Preparation of enantiomeric pure (-)-(3R,4S)-1-benzyl-3,4-epoxypiperidine and enriched (-)-(R)-1-Benzyl-3-hydroxy-1,2,3,6-tetrahydropyridine by kinetic separation of (±)-1-benzyl-3,4-epoxypiperidine under the action of chiral lithium amides

Article abstract of DOI:10.1134/s1070428008020164

Enantiomeric pure (-)-(3R,4S)-1-benzyl-3,4-epoxypiperidine and (-)-(R)-1-benzyl-3-hydroxy-1,2,3,6-tetrahydropyridine with enantiomeric excess 61.9% were obtained by kinetic separation of (±)-1-benzyl-3,4- epoxypiperidine under the action of lithium salt (

Full text of DOI:10.1134/s1070428008020164

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2008, Vol. 44, No. 2, pp. 282−287. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © G.V. Grishina, I.S. Veselov, V.A. Davankov, M.M. Il’in, N.S. Zefirov, 2008, published in Zhurnal Organicheskoi Khimii, 2008,
Vol. 44, No. 2, pp. 287−291.
Preparation of Enantiomeric Pure
(−)-(3R,4S)-1-Benzyl-3,4-epoxypiperidine and Enriched
(−)-(R)-1-Benzyl-3-hydroxy-1,2,3,6-tetrahydropyridine by Kinetic
Separation of (±)-1- Benzyl-3,4-epoxypiperidine
under the Action of Chiral Lithium Amides
G. V. Grishinaa, I. S. Veselova, V. A. Davankovb, M. M. Il’inb, and N. S. Zefirova
aLomonosov Moscow State University, Moscow, 119992 Russia
e-mail: grishina@org.chem.msu.ru
bNesmeyanov Institute of Organoelemental Compounds, Russian Academy of Sciences, Moscow, Russia
Received June 1, 2007
Abstract—Enantiomeric pure (–)-(3R,4S)-1-benzyl-3,4-epoxypiperidine and (–)-(R)-1-benzyl-3-hydroxy-1,2,3,6-
tetrahydropyridine with enantiomeric excess 61.9% were obtained by kinetic separation of (±)-1-benzyl-3,4-
epoxypiperidine under the action of lithium salt (+)-(S)-2-[(pyrrolidin-1-yl)methyl]pyrrolidine. The sterical direction
of the kinetic separation of (±)-1-benzyl-3,4-epoxypiperidine and absolute configurations of the target products
were established.
DOI: 10.1134/S1070428008020164
Recently the interest grew significantly to hydroxylat-
ed and aminohydroxylated piperidine derivatives, analogs
of polyhydroxylated piperidine alkaloids of castano-
spermine (I) family, deoxynojirimycin (II), fagomine (III)
[1, 2], and separated from sea organisms aminohydroxyl-
ated alkaloids pseudodistomines A−F IV–IX [3–5],
glucosidase inhibitors containing a hydroxy- and amino-
hydroxypiperidine fragment (Scheme 1).
Both subfamilies of hydroxylated alkaloids or imino-
sugars are strong inhibitors of a series of glucosidases
and related enzymes, they exhibit anticancer and
antiviral, in particular, anti-AIDS activity. The presence
of an amino group results in a selective inhibition of
definite types of glucosidases and hexaminidases. For
instance, aminodiol XI, aminoanalog of galactoiso-
fagomine (X), an inhibitor of β-galactosidase, exhibits
a selective inhibiting activity toward β-glucosidase [6].
Acetylamino derivative of 1- deoxynojirimycin (XII)
inhibits selectively only the β-N-acetylglucaminidase [7].
Scheme 1.
OH
OH
OH
HO
OH
OH
OH
H
OH
OH
HO
OH
N
H
N
H
N
I
II
III
Nowadays an intensive development continues of
efficient synthetic procedures for analogs of natural
iminosugars [8, 9], and the most promising way is the
OH
OH
OH
NH2
NH2
NH2
R
R
N
H
IV_VI
N
N
R
OH
OH
OH
OH
H2N
OH
AcHN
H
H
HO
OH
VII, VIII
IX
R = R1,8-cis (IV), R1,8-trans (V, VIII, IX); R = R2 (VI,
8
R1 =
,
VII);
N
H
X
N
H
XI
N
H
XII
R2 =
.
