Regio- and stereoselectivity in phenylselenoetherification of (Z)- and (E)-hex-4-en-1-ols

Article abstract of DOI:10.1055/s-0029-1217011

Δ⁴-Primary alkenols (Z)- and (E)-hex-4-en-l-ols underwent facile cyclization to the corresponding phenyl selenoethers using PhSeX (X = Cl, Br) in high yields and in good to excellent selectivities in the presence of some catalysts (Lewis acids

Full text of DOI:10.1055/s-0029-1217011

3684
PAPER
Regio- and Stereoselectivity in Phenylselenoetherification of (Z)- and
(E)-Hex-4-en-1-ols
R
egio- andStereos
e
electivityi
r
nPhenyl
a
selenoetherificati
M
on . Divac, Zorica M. Bugarčić*1
University of Kragujevac, Faculty of Science, Department of Chemistry, Radoja Domanovića 12, P.O. Box 60, 34 000 Kragujevac, Serbia
Fax +381(34)335040; E-mail: zoricab@kg.ac.rs
Received 17 June 2009; revised 6 July 2009
exo-trig attack of the hydroxy group (Scheme 1), the tet-
Abstract: D4-Primary alkenols (Z)- and (E)-hex-4-en-1-ols under-
rahydrofuran-type of ring will be produced. On the other
hand, if ring-closure is by 6-endo-trig attack then the tet-
rahydropyran-type of ring will be retrieved (Scheme 1).
went facile cyclization to the corresponding phenyl selenoethers us-
ing PhSeX (X = Cl, Br) in high yields and in good to excellent
selectivities in the presence of some catalysts (Lewis acids and
bases). The reaction succeeded in complete control of stereo- and
regioselectivity. The best results were obtained with triethylamine
and tin(II) chloride. Triethylamine as an additive in the reaction
with (Z)-hex-4-en-1-ol gives only erythro-2-[1-(phenylseleno)eth-
yl]tetrahydrofuran, while the E-isomer gives cis-2-methyl-3-(phe-
nylseleno)tetrahydropyran in large excess. Tin(II) chloride as an
additive in the reaction with (Z)-hex-4-en-1-ol gives threo-2-[1-
(phenylseleno)ethyl]tetrahydrofuran and with the E-isomer gives
trans-2-methyl-3-(phenylseleno)tetrahydropyran. The reactions
were performed under very mild experimental conditions, and the
obtained yields were almost quantitative.
-trig
exo
O
O
5-
OH
endo
Scheme 1
Key words: alcohol, cyclization, heterocycles, regioselectivity,
Lewis acids
By using various reagents in the cyclization reaction of
D4-alkenols, such as H2SO4, Hg(OAc)2, Pb(OAc)4, NBS,
peracid, BF3·OEt2, AlCl3, SnCl4, etc., a mixture of tetrahy-
drofuran- and tetrahydropyran-type ring products are pro-
duced in various ratios.1 Benzeneselenenyl halides are
very good electrophilic reagents in organic synthesis and
in reactions with olefin bonds they often produce regiose-
lective products, which is also the case for (Z)- and (E)-
hex-4-en-1-ols [(Z)-1 and (E)-1] (Scheme 2). Reaction of
benzeneselenenyl halides and (Z)-1 and (E)-1 affords a
Cyclization of unsaturated alcohols leading to cyclic
ethers, are well documented in the literature as convenient
pathways in the synthesis of natural products and related
compounds.1 Organoselenium-induced cyclization of un-
saturated alcohols has been widely explored in organic
synthesis over the last decade and depending on the nature
of the substrate, a variety of five- and six-membered ring
heterocycles can be prepared.2–6 It must be emphasized
that different regio- and stereoisomers can be produced in
some cases by simply choosing conditions favorable to ki-
netic or thermodynamic control. Selenium-induced cy-
clization has the advantage that the introduction of the
heteroatom, the manipulation of the obtained product, and
the removal of the function are facilitated by the simple
and mild conditions required, such as oxidation–syn-sele-
noxide elimination, hydrogenolytic removal, or nucleo-
philic substitution of the corresponding selenones.5–9 In
past years we have investigated the influence of different
types of additives on yields and selectivity of phenylsele-
nium-induced cyclization of alkenols bearing a hydroxy
group as an internal nucleophile.10–14 In this work we re-
port the regio- and stereocontrolled cyclization of two D4-
geometric isomers, (Z)- and (E)-hex-4-en-1-ols [(Z)- and
(E)-1].
seleniranium intermediate
A and C, respectively
(Scheme 2), which can be trapped by an external (halide
ion from reagent) or internal nucleophile (hydroxy group
from alcohol) to give two diastereoisomeric addition
products 4 and 5 or cyclization products 2 or 3, respective-
ly.
