Pyridyl and furyl epoxides of more than 99% enantiomeric purities: The use of a phosphazene base

Article abstract of DOI:10.1002/(SICI)1099-0690(200003)2000:6<1077::AID-EJOC1077>3.0.CO;2-4

trans-1-(2-Pyridyl)-, trans-1-(3-pyridyl)-, trans-1(2-furyl)-, and trans-1(3-furyl)-2-phenyl epoxides with enantiomeric purities ranging from 96.8 to 99.8% [in favor of the (+, EtOH)-isomer] are obtained in two steps from pure (R,R,R)-oxathiane which is recovered (85-90%) and reused. The chiral bidentate ligand 2-phenyl-(S)-1-(2-pyridyl)ethanol with 99.6% ee was obtained in three steps and 67% overall isolated yield.

Full text of DOI:10.1002/(SICI)1099-0690(200003)2000:6<1077::AID-EJOC1077>3.0.CO;2-4

FULL PAPER
Pyridyl and Furyl Epoxides of More Than 99% Enantiomeric Purities:
The Use of a Phosphazene Base
[a]
[a]
Arlette Solladie-Cavallo,* Marin Roje, Thomas Isarno,[a] Vitomir Sunjic,[b]
´
and Vladimir Vinkovic[b]
Keywords: Asymmetric synthesis / Phosphazene base / Pyridyl epoxide / Furyl epoxide / Pyridyl alcohol
trans-1-(2-Pyridyl)-, trans-1-(3-pyridyl)-, trans-1(2-furyl)-,
and trans-1(3-furyl)-2-phenyl epoxides with enantiomeric
purities ranging from 96.8 to 99.8% [in favor of the (+, EtOH)-
ane which is recovered (85–90%) and reused. The chiral bi-
dentate ligand 2-phenyl-(S)-1-(2-pyridyl)ethanol with 99.6%
ee was obtained in three steps and 67% overall isolated
isomer] are obtained in two steps from pure (R,R,R)-oxathi- yield.
Introduction
Chiral pyridyl alcohols have numerous applications, as
ligands in asymmetric metal catalysis,[1] as resolving
agents[2] or as starting materials for the preparation of more
advanced chiral ligands.[3] They can be prepared by enanti-
oselective reduction of the corresponding pyridyl ketones.
Similarly, chiral furyl hydroperoxides have been used for the
asymmetric epoxidation of allylic alcohols[4] and sulfides;[5]
they are obtained by direct enzymatic resolution.
As an application of our asymmetric synthesis of diaro-
matic epoxides,[6] which has been shown to give almost total
Scheme 1
enantioselectivities, we present here the asymmetric syn-
thesis and characterization of the pyridyl and furyl epoxides
1–4 as well as the first attempts of the ring-opening of 2-
pyridyl epoxide and 2-furyl epoxide.
epoxides ( 95%) and in much shorter reaction times ( 30
min instead of 1 to 2 days). The results are gathered in
Table 1.
When the reaction was complete, 1:1 mixtures of the
starting chiral auxiliary 6 and the desired epoxide (1–5)
1
were obtained (as seen from the H NMR spectrum of the
Results
crude product of the reaction) which could be separated by
chromatography. The chiral auxiliary was recovered pure in
85–90% yield and was reused.
Asymmetric Synthesis of Epoxides 1–4 with EtP2 as Base
Until now the 2- and 3-pyridyl epoxides 1 and 2 have
only been prepared in their racemic form,[7] as has the only
known 2-furyl epoxide 3.[8] Asymmetric synthesis of the ep-
oxides 1–4 has been successfully performed following our
method from the pure (R,R,R,SS)-(–)-sulfonium salt 7,[9]
the corresponding commercially available aldehydes
(Scheme 1) and a phosphazene base [EtP2 ϭ Et–Nϭ
P(NMe2)2(NϭP(NMe2)3] to generate the ylide.
