Catalytic asymmetric epoxidation of alkenes with arabinose-derived ketones containing a cyclohexane-1,2-diacetal

Article abstract of DOI:10.1016/j.tetasy.2009.02.039

The effect of diol blocking groups, cyclohexane-1,2-diacetal verses butane-1,2-diacetal, on the asymmetric epoxidation of trans- and cis-alkenes by arabinose-derived ketones is reported. The ketone catalysts with a cyclohexane-1,2-acetal display similar asymmetric induction as those catalysts with a butane-1,2-diacetal in most cases. For (E)-1-benzyloxy-4-hexene, the ee of the enantioselective epoxidation has reached 61% with the cyclohexane-1,2-dineopentyl acetal ketone catalyst.

Full text of DOI:10.1016/j.tetasy.2009.02.039

Tetrahedron: Asymmetry 20 (2009) 883–886
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Catalytic asymmetric epoxidation of alkenes with arabinose-derived
ketones containing a cyclohexane-1,2-diacetal
*
Tony K. M. Shing , To Luk
Department of Chemistry, The Chinese University of Hong Kong, Shatin, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of diol blocking groups, cyclohexane-1,2-diacetal verses butane-1,2-diacetal, on the asymmet-
ric epoxidation of trans- and cis-alkenes by arabinose-derived ketones is reported. The ketone catalysts
with a cyclohexane-1,2-acetal display similar asymmetric induction as those catalysts with a butane-
1,2-diacetal in most cases. For (E)-1-benzyloxy-4-hexene, the ee of the enantioselective epoxidation
has reached 61% with the cyclohexane-1,2-dineopentyl acetal ketone catalyst.
Received 4 February 2009
Accepted 23 February 2009
Available online 28 March 2009
Dedicated to George W. J. Fleet on the
occasion of his 65th birthday
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
tuted and cis-alkenes. For arabinose-derived 4-uloses, the enanti-
oselectivity increases with the size of the acetal alkoxy group for
The catalytic asymmetric epoxidation of olefins is an important
synthetic method to induce chirality into organic molecules; the
resultant epoxides therefore provide versatile intermediates for
natural products or pharmaceutical synthesis. Catalytic enantiose-
lective epoxidation using transition metal complexes is well estab-
lished and chiral oxiranes are obtained from allylic alcohols1 and
from unfunctionalized cis-alkenes2 with excellent asymmetric
induction. In recent years, chiral dioxiranes,3 generated in situ
from OxoneÒ and chiral ketones, have become promising reagents
for the asymmetric epoxidation of unfunctionalized alkenes. This
organocatalytic epoxidation using chiral ketone catalysts was pio-
neered by Curci in 1984.4 Subsequently, elegant 11-membered C2-
symmetry biaryl ketones developed by Yang et al.5 afforded
enantioselectivities which have stimulated worldwide investiga-
tions on the subject. There are now many research groups which
have achieved impressive results using different chiral ketone/Ox-
oneÒ systems.6 Our long-term interest in the application of carbo-
hydrates in asymmetric synthesis has prompted us to search for a
readily prepared ketone catalyst to induce chirality with high ee in
the asymmetric epoxidation. Arabinose, available commercially in
large quantities for both enantiomers, has been the first choice for
our studies. Over the years, we had described the use of arabinose-
derived alcohols as chiral auxiliaries in asymmetric Diels–Alder
reactions7 and in asymmetric Hosomi–Sakurai reaction.8 Our ef-
forts towards the enantioselective epoxidation of alkenes have
both trans-disubstituted and -trisubstituted alkenes.11b,c Interest-
ingly, an inverse relationship with the size of the alkoxy group
was observed for cis-alkenes.11a Herein, we report the effect of diol
blocking groups, cyclohexane-1,2-diacetal verses butane-1,2-diac-
etal, on the asymmetric epoxidation of trans- and cis-alkenes.
2. Results and discussion
In order to study the effect of a cyclic as opposed to an acyclic
diketone blocking group for the trans-1,2-diol on asymmetric
epoxidation, we employed 1,1,2,2-tetramethoxycyclohexane in-
stead of 2,2,3,3-tetramethoxybutane (TMB) in the acetalization
reaction. Two new uloses, 1 and 2, were prepared and the synthe-
ses are shown in Scheme 1 and 2, respectively.
