Asymmetric synthesis of 3,5-disubstituted indolizidines by intermolecular addition of an allylsilane on an N-acyliminium ion

Article abstract of DOI:10.1016/j.tetasy.2006.04.027

A diastereoselective synthesis of 3,5-disubstituted indolizidines based on an intermolecular addition of an allylsilane on an acyliminium ion derived from (S)-pyroglutamic acid is described. The synthetic potential of this methodology is demonstrated by t

Full text of DOI:10.1016/j.tetasy.2006.04.027

Tetrahedron: Asymmetry 17 (2006) 1253–1257
Asymmetric synthesis of 3,5-disubstituted indolizidines
by intermolecular addition of an allylsilane on an N-acyliminium ion
Elisabeth Conchon, Yvonne Gelas-Mialhe and Roland Remuson*
`
UMR 6504, CNRS, Universite´ Blaise Pascal (Clermont-Ferrand), 63177 Aubiere Cedex, France
Received 8 March 2006; accepted 24 April 2006
Abstract—A diastereoselective synthesis of 3,5-disubstituted indolizidines based on an intermolecular addition of an allylsilane on an
acyliminium ion derived from (S)-pyroglutamic acid is described. The synthetic potential of this methodology is demonstrated by the
enantioselective synthesis of (ꢀ)-indolizidine 195B, (ꢀ)-indolizidine 223AB, (+)-monomorine and (ꢀ)-3-butyl-5-propyl indolizidine.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
Herein, we report the enantioselective synthesis of (ꢀ)-indol-
izidine 195B, (+)-monomorine, (ꢀ)-indolizidine 223AB
and (ꢀ)-3-butyl-5-propyl indolizidine (Fig. 1) based on
the intermolecular addition of allylsilanes on an N-acyl-
iminium ion starting from L-pyroglutamic acid used as
the chiral precursor.
Indolizidine alkaloids constitute a very important class
of compounds, which are mainly isolated from the skin
extracts of neotropical frogs of the family Dendrobates.1,2
Most of them are disubstituted by alkyl chains at the 3,5
positions. These compounds have been attractive targets
for synthesis because of their potential biological activi-
ties.3 Accordingly, novel strategies for the preparation of
substituted indolizidines have received considerable
attention.4
As the disconnective analysis shows, our plan for the
synthesis of indolizidines 1 and 3 depends on the diastereo-
selectivitiy of steps A and C (Scheme 1).
Step A involves the reduction of iminium ion 5. A literature
survey shows that, whatever the reducing agent (H2/Pd or
hydrides)4d,e and C-3 configuration, the incoming C-5
hydrogen atom is mainly deduced to be syn to that of C-
8a. In fact, the stereocontrol of our synthesis depends on
the stereoselective addition of the allylsilyl group of alco-
hols 7 to the in situ generated N-acyliminium 6 (step C).
It has been well established that p-nucleophiles give,
selectively, syn addition to the N-acyliminium ion derived
The allylsilyl functional group is a weak carbon nucleo-
phile used for trapping N-acyliminium ions, thus provid-
ing a useful method for intramolecular carbon–carbon
bond formation.5 We have applied this methodology
towards the synthesis of quinolizidine and indolizidine
alkaloids.6 More recently, an intermolecular version
has been developed to access the racemic indolizidine
167B.7
H
H
Bu
Bu
N
R
R
2 R = Me (+)-monomorine
1 R = Me (-)-indolizidine 195B
3 R = Pr (-)-indolizidine 223AB
4 R = Pr (-)- (3R,5S,8aS)-3-butyl-5-propylindolizidine
Figure 1.
*
Corresponding author. Tel.: +33 4 73 40 76 45; fax: +33 4 73 40 77 17; e-mail: Roland.REMUSON@univ-bpclermont.fr
0957-4166/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.04.027

