthetic applications of complex 6, sub-
stitution reactions of optically active complex 6 and trialkyl-
aluminium were studied. The results are shown in Scheme 4.
Scheme 3 Reagents: i. Fe2(CO)9; ii. BzCl, pyridine.
Scheme 4
Reaction of (R,R)-6 (87% ee) with trimethylaluminium gave
monomethylated product 12 in 61% yield, and that with triethyl-
aluminium gave monoethylated 13 in 62% yield. The relative
stereochemistry between the benzyloxy and alkyl groups was
determined to be trans after conversion into the corresponding
compounds 14 and 15. In the NOESY spectrum of compound
14, correlation between C5–H and C6–Me was observed, and a
similar relationship was also observed in that of compound 15.
In addition, both products 12 and 13 were optically inactive,
which means that complete racemization took place during the
reaction. These results prompted us to study the reaction of
optically active 6 (78% ee) with several Lewis acids such as
BF3ؒOEt2, SnCl4 and TiCl4 at Ϫ15 ЊC in CH2Cl2 for 1.5 h. In all
cases, the recovered substrate in 80–95% yield was completely
racemized into 0% ee
Fig. 1 ORTEP drawing of the crystal structure of 11 with atom
numbering (ellipsoids at 50% probability).
the iron and oxygen atoms of the hydroxy groups might prefer-
entially act on diastereomeric face selection.5
Synthesis of the optically active complex
These results prompted to us synthesize the corresponding
complex in optically active form and to study its application as
a chiral source. Reaction of (R,R)-5 (97% ee) with Fe2(CO)9
(1 equiv.) in refluxing THF for 2 h gave optically active (R,R)-6
in 95% ee and 40% yield. To our knowledge, this is the first
example of the preparation of an optically active tricarbonyl-
iron complex without diastereoface selectivity because of its
These results led us to consider the reaction mechanisms as
shown in Fig. 2. At first, the Lewis acid coordinated the ether
oxygen opposite the Fe atom because of steric effects, then
elimination of the benzyloxy group afforded a prochiral dienyl
cation (A) as an intermediate. Finally, regeneration of the
C–(O-benzyl) bond opposite the Fe atom at the C4 or C6
position of (A) gave completely racemic 6. These reaction
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(hexane–ethyl acetate = 20 : 1) to give (Ϫ)-5 (715 mg, 49% yield)
as a colorless oil. [α]2D4 = Ϫ270.7 (c = 1.00, CHCl3) 97% ee.
δH (270 MHz, CDCl3) 7.38–7.26 (10H, m, Ph), 6.00–5.91 (4H,
m), 4.65 (4H, s), 4.44 (2H, d, J = 1.0); δC (68 MHz, CDCl3)
138.5 (s), 128.4 (d), 127.8 (d), 127.6 (d), 124.5 (d), 78.22 (d),
70.85 (t); νmax/ cmϪ1 (neat) 3040, 2850, 1490, 1440, 1080, 1060,
720, 680; m/z (FAB) 292 (Mϩ, 7.0), 91 (Bn, 100) [Calcd For
C20H20O2: M 292.1463. Found: Mϩ 292.1481 (EI)]. For (ϩ)-
(5S,6S )-5,6-bis(benzyloxy)cyclohexa-1,3-diene ((ϩ)-5); [α]1D8
ϩ263.0 (c = 1.04, CHCl3), 96% ee.
=
Fig. 2
mechanisms were also supported by the reaction of (R,R)-6
(87% ee) with BF3ؒOEt2 in the presence of MeOH to afford the
racemic 16 in 63% yield. In this reaction, formation of a bis-
(methoxy) product was not observed. For the formation of the
cationic intermediate A, the exo-benzyloxy group at the C6–
position was considered necessary based on the fact that the
bis-endo-(benzoyloxy) complex (11) did not react with trialkyl-
aluminium and/or Lewis acid. These obtained results using
optically active complex 6 might be important evidence of the
formation of the dienyl cation intermediate (A).