282

PREPARATION OF ENANTIOMERIC PURE (−)-(3R,4S)-1-BENZYL-3,4-EPOXYPIPERIDINE
283
preparation of optically pure synthetic hydroxylated
piperidines due to the opportunity of multifold increasing
various inhibiting activity of individual enantiomers [10].
enriched allyl alcohol XIV*, and the initial enantiomer
enriched epoxide XIII* would accumulate in the reaction
mixture (Scheme 2).
We report here on developing a convenient and
efficient synthesis of precursors of versatile optically
active 4-amino-3-hydroxylated piperidines while
selecting as a case in point optically pure 1-benzyl-3,4-
epoxypiperidine and enantiomer-enriched 1-benzyl-3-
hydroxy-1,2,3,6-tetrahydropyridine obtained by kinetic
separation of (±)-1-benzyl-3,4- epoxypiperidine under
the action of chiral lithium amides. We had showed
formerly [11] that a series of C- and N-substituted (±)-
3,4-epoxypiperidines by the treatment of lithium dialkyl-
amines had been cleanly rearranged into the correspond-
ing (±)-3-hydroxy-1,2,3,6-tetrahydropyridines, therefore
in this study we used (±)-1-benzyl-3,4-epoxypiperidine
(XIII) and (±)-1-benzyl-3-hydroxy-1,2,3,6-tetrahydro-
pyridine (XIV) as reference compounds.
The kinetic separation of (±)-1-benzyl-3,4-epoxy-
piperidine was performed applying chiral lithium amides
based on (+)-(S)-2-[(pyrrolidin-1-yl)methyl]pyrrolidine
(XV), (+)-(S)-2-[(piperidin-1-yl)methyl]pyrrolidine
(XVI), and (+)-(S)-2-[(morpholin-1-yl)methyl]pyr-
rolidine (XVII) [12] formerly marked as fairly efficient
bases in asymmetrical transformation of meso-epoxides
and in kinetic separation of racemic epoxides [13–15]
(Scheme 3).
We established by chromatographic monitoring the
operating temperature range for the kinetic separation
of epoxide XIII (1.00 equiv) at the action of lithium (+)-
(S)-2-[(pyrrolidin-1-yl)methyl]pyrrolidinide (Li-XV)
(0.20 equiv) in THF under an argon atmosphere: At
–70°C only the presence of initial (±)-epoxide XIII was
observed; gradual raising of the temperature of the
reaction mixture by 20°C steps and keeping at this
temperature for 40 min showed that the rearrangement
giving allyl alcohol XIV* started in the range –15÷–
10°C. The invariability of the mixture with respect to
chromatographic analysis was attained by its maintaining
at 5°C for 12 h. Further the reaction mixture was
hydrolyzed with a saturated ammonium chloride solution
till pH 9. The target epoxide XIII* and allyl alcohol
XIV* were extracted with dichloromethane, and
It is presumable that in the kinetic separation of
(±)-1-benzyl-3,4-epoxypiperidine (XIII) by the treatment
with chiral lithium amides both enantiomers of epoxide
XIII in the chiral environment form diastereomer
complexes A and B (Scheme 2) with a different reactivity.
Therefore in the course of the rearrangement the syn-
chronous transfer of the syn-proton to nitrogen atom and
of lithium ion to oxygen in the diastereomer complexes
A and B would occur with a different rate, and the more
reactive complex A would provide the enantiomer
Scheme 2.
(H2C)n
NH
(H2C)n
H
H
X
X
NH
Li
O
OH
N
Li
N
Bn
Bn
O
N
O
H
H
N
N
N
H
H
Bn
XIV*
Bn
XIII*
H
H
H
H
A
B
Scheme 3.
H
O
O
O
H
H
OH
Li-XV_XVII
N
N
N
THF
N
N
H
N
N
N
N
H
H
XV
XVI
XVII
Bn
Bn
XIII*
Bn
XIV*
XIII
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 2 2008

GRISHINA et al.
284
we estimated the e.e. values of the target compounds
applying these reference figures.
therewith the chiral amine XV remained practically
completely in the water phase and therefore was
recovered. The GC-MS analysis of the organic phase
showed the presence of epoxide XIII*, allyl alcohol
XIV*, and 4% of chiral amine XV. From the water phase
at pH 14 75% of amine XV was recovered and used in
repeated experiments.