The benzyl arabinopyranoside 311b was acetalized with 1,1,2,2-
tetramethoxy-cyclohexane12 and the trans-diequatorial diol moiety
in 3 was protected to afford cyclohexane-1,2-diacetal 4 in 51% yield
(Scheme 1). Oxidation of the free alcohol in dimethyl acetal 4 with
pyridinium dichromate (PDC) gave ketone 1 in excellent yield. The
methoxy substituents in dimethyl acetal 4 were exchanged to neo-
pentyl groups by transacetalization11b under acidic conditions with
neopentyl alcohol to give di-neopentyl acetal 5 in 87% yield
(Scheme 2). The PDC oxidation of the C-4 hydroxyl group in 5 fur-
nished cyclohexane-1,2-dineopentyl acetal ketone 2 in 92% yield.
Ketones 6 and 7 containing an acyclic butane diacetal were pre-
pared in a similar manner with 2,2,3,3-tetramethoxybutane and
transacetalization (for 7) as reported by us previously.11b Ketones
1, 2, 6 and 7 were used as catalysts in the asymmetric epoxidation
of different alkenes and the results are summarized in Table 1.
In the comparison of the enantioselectivities between ketones 6
and 1, it was found that changing the trans-diol-protecting group
from butane-1,2-diacetal to cyclohexane-1,2-diacetal, did not af-
fect the enantioselectivity appreciably (Table 1, entries 1,2; 5,6;
provided chiral ketone organocatalysts derived from
D
-glucose;9
L
-arabinose;10 and arabinose-
2-uloses and 3-uloses derived from
derived 4-uloses11 containing a tunable butane-2,3-diacetal12 as
the steric blocker. These ketone catalysts have been employed for
the asymmetric epoxidation of trans-disubstituted, trans-trisubsti-
* Corresponding author. Tel.: +852 2609 6367; fax: +852 2603 5057.
E-mail address: tonyshing@cuhk.edu.hk (T.K.M. Shing).
0957-4166/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2009.02.039

884
T. K. M. Shing, T. Luk / Tetrahedron: Asymmetry 20 (2009) 883–886
MeO OMe
O
OBn
O
MeO
OMe
O
O
OBn
OH
OH
OH
BnOH, AcCl, reflux
87%
HO
OMe
HO
HO
H+, MeOH, reflux
51%
O
OH
OH
MeO
3
4
L-Arabinose
O
OBn
O
O
OBn
O
PDC, 4Å MS
O
HO
CH2Cl2, rt, 96%
OMe
OMe
O
O
MeO
MeO
4
1
Scheme 1. Synthesis of ulose 1.
O
OBn
O
OBn
O
OBn
OH
, H+
PDC, 4Å MS
CH2Cl2, rt, 92%
HO
O
HO
O
O
O
O
PhH,
OMe
O
O
O
O
O
O
Dean & stark
87%
MeO
4
5
2
Scheme 2. Synthesis of ulose 2.
9,10; 13,14; 17,18). Similar asymmetric induction was obtained in
all cases.
96%) of epoxides in the catalytic asymmetric epoxidation of al-
kenes. The new cyclohexane-1,2-dineopentyl acetal ketone 2 was
found to be the best enantioselective catalyst for the epoxidation
of trans-alkenes. The more rigid structure of 2 is believed to cause
a stronger steric interaction with long-chain disubstituted trans-al-
kenes, rather than with simple trans-alkenes. Hence, a good ee of
61% was obtained for (E)-1-benzyloxy-4-hexene that compares
favourably with the best ee of 68% reported for this substrate. In
line with our previous finding, the ee of the asymmetric epoxida-
tion of cis-alkenes decreased with the size of the acetal-blocking
group.
In the comparison of the enantioselectivity shown between ke-
tones 7 and 2, it was also found that changing the trans-diol protect-
ing group from butane-1,2-diacetal to cyclohexane-1,2-diacetal did
not affect the enantioselectivity [Table 1, (entries 3,4; 11,12; 15,16;
19,20) except for (E)-1-benzyloxy-4-hexene (entries 7 and 8)].