However, the presence of the nucleophilic halide anions is
sometimes responsible for some undesirable processes
such as addition of the halide ion to give 4 or 5 (which
gives a low yield of the cyclic ether product) and decrease
in stereoselectivity especially in the case of bromide as the
anion (Table 3, without additive). In order to decrease the
side reaction and to increase the yields of cyclic products,
we performed experiments with two different sets of addi-
tives: Lewis acids (SnCl2, CoCl2, AlCl3) and Lewis bases
(Et3N, pyridine, quinoline, 2,2¢-bipyridine). Results of
these investigations are given in the Tables 1 and 2 and
shows that all reactions proceed with good yields and with
good to excellent stereoselectivites. From data in Table 1
(with Lewis acids as additives) it can be seen that (Z)-1
gives threo-2 as the main product, and in the case of (E)-
1 the predominate product is trans-3. Best results with
both (Z)-1 and (E)-1 and both reagents (PhSeCl and
In cyclization reaction of D4-alkenols, two regioisomeric
products can be obtained. If ring-closure reaction is by 5-
SYNTHESIS 2009, No. 21, pp 3684–3688
Advanced online publication: 23.09.2009
DOI: 10.1055/s-0029-1217011; Art ID: Z12609SS
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x
.
2
0
0
9

PAPER
Regio- and Stereoselectivity in Phenylselenoetherification
3685
X–
Se
Ph
– H+
PhSeX
SePh
SePh
+
+
(X = Cl, Br)
O
O
O
O
H
H
(Z)-1
H
erythro/threo-2
B
A
X
SePh
+
OH
OH
SePh
X
4
5
SePh
SePh
Ph
+Se
X–
– H+
PhSeX
+
O
(X = Cl, Br)
O
O
H
O
cis/trans-3
H
H
D
(E)-1
C
X
SePh
+
OH
OH
SePh
X
4
5
Scheme 2
PhSeBr) were obtained by using tin(II) chloride as an ad- best results were obtained in the presence of triethyl-
ditive (Table 1); cyclic ethers were obtained as the prod- amine. If the reaction of (Z)-1 is performed in the presence
uct in almost quantitative yields and (Z)-1 gave threo-2 in of triethylamine a single erythro-stereoisomer, erythro-2,
large excess, while (E)-1 gave trans-3 as the main prod- is obtained (Table 2), while in the case of other bases
uct. Cobalt(II) chloride as the additive (Table 1) also gave erythro-2 is obtained in excess (Table 2). With (E)-1 the
cyclic ether products in high yields, but in the reaction unexpected product, cis-3, was indeed obtained in good
with (Z)-1 the stereoselectivity was lower than with tin(II) yield, but this was accompanied by considerable amounts
chloride as an additive. Not so good results were obtained of trans-3.
in the reactions with aluminum(III) chloride, which is a
strong Lewis acid (Table 1). This could be explained by
the high affinity of the additive for attack at the double
bond of hex-4-en-1-ol when it forms a mixture of various
In the Table 3 and Scheme 3 the results with additives that
gave the best stereoselectivity in the reactions of (Z)-1 and
(E)-1 are given and compared with reactions where no ad-
ditive was present.