To raise the efficiency of the kinetic separation we
carried out a series of experiments varying the amount
of amide Li-(XV) used with 1 equiv of (±)-epoxide XIII.
Some of the most important results are presented in the
table (runs nos. 2–4). At increasing the molar fraction of
amide Li-(XV) as expected grew the e.e. of epoxide
XIII* and decreased the e.e. of allyl alcohol XIV*. At
the use of 0.80 equiv of amide Li-(XV) and the conver-
sion 75% the e.e. values of epoxide XIII* and alcohol
XIV* were 83.1 and 28.1% respectively (see the table,
run no. 4). A considerable effect of supplemental co-
ordination was attained by addition of DBU (see the table,
runs nos. 5–7): At applying equivalent amounts of DBU
and amide Li-(XV) the e.e. of alcohol XIV* grew to
61.9 %. Even at ω 31% (see the table, run no. 5) the e.e.
of allyl alcohol XIV* proved to exceed by 2% the value
obtained without DBU addition at ω 16% (see the table,
run no. 1). Optically pure epoxide XIII*, e.e. 98.7%,
was obtained at the use of 0.95 equiv of Li-(XV) and
0.95 equiv of DBU (ù 84%) (see the table, run no. 7).
The kinetic separation of (±)-epoxide XIII applying
lithium salts of diamines XVI and XVII turned out to be
less efficient and resulted in lower e.e. values of target
compounds XIII* and XIV* (see the table, runs nos. 8–
11). In all experiments chiral diamines XV–XVII were
recovered for repeated use.
The ratio epoxide XIII* –allyl alcohol XIV* in the
1
reaction mixture according to H NMR spectrum was
5.25:1 measured by comparing the integral intensity of
the signals of benzyl protons of the epoxide (δ 3.44 ppm)
and allyl alcohol (δ 3.58 ppm) and corresponded to the
conversion of 16% (ω). In approximately the same weight
ratio (5.27:1) in overall yield 94% the chemically
individual epoxide XIII* and allyl alcohol XIV* were
isolated by column chromatography on silica gel. The
epoxide XIII* and allyl alcohol XIV* obtained by the
kinetic separation of 1 equiv of racemic epoxide XIII by
the treatment with 0.20 equiv of amide Li-(XV) exhibited
a specific rotation [α]D20 –0.52 and –52.9° respectively
(see the table, run no. 1).
Applying HPLC on columns with chiral phases we
established the enantiomeric excess (e.e.) of epoxide
XIII* with [α]D20 –3.87° equal 83.1% (see the table, run
no. 4) and e.e. of allyl alcohol XIV* with [α]D20 –52.9°
equal 59.8%, (see the table, run no. 1). The specific
rotations for the optically pure epoxide [(–)-XIII*] and
alcohol [(–)-XIV*] calculated from these data amounted
to –88.5 and –4.66° respectively. In further experiments
Kinetic separation of 1 equiv of (±)-epoxide XIII under the action of chiral lithium amides Li-(XV–XVII)
a Conversion ω was estimated from the ratio of integral intensities of the benzyl protons signal of allyl alcohol XIV* and epoxide XIII* in
the 1H NMR spectrum of the reaction mixture.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 2 2008

PREPARATION OF ENANTIOMERIC PURE (−)-(3R,4S)-1-BENZYL-3,4-EPOXYPIPERIDINE
285
developed consisting in kinetic separation of (±)-1-
benzyl-3,4-epoxypiperidine by treatment with the most
efficient chiral lithium amide Li-(XV). Both enantiomeric
pure epoxide XIII* and enantiomer-enriched allyl
alcohol XIV* are promising chiral precursors for
preparation of 3,4-aminoalcohols of the piperidine series,
analogs of pseudodistomine alkaloids A−F and other
natural iminosugars.