Typical procedure for complex formation
(؊)-(5R,6R)-[5,6-Bis(benzyloxy)cyclohexa-1,3-dienyl]-
tricarbonyliron (؊)-6
To a solution of (Ϫ)-5 (300 mg, 1.0 mmol) in THF (25 ml), iron
nonacarbonyl (730 mg, 2 mmol) was added portionwise at
room temperature. The reaction was refluxed for 2 h after which
time it was passed through a short silica-gel column (ether), and
concentrated in vacuo. The residue was purified by silica-gel
column chromatography (hexane–ethyl acetate = 15 : 1) to give
(Ϫ)-6 (250 mg, 58% yield) of 95% ee as a yellowish oil; [α]2D0
=
Experimental
Ϫ71.9 (c = 0.91, CHCl3); δH (270 MHz, CDCl3) 7.35–7.23 (10H,
m, Ph), 5.49–5.41 (2H, m), 4.54 (2H, s), 4.51 (1H, d, J = 11.7),
4.37 (1H, d, J = 11.7), 3.84 (1H, dd, J = 3.63, 0.99), 3.53 (1H, dd,
J = 3.63, 0.99), 3.10 (1H, m), 3.02 (1H, d, J = 6.27); δC (68 MHz,
CDCl3) 210.6 (s), 138.4 (s), 138.3 (s), 128.4 (d), 128.3 (d), 127.7
(d), 127.6 (d), 127.5 (d), 85.0 (d), 84.4 (d), 84.3 (d), 78.4 (d), 71.0
(t), 70.2 (t), 58.97 (d), 58.40 (d); νmax /cmϪ1 (neat) 3025, 2850,
2920, 2045, 1970 (br), 1440, 1340, 1080, 1060, 730, 690; m/z
(FAB) 431.1 ([M Ϫ H]ϩ, 1.6), 241.1 ([M–OBn–(CO)3]ϩ, 100)
[Calcd for C23H19O5Fe: M Ϫ H 431.0582. Found: (M Ϫ H)ϩ
431.0543 (FAB)]; For (ϩ)-6 of 95% ee; [α]2D0 = ϩ72.1 (c = 0.90,
CHCl3).
IR spectra were measured with a JASCO A-202 spectrometer or
a ThermoNicolet Avator 320 KYD spectrometer. 1H NMR and
13C NMR spectra were measured by a JEOL JNM-GX 270 or
JNM-GX 500 spectrometer using CDCl3 as a solvent and
tetramethylsilane (TMS) as an internal standard. Coupling
constants (J) are given in Hz. Mass spectra were measured on a
JEOL JMS-600H or JMS-SX/SX102A spectrometer. Optical
rotations were measured on a JASCO DIP-360 polarimeter in
units of 10Ϫ1 deg cm2 gϪ1. HPLC were performed using a
HITACHI L-7100F with UV-detection, and Chiralpak AD was
used as a chiral column. For column chromatography, silica gel
(Nakarai Tesque, Silica Gel 60, 230–400 mesh) was used. All
organic solvent extracts were dried over anhydrous MgSO4.
(1RS,2SR,5RS )-(1-Hydroxycyclohexa-2,4-dienyl)tricarbonyliron
( )-7
(؊)-(1R,2R,4R,5R)-4,5-Bis(phenylmethoxy)-1,2-dibromocyclo-
hexane (؊)-4
Yellow solid; δH (270 MHz, CDCl3) 5.31–5.24 (2H, m), 3.80
(1H, m), 3.22 (1H, m), 3.14 (1H, ddd, J = 2.31, 3.62, 11.87), 2.07
(1H, ddd, J = 2.31, 6.39, 15.84), 1.56–1.48 (1H, m); νmax/cmϪ1
(KBr) 3256, 2943, 2895, 2038, 1942 (br), 1455, 1424, 1378, 1326,
1292, 1256, 1078, 1059, 1026, 935, 902, 622, 560,; m/z (FAB) 235
([M Ϫ H]ϩ).
A solution of (1R,2R,4R,5R)-4,5-dibromocyclohexane-1,2-diol
(2, 2.74 g, 10 mmol) in DMF (10 ml) was added to a stirred
mixture of 60% sodium hydride (1.3 g, 30 mmol, 3 eq.) in DMF
(50 ml) at Ϫ15 ЊC under an Ar atmosphere. Benzyl bromide
(4.5 ml, 70 mmol) was then added dropwise at the same
temperature. The mixture was stirred for 5 h at room tem-
perature. The reaction was quenched by addition of sat. aque-
ous NH4Cl and was extracted with ether. The organic layer
was washed with brine and dried. After removal of the sol-
vent, the residue was purified by silica-gel column chromato-
graphy (hexane–ethyl acetate = 30 : 1) to afford (Ϫ)-4 (4.4 g,
98% yield) as a colorless oil. [α]2D4 = 44.9 (c = 1.09, CHCl3).