products that are difficult to separate. From data in
The role of Lewis acids in these reactions is to increase the
Table 2 (with Lewis bases as additives), the opposite re-
electrophilicity of the reagent (PhSeX) and to decrease the
possibility of the addition of halides by removing the an-
ion from the reagent and, in this way, to improve yields of
sults were noted. The main product of the cyclization of
(Z)-1 was erythro-2 and (E)-1 gives cis-3 in excess. The
Table 1 Phenylselenoetherification of (Z)- and (E)-Hex-4-en-1-ols in the Presence of Lewis Acids
Substrate Product
Yield (%) [ratio erythro/threo or cis/trans]
PhSeCla
PhSeBra
SnCl2
SnCl2
CoCl2
AlCl3
CoCl2
AlCl3
(Z)-1
(E)-1
erythro/threo-2
cis/trans-3
100 [3:97]
96 [3:97]
98.5 [10:90]
97 [3:97]
91.7 [20:80]
73 [16:84]
99 [16:84]
100 [3:97]
95 [16:84]
98 [3:97]
83.7 [21:79]
79.6 [11:89]
a Additive as indicated.
Table 2 Phenylselenoetherification of (Z)- and (E)-Hex-4-en-1-ols in the Presence of Lewis Bases
Substrate Product
Yield (%) [ratio erythro/threo or cis/trans]
PhSeCla
PhSeBra
Et3N
Et3N
Bipy
Py
Quinoline
Bipy
Py
95.8 [98:2] 98 [96:4]
100 [81:19] 99.3 [80:20] 96 [80:20] 98.4 [79:21] 100 [85:15] 99.7 [85:15] 99.5 [82:18] 99.6 [80:20]
Quinoline
(Z)-1
(E)-1
erythro/threo-2 100 [100:0] 97.9 [99:1] 97.5 [99:1] 97.4 [98:2] 100 [100:0] 98.3 [99:1]
cis/trans-3
a Additive as indicated.
Synthesis 2009, No. 21, 3684–3688 © Thieme Stuttgart · New York

3686
V. M. Divac, Z. M. Bugarčić
PAPER
Table 3 Phenylselenoetherification of (Z)- and (E)-Hex-4-en-1-ols in the Presence of Et3N and SnCl2
Substrate Product
Yield (%) [ratio erythro/threo or cis/trans]
PhSeCla
PhSeBra
without
without
SnCl2
Et3N
SnCl2
Et3N
(Z)-1
(E)-1
erythro/threo-2
cis/trans-3
72 [30:70]
81 [31:69]
100 [3:97]
96 [3:97]
100 [100:0]
100 [81:19]
75 [70:30]
65 [35:65]
99 [16:84]
100 [3:97]
100 [100:0]
100 [85:15]
a Additive as indicated.
the desired products. The possible role of Lewis bases is
to remove a proton from the oxonium ions B and D
(Scheme 1) and alleviate the formation of the final cyclic
product. That the specific distribution of cyclic ethers
erythro/threo-2 and cis/trans-3 depends of the additive
used can be explained by the nature of the additive itself.
R1
R2
Se
H
O
N
δ–
δ+
H
trans-Isomer trans-3 is formed in excess in the reactions
of (E)-hex-4-en-1-ol [(E)-1] with PhSeX in the presence
of Lewis acids due to the greater stability of the trans-iso-
mer over the cis-isomer (thermodynamically controlled
product). Unexpected formation of cis-3 in the reactions
with Lewis bases present (Table 2) can be explained by
steric hindrance during the ring-closure process by these
additives in the cyclization step. Since, the base forms a
hydrogen bond with the proton from the hydroxy group,
in this way they are bonded together, hence, they make the
approach of oxygen in the ring-closure reaction difficult
and the attack of the hydroxy group occurs from the oppo-
site side from that expected (Figure 1).
(Z)-1 R1 = Me; R2 = H
(E)-1 R1 = H; R2 = Me
Figure 1
eficial to the isomerization process and this is most likely
due to their elimination of the possibility of forming an
acidic medium which is responsible for ring opening and
isomerization.
Reactions with benzeneselenenyl chloride as a reagent
were faster than with benzeneselenenyl bromide. All reac-
tions were faster with an additive than without, but there
were some differences with different reagents. When ben-
zeneselenenyl chloride was used as a reagent, in the pres-
ence of additives, the reaction went to completion in only
a few minutes, while when benzeneselenenyl bromide
was used the reaction could took up to 30 minutes.
