Oxidation of (1S,5R,7R,S)-(4,7-Dimethyl-6-oxabicyclo[3.2.1]oct-3-en-7-yl) methanol with pyridinium chlorochromate

Article abstract of DOI:10.1134/S1070428011050058

The oxidation of (1S,5R,7R,S)-(4,7-dimethyl-6-oxabicyclo[3.2.1]oct-3-en-7- yl)methanol epimeric at the C7 atom resulted in scalemic (5R)-5-acetyl-2- methylcyclohex-2-en-1-one. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S1070428011050058

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2011, Vol. 47, No. 5, pp. 682−686. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © S.A. Torosyan, F.A. Gimalova, R.F. Valeev, M.S. Miftakhov, 2011, published in Zhurnal Organicheskoi Khimii, 2011, Vol. 47, No. 5,
pp. 680−684.
Oxidation
of (1S,5R,7R,S)-(4,7-Dimethyl-6-oxabicyclo[3.2.1]oct-3-en-7-yl)
methanol with Pyridinium Chlorochromate
S. A. Torosyan, F. A. Gimalova, R. F. Valeev, and M. S. Miftakhov
Institute of Organic Chemistry, Ufa Scientific Center, Russian Academy of Sciences
Ufa, 450054 Russia; e-mail: bioreg@anrb.ru
Received July 14, 2010
Abstract—The oxidation of (1S,5R,7R,S)-(4,7-dimethyl-6-oxabicyclo[3.2.1]oct-3-en-7-yl)methanol epimeric at
the C7 atom resulted in scalemic (5R)-5-acetyl-2-methylcyclohex-2-en-1-one.
DOI: 10.1134/S1070428011050058
Corey’s reagent C5H6N+CrO3Cl– (pyridinium chloro-
chromate, PCC) [1] is a stable and convenient for han-
dling oxidant for alcohols, series of S-, N-, B-containing
compounds, for activated C–H and C=C bonds [2].
Among the synthetically valuable PCC applications the
oxidation of allyl alcohols should be mentioned provid-
ing rearranged α,β-unsaturated aldehydes and ketones [3,
4]. We observed an interesting example of involvement
into similar succession of conversions in the course of
the study of oxidation with PCC of a mixture of alcohol
I epimeric at C7-atom (~6 : 5) and containing an allyl-
ether fragment.As a result of this reaction we obtained in
a good yield cyclohexenone II with [α]D20 –2.5° (C 0.09,
MeOH) (Scheme 1). The rotation angle of the sample
(–)-(II) obtained by the splitting of the epoxide III with
periodic acid proved to be –53.0° (C 0.26, MeOH). The
difference in the rotation angles of the samples of cyclo-
hexenone II from two experiments on the oxidation of
alcohol I with PCC showed the materially total loss of the
opical activity in the course of oxidation, consequently,
on the formation of commeasurable amounts of (–)- and
(+)-diketone II.
epimerization at the center C5 of diketone II under the
conditions of oxidation of alcohol I with PCC is ruled
out. We presume that the formation of approximately
equal amounts of (–)- and (+)-enantiomers of diketone
II is due to the streochemical heterogeneity of compound
I governing different directions of the oxidative splitting
with pyridinium chlorochromate.
Inasmuch as alcohol I used in the study was a mixture
of 7S- and 7R-hydroxymethyl-epimers in the ratio ~6 :
5, the necessary condition of the essential racemiza-
Scheme 1.
O
2 equiv. PCC
CH2Cl2, 20°C
O
OH
O
II
I
O
O
O
HIO4
THF_H2O
PCC is known to be of weakly acidic character. The
R-(–)carvone derivatives with the C4-acetyl function
resembling compound II are not prone to epimerization
in the H+-containing environment [5, 6]. Therefore the
O
O
(+)-II
O
(_)-II
III
682

683
OXIDATION OF (1S,5R,7R,S)-(4,7-DIMETHYL-6-OXABICYCLO[3.2.1]OCT-3-EN-7-YL)
tion should be the process where each of these epimers demonstrated new versions of transformations initiated
would react with PCC giving a single enantiomer. This by PCC, including the oxidation in the tertiary allyl site
statement is made clear by the possible versions of the of the substrate along the “1,3-transition” pathway and
successive transformations of alcohol I epimers shown fragmentation in the quaternary center containing a hy-
in Scheme 2. In keeping with the common concepts on droxymethyl group with the generation of a keto function.
