Acceleration of bromide mediated benzoylperoxide oxidations of secondary alcohols in aqueous organic solvents

Article abstract of DOI:10.1016/j.tetlet.2008.03.098

The efficiency of the bromide mediated benzoylperoxide oxidation of 2° alcohols to ketones was greatly improved by the addition of water. The aqueous oxidation protocol allows also the direct use of off-the-shelf benzoylperoxide reagent without an otherwise necessary and potentially dangerous drying procedure. The oxidation of cyclopentanol, cyclohexanol, 1-phenyl-ethanol and three menthol isomers occurred in good to excellent yields. The oxidation reaction tolerated N,N-dimethylacetamide (DMA) as the solvent, which resulted in a slightly lower oxidation rate than acetonitrile. Chemoselective oxidation of vicinal diols to α-hydroxy ketones did not succeed under the aqueous organic conditions employed as over-oxidation and bromination side-reactions were observed. The impact of water content, solvent, oxidant source and type of alcohol substrates employed was investigated.

Full text of DOI:10.1016/j.tetlet.2008.03.098

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 3199–3203
Acceleration of bromide mediated benzoylperoxide oxidations
of secondary alcohols in aqueous organic solvents
a
a
a,b,
*
Jennessa Ji Youn Youm , Marcel Schlaf , Matthias Bierenstiel
a
University of Guelph, Department of Chemistry, Guelph, Ontario, Canada N1E 2W1
Cape Breton University, Department of Chemistry, Sydney, Nova Scotia, Canada B1P 6L2
b
Received 4 March 2008; revised 18 March 2008; accepted 19 March 2008
Available online 23 March 2008
Abstract
The efficiency of the bromide mediated benzoylperoxide oxidation of 2° alcohols to ketones was greatly improved by the addition of
water. The aqueous oxidation protocol allows also the direct use of off-the-shelf benzoylperoxide reagent without an otherwise necessary
and potentially dangerous drying procedure. The oxidation of cyclopentanol, cyclohexanol, 1-phenyl-ethanol and three menthol isomers
occurred in good to excellent yields. The oxidation reaction tolerated N,N-dimethylacetamide (DMA) as the solvent, which resulted in a
slightly lower oxidation rate than acetonitrile. Chemoselective oxidation of vicinal diols to a-hydroxy ketones did not succeed under the
aqueous organic conditions employed as over-oxidation and bromination side-reactions were observed. The impact of water content,
solvent, oxidant source and type of alcohol substrates employed was investigated.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Hypobromous acid; Benzoyl hypobromite; Selective oxidation; Mechanistic studies; Vicinal diol
5
,6
1
. Introduction
vic-diols to a-hydroxy ketones. This remarkable method
generates the active oxidant hypobromous acid, HOBr, at a
low concentration that is steadily replenished through a
cascade of comproportionation and disproportionation
a-Hydroxy ketones (a-ketols) are an important func-
1
–3
tional group in natural products and have been synthe-
sized by many different methods. However, only little is
4
6
reactions.
known about the selective oxidation of vicinal diols to
a-hydroxy ketones as over-oxidation to vic-diones and carb-
oxylic acids is thermodynamically favoured (Scheme 1). We
reported earlier the effectiveness and the chemoselectivity
of Ishii’s NaBrO /NaHSO reagent for the oxidation of
Doyle et al. reported a mild method for the oxidation of
sterically hindered 2° alcohols to ketones by a bromide
mediated benzoylperoxide (Bz O ) oxidation mechanism
2
2
generating benzoyl hypobromite, PhC(O)OBr, as the oxi-
7
–11
dant.
exhibited excellent regioselectivity for the oxidation of
,2-disubstituted-1,4-butanediols to c-lactones by the for-
In the presence of nickel(II) salts, the method
3
3
2
mation of a seven-membered nickel(II)-1,4-butanediol
complex intermediate. These results prompted us to exam-
OH
OH
O
O
O
COOH
COOH
8
OH
ine Doyle’s method for the oxidation of 2° alcohol func-
tions of vic-diols under polar, aprotic conditions, which
we considered potentially interesting for the synthesis of
usoliduloses (‘oxo’-sugars) in which a 2° alcohol is oxidized
vic-diol
α-hydroxy ketone vic-dione α,ω-dicarboxylic acid
Scheme 1. Oxidation of cis-/trans-1,2-cyclohexanediol.