That the stereoselectivity for erythro- or threo-isomers 2
from (Z)-1 depends on the additive present was harder to
explain. Nevertheless, it was noticed that, if the reaction
mixture (when reactions were performed without addi-
tive) was left without purification for a few days, the ratio
of isomers erythro/threo-2 changed, that is, isomerization
occurred (which did not happen with additive present).
This observation may be ascribed to the role of the addi-
tives. It appears that the presence of some additives is ben-
1H NMR spectroscopy was used to investigate the process
of conversion of (E)-1 and (Z)-1 to cyclic ether products 2
and 3. Due to the high rate of these reactions only the re-
actions with benzeneselenenyl bromide and no additive
present were of experimental relevance in the NMR study
i.e. they were slow enough for this method. In all other
cases by the time when the first spectrum was taken
isomerization reaction was already in progress or at the
end. The useful signals were the doublets from methyl
groups in ether products marked as I and II in Figure 2.
The equimolar concentrations of (Z)-1 and benzenesele-
nenyl bromide were mixed in an NMR tube and first spec-
trum was taken after 15 minutes. Only the doublet at
d = 1.41 from the methyl group of erythro-2 appeared
(Figure 2, spectrum A). The second spectrum was taken
after one hour and pairs of two doublets at d = 1.41 and
d = 1.33 showed that the isomerization process had oc-
curred. The doublet from erythro-2 decrease in size while
the doublet from threo-2 began to appear. During this pro-
cess of isomerization some peaks from the addition prod-
ucts begin to appear (Figure 2, spectrum B). The third
spectrum, which was taken after 24 hour from the start of
the reaction, showed that the isomerization process was
SePh
H
Me
O
H
threo-2
PhSeX
+
OH
(Z)-hex-4-en-1-ol
(Z)-1
SePh
H
H
O
Me
erythro-2
SePh
SePh
O
Me
trans-3
PhSeX
OH +
(E)-hex-4-en-1-ol
Me
(E)-1
O
cis-3
Scheme 3
Synthesis 2009, No. 21, 3684–3688 © Thieme Stuttgart · New York

PAPER
Regio- and Stereoselectivity in Phenylselenoetherification
3687
Figure 2
with the calculated values. TLC was carried out on 0.25 mm E.
Merck precoated silica gel plates (60F-254) using UV light for vi-
sualization. Column chromatography used E. Merck silica gel (60,
particle size 0.063–0.200 mm). Olefinic alcohols used as substrates
are commercially available. Reagents (PhSeCl and PhSeBr) were
used as supplied by Aldrich. CH2Cl2 was distilled from CaH2.
finished and threo-2 was present in excess (Figure 2,
spectrum C). From this experiment it can be concluded
that erythro-2 is the kinetically controlled product, and
threo-2 is the thermodynamically controlled product. The
same experiment was repeated with (E)-1 and the tetrahy-
dropyran products 3 were formed in a ratio trans/cis of
70:30; during the 24 hour experiment no isomerization
was noticed.
Phenylselenoetherification of (E)- and (Z)-Hex-4-enols; General
Procedure
All reactions were carried out on a 1-mmol scale. To a magnetically
stirred soln of alkenol 1 (0.1 g, 1 mmol) and Et3N (0.101 g, 1 mmol)
[or SnCl2 (0.19 g, 1 mmol), 2,2¢-bipyridine (0.156 g, 1 mmol), py-
ridine (0.079 g, 1 mmol), quinoline (0.129 g, 1 mmol), AlCl3 (0.133
g, 1 mmol), or CoCl2 (0.065 g, 0.5 mmol)] in anhyd CH2Cl2 (5 mL)
was added solid PhSeCl (0.212 g, 1.1 mmol) or PhSeBr (0.260 g,
1.1 mmol) at r.t. until the solid dissolved. The reaction went to com-
pletion virtually instantaneously. The pale yellow soln was washed
with 2 M HCl (only when the additive was a base), then sat. aq
NaHCO3 soln, and brine. The organic layer was dried (Na2SO4),