Alkylation of pure (R,R,R)-(ϩ)-oxathiane 6 was per-
formed in 90% conversion (68% isolated yield) using
Vedej’s triflate method.[10] Use of the phosphazene base
EtP2 instead of NaH to generate the ylide gave, even at –
78 °C, a very high percentage of conversion into the desired
It is worth noting that only the trans-epoxide was formed,
in agreement with previous results concerning diaromatic
epoxides[6,11] obtained with NaH as base; only in the case
of 2-pyridyl epoxide (1) was 12% of the cis-isomer obtained.
The 2-furyl epoxide (3) is very reactive/unstable and was
lost on the column during chromatography, although
enough compound was recovered to measure the optical
rotation. However, in situ ring-opening of this epoxide (in
the presence of the oxathiane and before purification) can
be performed (cf. below) which allows to overcome this dif-
ficulty. The known epoxide 5 was reprepared as a reference.
Enantiomeric Purities and Absolute Configurations of
Epoxides 1–5
[a]
´ ´
´
Laboratoire de Stereochimie Organometallique,
´
ECPM/Universite L. Pasteur,
25 rue Becquerel, F-67087-Strasbourg, France
Department of Organic Chemistry and Biochemistry,
Rudjer Boskovic Institut,
The enantiomeric purities (Table 1) were determined by
chiral HPLC and the conditions used are gathered in Table
3 (Experimental Section). It is interesting to note that EtP2
[b]
Bijenicka 54, HR-10000, Zagreb, Croatia
Eur. J. Org. Chem. 2000, 1077Ϫ1080
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ193X/00/0306Ϫ1077 $ 17.50ϩ.50/0
1077

´
A. Solladie-Cavallo, M. Roje, T. Isarno, V. Sunjic, V. Vinkovic
FULL PAPER
Table 1. Asymmetric synthesis of epoxides 1–5 from the sulfonium salt 7 using EtP2 as base; reaction time ϭ 30 min
Epox.
Temp.
(°C)
Conv.[a]
(%)
Yield[b]
(%)
trans/cis[a]
ee-trans[c]
(config.)
(%)
ee-cis[c] (config.)
(%)
[α]2D0, (c)[d]
trans
1
2
3
4
5
–78
–78
–78
–78
–40
Ͼ95
Ͼ95
Ͼ95
Ͼ95
85
94
81
11
82
69
88/12
100/0
100/0
100/0
100/0
99.2 (1R,2R)
96.8 (1R,2R)
99.2 (1S,2R)
99.8 (1R,2R)
97.0 (1R,2R)
99.9 (1S,2R)
ϩ285(1)
ϩ285.5 (1)
ϩ246 (0.18)
ϩ199(1)
ϩ281 (1)
[a]
[b]
Determined by 1H NMR spectroscopy of the crude product of the reaction. –
After preparative chromatography of quantities
[d]
[c]
ranging from 500 mg to 2 g (silica gel or Alox). – Determined by chiral HPLC (cf. below). – Measured in EtOH, c ϭ concentration
in g/100 mL.
also provides exceptionally high enantioselectivities: Ͼ99%
ee for epoxides 1,3,4 and 97% ee for epoxides 2 and 5.
The R,R-configuration was assigned to the trans-stilbene
oxide 5 on the basis of the positive sign of its optical rota-
tion in EtOH and at 589 nm as in our previous papers.[6,9,11]
It thus appeared that EtP2 leads to the same configuration
as NaH and one can postulate that the model[9,12] shown in
Figure 1 holds. Therefore the same structure was assigned
to the (؉, EtOH)-isomers obtained for the new and un-
known trans-epoxides 1,2,3 and 4: (1R,2R)-1, (1R,2R)-2,
(1S,2R)-3 and (1R,2R)-4. Similarly the configuration at C-
Scheme 2
2 being R due to the preferred direction of approach (cf.
Model in Figure 1), it can be concluded that the configura-
tion at C-1 is S in the cis-isomer (Table 1, line 1).