1
2–17
to a carbonyl group.
Here, we report an improvement
*
on and mechanistic insights into Doyle’s bromide mediated
peroxide oxidation method of alcohols.
Corresponding author. Tel.: +1 902 563 1391; fax: +1 902 563 1880.
E-mail address: matthias_bierenstiel@cbu.ca (M. Bierenstiel).
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.03.098

3200
J. J. Y. Youm et al. / Tetrahedron Letters 49 (2008) 3199–3203
2
. Results and discussion
is concentration dependent at low water levels and is little
affected once an elevated water content has been reached.
Doyle’s oxidation method was studied for cyclic 2° alco-
In order to oxidize carbohydrates CH CN was changed
3
9,20
1
hols such as cyclopentanol and cyclohexanol, which were
employed as simple furanose and pyranose sugar model
systems. In anhydrous acetonitrile medium at 60 °C, com-
plete conversion to cyclopentanone and cyclohexanone was
detected by GC after 17 h and 12 h reaction time, respec-
tively. A dramatic increase of the reaction rate was discov-
ered when wet Bz O was used instead of anhydrous
to N,N-dimethylacetamide (DMA).
Aprotic, polar
solvents, such as DMA, are uniquely suited to dissolve
partially protected sugars and free monosaccharides.
1-Phenyl-ethanol, cyclopentanol and cyclohexanol were
oxidized to the corresponding ketones in DMA after 24 h
at 60 °C in 92%, 74% and 80%, respectively. The lower
reaction rate in DMA relative to acetonitrile is presumably
due to increased peroxide deactivation, which is more
prominent in solvents with higher polarity, by the loss of
2
2
Bz O . Commercially available benzoylperoxide contains
2
2
approximately 30 wt % water and requires the removal of
1
8
the water by an appropriate drying method. (Caution—
CO via pathways of carboxy inversion and subsequent
2
2
1
Drying of Bz O can cause dangerous explosions and
side reactions. In order to decrease the thermal decompo-
sition of Bz O , the oxidation of 1-phenyl-ethanol was car-
2
2
should only be conducted on a small scale.) After 1 h reac-
tion time, the oxidation reaction of cyclohexanol had a
cyclohexanone content of 38%, detected by GC, when
wet Bz O was used, compared to 18% under anhydrous
2
2
ried out at ambient temperature. In DMA and acetonitrile
solvent, the oxidation rate is very slow at ambient temper-
ature with only 10% acetophenone product after 48 h.
Quantitative conversions were confirmed by GC after
12 h when a second addition of 1.5 equiv of Bz O had
2
2
conditions. The water content of 432 mg of wet benzoyl-
peroxide (30 wt % water) in 6.5 mL of acetonitrile gave a
.1 vol % aqueous acetonitrile solution. The influence of
2
2
2
2,23
1
been added after 6 h.
We also conclude that the accel-
the water was thus investigated by varying the water con-
tent of acetonitrile solutions (Table 1). A 20 vol % and
eration of the oxidation reaction is not a solvent effect as
similar reactivity profiles have been observed in different
3
0 vol % water content resulted in 95% and 98% cyclo-
hexanone yields after only 1 h at 60 °C, that is, a more than
0-fold acceleration compared to the anhydrous condition.
solvents, DMA and CH CN, only in the presence of water.
3
Further insights into the oxidation mechanism were
obtained by a series of experiments varying nickel salts,
oxidants and substrates. Nickel salts with various counter
ions were investigated for the oxidation of cyclohexanol
1
Additional water, however, decreased the yield. Oxidation
in water only, that is, in absence of acetonitrile, was not
possible due to the insolubility of benzoylperoxide in water.