the oxidation with chromates [7] similar intermediates
A and A' are generated in the first stage. In the first case
the oxidative fragmentation of A initiated by PCC oc-
curs through intermediate B [8] and results in diketone
(–)-(II). In intermediate A' owing to the favorable spatial
orientation of the Cr=O bond the substitution of SN2’type
in the allyl-ether fragment gets probable. Intermediate
B’ formed in this process suffers the fragmentation into
ketone (+)-(II).
Alcohol I used in the study was synthesized from R-
(–)-carvone. First epoxide III was obtained by the reaction
with m-CPBA (3-chloroperoxybenzoic acid) in CH2Cl2
at 20°C. This reaction is exclusively regioselective but
not stereoselective. Epoxide III is an epimer mixture in
the ratio ~6 : 5 with respect to the epoxy-bearing center.
At its purification on silica gel insignificant amounts of
aldehyde IV were found. The same compound was ob-
tained in high yield at keeping epoxide III in CH2Cl2 in
Thus by the example of oxidation of alcohol I we the presence of BF3·Et2O.
Scheme 2.
7S-(I)
CrO3
7R-(I)
CrO3
O
O
O
.
BF3 Et2O
m-CPBA
CH2Cl2
CH2Cl2
O
O
IV
III
O
O
O
O
OCr O
OH
O
OCr
The reduction of epoxyketone III by the system
NaBH4–CeCl3 in MeOH at 20°C was carried out till com-
plete consumption of compound III (TLC monitoring
OH
A'
A
)
(Scheme 3). Like in similar processes [6], the stereochem-
ical result of the reduction of epoxide III was governed
by the steric control of C5-substituent: The predominant
formation of β-alcohol Va was observed. However this
reaction was complicated by the simultaneous opening of
the epoxide by the alkoxy anion in alcohol Va resulting
in bicycle I. As a result formed a mixture of epoxy- (V)
and bicyclic (I) alcohols in the ratio 2 : 1 and in overall
yield 80%. This mixture by repeated chromatography on
SiO2 was separated in two fractions of different polarity.
HO
O
O
Cr O
O
O
B
B'
CrO3
(+)-II
CrO3
(_)-II
Scheme 3.
H3C
H3C
4
5
6O
+ Vа, Vb
1
2
7
CH3
OH
HO
_
NaBH4 CeCl3
CH2Cl2, 20oC
SiO2
Iа
+
III
O
O
Compounds
OH
O
+
of unknown
structure
Vа, Vb
Iа, Ib
OH
CH3
Ib
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 5 2011

TOROSYAN et al.
684
Therewith the good separation of bicycle I diastereomers
was attained, but each diastereomer contained a minor
side product. The less polar fraction was composed of
the mixture of β-epoxyalcohol Va,Vb and diastereomer
Ia, ~1 : 1, and the more polar fraction was the mixture of
diastereomer Ib and compound of unidentified structure in
the ratio 3 : 1. The characteristic signals in the 1H NMR
spectra used for the assignment of diastereomers Ia, Ib
are the peaks of the protons of the methyl groups in the
position 7 and of CH2O respectively: Due to the steric
hindrances they are located more upfield.
drous THF was added by portions 0.16 g (0.51 mmol)
of H-Bu4N+F–, the reaction mixture was stirred at room
temperature till complete consumption of the initial
compound (TLC monitoring). The residue after the
evaporation in a vacuum was purified by column chroma-
tography on SiO2 (eluent EtOAc–petroleum ether, 1 : 1).