Ring opening of the 2-furyl epoxide 3 was performed on
the crude oxathiane/epoxide mixture and provided the alco-
hol in high isolated yield (80%), while 90% of the oxathiane
was recovered. It is worth noting that although lower regio-
selectivities were expected, the observed inversion of the re-
gioselectivity in favor of isomers II (9II ϭ major) was sur-
prising. Both alcohols have, of course, high enantioselectivi-
ties (99% ee). The structure of 9II was determined from the
unambiguous preparation of racemic-9I (addition of benzyl
Grignard reagent to 2-furylaldehyde).
In conclusion, 1-(2-pyridyl), 1-(3-pyridyl), 1-(2-furyl) and
1-(3-furyl)-2-phenylepoxides 1-4-trans with enantiomeric
purities ranging from 96.8 to 99.8% [in favor of the (ϩ,
EtOH) isomer] have been obtained for the first time in only
two steps, the chiral auxiliary being recovered in 85–90%
yield and reused.
Figure 1
Ring Opening of Epoxides 1 and 2
The first attempts at ring opening of these epoxides were
performed with LiAlH4 in Et2O (Scheme 2 and Table 2).
trans-Epoxide 1 (isolated by chromatography) is opened
regioselectively to give alcohol 8I (Scheme 2) with 99.6%
enantiomeric purity. From the known configuration of the
starting epoxide (1R,2R, cf. Table 1), the absolute config-
uration of the resulting alcohol 8I was assigned to be: (–,
CHCl3) ϭ (S). The structure of alcohol 8I was confirmed
A potentially important bidentate chiral ligand, the (S)-
through the nonambiguous synthesis of alcohol 8II (addi- 2-pyridyl alcohol 8I with 99.6% ee, was obtained in three
tion of α-picolyllithium to benzaldehyde).
steps and 67% overall isolated yield.
Table 2. Opening of epoxides 1 and 3 with LiAlH4 in Et2O
Alcohol
T
(°C)
React.
time
Yield
(%)
I/II
I
I
II
ee (conf.)
[α]2D2
ee (conf.)
[a]
8 (2-pyr)
9 (2-fur)
Room temp.
Room temp.
30 min
1 h
81
91
100/0
21/79
99.6 (S)
-17.5
-
99.0 (S)[b]
99.0 (S)[b]
[a]
[b]
c ϭ 1, CHCl3. – Same conditions as for 8I (cf. Table 3, experimental section).
1078
Eur. J. Org. Chem. 2000, 1077Ϫ1080

Pyridyl and Furyl Epoxides
FULL PAPER
EtP2Hϩ/TfO– was recovered by varying the solvent po-
larity;[13] the expensive base EtP2 can thus be regenerated.
It is worth noting that the epoxides 1–4 are not available
from direct asymmetric epoxidation[14] of the corresponding
olefins because of side oxidation reactions of the heterocyc-
lic rings. Attempts to make this method catalytic[15] through
the use of phenyldiazomethane (dangerous on large scale)
have been done, however, because of the possible direct re-
action of the diazo compound with the aldehyde prior to
formation of the desired chiral ylide, the enantiomeric pur-
ity obtained is often lower.
ucts were found to be 1:1 mixtures of the epoxide 1 (or 2, 3, 4 or
5) and the oxathiane 6 (as seen from H NMR spectroscopy).
1
[trans-(1R,2R)]-1: Chromatographic purification on silica gel
(CHCl3/Et2O, 7:3; Rf ϭ 0.56) of the crude product yielded 94% of
the pure epoxide as pale yellow crystals, m.p. 51–53 °C. – [α]2D0
ϭ
ϩ285.2 (c ϭ 1, EtOH); [α]2D0 ϭ ϩ326 (c ϭ 1, benzene); ee ϭ 99.2%
(cf. Table 1 and 2). – 1H NMR (CDCl3/TMS): δ ϭ 4.05 (br. s, 2
H), 7.25 (m, 2 H), 7.35 (br. m, 5 H), 7.70 (td, 3J ϭ 7 Hz, 7 Hz,
4J ϭ 2 Hz, 1 H), 8.60 (dd, 3J ϭ 5 Hz, 4J ϭ 2 Hz, 1 H). – 13C NMR
(CDCl3/TMS): δ ϭ 61.8, 62.9 (CH epox), 120.2, 123.3, 125.8,
128.5, 128.6, 136.9, 149.6 (CH rings), 136.8, 156.4 ( C).