concentrated, and chromatographed. TLC and GM analysis as well
as NMR spectra showed complete conversion of starting alkenol
into cyclic ether products. The products were obtained after the elu-
tion of the traces of (PhSe)2 on a column (silica gel, CH2Cl2). All
products were characterized and identified on the basis of their
spectral data. Cyclic ether products were known compounds and
their spectral data have been presented previously.10
In this work we have demonstrated a practical, fast, inex-
pensive, and efficient process for the preparation of cyclic
ethers under mild reaction condition. The additives used
not only provide selectivity, they also increase the yields
to almost quantitative. The correct choice of alkenol 1 and
catalysts (Lewis acid or base) enables formation of the de-
sired regio- or stereocontrolled product. The process can
be extended to other compounds with a hydroxy group
and double bond. The methodology should be easily ap-
plicable to the synthesis of a large number of biologically
active compounds containing an oxacyclic moiety. These
units can be found in monocyclic or polycyclic com-
pounds, fused with other cyclic ethers or forming spiro
systems.15 The presence of molecules with oxygenated
heterocycles in nature receives considerable attention due
to their capacity to modify of the transport of Na+, K+, and
NMR Measurements
Ca2+ cations through lipid membranes.16–19 This activity is All reactions were carried out in NMR tubes and CDCl3 was used
as the solvent. 1H NMR chemical shifts, d, were calibrated to TMS
responsible for their antibiotic,16 neurotoxic,20,21 antivi-
ral,22 and cytotoxic action23,24 and as growth
regulators16,25,26 or inhibitors of the level of cholesterol in
blood.27
1
as standard. The H NMR spectra were recorded every 15 min, at
the beginning of the reactions, and later every 30 min, 1 h, 2 h, and
4 h. The reaction began with mixing alkenol (1.50 mg) with PhSeBr
(3.54 mg, 1 equiv) in CDCl3 (3 mL). The real concentration of the
reagents was 5 × 10–3 M.
GM analysis were obtained with an Agilent Technologies instru-
ment, model 6890 N with HP-5NS columns (5% phenyl-, 95%
methyl-polysiloxane). 1H and 13C NMR spectra were run in CDCl3
on Varian Gemini 200 MHz and Bruker WH 400 MHz NMR spec-
trometers. IR spectra were obtained with Perkin-Elmer Model 137B
and Nicolet 7000 FT spectrophotometers. Microanalyses were per-
formed by Dornis and Colbe and found to be in good agreement
threo-2-[1-(Phenylseleno)ethyl]tetrahydrofuran (threo-2)
1H NMR (360 MHz, CDCl3): d = 1.33 (d, J = 7 Hz, 3 H, CH3), 2.00–
2.20 (m, 4 H, H3, H4), 3.37 (m, J = 7 Hz, 1 H, H2), 3.51 (dt, J = 3,
12 Hz, 1 H, H5cis to H2), 3.66 (dq, J = 7, 1.5 Hz, 1 H, CHSe), 4.00
(br d, J = 12 Hz, 1 H, H5trans to H2), 7.22–7.30 (m, 3 H, HPh), 7.52–
7.60 (m, 2 H, HPh).
Synthesis 2009, No. 21, 3684–3688 © Thieme Stuttgart · New York

3688
V. M. Divac, Z. M. Bugarčić
PAPER
13C NMR (50.32 MHz, CDCl3): d = 18.6, 26.0, 30.1, 44.0, 68.2,
(5) (a) Paulimer, C. In Selenium Reagents and Intermediates in
Organic Synthesis, Vol. 4; Baldwin, J. E., Ed.; Pergamon:
New York, 1986. (b) The Chemistry of Organic Selenium
and Tellurium Compounds, Vol. 2; Patai, S.; Rappoport, Z.,
Eds.; Wiley: New York, 1987.
82.6, 127.2, 128.7, 130.7, 134.9.
erythro-2-[1-(Phenylseleno)ethyl]tetrahydrofuran (erythro-2)
1H NMR (360 MHz, CDCl3): d = 1.41 (d, J = 7 Hz, 3 H, CH3), 1.68–
2.07 (m, 4 H, H3, H4), 3.41 (quint, J = 7 Hz, 1 H, CHSe), 3.77 (q,
J = 7 Hz, 1 H, H5cis to H2), 3.90 and 3.95 (2 q, J = 7 Hz, 2 H, H5trans
to H2 and H2), 7.22–7.30 (m, 3 H, HPh), 7.55–7.62 (m, 2 H, HPh).