The water content–ketone yield correlation can be
explained by an acceleration of the reaction rate with
water: The steep initial increase reaches a maximum at
around 30 vol % water as benzoylperoxide becomes less
soluble in the solvent mixture, which results in an overall
decline in the reaction rate by either a reduction of the
effective concentration of benzoylperoxide or a change in
the reaction mechanism. The large increase in reaction rate
from 1.5 vol % to 20 vol % and the little increase from
2
+
as Ni influenced the regioselectivity of the oxidation of
2,2-disubstituted-1,4-butanediols to c-lactones dramati-
7
,8
cally. The reactions were conducted with 2.5 equiv of
Bz O in 20 vol % aqueous acetonitrile solvent at 60 °C
2
2
and were monitored by GC. A snapshot of the cyclohexa-
none content after 2 h was chosen in order to compare the
influences of the nickel salts. No oxidation products were
detected when the nickel salts lacked halogenides, that is,
2
4
NiSO , Ni(NO ) , Ni(ClO ) , Ni(OAc) and NiO . The
4
3 2
4 2
2
2
best result was obtained with NiBr with a 47% cyclohexa-
2
2
0 vol % to 30 vol % is an indication that the reaction rate
none content after 2 h. NiBr (PPh ) showed a slightly
2
3 2
decreased oxidation activity, 37% cyclohexanone after
h, presumably by the interference of triphenyl phosphine
2
Table 1
or by reduced Ni–Br dissociation, thereby lowering the
effective bromide ion concentration. The presence of tri-
phenyl phosphine also interfered with the isolation of the
ketones. NiCl and NiI produced a cyclohexanone content
a
Influence of water content on the oxidation of cyclohexanol
Entry Volume of Volume of
vol % of Cyclohexanone
b
H
2
O (mL)
CH
3
CN (mL) water
content after 1 h (%)
2
2
1
2
3
4
5
6
—
6.5
6.4
5.2
4.5
3.25
—
—
1.5
20
30
50
18
38
95
98
83
0
of 23% and 20% after 2 h.
c
0.1
1.3
2.0
3.25
6.5
Table 2 lists the result of the oxidation of 2° alcohols in
DMA. Cyclohexanol was oxidized completely to cyclo-
hexanone within 1 h, while cyclopentanol required 3 h for
quantitative oxidation. The investigation of steric influ-
ences in a series of the menthol isomers showed that the
oxidation of neomenthol was completed after 1 h, while
the menthone and isomenthone contents produced for
menthol and isomenthol were 65% and 36% yields. There
was a clear preference for the oxidation of OH in axial
versus equatorial positions as the reactivity decreases from
neomenthol > menthol > isomenthol. This reactivity effect
d
100
a
Cyclohexanol (0.50 mmol, 100 mg), anhydrous benzoylperoxide
1.25 mmol, 303 mg) and NiBr (0.125 mmol, 27.4 mg) in 6.5 mL at 60°C.
Determined by GC; calibrated against methyl sulfone as internal
(
2
b
standard. Typical error range less than ± 2%.
c
The result was identical to a 1.1 vol % solution with wet benzoyl-per-
oxide (1.25 mmol, 432 mg wet Bz
CH CN.
2
O2; 30 wt % water) in 6.5 mL of dry
3
d
Vigorous stirring of the reaction mixture.

J. J. Y. Youm et al. / Tetrahedron Letters 49 (2008) 3199–3203
3201
Table 2
substitution of OH with Br. The presence of Bz O could
2 2
a
Oxidation of secondary alcohol substrates
allow a Prevost type reaction mechanism in which a
hydroxy group is substituted by a halogenide via a benzoyl-
Entry Reactant
Product
Yield after Yield after
1
h (%)
100
Cyclopentanone 23
3 h (%)
25
ester intermediate. In an attempt to decrease the amount
1
2
3
Cyclohexanol
Cyclopentanol
Cyclohexanone
—
of brominated by-products the reaction temperature was
lowered from 60 °C to room temperature slowing the oxi-
dation reaction, however, the amounts of brominated com-
pounds generated did not change further suggesting that
they are formed through radical processes with low activa-
tion barriers.
The sugar substrates methyl 4,6-O-benzylidene-a-D-
glucopyranoside, phenyl 4,6-O-benzylidene-a-D-gluco-
pyranoside and methyl 4,6-O-isopropylidene-D-mannopyr-
annoside were tested in aqueous DMA at 60 °C. After
work-up, the product mixtures had a syrupy consistency,
and an intractable mixture of a multitude of products
was detected by NMR that indicates non-selective decom-
position of the sugars. The decomposition of the sugar
compounds can be caused by thermal stress, low pH gener-
ated by the benzoic acid products and side reactions of oxi-
dant. Attempts to increase the chemoselectivity by
100
100
1-Phenyl-ethanol Acetophenone
94
HO
4
5
65
86
—
O
(
+)-menthol
(−)-menthone
100
36
O
OH
(
−)-menthone
(
+)-neomenthol
6
6
46
HO
O
(
+)-isomenthol
(+)-isomenthone
a
Alcohol (1.0 mmol), NiBr
2
(0.25 mmol), wet Bz
2
O
2
(2.5 mmol) in
.5 mL of DMA at 60 °C with dimethyl sulfone as internal GC standard.
can be explained by a preferential attack of the oxidant,
benzoyl hypobromite or hypobromous acid (vide infra),
of an equatorial C–H of the 2° alcohol, that is, an axial
OH group, as the hydrogen is less sterically shielded by
buffering of the reaction mixture with Na HPO failed as
2
4
again only decomposition products could be detected.