δH (270 MHz, CDCl3) 7.39–7.27 (10H, m, Ph), 4.60 (4H, s), 4.45
(2H, t, J = 3.47), 3.79 (2H, t, J = 2.64), 2.60–2.50 (2H, m), 2.41–
2.32 (2H, m); δC(68 MHz, CDCl3) 138.1 (s), 128.4 (d), 127.7 (d),
127.5 (d), 76.17 (d), 71.83 (t), 52.30 (d), 36.02 (t); νmax/cmϪ1
(neat) 3020, 2850, 1440, 1170, 1080, 1060, 1020, 720, 680; m/z
(FAB) 453 ([M ϩ H]ϩ 33.3), 181 (100) [Calcd For C20H23O2Br2:
M ϩ H 453.0065. Found: (M ϩ H)ϩ 453.0036 (FAB)]. For
(ϩ)-(1S,2S,4S,5S )-4,5-bis(benzyloxy)-1,2-dibromocyclohexane
(ϩ)-4 [α]2D2 = ϩ44.4 (c = 1.20, CHCl3).
(1SR,2RS,3RS,6SR)-(1,2-Dihydroxycyclohexa-3,5-dienyl)-
tricarbonyliron 10
Compound 10 was obtained as a crude yellow solid; δH (270
MHz, CDCl3) 5.29 (2H, dd, J = 2.64, 5.28), 3.84 (2H, s), 3.15
(2H, t, J = 3.29), 2.64 (2H, br s); νmax/cmϪ1 (KBr) 3250, 2030,
1970, 1940, 1320, 1285, 1220, 1070, 1040; m/z (FAB) 252.0
(Mϩ).
(1RS,2SR,5RS )-(1-Benzoyloxycyclohexa-2,4-dienyl)tricarbonyl-
iron ( )-8
To a stirred solution of ( )-7 (126 mg, 0.53 mmol) and pyrid-
ine (0.1 ml) in CH2Cl2 (0.5 ml)) was added benzoyl chloride
(0.15 ml, 1.25 mmol) at 0 ЊC. The reaction was stirred at room
temperature for 1 h. After addition of water, the reaction was
extracted with ether. The organic layer was dried and concen-
trated in vacuo. The residue was purified by silica gel column
chromatography (hexane–ethyl acetate = 10 : 1) to afford ( )-8
(169 mg, 91% yield) as yellowish needles; mp 113.4–114.4 ЊC
(acetone); δH (270 MHz, CDCl3) 8.10 (2H, m), 7.57 (1H, m),
7.46 (2H, m), 5.38 (1H, m), 4.99 (1H, m), 3.33 (1H, m), 3.19
(1H, m), 2.23 (1H, ddd, J = 2.31, 6.93, 15.84), 1.81 (1H, dt,
J = 2.64, 15.84); νmax/cmϪ1 (KBr) 3059, 3032, 2923, 2939, 2833,
2049, 1958, (br), 1704, 1488, 1448, 1283, 1263, 1111, 711, 620,
(؊)-(5R,6R)-5,6-Bis(benzyloxy)cyclohexa-1,3-diene (؊)-5
To a solution of (Ϫ)-4 (2.27 g, 5 mmol) in benzene (20 ml),
DBU (3 ml, 20 mmol) was added at room temperature. The
reaction was warmed to 50 ЊC and stirred for 5 h. After addition
of water to the reaction mixture, it was extracted with benzene.
The organic layer was dried and concentrated in vacuo. The
residue was purified by silica-gel column chromatography
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558; m/z (FAB) 354 (Mϩ); Anal. calcd for C16H12FeO5: C, 56.50;
H, 3.56. Found: C, 56.56; H, 3.52%.
CH3); νmax/cmϪ1 (neat) 3035, 2970, 2920, 2850, 2045, 1980–1950
(br), 1445, 1335, 1280, 1090, 1070, 1020, 855, 730; m/z (FAB)
354.1 (Mϩ) [Calcd for C18H18O4Fe: M 354.0555. Found: Mϩ
354.0564 (FAB)].