Cyclohexane-1,2-dineopentyl acetal ketone catalyst 2 improves
the ee of the asymmetric epoxidation from 45% to 61% for this
long-chain disubstituted trans-alkene (entry 8). This finding is close
to the best result reported by Denmark (68% ee).17 We suggest that,
in ketone 2, the cyclohexyl group causes the branched neopentyl
acetal to be more rigid than that in ketone 7. Hence, the spatial dis-
order of the neopentyl acetal moiety in ketone 2 is smaller than that
in ketone 7. This parameter may cause a stronger steric effect for
long-chain disubstituted alkenes rather than for simple alkenes.
Further investigation in this direction is currently in progress.
We previously reported11a that in the epoxidation of cis-al-
kenes, the ee increased inversely with the size of the blocking
groups in our arabinose-derived ketones. In the present study,
cyclohexane-1,2-dineopentyl acetal ketone 2 has the more bulky
blocking group and displays the poorest ee with cis-alkenes (en-
tries 16 and 20).
4. Experimental
4.1. General
Melting points were reported in degrees Celsius and are uncor-
rected. Optical rotations were measured at 589 nm. Infrared (IR)
spectra were recorded on an FT-IR spectrophotometer as thin films
on KBr discs. Nuclear magnetic resonance (NMR) spectra were
measured at 300.13 MHz (1H) or at 75.47 MHz (13C) in CDCl3 solu-
tions, unless stated otherwise. All chemical shifts were recorded in
ppm relative to tetramethylsilane (d = 0.0). Spin–spin coupling
constants (J) in hertz were measured directly from the spectra.
MS and HRMS were performed at the Department of Chemistry,
The Chinese University of Hong Kong, Hong Kong, China. Carbon
and hydrogen elemental analyses were carried out by the Depart-
ment of Chemistry, Brunel University, Uxbridge, U.K. All reactions
3. Conclusion
In conclusion, all the ketone catalysts were easily prepared from
L-arabinose in four steps and afforded good chemical yields (81–

T. K. M. Shing, T. Luk / Tetrahedron: Asymmetry 20 (2009) 883–886
885
Table 1
The effect of trans-diequatorial diol blocking group on asymmetric epoxidation of alkenes catalyzed by ketones 1, 2, 6 and 7
10 mol% catalyst
Oxone / NaHCO3
R1
R2
R3
R4
R1 O R3
CH3CN / H2O
0 ºC, pH 7-7.5
R2
R4
Catalysts:
O
O
OBn
O
O
OBn
O
OBn
OBn
O
O
O
O
O
O
O
O
O
OMe
OMe
O
O
O
O
O
O
MeO
MeO
6
1
2
7
Substrates:
Ph
Ph
OBn
Ph
Ph
Ph
CO2Et
d
a
c
b
e
Entrya
Catalysts
Substrates
Yieldb (%)
ee (%)
Config.e
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
6
1
7
2
6
1
7
2
6
1
7
2
6
1
7
2
6
1
7
2
a
a
a
a
b
b
b
b
c
96
85
91
91
84
93
88
92
80
92
84
89
96
94
94
87
93
82
85
81
47c
46d
83d
77d
10d
13d
45d
61d
75c
72d
88d
89d
63c
64c
36c
34c
32c
27c
11c
7c
(ꢁ)-(S,S)13
(ꢁ)-(S,S)13
(ꢁ)-(S,S)13
(ꢁ)-(S,S)13
(ꢁ)-(S,S)f
(ꢁ)-(S,S)f
(ꢁ)-(S,S)f
(ꢁ)-(S,S)f
(ꢁ)-(S,S)14
(ꢁ)-(S,S)14
(ꢁ)-(S,S)14
(ꢁ)-(S,S)14
(+)-(2R,3R)15
(+)-(2R,3R)15
(+)-(2R,3R)15
(+)-(2R,3R)15
(ꢁ)-(1S,2R)16
(ꢁ)-(1S,2R)16
(ꢁ)-(1S,2R)16
(ꢁ)-(1S,2R)16
c
c
c
d
d
d
d
e
e
e
e
a
Procedures are described in Section 4.
Isolated yield.
Enantioselectivity was determined by HPLC using a Chiralcel OD-H column.
b
c
d
e
f
Enantioselectivity was determined by 1H NMR analysis of the epoxide products directly with the shift reagent Eu(hfc)3.