[cis-(1S,2R)]-1: (Rf ϭ 0.34, CHCl3/Et2O 7:3) was obtained in 86%
yield as pale yellow crystals, m.p. 30–32 °C. – [α]2D0 ϭ ϩ63 (c ϭ
0.36, EtOH); [α]2D0 ϭ ϩ82 (c ϭ 0.36, benzene); ee ϭ 99.9% (cf.
Table 1 and 2). – 1H NMR (CDCl3/TMS): δ ϭ 4.45 (br. s, 2 H),
7.05 (m, 2 H), 7.15 (bm, 6 H), 7.45 (td, 3J ϭ 7 Hz, 7 Hz, 4J ϭ
2 Hz, 1 H), 8.40 (dd, 3J ϭ 5 Hz, 4J ϭ 2 Hz, 1 H). – 13C NMR
(CDCl3/TMS): δ ϭ 59.9, 60.0 (CH epox), 121.3, 122.4, 135.2, 147.8
(CH pyr), 126.3, 127.0 (CH arom), 133.3, 154.0 ( C).
Experimental Section
1
General: H and 13C NMR spectra were recorded on a Bruker AC
200 (200 MHz) or a Bruker Avance (400 MHz) spectrometer with
CDCl3 as the solvent. Chemical shifts (δ) are given in ppm down-
field from an internal standard of TMS. – Optical rotation meas-
urements were carried out with a Perkin–Elmer 241 MC polari-
meter. – Melting points were determined with a Reichert melting
point apparatus, and are not corrected. – TLC was performed on
Merck glass plates with silica gel 60 F254. Silica gel for column
chromatography (Merck) was used for the chromatographic puri-
fications, as was aluminium oxide (Fluka, type 5016A basic). –
HPLC was performed with a Knauer HPLC pump 64 and Knauer
Variable wavelength monitor, equipped with a PC. – Analytical
chiral columns were: Chiralcel OD-H, Chiralpak AS (all
25 cm ϫ 4.6 mm I.D., Daicel, Japan). – The (R,R,R)-oxathiane 6
was prepared from (ϩ)-pulegone following Eliel’s method:[16]
[α]2D2 ϭ ϩ12 (c ϭ 2.1, acetone). The sulfonium salt 7 was prepared
[trans-(1R,2R)]-2: Chromatographic purification on silica gel
(EtOAc/Et2O, 1:1; Rf ϭ 0.49) of the crude product yielded 81% of
the pure epoxide as white crystals, m.p. 47–50 °C. – [α]2D0 ϭ ϩ285.5
(c ϭ 1, EtOH); [α]2D0 ϭ ϩ365.9 (c ϭ 1, benzene); ee ϭ 96.8% (cf.
Table 1 and 2). – 1H NMR (CDCl3/TMS): δ ϭ 3.90 (br. s, 2 H),
4
7.25 (m, 1 H), 7.35 (bm, 5 H), 7.62 (dt, 3J ϭ 7 Hz, J ϭ 1.5 Hz,
1.5 Hz, 1 H), 8.59 (dd, 3J ϭ 5 Hz, 4J ϭ 1.5 Hz, 1 H), 8.63 (d, 4J ϭ
1.5 Hz, 1 H). – 13C NMR (CDCl3/TMS): δ ϭ 60.7, 62.8 (CH epox),
123.6, 125.6, 128.7, 132.8, 147.9, 149.8 (CH), 132.8, 136.5 ( C).