(1RS,2SR,3RS,6SR)-[1,2-Bis(benzoyloxy)cyclohexa-3,5-
dienyl]tricarbonyliron 11
(5RS,6SR)-5-Benzyloxy-6-methylcyclohexa-1,3-diene ( )-14
By a similar manner to that described for the preparation of 8,
compound 10 was converted into bis-benzoate 11 in 94% yield;
yellow prisms; mp 156.5–157.5 ЊC (EtOH); δH (270 MHz,
CDCI3) 7.94 (4H, dd, J = 1.32, 8.58), 7.47 (2H, tt, J = 1.65,
7.59), 7.26 (4H, t, J = 7.59), 5.45 (2H, dd, J = 3.30, 5.28), 5.28
(2H, s), 3.26 (2H, s), 2.64 (2H, tt, J = 1.32, 3.96); νmax/cmϪ1
(KBr) 3380, 3330, 2020, 1980, 1950, 1710, 1250, 1100, 1060,
1010, 920, 930, 880, 795, 695; m/z (FAB) 458.9 ((M Ϫ H)ϩ, 0.9);
Anal. calcd for C23H16FeO7: C, 60.30; H, 3.50. Found: C, 60.08;
H, 3.50%.
X-Ray analysis†: The yellow prisms of 11 were grown from
acetone–EtOH. Data collection was performed on a Rigaku-
RAXIS-RAPID Imaging Plate diffractometer, graphite
monochromated Mo–Kα radiation. Crystal and collection
parameters are listed in Table 2. The crystal remained stable
during the X-ray data collection. The structure was solved by
direct methods with SIR926 and expanded by Fourier tech-
niques.7 All non-H-atoms were given anisotropic thermal
parameters, and H-atoms were refined isotropically. The final
cycle of full-matrix least-squares refinement of 11 gave an R
factor of 0.047 (Rw = 0.070) based on 2583 (I > Ϫ10.00 σ(I))
reflections and R1 factor of 0.028 based on 2360 (I > 2.0 σ(I))
reflections, and the largest peak and hole in the final difference
Fourier map were 0.36 and Ϫ0.36 eÅϪ3. All calculations were
performed by means of the teXsan8 crystallographic package.
The absolute configuration could not be determined from the
refinement.