The modified Doyle’s oxidation method is therefore only
suitable for simple 2° alcohols.
1
,3-diaxial interactions (Fig. 1). The bulky isopropyl group
Doyle proposed that the oxidation is based on the
formation of benzoyl hypobromite, which is generated by
of the menthol isomers influences the conformation of the
cyclohexane ring and thus directs the hydroxyl group to an
equatorial position in menthol, to an intermediate equato-
rial–axial position in isomenthol and to an axial position in
neomenthol affecting the oxidation rates of the respective
isomer. The activated benzylic C–H group in 1-phenyl-
ethanol was less favoured for the oxidant attack than the
aliphatic equatorial C–H in neomenthol. A similar oxida-
tion reactivity profile has been obtained for the menthol
7
,8
the reaction of bromide with Bz O (Scheme 2).
2
2
Although the stoichiometry of the reaction is known, the
actual oxidation mechanism of the reaction of benzoyl
hypobromite with a 2° alcohol is not well understood. It
+
is assumed that a formal Br cation, or the positively
charged Br in benzoyl hypobromite, attacks the geminal
C–H of the 2° alcohol (Fig. 1). A detailed investigation
of the formation of benzoyl hypobromite as intermediate
in anhydrous acetonitrile in the presence of 18-crown-6
6
series with the NaBrO /NaHSO reagent.
3
3
cis- and trans-Configurations of 1,2-cyclohexanediol
and 1,2-cyclopentanediol were oxidized in 20 vol % aque-
ous DMA at 60 °C. The oxidations of vic-diols resulted
in unselective side reactions as a multitude of compounds
were observed by GC. GC–MS analysis confirmed the
presence of monobrominated compounds based on the
characteristic M and M+2 m/z signal pattern for bromine
isotopes. However, it was impossible to identify the prod-
ucts of these complex reaction mixtures. The brominated
compounds had molecular masses that indicated a formal
2
6
ether and KBr was reported by Opeida et al. Bunce
et al. reported that the related benzoyl hypochlorite was
very sensitive towards water as it quenched a radical reac-
2
7,28
tion pathway through the hydrolysis.
Based on the
results of the studies involving nickel salts, oxidant sources,
water content and solvent we would like to postulate that
the rate acceleration under wet conditions is based on the
in situ generation of hypobromous acid, HOBr, which is
generated by the hydrolysis of the benzoyl hypobromite
intermediate (Scheme 3). An explanation for the accelera-
tion of the oxidation rate is the smaller size of HOBr and
d+
axial OH-group
equatorial OH-group
the greater polarization of Br in HOBr compared to
+
H
+
Br
OR
δ
H
NiBr2
+
[C H C(O)O]
[C H C(O)O]NiBr + C H C(O)OBr Eq. 1
6 5 6 5
6
5
2
H
O
+
δ
+
H
C H C(O)OBr + RR'CHOH
RR'C=O + C H COOH + HBr
Eq. 2
Eq. 3
Br
6
5
6 5
O
H
OR
H
[C6H5C(O)O]NiBr + HBr
6 5
NiBr2 + C H COOH
equatorial attack of BrOR
R = H or benzoyl
1,3-diaxial shielding
of axial BrOR attack
[
NiBr2]
RR'CHOH + [C6H5C(O)O]2
RR'C=O + 2 CH H COOH
Eq. 4
6
5
Fig. 1. Proposed equatorial versus axial attack of BrOR (R = H or
benzoyl).
2 2 2
Scheme 2. NiBr mediated Bz O oxidation of a 2° alcohol.

3202
J. J. Y. Youm et al. / Tetrahedron Letters 49 (2008) 3199–3203
O
O
2 M NaHCO solution and the aqueous layer is extracted
3
Br
by 3 Â 20 mL of diethyl ether. The combined organic layers
O
OH
+
H O
+ HOBr
2
were washed with 25 mL of brine, dried over anhydrous
Na SO , filtered and the solvent was removed under
2
4
Scheme 3. Hydrolysis of benzoyl hypobromite to generate the proposed
actual oxidant HOBr.
reduced pressure.