1254
E. Conchon et al. / Tetrahedron: Asymmetry 17 (2006) 1253–1257
H
H
H
O
8a
3
A
B
C
+
Bu
N
Bu
N
Bu
N
CBz
5
R
R
1 (R = Me)
3 (R = Pr)
5
R
OH
+
+
O
HO2C
N
H
Bu
N
R
CBz
SiMe3
6
7
(S)-pyroglutamic acid
Scheme 1. Retrosynthetic analysis.
from (S)-proline8 and, despite some previous examples of
cis-addition of the allylsilane to substituted five-membered
N-acyliminium ions,9 we reasoned that the steric require-
ment of the n-butyl side chain would direct the approach
of the allylsilyl moiety to the re-face of 6. This selectivity
was registered by Pilli during the total synthesis of (ꢀ)-indol-
izidine 223AB.4e
pyroglutamic acid according to a previously described
procedure.4e,12
Next, lactam 8 was protected (n-BuLi, benzyl chloroformate),
then converted to ethoxycarbamate 9, isolated as a mixture
of two diastereomers in 84% yield according to Hiemstra’s
procedure.13 Condensation of allylsilanes 7a7 (R = Me)
and 7b14 (R = Pr) onto iminium ion 6 generated in situ
by treatment of 9 with stannous chloride led to compounds
10a and 10b in 40% and 30% yields, respectively.
Compounds 10a and 10b were obtained as a mixture of
isomers, which could not be separated. Spectroscopic data
did not allow us to establish the stereochemical pattern of
each isomer; their ratios were not determined at this step,
but would be during the final step on the natural products
possessing known configurations.
Moreover, according to the literature, additions of cup-
rates10 and enol silylated ethers4e onto iminium ions were
performed to give a majoritary of trans products. No
results concerning the addition of allylsilanes on iminium
ions type 5 were described except an example of addition
of an allylsilane on the iminium ion derived from
oxazolidinones.11
2. Results and discussion
The next two steps were straightforward: oxidation (pyrid-
inium dichromate, CH2Cl2) of 10a and 10b afforded a,b-
ethylenic ketones 11a and 11b in 60% and 84% yields,
respectively, and 11a and 11b were obtained as a mixture
of two isomers, which could not be separated.
The synthesis of indolizidines 1, 2, 3 and 4 was carried out
as shown in Scheme 2. Preparation of lactam 8 was accom-
plished starting from the commercially available S-(ꢀ)-
a, b, c
d, e
f
+
O
O
HO2C
N
H
Bu
N
H
8
Bu
Bu
N
OEt
H
Bu
Bu
N
CBz
CBz
9
6
(S)-pyroglutamic acid
H
H
O
H
+
g
h
Bu
N
CBz
Bu
N
CBz
N
N
R
R
1, 3
OH
R
R
+
10a (R = Me)
10b (R = Pr)
11a
11b
5a
5b
H
Bu
N
R
2, 4
Scheme 2. Reagents and conditions: (a) SOCl2, NaBH4; (b) TsCl, Et3N; (c) Pr2CuLi, ether, ꢀ20 °C; (d) n-BuLi, benzylchloroformate; (e) NaBH4, H2SO4,
EtOH; (f) SnCl4, RCH(OH)CH(SiMe3)CH@CH2 7; (g) PDC, CH2Cl2, 25 °C; (h) H2, Pd/C, MeOH.