To establish the sterical direction of the kinetic
separation of (±)-epoxide XIII it was necessary to
determine the absolute configuration of allyl alcohol
XIV*. To this end we obtained from allyl alcohol XIV*,
[α]D20 –49.9° (e.e. 56.4 %), in 92% yield (+)-enantiomer
of 3-hydroxypiperidine [(+)-(XVIII)], [α]D20 +4.2°, by
means of removing 1-benzyl group by hydrogenolysis
on palladium on carbon. The stereochemical comparison
of the sign and the value of specific rotation of (+)-3-
hydroxypiperidine we obtained with the (–)-enantiomer
of (S)-3-hydroxypiperidine [(–)-(XVIII)] described in
[16] led to the conclusion that 3-hydroxypiperidine [(+)-
(XVIII)] possessed R-configuration. The hydrogenation
of (–)-allyl alcohol XIV* did not affect the stereocenter
at C3 of the piperidine ring, consequently, (–)-allyl
alcohol XIV* also possessed the R-configuration
(Scheme 4).
EXPRIMENTAL
NMR spectra were registered on a spectrometer
Varian VXR-400 at operating frequancy 400MHz,
internal reference TMS in CDCl3. GC-MS measurements
were performed on a Finnigan SSQ7000 instrument,
electron impact, 70 eV, column DB5 25 m. TLC was
performed on Silufol plates (Merck, Germany). Column
chromatography was carried out on a column packed by
wet procedure with SiliCagel 60-40 (Merck, Germany).
The specific rotation was measured on a polarimeter
Perkin-Elmer 241; cell V 1 ml, l 10 cm.
These data make evident the sterical direction of the
kinetic separation:
(–)-(R)-enantiomer of allyl alcohol [(R)-(XIV*)]
forms from (3S,4R)-enantiomer of epoxide XIII*, and
less reactive (–)-(3R,4S)-enantiomer of epoxide XIII*
accumulates in the reaction mixture; consequently, (–)-
1-benzyl-3,4epoxypiperidine [(–)-(XIII*)] has (3R,4S)-
configuration (Scheme 5).
For separation of enantiomers of epoxide XIII* and
allyl alcohol XIV* a series of chiral columns for HPLC
was tested in combination with various eluents. Finally
the estimation of the enantiomeric purity of epoxide
XIII* was carried out on a column Chiracel OJ-H
(0.46 × 25 cm), e.e. of allyl alcohol XIV*, on a column
Chiracel OD-H (0.46 × 25 cm), eluent hexane–2-
propanol, 9:1, 1 ml/min, detector UV, 254 nm.
Hence an economically feasible method of
preparation of enantiomeric pure epoxide [(–)-(3R, 4S)-
(XIII*)], e.e. 98.6 %, and enantiomer-enriched allyl
alcohol [(–)-(R)-(XIV*)], e.e. up to 61.9%, was
Synteses of (+)-(S)-2-[(pyrrolidine-1-yl)methyl]-
pyrrolidine (XV), [α]D20 +7.2° (C 0.18, C2H5OH), (+)-
(S)-2-[(piperidin-1-yl)methyl]pyrrolidine (XVI), [α]D23
+17.9° (C 5.00, C2H5OH), and (+)-(S)-2-[(morpholin-4-
yl)methyl]pyrrolidine (XVII), [α]D23 +17.6° (without
solvent) were carried out as in [12].
Scheme 4.
R
S
OH
R
OH
OH
10% Pd/C
H2, MeOH
N
N
N
Standard procedure of kinetic separation (see the
table, run no. 1). Into a round-bottom flask of 25 ml
capacity equipped with a magnetic stirrer and septum
H
H
Bn
(_)-XVIII
(_)-XIV*
(+)-XVIII
Scheme 5.
H
*
H
*
H
*
O
H
O
O
H
H
S
S
R
H
OH
*
*
R
*
*
R
R
S
N
N
N
N
Bn
Bn
(_)-(3R,4S)-XIII
Bn
(+)-(3R,4S)-XIII
Bn
(_)-(R)-XIV
(_)-(3R,4S)-XIII
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 2 2008

GRISHINA et al.