Yield 0.05 g (85%), isomers mixture. IR spectrum, cm–1:
3414, 2960, 2927, 2873, 1700, 1629, 1448, 1410, 1375,
1159, 1130, 1099, 1070, 1032, 991, 914, 829, 756. Mass
spectrum, m/z (Irel, %): 168 [M]+ (1), 137 [M – CH2OH]+
(90), 93 (100), 43 [CH3CO]+ (98). Found, %: C 71.97;
H 9.77. M+ 168.10. C10H16O2. Calculated, %: C 71.39;
H 9.59. M 168.23.
Hence the products of this reaction are isomer pairs of
compounds V and I with a little prevalence of bicycle I. It
was formerly [9] indicated the possibility of formation of
epimeric mixture Ia, Ib at the reduction of epoxyketone
III with Luche reagent.
Oxidation of compound I with pyridinium chloro-
chromate. To the stirred dispersion of 0.27 g (1.25 mmol)
of PCC in 4 ml of anhydrous CH2Cl2 at 0°C was added in
one portion a solution of 0.12 g (0.71 mmol) of compound
I in 2 ml of CH2Cl2. The reaction mixture was stirred
for 2 h at 0°C, then at room temperature. It was filtered
through a thin bed of silica gel, the solbent was washed
with CH2Cl2 (5 × 3 ml), the combined extracts were dried
with MgSO4 and evaporated. The residue was subjected
to chromatography on a column packed with SiO2 (ethyl
acetate–petroleum ether, 10 : 3) to obtain 0.04 g (55%) of
oily mixture of enantiomers of (5R,S)-5-acetyl-2-meth-
ylcyclohex-2-en-1-one (II), [α]D20 –15.0±5.0° (C 0.06,
CHCl3), [α]D20 –2.5° (C 0.09, MeOH). 1H NMR spectrum,
δ, ppm: 1.79 s (3H, CH3), 2.19 s (3H, CH3), 2.51–2.56 m
(3H, CH2), 2.69 d.d (1H, CH2, J 4.1, 16.4 Hz), 3.11 m
(1H, CH), 6.70 m (1H, =CH). 13C NMR spectrum, δ, ppm:
15.68 (CH3), 27.46 (C4), 27.83 (CH3), 39.41 (C6), 48.13
(C5), 135.90 (C2), 142.61 (C3), 197.46 (C=O), 207.82
(C1). Found, %: C 71.55; H 8.02. C9H16O2. Calculated,
%: C 71.03; H 7.95.
Then the mixture of compounds V and I was subjected
to silylation in the system TBSCl–Im in CH2Cl2 [10].
Therewith virtually complete conversion was observed
of the components of this mixture into tert-butyldimeth-
ylsilyl-protected (TBS) derivative VI, and the inherent to
epoxide III epimeric composition (6 : 5) was conserved
in TBS-ether VI. Finally the removal of the protective
group in ether VI by the fluoride ion led to the formation
of bicyclic alcohol I used in the study also as a mixture
of epimers
.
EXPERIMENTAL
IR spectra were recorded on a spectrophotometer Shi-
madzu IR Prestige-21 from thin films. 1H and 13C NMR
spectra were registered on a spectrometer Bruker AM-
300 at operating frequencies 300.13 and 75.47 MHz
respectively in CDCl3, internal reference TMS. The
rotation angles were measured on an instrument Perkin
Elmer-341. Mass spectra were taken on an instrument
Thermo Finnigan MAT 95XP, ionizing energy 70 eV,
temperature of ionizing chamber 200°C, that of sample
admission, 5–270°C, heating rate 22 deg/min. The reac-
tion progress was monitored on Sorbfil plates, develop-
ment with a solution of anise aldehyde and sulfuric acid
in ethanol with subsequent heating to 120–150°C. The
products of the synthesis were isolated by column chro-
matography on silica gel (30–60 g of sorbent per 1 g of
substance), as eluents freshly distilled solvents were used.