4.2. General procedure for large scale oxidations
benzoyl hypobromite. Thus, HOBr can more easily attack
the geminal C–H of the 2° alcohol group (Fig. 1).
In a 100 mL round-bottom flask, alcohol (2.0 mmol)
benzoylperoxide (5 mmol, 1.21 g) and nickel(II)bromide
1.0 mmol, 218 mg) in 5 mL of water and 20 mL of DMA
(
3
. Conclusion
or CH CN were stirred at 60 °C. After completion, the
3
reaction mixture is quenched with 30 mL 1.0 M sodium
The utility of the NiBr2 mediated benzoylperoxide
thiosulfate/2 M NaHCO solution and the aqueous layer
3
method for the oxidation of 2° alcohols to ketones was sub-
stantially improved by the addition of water. The aqueous
oxidation protocol allows the direct use of commercially
available benzoylperoxide off-the-shelf without a poten-
tially dangerous drying procedure and also results in
shorter reaction times. The modified oxidation method
tolerates also DMA solvent, however, the oxidation
proceeds at a slightly slower rate than in acetonitrile. The
oxidation of vicinal-diol compounds failed as over-oxida-
tion and bromination side reactions were observed.
is extracted with 3 Â 50 mL diethyl ether. The combined
organic layers were washed with 50 mL brine, dried over
anhydrous Na SO , filtered and the solvent was removed
under reduced pressure. Without further purification, the
ketone products were characterized by GC–MS and
NMR spectroscopy and found to be congruent to literature
values.
2
4
Acknowledgements
Support of this project by the Canadian Natural Science
and Engineering Research Council (NSERC) and the
Canadian Foundation for Innovation (CFI) and the Ontar-
io Innovation Trust Fund (OIT) is gratefully acknow-
ledged. M.B. would like to thank the Schering Research
Foundation, Berlin, Germany, for a doctoral fellowship.
4
. Experimental
NMR spectra (400 MHz/ H; 100 MHz/ C) were mea-
1
13
1
13
sured in deuterated chloroform (d 7.24, H; d 77.0, C)
as the internal reference. GC analyses were performed on
a 30 m  0.25 mm PEG column. The GC-FID was cali-
brated for cyclohexanol, cyclopentanol, cis- and trans-
Supplementary data
1
,2-cyclohexanediol, cis- and trans-1,2-cyclopentanediol,
a-hydroxy cyclohexanone and 1,2-cyclohexadione with
dimethyl sulfone as an internal standard. All chemicals
were purchased from commercial sources and were reagent
grade and used as obtained without further purification
unless otherwise noted. Oil-pump vacua applied in the
isolation of the ketones were 630 mTorr. Caution: The
handling of solid peroxides—particularly anhydrous—can
Supplementary data (additional oxidant source studies,
CAS registry numbers of organic compounds and IR spec-
troscopic data) associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.03.098.
References and notes
1
8
lead to potential explosions. All experiments should be
performed in a fume hood. Excess peroxide is to be
destroyed with aqueous saturated sodium thiosulfate
1
2
. The Merck Index, 13th ed.; Merck: Whitehouse Station, NJ, 2001.
. Larock, R. C. Comprehensive Organic Transformation, 2nd ed.; Wiley-
VCH: New York, 1999.
1
8
3
. Hudlicky, M. In Oxidations in Organic Chemistry. In ACS Mono-
graph; American Chemical Society: Washington, DC, 1990.
. Acyloin condensation of diesters (a) Finley, K. T. Chem. Rev. 1964,
64, 573–589. Oxidation of alkenes: (b) Murahashi, S.-I.; Saito, T.;
Hanaoka, H.; Murakami, Y.; Naota, T.; Kumobayashi, H.; Akuta-
gawa, S. J. Org. Chem. 1993, 58, 2929–2930; (c) Sakata, Y.;
Katayama, Y.; Ishii, Y. Chem. Lett. 1992, 671–674. a-Hydroxylation
of ketones: (d) El-Qisairi, A. K.; Qaseer, H. A. J. Organomet. Chem.
solution.