13C NMR (50.32 MHz, CDCl3): d = 18.3, 26.1, 29.0, 43.9, 68.5,
82.2, 127.2, 128.7, 129.1, 134.6.
(6) Electrophilic selenium, selenocyclizations, see: Tiecco, M.
Top. Curr. Chem. 2000, 208, 7.
(7) Nicolaou, K. C.; Seitz, S. P.; Sipio, W. J.; Blount, J. F. J. Am.
Chem. Soc. 1979, 101, 3884.
(8) Nikolaou, K. C. Tetrahedron 1981, 37, 4097.
(9) Schimdt, G. H.; Garratt, D. G. In The Chemistry of Double
Bonded Functional Groups: Supplement A, Vol. 2; Patai, S.,
Ed.; Wiley: New York, 1977.
(10) Konstantinovic, S.; Bugarcic, Z.; Milosavljevic, S.; Schroth,
G.; Mihailovic, M. Lj. Liebigs Ann. Chem. 1992, 261.
(11) Mojsilovic, B.; Bugarcic, Z. Heteroat. Chem. 2001, 12, 475.
(12) Bugarcic, Z.; Gavrilovic, M. Monatsh. Chem. 2003, 134,
1359.
(13) Bugarcic, Z.; Mojsilovic, B. Heteroat. Chem. 2004, 15, 146.
(14) (a) Bugarcic, Z.; Mojsilovic, B.; Divac, V. J. Mol. Catal. A:
Chem. 2007, 272, 288. (b) Divac, V. M.; Rvovic, M. D.;
Bugarcic, Z. M. Monatsh. Chem. 2008, 139, 1373.
(15) Faulkner, D. J. Nat. Prod. Rep. 1997, 14, 259.
(16) Polyether Antibiotics Naturally Occurring Acid Ionophores,
Vols. 1 & 2; Wesley, J. W., Ed.; Dekker: New York, 1982.
(17) Painter, G. R.; Presman, B. C. Top. Curr. Chem. 1982, 101,
83.
trans-2-Methyl-3-(phenylseleno)tetrahydropyran (trans-3)
1H NMR (200 MHz, CDCl3): d = 1.35 (d, J = 6.1 Hz, 3 H, CH3),
1.65–2.20 (m, 4 H, H4, H5), 2.95 (sym. m, 1 H, CHSe), 3.30–3.50
(m, 2 H, H2 and Hax of CH2O), 3.92 (d, J = 11.3 Hz, 1 H, Heq of
CH2O), 7.27 (m, 3 H, HPh), 7.56 (m, 2 H, HPh).
13C NMR (50.32 MHz, CDCl3): d = 21.0, 27.9, 32.2, 46.9, 67.9,
78.1, 127.6, 127.9, 128.8, 135.3.
cis-2-Methyl-3-(phenylseleno)tetrahydropyran (cis-3)
1H NMR (200 MHz, CDCl3): d = 1.43 (d, J = 6.9 Hz, 3 H, CH3),
1.65–2.20 (m, 4 H, H4, 5-H5), 3.32 (quint, J = 6.9 Hz, 1 H, H2),
3.60–4.05 (m, 3 H, H3, H6), 7.27 (m, 3 H, HPh), 7.56 (m, 2 H, HPh).
13C NMR (50.32 MHz, CDCl3): d = 21.0, 22.2, 30.8, 50.5, 68.2,
76.0, 126.9, 128.9, 130.5, 133.8.
(18) Still, W. C.; Hauck, P.; Kempf, D. Tetrahedron Lett. 1987,
28, 2817.
(19) Smith, P. W.; Still, W. C. J. Am. Chem. Soc. 1988, 110,
7917.
(20) Shimizu, Y. Marine Natural Products: Chemical and
Biological Perspectives, Vol 1; Scheuer, P. J., Ed.;
Academic: New York, 1978, 1.
Acknowledgment
This work was funded by the Ministry of Science, Technology and
Development of the Republic of Serbia (Grant: 142008).
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Synthesis 2009, No. 21, 3684–3688 © Thieme Stuttgart · New York