Oxidation of epoxide III with periodic acid. To
a solution of 0.16 g (0.9 mmol) of epoxide III in 30 ml of
anhydrous ethyl ether at 0°C under an argon atmosphere
was added 0.33 g (1.44 mmol) of periodic acid, and the
mixture was stirred for 1 h at. Then the temperature was
raised to ambient, and the mixture was stirred till com-
plete disappearance of the epoxide (TLC monitoring).Af-
terwards the reaction mixture was washed with saturated
solution of NaHCO3, the water layer was extracted with
ether (3 × 10 ml). The combined organic solutions were
dried with MgSO4, evaporated, the residue was subjected
to chromatography on a column packed with SiO2 (ethyl
acetate–petroleum ether, 10 : 3) to obtain 0.12 g (82%)
of individual 5R-isomer of diketone II.
(1S,5R,7R,S)-(4,7-Dimethyl-6-oxabicyclo[3.2.1]-
oct-3-en-7-yl)methanol (Ia, Ib). To the stirred solution
of 0.12 g (0.43 mmol) of silyl ether VI in 6 ml of anhy-
(5R)-5-Acetyl-2-methylcyclohex-2-en-1-one (–)-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 5 2011

OXIDATION OF (1S,5R,7R,S)-(4,7-DIMETHYL-6-OXABICYCLO[3.2.1]OCT-3-EN-7-YL)
685
(II). [α]D20 –75.1° (C 1.24, CHCl3), [α]D20 –53° (C 0.26,
MeOH). Spectral characteristics coincide with those
described above for the product obtained by the oxida-
tion with PCC.
yield 80%) of a mixture of epimers of epoxyalcohol V
and bicycle I in a ratio ~1 : 2 (according to the intensity
of olefin protons in the 1H NMR spectrum of the mixture).
The mixture was chromatographed on SiO2 to separate
in two pairs of mixtures: Va, Vb + Ia, 1 : 1, and Ib +
unidentified compounds, 3 : 1. The products were char-
acterized by NMR spectra.
(5R)-2-Methyl-5-(2-methyloxiran-2-yl)cyclohex-
2-en-1-one (III). To a dispersion of 0.44 g (2.55 mmol)
of m-CPBA in 20 ml of anhydrous CH2Cl2 was added
dropwise a solution of 0.25 g (1.7 mmol)of R-carvone in
5 ml of CH2Cl2, and the reaction mixture was stirred at
room temperature till complete consumption of the initial
compound (TLC monitoring). The reaction mixture was
diluted with the saturated solution of NaHCO3, the reac-
tion products were extracted into CH2Cl2 (3 × 20 ml),
the combined organic solutions were washed with brine,
dried with MgSO4, evaporated, the residue was subjected
to chromatography on a column packed with SiO2 (elu-
ent EtOAc–petroleum ether, 1 : 2). Yield 0.18 g (72%),
isomers mixture in a ratio 6 : 5 (according to the intensity
of different doublet signals of the epoxide proton in the
1H NMR spectrum).
(1S,5R,7R)-(4,7-Dimethyl-6-oxabicyclo[3.2.1]-oct-
1
3-en-7-yl)methanol (Ia). H NMR spectrum, δ, ppm:
1.20 s (3H, CH3), 1.69 d (3H, CH3, J 1.7 Hz), 1.85 d (1H,
J 10.5 Hz) and 2.17–2.23 m (5H, CH, CH2), 3.56 d (1H,
J 10.5 Hz) and 3.69 d (1H, OCH2, J 10.8 Hz), 4.04 d (1H,
C5H, J 4.2 Hz), 5.24 br.s (1H, C3H). 13C NMR spectrum,
δ, ppm: 19.78 (CH3), 21.38 (CH3), 29.46 (C2), 34.88 (C8),
40.57 (C1), 69.97 (OCH2), 76.62 (C5), 84.76 (C7), 121.04
(C3), 139.32 (C4).
(1R,5R,2’RS)-2-Methyl-5-(2-methyloxiran-2-yl)
cyclohex-2-en-1-ol (Va, Vb). Mixture of diastereomers,
6 : 5. 1H NMR spectrum, δ, ppm: 1.23 (1.24) s (3H, CH3),
1.74 (1.76) s (3H, CH3), 1.80–2.40 (6H, CH, CH2, OH),
2.48–2.70 m (2H, OCH2), 4.17 m (1H, C1H), 5.50 br.s
Main isomer. 1H NMR spectrum, δ, ppm: 1.31 s (3H,
CH3), 1.76 s (3H, CH3), 2.00–2.60 m (6H, CH2, CH,
OCH2), 2.65 d (1H, OCH2, J 4.6 Hz), 6.72 br.s (1H, =CH).