To a stirred solution of NaOH (200 mg, 6 mmol) and 30% H2O2
(0.7 ml, 6 mmol) in MeOH (8 ml), a solution of ( )-12 (340 mg,
1 mmol) in MeOH (2 ml) was added at 0 ЊC. After being stirred
for 1 h, the reaction was diluted with ether, and washed with 1%
aqueous HCl, saturated aqueous NaHCO3 and brine, and
dried. After removal of the solvent in vacuo, the residue was
purified by silica-gel column chromatography (hexane–ethyl
acetate = 20 : 1) to give ( )-14 (148 mg, 74% yield). Colorless
oil; δH (270 MHz, CDCl3) 7.37–7.26 (5H, m, Ph), 6.05 (1H, dd,
J = 9.61, 5.03), 5.91–5.80 (3H, m), 4.59 (1H, d, J = 11.9), 4.52
(1H, d, J = 11.9), 3.88 (1H, m), 2.62 (1H, m,), 1.02 (3H, d,
J = 7.26); δC (68 MHz, CDCl3) 138.9 (s), 133.3 (d), 128.3 (d),
127.7 (d), 127.4 (d), 125.7 (d), 124.4 (d), 121.6 (d), 77.47 (d),
69.38 (t), 34.43 (d), 17.79 (q); νmax/cmϪ1 (neat) 3060, 3040, 2960,
2925, 2870, 1495, 1450, 1380, 1025, 1060, 730, 1020, 690; m/z
(FAB) 200 (Mϩ, 16), 91 (Bn, 100); [Calcd for C14H16O: M
200.1201. Found: Mϩ 200.1166 (FAB)].
(5RS,6SR)-5-Benzyloxy-6-ethylcyclohexa-1,3-diene ( )-15
A similar reaction of ( )-13 gave ( )-15 in 69% yield; colorless
oil; δH (500 MHz, CDCl3) 7.38–7.30 (4H, m, Ph), 7.28–7.24
(1H, m, Ph), 6.06 (1H, ddd, J = 0.91, 5.03, 8.69), 5.94 (1H, dt,
J = 1.14, 5.03), 5.90–5.86 (2H, m), 4.57 (1H, d, J = 11.89,
O–CH2–Ph), 4.52 (1H, d, J = 11.89, O–CH2–Ph), 3.96 (1H, t,
J = 5.03, C1–H), 2.47 (1H, m, C2–H), 1.50–1.35 (2H, m, –CH2–
Me), 0.93 (3H, t, J = 7.55, –CH2–CH3); νmax/cmϪ1 (neat) 3060,
3040, 2960, 2925, 2870, 1495, 1450, 1380, 1025, 1060, 730, 1020,
690; m/z (FAB) 214.2 (Mϩ) [Calcd for C15H18O: M 214.1358.
Found: Mϩ 214.1319 (FAB)].
Reaction of 6 with trialkylaluminium
To a solution of (R,R)-6 of 87% ee (432 mg, 1 mmol) in CH2Cl2
(10 ml), 1.0 M trialkylaluminium (2 ml, 2 mmol) was added at
Ϫ78 ЊC under an Ar atmosphere. After being stirred for 2 h at
the same temperature, the reaction was warmed slowly to room
temperature. The reaction was quenched by addition of 1%
aqueous HCl, and the reaction mixture was extracted with
ether. The organic layer was washed with saturated aqueous
NaHCO3, brine and dried. After removal of the solvent in
vacuo, the residue was purified by silica-gel column chromato-
graphy (hexane–ethyl acetate = 50 : 1) to give ( )-12 or -13.
(1RS,4SR,5RS,6RS )-5-Benzyloxy-6-methoxycyclohexa-1,3-
dienyltricarbonyliron 16
To a stirred solution of ( )-6 (432 mg, 1 mmol) and MeOH
(0.2 ml) in CH2Cl2 (10 ml), BF3ؒOEt2 (0.2 ml, 2 mmol) was
added at 0 ЊC. The reaction was stirred at the same temperature
for 1 h. The reaction mixture was poured into aqueous satur-
ated NaHCO3, and extracted with ether. The combined extracts
were dried and concentrated in vacuo. The residue was purified
by silica-gel column chromatogtaphy (hexane–ethyl acetate =
50 : 1) gave ( )-16 (157 mg, 63% yield as a yellow oil. νmax/cmϪ1
(neat) 3025, 2930, 2870, 2050, 1970, 1170, 1025, 1000, 940, 930,
905, 860, 735, 695; δH (500 MHz, CDCl3) 7.39–7.25 (5H, m,
Ph), 5.43 (2H, C5,6–H), 4.56 (2H, s, O–CH2–Ph), 3.61, 3.43,
3.08, 3.00 (1H each, olefinic H), 3.25 (3H, s, OMe); m/z (FAB)
355 (Mϩ Ϫ H).
(1RS,4SR,5SR,6RS )-5-Benzyloxy-6-methylcyclohexa-1,3-
dienyltricarbonyliron ( )-12
Yield: 54%; yellow oil; δH (270 MHz, CDCl3) 7.38–7.25 (5H, m,
Ph), 5.32 (1H, m), 5.26 (1H, m), 4.54 (1H, d, J = 11.9), 4.49 (1H,
d, J = 11.9), 3.15 (1H, d, J = 6.6), 3.10 (1H, s), 3.05 (1H, m), 2.08
(1H, m), 0.95 (3H, d, J = 7.26); δC (68 MHz, CDCl3) 211.5 (s),
138.7 (s), 128.3 (d), 127.5 (d), 127.4 (d), 84.18 (d), 83.06 (d),
80.38 (d), 69.90 (t), 67.28 (d), 63.23 (d), 42.74 (d), 23.97 (q);
νmax/cmϪ1 (neat) 3040, 2960, 2925, 2860, 2050, 1990–1940 (br),
1450, 1335, 1260, 1090, 1060, 1020, 860, 730, 690; m/z (FAB)
340.1 (Mϩ, 8.2) [Calcd for C17H16O4Fe: M 340.0398. Found: M
Ϫ Hϩ 340.0392 (FAB)].
Acknowledgements
We gratefully acknowledge Mr K. Nakazawa for experimental
support and Dr M. Tanaka for solving the X-ray structure.
(1RS,4SR,5SR,6RS )-5-Benzyloxy-6-ethylcyclohexa-1,3-dienyl-
tricarbonyliron ( )-13
References
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