[trans-(1S,2R)]-3: Chromatographic isolation was done on Al2O3
(n-hexane/Et2O, 9:1) of the crude product (see above) yield: 11% of
pure epoxide as pale yellow crystals, m.p. 29–33 °C. – [α]2D0 ϭ ϩ246
following Vedej’s method:[10] white powder, m.p. 136–139 °C. – (c ϭ 0.18, EtOH), [α]2D0 ϭ ϩ280 (c ϭ 0.18, benzene); ee ϭ 99.2%
1
3
[α]2D2 ϭ –222 (c ϭ 1.04, CHCl3), one diastereomer by 1H NMR
(cf. Table 1 and 2). – H NMR (CDCl3/TMS): δ ϭ 3.88 (d, J ϭ
spectroscopy.[6,11]
3 Hz, 1 H), 4.35 (d, 3J ϭ 3 Hz, 1 H), 6.43 (AB of an ABX, ∆νAB ϭ
Table 3. Conditions and parameters for HPLC analyses of epoxides 1–4 on chiral columns (for analysis of epoxide 5 see ref.[6b]
)
1 trans
1 cis
2
3
4
8I
Column
Chiralcel
OD-H
10% iPrOH,
90% hexane
1.0
Chiralpak
AS
Chiralcel
OD-H
10% iPrOH,
90% hexane
1.0
Chiralcel
OD-H
10% iPrOH,
90% hexane
0.5
Chiralcel
OD-H
10% iPrOH,
90% hexane
1.0
Chiralcel
OD-H
20% iPrOH,
80% hexane
1.0
Mobile phase
10% iPrOH,
90% hexane
1.0
Flow (mL/min)
Rt (min), major
16.9
7.7
23.6
15.9
14.1
7.4
Rt (min), minor
9.9
11.5
26.0
12.4
17.7
8.8
k1’
k2’
α
2.83
2.20
4.90
0.59
3.70
1.11
4.83
3.29
5.50
1.01
4.80
1.51
1.71
1.50
1.12
1.71
1.30
1.36
Rs
6.67
6.13
1.78
4.12
2.06
1.86
3
General Procedure for the Synthesis of Epoxides 1–5: To a stirred
solution of the benzylic sulfonium salt 7 (1 equiv., 1.5 mmol) in
anhydrous CH2Cl2 (5 mL) was added 1 equiv. of the commercially
available phosphazene base EtP2 at –78 °C. After stirring for 10 to
15 min the desired aldehyde (1 equiv.) was added dropwise. The
reaction was then stirred for 30 min at –78 °C. After addition of a
17 Hz, JAB ϭ 4 Hz, 3J ϭ 1.5 Hz, 4J ϭ 0, 2 H), 7.35 (bm, 5 H),
7.42 (X of the ABX, 1 H). – 13C NMR (CDCl3/TMS): δ ϭ 56.6,
60.0 (CH epox), 110.2, 111.2, 143.3 (CH furyl), 126.0, 128.8, 128.9
(CH arom), 136.9, 150.3 ( C).
[trans-(1R,2R]-4: Chromatographic purification on silica gel
saturated solution of NaCl in water (1 mL), the organic phase was (CH2Cl2; Rf ϭ 0.85) of the crude product yielded 82% of the pure
separated and the aqueous phase extracted with CH2Cl2
(3 ϫ 5 mL). The organic phases were combined, dried over Na2SO4
and concentrated under vacuum. The remaining phosphazene-base
epoxide as pale yellow crystals, m.p. 49–52 °C. – [α]2D0 ϭ ϩ199 (c ϭ
1, EtOH); [α]2D0 ϭ ϩ267 (c ϭ 1, benzene); ee ϭ 99.8% (cf. Table 1
1
3
and 2). – H NMR (CDCl3/TMS): δ ϭ 3.80 (d, J ϭ 3 Hz, 1 H),
salt EtP2HϩTfO– was precipitated by addition of a 1:1 mixture of 3.98 (d, 3J ϭ 3 Hz, 1 H), 6.40 (br. s, 1 H), 7.35 (bm, 5 H), 7.43 (br.