The absolute configurations were determined by comparing the measured specific rotations with the reported ones.
The absolute configuration was tentatively assigned by analogy on the basis of the spiro transition state.11
were monitored by analytical thin-layer chromatography (TLC) on
aluminium-precoated plates of silica gel 60 F254 and compounds
were visualized with a spray of 5% w/v dodecamolybdophosphoric
acid in EtOH and subsequent heating. Silica gel 60 (230–400 mesh)
was used for all flash column chromatographies. The reaction pH
was monitored by a pH meter with a pH triode electrode. All sol-
vents were reagent grade unless otherwise stated. Toluene, ben-
zene and THF were fleshly distilled from Na/benzophenone ketyl
under nitrogen. CH2Cl2 was freshly distilled from CaH2 under N2.
Other reagents were purchased from commercial suppliers and
used without purification. All hexanes used are n-hexane.
resulting solution was cooled to 0 °C (bath temperature). A solu-
tion of OxoneÒ (307 mg, 0.5 mmol) in aqueous EDTA (5 mL,
4 ꢀ 10ꢁ4 M) and a solution of NaHCO3 (252 mg, 3.0 mmol) in H2O
(5 mL) were added dropwise concomitantly via two dropping fun-
nels. The pH of the mixture was maintained at about 7–7.5 over a
period of 24 h. The reaction mixture was then poured into water
(10 mL) and extracted with Et2O (3ꢀ). The combined extracts were
dried with anhydrous magnesium sulfate and filtered. The filtrate
was concentrated under reduced pressure to give a residue that
was purified by flash column chromatography to give the epoxide.
4.1.2. Cyclohexane-dimethyl acetal 4
4.1.1. General in situ epoxidation procedure at 0 °C
To a solution of the benzyl b-L-arabinopyranoside 3 (90 mg,
To a stirred solution of 1,2-dihydronaphthalene (0.1 mmol), ke-
tone (10 mol %) and n-Bu4NHSO4 (0.5 mg) in CH3CN (10 mL) was
added an aqueous buffer (5 mL, 4 ꢀ 10ꢁ4 M aqueous EDTA). The
0.42 mmol), trimethyl orthoformate (0.23 mL, 2.09 mmol),
1,1,2,2-tetramethoxycyclohexane12 (170 mg, 0.83 mmol), in meth-
anol (10 mL) was added 10-camphorsulfonic acid (10 mg,

886
T. K. M. Shing, T. Luk / Tetrahedron: Asymmetry 20 (2009) 883–886
10 mol %). Upon refluxing for 12 h, the cooled reaction mixture was
neutralized with powdered NaHCO3 (200 mg) and filtered. The fil-
trate was concentrated under reduced pressure. The crude residue
was purified by flash column chromatography to afford cyclohex-
ane-dimethyl acetal 4 as a colourless syrup (81 mg, 51%): Rf 0.36
intensity) 515 ([M+Na]+, 100), 516 (25); HRMS (ESI) calcd for
C28H44O7 [M+Na]+ 515.2979, found 515.2990.
4.1.5. Cyclohexane-dineopentyl acetal ketone 2
To a solution of alcohol 5 (100 mg, 0.20 mmol) in dry CH2Cl2
(10 mL) were added slowly PDC (110 mg, 0.30 mmol) and pow-
dered 4 Å molecular sieves (110 mg). The mixture was stirred at
room temperature for 12 h. The mixture was suction filtered
through a pad of silica gel and the filtrate was concentrated under
reduced pressure. The crude residue was purified by flash column
chromatography to afford cyclohexane-dineopentyl ketone 2 as a
colourless syrup (90 mg, 92%): Rf 0.20 (hexane–Et2O, 2:1);
(hexane–EtOAc, 1:1); ½a D20
¼ þ38:7 (c 0.83, CHCl3); IR (thin film)
ꢂ
3472 (OH) cmꢁ1 1H NMR (CDCl3) d 7.40–7.25 (5H, m), 4.97 (1H,
;
d, J = 3.0 Hz), 4.76 (1H, d, J = 12.3 Hz), 4.69 (1H, d, J = 12.3 Hz),
4.41 (1H, dd, J = 10.8, 3.3 Hz), 4.33 (1H, dd, J = 10.8, 3.3 Hz), 3.95
(1H, t, J = 1.2 Hz), 3.83 (1H, dd, J = 12.8, 1.2 Hz), 3.72 (1H, dd,
J = 12.6, 1.8 Hz), 3.21 (3H, s), 3.17 (3H, s), 3.04 (1H, br s), 1.81–
1.77 (4H, m), 1.49–1.38 (2H, m), 1.38–1.35 (2H, m); 13C NMR
(CDCl3) d 137.9, 128.5, 128.2, 127.8, 99.3, 99.2, 97.5, 69.6, 68.6,
66.9, 66.1, 63.7, 47.1, 47.0, 27.2, 21.6; MS (EI) m/z (relative inten-
sity) 380 ([M]+, 100), 381 (25); HRMS (EI) calcd for C20H28O7
[M]+ 380.1830, found 380.1833; Anal. Calcd for C20H28O7: C,
63.14; H, 7.42. Found: C, 63.25; H, 7.65.