E. Conchon et al. / Tetrahedron: Asymmetry 17 (2006) 1253–1257
1255
On hydrogenation over palladium on carbon, 11a gave a
mixture of indolizidines 1 and 2 in an 8:2 ratio. These com-
pounds were separated by flash column chromatography.
They were identified as (ꢀ)-indolizidine 195B and (+)-
reported. Optical rotations were measured on a Jasco
DIP-370 polarimeter at 589 nm (Na D-line).
Nuclear magnetic resonance (NMR) spectra were recorded
on a Bruker Avance 400 spectrometer at operating frequen-
cies of 400 MHz (1H NMR) or 100 MHz (13C NMR).
Chemical shifts (d) are given in parts per million relative
to residual solvent (d = 7.27 ppm for 1H, d = 77.1 ppm
for 13C) and coupling constants (J) in hertz. Multiplicity
is tabulated as s for singlet, d for doublet, t for triplet, q
for quadruplet, m for multiplet; the prefix br is applied
when the signal in question is broadened. Microanalysis
was carried out at the Laboratoire Central de Microanalyse
du CNRS (Vernaison, France).
1
monomorine, respectively, by comparison of their H and
13C NMR spectra with the literature.15–17 The specific rota-
22
tions ½aꢁD were determined to be ꢀ90.2 (c 0.75, MeOH) for
1 and +17.6 (c 0.21, hexane) for 2. These values compare
favourably with those obtained previously for synthetic
samples.10,15,17
In the same manner, the hydrogenation of 11b provided a
mixture of isomers 3 and 4 in a 7:3 ratio. These could be
separated by flash column chromatography. Spectroscopic
data and the specific rotation of 3 are in agreement with
those described for (ꢀ)-indolizidine 223AB.4d Indolizidine
4 was identified as the stereoisomer of 3.
4.2. General procedure for preparation of 7
To a solution of s-BuLi (c 1.3 M, 73 mL, 95 mmol) in THF
(120 mL) at ꢀ78 °C under argon was added tetramethylen-
ethylendiamine (TMEDA) (11.04 g, 95 mmol). To this mix-
ture, allyltrimethylsilane (10.8 g, 95 mmol) was added
dropwise. The temperature was then raised to ꢀ30 °C after
which the solution was stirred at this temperature for
30 min. After cooling to ꢀ78 °C, titanium(IV) isopropox-
ide (27 g, 95 mmol) was added and stirred for 1 h.
Next, the aldehyde (0.9 equiv) was added and the solution
stirred for 2 h. The mixture was poured into an aqueous
solution of HCl 5% (250 mL). Ether was added and the
aqueous phase extracted with ether. The organic phases
were combined and dried over MgSO4. The crude
compound was purified by flash chromatography to afford
compounds 7.
This reaction involved the simultaneous hydrogenolysis of
the CBz group, and cyclisation to generate the iminium
ion, which was reduced to give the indolizidines. It is well
known that the reduction (H2, Pd/C) of iminium ions type
5a or 5b leads to a single isomer, resulting from the addi-
tion of the C-5 hydrogen atom syn to that of C-8a.4d,18
We have also observed this selectivity during the synthesis
of indolizidine 167B.7 Consequently, the observed ratio of
indolizidine isomers results from the diastereoselectivity of
the addition of the allylsilyl functional group of alcohols 7a
and 7b onto the iminium ion 6. This selectivity is attributed
to the n-Bu group, which preferentially orients the addition
of the allylsilyl functional group in a trans position.
3. Conclusion
4.2.1. 3-Trimethylsilylpent-4-en-2-ol 7a. Yield: 88%; 1H
NMR d 0.05 (s, 9H), 1.23 (d, 3H, J = 6.3 Hz), 1.62 (dd,
1H, J = 10.6 and 6.9 Hz), 1.87 (s, 1H), 3.95 (m, 1H), 4.95
(dd, 1H, J = 17.1 and 2.0 Hz), 5.05 (dd, 1H, J = 10.3 and
2.0 Hz), 5.80 (td, 1H, J = 17.1 and 10.3 Hz); 13C NMR d
0.0, 23.4, 45.1, 67.6, 115.3, 136.6.
We have developed a new diastereoselective method for the
preparation of 3,5-dialkylindolizidines. These syntheses
have been achieved starting from a common pyrrolidone
8, which is readily available from natural (S)-pyroglutamic
acid. Indolizidines were obtained in five steps with overall
yields of about 8%.
4.2.2. 3-Trimethylsilylhept-1-en-4-ol 7b. Yield: 76%; 1H
NMR: d 0.04 (s, 9H), 0.85 (t, J = 7 Hz), 1.25 (m, 2H),
1.45 (m, 2H), 1.57 (s, 1H), 1.65 (dd, J = 10.6 and 5.8,
1H), 3.77 (m, 1H), 4.91 (dd, 1H, J = 2.0 and 17.1 Hz),
5.04 (dd, J = 2.0 and 10.4 Hz, 1H), 5.8 (td, J = 10.4 and
17.1 Hz, 1H); 13C NMR d 0.0, 14.0, 19.0, 39.5, 42.5, 71.2,
114.7, 135.9.
4. Experimental
4.1. General
Commercially available materials were used without
further purification. THF, which was used for moisture
sensitive operations, was distilled from potassium/benzo-
phenone under an argon atmosphere. All moisture sensitive
reactions were carried out in flame-dried glassware under
an argon atmosphere. Analytical thin layer chromatogra-
phy (TLC) was performed on Merck silica gel 60 F254
plates. Visualisation on TLC was achieved by the use of
UV light (254 nm), iodide, ninhydrin or vanillin followed
by heating. Flash chromatography was performed using
Merck Kieselgel 40–60 lm silica gel.
4.3. General procedure for the preparation of 10
To a solution of ethoxypyrrolidine 9 (1 g, 3.3 mmol) in
anhydrous CH2Cl2 (65 mL), cooled at ꢀ78 °C under Ar,
was added a solution of SnCl4 in CH2Cl2 (c 1 M, 3.3 mL,
3.3 mmol) dropwise. The mixture was stirred until the com-
plete consumption of 9. Next, the temperature was raised
to ꢀ20 °C, and then silylated alcohols 7 (1.1 equiv) were
added dropwise and stirred for a further 90 min. The mix-
ture was then poured into a saturated solution of NaHCO3.
The aqueous phases were extracted with CH2Cl2, and
organic extracts were evaporated. The crude product was
purified by flash chromatography to afford pure 10.
Infrared spectra were recorded on a Perkin–Elmer 881
(spectra in solution) or on a Perkin–Elmer FTIR Spec-
trometer Aragon 500 (film), only selected absorbances are