ether/n-hexane and filtered off. After evaporation the crude prod-
Eur. J. Org. Chem. 2000, 1077Ϫ1080
s, 1 H), 7.57 (br. s, 1 H). – 13C NMR (CDCl3/TMS): δ ϭ 56.6, 61.3
1079

´
A. Solladie-Cavallo, M. Roje, T. Isarno, V. Sunjic, V. Vinkovic
FULL PAPER
(CH epox), 108.1, 141.4, 143.7, (CH furyl), 125.6, 128.5, 128.7 (CH The enantiomeric purity of 9I and 9II was determined from the
arom), 123.0, 137.0 ( C).
mixture using a Chiralcel OD-H column (20% 2-propanol/80%
hexane, flow rate ϭ 1.0 mL/min).
General Procedure for LiAlH4 Opening of Epoxides: To a solution
of 1 equiv. of the desired epoxide (pure or oxathiane/epoxide mix-
ture) in 2 mL of dry ether (under argon) was added dropwise 1.5
equiv. of LiAlH4 (1  in ether), the mixture was stirred at the de-
sired temperature and the reaction monitored by TLC. After com-
pletion of the reaction, water was added dropwise and the precipit-
ate was filtered off and washed with ether. The filtrate was dried
over Na2SO4 and the solvent was evaporated under vacuum.
[1] [1a]
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1142.
1
[5] [5a]
1, CHCl3), ee ϭ 99.6%, m.p. 89–94 °C. – H NMR (CDCl3/TMS):
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3
3J ϭ 7 Hz, 7 Hz, J ϭ 1 Hz, 1 H), 8.52 (dd, J ϭ 5 Hz, 4J ϭ 1 Hz,
1 H). – 13C NMR (CDCl3/TMS): δ ϭ 45.5 (CH2), 74.6 (CH), 121.1,
122.2, 136.8, 148.8 (CH pyr), 126.8, 128.7, 130.0 (CH arom), 138.2,
161.8 (C).
[6b]
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[8] [8a]
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3
3
(bd, J ϭ 7 Hz, 1 H), 7.30 (bm, 6 H), 7.62 (td, J ϭ 7 Hz, 7 Hz,
4J ϭ 1 Hz, 1 H), 8.52 (dd, J ϭ 5 Hz, J ϭ 1 Hz, 1 H).
Addition of racemic-8II thus prepared into a sample of (S)-8I ob-
tained from opening of the epoxide lead to a spectrum which was a
superposition of both spectra described above, mainly two different
ABX systems.
3
4
[10]
[11]
[12]
´
´
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included references. For the first use of oxathiane in synthesis
1
(S)-2-Furyl Alcohol 9I/9II (9I/9II, 21:79). – 9II (major): H NMR
(CDCl3/TMS): δ ϭ 3.05 (A2, d, 3J ϭ 7 Hz, 2 H), 5.03 (t, 3J ϭ 7 Hz,
1 H), 6.15 (br. s, 1 H), 6.35 (br. s, 1 H), 7.35 (bm, 6 H). – 9I (minor):
[15]
[16]
2
1H NMR (CDCl3/TMS): 3.19 (AB of an ABX, ∆ν ϭ 33 Hz, J ϭ
3
14 Hz, J ϭ 8 Hz, 5.5 Hz, 2 H), 4.94 (X of the ABX, bdd, 1 H),
6.29 (br. s, 1 H), 6.35 (br. s, 1 H), 7.35 (bm, 6 H).
of epoxides see ref.[11]
.
E. L. Eliel, J. E. Lynch, F. Kume, S. V. Frye, Organic Synthesis
Identification of 9I and 9II was done by addition of racemic-9I
(prepared by addition of benzylmagnesium bromide to 2-furyl alde-
hyde) to the mixture.
1987, 65, 215–221.
Received July 12, 1999
[O99433]
1080
Eur. J. Org. Chem. 2000, 1077Ϫ1080