286
2
3
2.53 d.d.d (1H, H2, J2,2' 11.4, J2,3 3.6, J 0.6 Hz),
2.71 d.d (1H, H2', 2J2',2 11.4, 3J2',3 3.8 Hz), 2.76 m (1H,
H6, 2J6,6' 16.7 Hz), 2.93 br.s (1H, OH), 3.05 m (1H, H6',
2J6',6 16.7 Hz), 3.59 d (1H, PhCH2, 2J 13.2 Hz), 3.57 d
(1H, PhCH2, 2J 13.2 Hz), 4.05 m (1H, H3), 5.76 m (1H,
H5, 3J5,4 10.0 Hz), 5.85 m (1H, H4, 3J4,5 10.0 Hz), 7.21–
7.33 m (5H, Ph). 13C NMR spectrum (100 MHz, CDCl3),
δ, ppm: 52.43, 57.49, 62.31 (C2, C6, PhCH2), 64.37 (C3),
127.93, 128.25 (C4, C5), 127.11, 128.16 (2C), 128.99
(2C), 137.47 (Ph).
was charged under an argon atmosphere 0.250 g (1.60
mmol) of (S)-(+)-2-[(1-pyrrolidyl)methyl]pyrrolidine
(XV) in 12 ml of anhydrous THF. The mixture was cooled
to–20°C, and 1.0 ml (1.60 mmol) of 1.6 M n-butyllithium
solution in hexane was added, and the mixture was stirred
for 40 min at –10°C, then the temperature was lowered
to –70°C, and 1.510 g (8.00 mmol) of 1-benzyl-3,4-
epoxypiperidine (XIII) in 4 ml of anhydrous THF was
added. The reaction mixture was warmed to 5°C within
3 h and it was left standing at this temperature for 12 h.
Then the reaction mixture was cooled to –10°C, was
treated with a saturated ammonium chloride solution
(5 ml) till pH 9, and extracted with dichloromethane
(5× 10 ml). The combined organic extracts were dried
with sodium sulfate, the solvent was removed on a rotary
evaporator, the crude reaction product (1.50 g) was
applied to a column packed with silica gel (20 g) in
hexane, gradient elution with a system hexane–ethyl
acetate, the content of ethyl acetate from 0 to 25%. The
chromatographically uniform fractions were combined.
(+)-(R)-3-Hydroxypiperidine (XVIII). In a round-
bottom flask of 25 ml capacity equipped with a magnetic
stirrer and adapter for hydrogen input 0.530 g
(2.80 mmol) of (–)-1-benzyl-3-hydroxy-1,2,3,6-tetra-
hydropyridine ([α]D20 –49.9°, e.e. 56.4%) in 25 ml of
MeOH was subjected to hydrogenation in the presence
of 0.050 g (10%) of Pd/C at room temperature while
vigorous stirring for 15 h. The completion of the process
was monitored by TLC. The catalyst was filtered off,
the solvent was evaporated. Yield 0.260 g (92%) of
chromatographically individual compound [(+)-(XVIII)],
viscous oily substance, Rf 0.1 (Silufol, hexane–acetone,
(–)-(3R,4S)-1-Benzyl-3,4-epoxypiperidine (XIII*).
Yield 1.19 g (79%), [α]D20 –0.52° (C 0.20, CHCl3), e.e.
11.1%. Spectral characteristics identical to those of
racemic 1-benzyl-3,4-epoxypiperidine (XIII) [11].
Colorless oily substance, Rf 0.7 [Silufol (Merck),
hexane–acetone, 5:1]. Mass spectrum (electron impact,
.
2:1). Cromato-mass spectrum, m/z (Irel, %): 101 (3) [Μ]+ .
C5H11NO. Μcalc 101. Chemical purity 97%, [α]D20 +4.2°
(C 0.8, ΜeOH). For (–)-(S)-enantiomer XVIII [α]D20 –7.5°
(C 2, ΜeOH) [16].
.
70 eV), m/z (Irel, %): 189 [Μ]+ (17), 188 (5), 172 (4),
The study was carried out under a financial support
of the Russian Foundation for Basic Research (grant no.
05-03-32658a).
160 (2), 146 (2), 133 (19), 132 (10), 118 (3), 112 (4),
104 (3), 98 (15), 92 (13), 91 (100). 1H NMR spectrum
(400 MHz, CDCl3), δ, ppm: 1.99 m (1H, H5a, 2J5a,5e 14.6,
3J5a,6a 9.4, 3J5a,6e 5.5, 3J5a,4 2.6 Hz), 2.03 d.d.t (1H, H5e,
2J5e,5a 14.6, 3J5e,6a 4.4, 3J5e,6e 4.4, 3J5ε,4 1.5 Hz), 2.19 d.d.d
(1H, H6a, 2J6a,6e 11.8, 3J6a,5a 9.2, 3J6a,5e 4.4 Hz), 2.32 m
(1H, H6e, 2J6e,6a 11.8, 3J6e,5a 5.8, 3J6e,5e 4.2, 4J6e,2e 1.2 Hz),
REFERENCES
1. Huryn, S.K. and Okabe, M., Chem. Rev., 1992, vol. 92,
p. 1745.