4
4
.1. General procedure for small-scale oxidations
In an 8 mL screw top vial, alcohol (0.50 mmol),
nickel(II)bromide (0.25 mmol = 0.50 equiv with respect to
available bromide and alcohol, 54 mg) and dimethyl sul-
2
002, 659, 50–55; (e) Moriarty, R. M.; Hu, H.; Gupta, S. C.
fone (ꢀ50 mg) were dissolved in 5.2 mL DMA or CH CN
Tetrahedron Lett. 1981, 22, 1283–1286. Enol ether: (f) Hashiyama, T.;
Morikawa, K.; Sharpless, K. B. J. Org. Chem. 1992, 57, 5067–5068.
Silylenol ethers: (g) Takai, T.; Yamada, T.; Rhode, O.; Mukaiyama,
T. Chem. Lett. 1991, 281–284.
3
and 1.3 mL water at 60 °C. The reactions were conducted
in air. After the addition of solid benzoyl peroxide
(
Bz O ; 1.25 mmol, 303 mg) the reaction mixture is stirred
2 2
5
. Sakaguchi, S.; Kikuchi, D.; Ishii, Y. Bull. Chem. Soc. Jpn. 1997, 70,
at 60 °C for 1–24 h and monitored by GC analysis of the
crude reaction mixture. After completion, the reaction mix-
ture is quenched with 10 mL of 1.0 M sodium thiosulfate/
2
561–2566.
6. Bierenstiel, M.; D’Hondt, P. J.; Schlaf, M. Tetrahedron 2005, 61,
911–4917.
4

J. J. Y. Youm et al. / Tetrahedron Letters 49 (2008) 3199–3203
3203
7
8
. Doyle, M. P.; Bagheri, V. J. Org. Chem. 1981, 46, 4806–4808.
. Doyle, M. P.; Dow, R. L.; Bagheri, V.; Patrie, W. J. J. Org. Chem.
17. Giffhorn, F.; K o¨ pper, S.; Huwig, A.; Freimund, S. Enzyme Microb.
Technol. 2000, 27, 734–742.
1
982, 48, 476–480.
18. Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory
Chemicals; Butterworth-Heinemann: Amsterdam, NL, 2003.
19. Collins, P.; Ferrier, R. Monosaccharides; John Wiley & Sons:
Chichester, UK, 1995.
20. Bierenstiel, M.; Schlaf, M. Eur. J. Org. Chem. 2004, 1474–1481.
21. Tarbell, D. S. Acc. Chem. Res. 1969, 2, 296–300.
22. Hawkins, E. Organic Peroxides, Their Formation and Reactions; E.
and F. Spon: London, UK, 1961.
9
. Doyle, M. P.; Dow, R. L. Synth. Commun. 1980, 10, 881–888.
1
1
1
1
1
0. Doyle, M. P.; Dow, R. L.; Bagheri, V.; Patrie, W. J. Tetrahedron Lett.
980, 21, 2795–2798.
1. Doyle, M. P.; Partie, W. J.; Williams, S. B. J. Org. Chem. 1979, 44,
955–2956.
1
2
2. Tsuda, Y.; Hanajima, M.; Matsuhira, N.; Okuna, Y.; Kanemitsu, K.
Chem. Pharm. Bull. 1989, 39, 2344–2350.
3. Liu, H.-M.; Sato, Y.; Tsuda, Y. Chem. Pharm. Bull. 1993, 41, 491–
23. Davies, A. G. Organic Peroxides; Butterworth: London, UK, 1961.
24. Even after 48 h reaction time.
25. Wilson, C. V. Org. React. 1957, 9, 350–352.
26. Opeida, I. A.; Turovskii, N. A.; Maksyuta, N. V. Russ. J. Org. Chem.
2000, 36, 1646–1649.
27. Bunce, N. J.; Tanner, D. D. J. Am. Chem. Soc. 1969, 91, 6096–6102.
28. Bunce, N. J.; Urban, L. O. Can. J. Chem. 1971, 49, 821–827.
5
01.
4. Gilles, M. K.; Polak, M. L.; Lineberger, W. C. J. Chem. Phys. 1992,
6, 8012–8020.
9
15. Freimund, S.; K o¨ pper, S. Carbohydr. Res. 1998, 308, 195–200.
6. Freimund, S.; Huwig, A.; Giffhorn, F.; K o¨ pper, S. Chem. Eur. J.
1
1998, 4, 2442–2455.