½
a 2D0
ꢂ
¼ þ18:75 (c 1.03, CHCl3); IR (thin film) 1744 (C@O) cmꢁ1
;
1H NMR (CDCl3) d 7.39–7.26 (5H, m), 5.21 (1H, d, J = 11.4 Hz),
5.13 (1H, d, J = 3.0 Hz), 4.80 (1H, d, J = 11.7 Hz), 4.75 (1H, d,
J = 11.7 Hz), 4.23 (1H, dd, J = 11.4, 3.0 Hz), 4.12 (1H, d,
J = 14.7 Hz), 3.91 (1H, d, J = 15.0 Hz), 3.03 (2H, dd, J = 8.4, 1.5 Hz),
2.98 (2H, d, J = 8.7 Hz), 1.89–1.83 (4H, m), 1.52–1.50 (2H, m),
1.36–1.25 (2H, m) 0.98 (9H, s), 0.94 (9H, s); 13C NMR (CDCl3) d
200.7, 137.5, 128.6, 128.1, 128.0, 99.0, 98.4, 97.9, 71.8, 70.9, 70.6,
69.3, 69.2, 67.7, 32.0, 27.7, 27.4, 27.3, 21.5; MS (ESI) m/z (relative
intensity) 513 ([M+Na]+, 100), 514 (30); HRMS (ESI) calcd for
C28H42O7 [M+Na]+ 513.2823, found 513.2829.
4.1.3. Cyclohexane-dimethyl acetal ketone 1
To a solution of alcohol 4 (50 mg, 0.13 mmol) in dry CH2Cl2
(10 mL) were added slowly PDC (74 mg, 0.20 mmol) and powdered
4 Å molecular sieves (80 mg). The mixture was stirred at room
temperature for 12 h. The mixture was suction filtered through a
pad of silica gel and the filtrate was concentrated under reduced
pressure. The crude residue was purified by flash column chroma-
tography to afford cyclohexane–dimethyl acetal ketone 1 as a col-
ourless syrup (47 mg, 96%): Rf 0.32 (hexane–EtOAc, 1:2);
References
1. (a) Katsuki, T.; Martin, V. S. Org. React 1996, 48, 1; (b) Johnson, R. A.; Sharpless,
K. B. In Catalytic Asymmetric Synthesis; Ojima, I., Ed.; VCH: New York, 1993.
Chapter 4.1; (c) Rossiter, B. E.; Katsuki, T.; Sharpless, K. B. J. Am. Chem. Soc. 1981,
103, 464; (d) Katsuki, T.; Sharpless, K. B. J. Am. Chem. Soc. 1980, 102, 5974.
2. (a) Jacobsen, E. N. In Catalytic Asymmetric Synthesis; Ojima, I., Ed.; VCH: New
York, 1993. Chapter 4.2; (b) Collman, J. P.; Zhang, X.; Lee, V. J.; Uffelman, E. S.;
Brauman, J. I. Science 1993, 261, 1404; (c) Mukaiyama, T. Aldrichim. Acta 1996,
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5831; (b) Curci, R.; Fiorentino, M.; Serio, M. R. J. Chem. Soc., Chem. Commun.
1984, 155.