2. Stutz, A.E., Iminosugars as Inhibitors: Nojirimycin and
Beyond, Verlag GmbH, 2002.
3. Ishibashi, M., Ohizumi, Y., Sasaki, T., Nakamura, H.,
Hirata, Y., and Kobayashi, J., J. Org. Chem., 1987,
vol. 52, p. 450.
4. Freyer, A.J., Patil, A.D., Killmer, L., Troupe, N., Men-
tzer, M., Carte, B., Faucette, L., Johnson, R.K., J. Nat.
Prod., 1997, vol. 60, p. 986.
2
2.67 d (1H, H2a, J2a,2e 13.5 Hz), 3.01 d.d.d (1H, H2e,
2J2e,2a 13.5, 3J2e,3 4.1, 4J2e,6e 1.5 Hz), 3.18–3.23 m (2H,
H3, H4), 3.44 s (2H, PhCH2), 7.21–7.32 m (5H, Ph).
13C NMR spectrum (100 MHz, CDCl3), δ, ppm: 25.51
(C5), 45.71, 50.54, 51.19, 52.25, 62.24 (C2, C3, C4,
C6, PhCH2), 127.00, 128.14 (2C), 128.92 (2C), 137.88
(Ar-C).
(–)-(R)-1-Benzyl-3-hydroxy-1,2,3,5-tetrahydro-
pyridine (XIV*). Yield 0.23 g (15%), [α]D20 –52.9°
(C 0.27, CHCl3), e.e. 59.8%. Spectral characteristics
identical to those of (±)-1-benzyl-3-hydroxy-1,2,3,5-
tetrahydropyridine (XIV) [11]. Colorless oily substance,
Rf 0.4 [Silufol (Merck), hexane–acetone, 5:1]. IR
spectrum (thin film), ν, cm–1: 1660 (C=C), 3410 (OH
bound). 1H NMR spectrum (400 MHz, CDCl3), δ, ppm:
5. Kobayashi, J., Naitoh, K., Doi, Y., Deki, K., and Ishiba-
shi, M., J. Org. Chem., 1995, vol. 60, p. 6941.
6. Lohse, A., Jensen, H., Bach, P., and Bols, M., J. Chem.
Soc., Perkin Trans. I, 2000, p. 659.
7. Bernotas, R.C. and Ganem, B., Carbohydr. Res., 1987,
vol. 167, p. 312.
8. Guanti, G. and Riva, R., Tetrahedron lett., 2003, vol. 44,
p. 357.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 2 2008

PREPARATION OF ENANTIOMERIC PURE (−)-(3R,4S)-1-BENZYL-3,4-EPOXYPIPERIDINE
287
9. Banda, Y., Abe, C., Nemoto, H., Kato, A., Adachi, I., and
Takahata, H., Tetrahedron: Asymmetry, 2001, vol. 12,
p. 817.
10. Ernholt, B.V., Thomson, I.B., Jensen, H.B., and Bols, M.,
Synlett., 1999, vol. 6, p. 701.
11. Grishina, G.V., Borisenko, A.A., Veselov, I.S., and
Petrenko, A.M., Zh. Org. Khim., 2005, vol. 41, p. 281.
12. Hendrie, S.K. and Leonard, J., Tetrahedron, 1987,
vol. 43, p. 3289.
13. Asami, M., Bull. Chem. Soc. Jpn., 1990, vol. 63, p. 721.
14. Asami, M. and Kanemaki, N., Tetrahedron Lett., 1989,
vol. 30, p. 2125.
15. Asami, M. and Inoue, S., Tetrahedron, 1995, vol. 51,
p. 11725.
16. Deane, C.C. and Inch, T. D., Chem. Commun., 1969,
p. 813.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 2 2008