13C NMR spectrum, δ, ppm: 15.50 (CH3), 18.22 (CH3),
27.55 (C4), 39.74 (C6), 40.57 (C5), 52.27 (C3’), 57.73
(C2'), 135.38 (C2), 144.11 (C3), 198.73 (C1).
13
(1H, =CH). C NMR spectrum, δ, ppm: 17.93 (18.56)
(CH3), 18.75 (18.86) (CH3), 27.61 (27.74) (C4), 35.13
(35.20) (C6), 39.19 (39.83) (C5), 53.20 (53.61) (C3’),
58.73 (58.82) (C2’), 70.16 (70.33) (C1), 122.78 (123.02)
(C3), 136.70 (136.81) (C2).
1
Minor isomer. H NMR spectrum, δ, ppm: 1.29 C
(1S,5R,7S)-(4,7-Dimethyl-6-oxabicyclo[3.2.1]-oct-
3-en-7-yl)methanol (Ib). H NMR spectrum, δ, ppm:
1
(3H, CH3), 1.76 s (3H, CH3), 2.00–2.60 m (6H, CH2,
CH, OCH2), 2.69 d (1H, OCH2, J 4.6 Hz), 6.72 br.s (1H,
=CH). 13C NMR spectrum, δ, ppm: 15.50 (CH3), 18.77
(CH3), 27.75 (C4), 40.14 (C6), 41.13 (C5), 52.73 (C3’),
57.84 (C2’), 134.34 (C2), 143.87 (C3), 198.73 (C1). Found,
%: C 72.88; H 8.59. C10H14O2. Calculated, %: C 72.26;
H 8.46.
1.30 s (3H, CH3), 1.71 d (3H, CH3, J 1.8 Hz), 1.87 d (1H,
J 10.0 Hz) and 2.00–2.23 m (6H, CH, CH2, OH), 3.34 d
(1H, OCH2, J 10.8 Hz), 3.40 d (1H, OCH2, J 10.8 Hz),
3.99 d (1H, C5H, J 4.1 Hz), 5.24 br.s (1H, C3H). 13C NMR
spectrum, δ, ppm: 21.43 (CH3), 25.14 (CH3), 30.13 (C2),
34.53 (C8), 38.40 (C1), 67.64 (OCH2), 76.96 (C5), 84.76
(C7), 120.04 (C3), 139.65 (C4).
Reduction of epoxide III. To a solution of 0.50 g
(3.0 mmol) of epoxide III in 10 ml of anhydrous MeOH
was added 1.24 g (5.0 mmol) of CeCl3·7H2O, and the
mixture was stirred till it turned homogenic. Then at
–10°C 0.125 g (3.00 mmol) of NaBH4 was added, the
temperature of the mixture was raised to ambient, and
the stirring was continued till the disappearance of the
initial ketone (TLC monitoring). The reaction mixture
was evaporated, 5 ml of water was added, the reaction
product was extracted into EtOAc (3 × 20 ml), the com-
bined organic solutions were washed with brine, dried
with MgSO4, evaporated, the residue was subjected to
chromatography on a column packed with SiO2 (eluent
EtOAc–petroleum ether, 1 : 1) to obtain 0.44 g (overall
(5R)-2-Methyl-5-(1-oxo-2-propyl)cyclohex-2-en-1-
one (IV). To a solution of 0.13 g (0.78 mmol) of epoxide
III in 10 ml of anhydrous CH2Cl2 was added dropwise
1 ml of BF3·Et2O, the reaction mixture was stirred at room
temperature till complete consumption of the initial com-
pound (TLC monitoring). To the reaction mixture 5 ml
of saturated solution of NH4Cl was added, the organic
layer was separated, the water layer was extracted with
CH2Cl2 (3 × 3 ml). The combined organic solutions were
washed with brine, dried with MgSO4, and evaporated.