½
a 2D0
ꢂ
¼ þ34:2 (c 1.42, CHCl3); IR (thin film) 1743 (C@O) cmꢁ1
;
1H
NMR (CDCl3) d 7.43–7.26 (5H, m), 5.11 (1H, d, J = 3.0 Hz), 5.10
(1H, d, J = 10.8 Hz), 4.80 (2H, s), 4.26 (1H, dd, J = 11.2, 3.3 Hz),
4.13 (1H, d, J = 15 Hz), 3.90 (1H, d, J = 14.7 Hz), 3.23 (3H, s), 3.18
(3H, s), 1.88–1.80 (4H, m), 1.56–1.53 (2H, m), 1.37–1.36 (2H, m);
13C NMR (CDCl3) d 200.2, 137.3, 128.8, 128.6, 128.3, 99.6, 98.9,
97.1, 71.9, 70.8, 70.6, 67.6, 47.5, 47.2, 27.3, 27.2, 21.6; MS (ESI)
m/z (relative intensity) 401 ([M+Na]+, 100), 402 (15); HRMS (ESI)
calcd for C20H26O7 [M+Na]+ 401.1571, found 401.1564.
4.1.4. Cyclohexane-dineopentyl acetal 5
A
solution of cyclohexane-dimethyl acetal
4
(100 mg,
0.26 mmol) in benzene (20 mL) containing neopentyl alcohol
(93 mg, 1.0 mmol) and p-TsOH (2 mg) was heated at reflux with
a Dean-Stark trap for 12 h. The cooled reaction mixture was then
treated with saturated aqueous NaHCO3 and extracted with Et2O
(3 ꢀ 20 mL). The combined organic extracts were dried over anhy-
drous MgSO4, and filtered. The filtrate was concentrated under re-
duced pressure. The crude residue was purified by flash column
chromatography to afford cyclohexane-dineopentyl acetal 5 as a
colourless syrup (111 mg, 87%): Rf 0.33 (hexane–Et2O, 2:1);
5. (a) Yang, D.; Yip, Y.-C.; Tang, M.-W.; Wong, M.-K.; Zheng, J.-H.; Cheung, K.-K. J.
Am. Chem. Soc. 1996, 118, 491; (b) Yang, D.; Wang, X.-C.; Wong, M.-K.; Yip, Y.-
C.; Tang, M.-W. J. Am. Chem. Soc 1996, 118, 11311.
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2004, 37, 497; (c) Shi, Y. Acc. Chem. Res. 2004, 37, 488.
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(b) Shing, T. K. M.; Lloyd-Williams, P. J. Chem. Soc., Chem. Commun. 1987, 423.
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9. Shing, T. K. M.; Leung, G. Y. C. Tetrahedron 2002, 58, 7545.
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C.; Yeung, K. W. Tetrahedron Lett. 2003, 44, 9225.
½
a 2D0
ꢂ
¼ þ36:4 (c 0.66, CHCl3); IR (thin film) 3492 (OH) cmꢁ1
;
1H
NMR (CDCl3) d 7.34–7.25 (5H, m), 5.01 (1H, d, J = 1.8 Hz), 4.76
(1H, d, J = 12.0 Hz), 4.61 (1H, d, J = 12.0 Hz), 4.42 (2H, m), 3.91
(1H, br s), 3.81 (1H, dd, J = 12.6, 1.5 Hz), 3.79 (1H, dd, J = 12.6,
1.5 Hz), 3.02 (2H, br s), 2.99 (1H, dd, J = 11.4, 8.1 Hz), 2.98 (1H,
dd, J = 11.4, 8.1 Hz), 2.01 (1H, br s), 1.90–1.77 (4H, m), 1.49–1.48
(2H, m), 1.38–1.25 (2H, m), 0.99 (9H, s), 0.93 (9H, s); 13C NMR
(CDCl3) d 138.4, 128.4, 127.7, 127.6, 98.8, 98.7, 98.5, 69.8, 69.0,
66.5, 66.1, 63.7, 32.0, 27.8, 27.5, 27.4, 21.5; MS (ESI) m/z (relative
12. For a review on 1,2-diacetals see: Ley, S. V.; Baeschlin, D. K.; Dixon, D. J.; Foster,
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