The residue was purified by column chromatography
on SiO2 (eluent EtOAc–petroleum ether, 1 : 2). Yield
0.114 g(88%), mixture of diastereomers. Colorless oily
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 5 2011

TOROSYAN et al.
686
Minor isomer. 1H NMR spectrum, δ, ppm: 0.01 s (6H,
SiMe2), 0.85 s (9H, SiCMe3), 1.25 s (3H, CH3), 1.70 d
(3H, CH3, J 1.8 Hz), 1.80 m (1H) and 2.10–2.40 m (4H,
CH, CH2), 3.55 d.d (2H, OCH2, J 4.6 and 9.3 Hz), 4.02 d
(1H, C5H, J 4.4 Hz), 5.20 br.s (1H, C3H). 13C NMR spec-
trum, δ, ppm: –0.97 (SiCH3), 18.26 (CMe3), 21.50 (CH3),
25.63 (CH3), 25.91 [C(CH3)3], 30.34 (C8), 34.18 (C2),
40.02 (C1), 69.56 (OCH2), 76.87 (C5), 85.01 (C7), 120.90
(C3), 139.67 (C4). Found, %: C 68.77; H 10.55; Si 10.22.
C16H30O2Si. Calculated, %: C 68.03; H 10.70; Si 9.94.
substance, [α]D20 –39.7° (C 4.28, CHCl3). IR spectrum,
cm–1: 2974, 2922, 2885, 2717, 1724, 1674, 1452, 1433,
1
1367, 1147, 1107, 1074, 904, 754. H NMR spectrum,
δ, ppm: 0.99 t (1.02) (3H, CH3, J 2.2 Hz), 1.69 s (3H,
CH3), 2.09–2.45 m (6H, CH, CH2), 6.66 br.s (1H, =CH),
9.58 (9.60) d (1H, CHO, J 1.8 Hz). 13C NMR spectrum,
δ, ppm: 10.20 (9.94) (CH3), 15.49 (CH3), 30.22 (28.40)
(C4), 35.39 (C5), 40.62 (41.70) (C6), 49.92 (C2’), 135.50
(C2), 144.08 (C3), 198.56 (C1), 203.32 (CHO). Found, %:
C 71.49; H 8.57. C10H14O2. Calculated, %: C 72.26; H 8.49.
tert-Butyl{[(1S,5R,7RS)-4,7-dimethyl-6-oxabicyc-
lo[3.2.1]oct-3-en-7-yl]methoxy}dimethylsilane (VI).
To a solution of 0.12 g (0.72 mmol) of the mixture of
epoxyalcohol V and bicycle I in 10 ml of anhydrous
CH2Cl2 was added 0.03 g (0.44 mmol) of imidazole,
0.11 g (0.72 mmol) of TBSCl, and a catalytic amount of
DMAP. The reaction mixture was stirred till complete
consumption of the initial compound (TLC monitoring),
then it was diluted with CH2Cl2, washed with brine, dried
with MgSO4, and evaporated. The residue was subjected
to column chromatography on SiO2 (eluent EtOAc–petro-
leum ether, 1 : 1). Yield 0.15 g (75%). Epimers mixture.
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SiMe2), 0.85 s (9H, SiCMe3), 1.20 s (3H, CH3), 1.68 d
(3H, CH3, J 1.8 Hz), 1.80 m (1H) and 2.10–2.40 m (4H,
CH, CH2), 3.30 d.d (2H, OCH2, J 4.6 and 9.8 Hz), 3.93
d (1H, C5H, J 4.6 Hz), 5.20 br.s (1H, C3H). 13C NMR
spectrum, δ, ppm: –0.97 (SiCH3), 18.12 (CMe3), 21.44
(CH3), 25.51 (CH3), 25.81 [C(CH3)3], 29.36 (C8), 34.06
(C2), 37.24 (C1), 67.51 (OCH2), 76.24 (C5), 84.87 (C7),
120.95 (C3), 139.58 (C4).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 5 2011