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E. Conchon et al. / Tetrahedron: Asymmetry 17 (2006) 1253–1257
22
4.3.1. N-(Benzyloxycarbonyl)-2-(4-hydroxypent-2-enyl)-5-
butyl pyrrolidine 10a. Yield: 30%; 1H NMR d 0.83 (t,
3H, J = 6.7 Hz), 1.23 (d, 3H, J = 6.2 Hz), 1.25–1.36 (m,
8H), 1.63–1.93 (m, 5H), 3.83 (m, 2H), 4.2 (m, 1H), 5.13
(m, 2H), 5.45–5.57 (m, 2H), 7.3–7.4 (m, 5H); 13C NMR d
14.4, 14.5, 23.1, 26.9, 28.0, 29.2, 57.8, 57.9, 58.4, 58.9,
67.0, 66.9, 69.0, 69.1, 127.4, 128.9, 137.2, 137.3, 137.4,
155.0. Anal. Calcd for C21H31NO3: C, 73.01; H, 9.04; N,
4.05. Found: C, 73.08; H, 9.32; N, 4.11.
Compound 1: 36% yield; ½aꢁD ¼ ꢀ90:2 (c 0.75, MeOH);
22
1
lit.10 ½aꢁD ¼ ꢀ99 (c 0.21, MeOH). H NMR d 0.9 (t, 3H),
1.1 (d, 3H), 1.15–1.6 (m, 12H), 1.75–1.9 (m, 4H), 2.4 (m,
1H), 2.55 (m, 1H), 3.3 (br s, 1H); 13C NMR d 14.3, 20.5,
23.1, 24.8, 25.9, 26.4, 29.4, 30.1, 32.4, 34.5, 52.1, 58.9, 59.0.
22
Compound 2: 7% yield; ½aꢁD ¼ þ17:6 (c 0.21, hexane); lit.17
22
½aꢁD ¼ þ29 (c 0.80, hexane). 1H NMR d 0.9 (t, 3H), 1.15 (d,
3H), 1.2–1.8 (m, 16H), 2.05–2.25 (m, 2H), 2.5 (br t, 1H);
13C NMR d 14.25, 20.4, 22.9, 24.9, 29.4, 29.8, 30.4, 31.0,
35.9, 38.8, 60.3, 62.9, 67.3.
4.3.2. N-(Benzyloxycarbonyl)-2-(4-hydroxyhept-2-enyl)-5-
1
butylpyrrolidine 10b. Yield: 40%; H NMR d 0.83–0.92
22
Compound 3: 34% yield; ½aꢁD ¼ ꢀ98 (c 2.1, MeOH); lit.4d
(m, 6H), 1.25–1.36 (m, 10H), 1.65–1.93 (m, 6H), 4.03 (m,
3H), 4.06 (m, 1H), 5.3 (m, 2H), 5.48–5.57 (m, 2H), 7.3–
7.4 (m, 5H). Anal. Calcd for C23H35NO3: C, 73.95; H,
9.44; N, 3.75. Found: C, 74.05; H, 9.62; N, 3.95; 13C
NMR d 14.4, 14.5, 19.0, 19.1, 22.9, 23.1, 28.9, 28.93,
29.1, 29.2, 32.7, 34.0, 39.8, 39.9, 57.8, 58.4, 58.5, 58.9,
66.8, 66.9, 72.9, 73.0, 128.1, 128.9, 136.2, 136.4, 137.3,
137.4, 154.7, 154.8.
22
1
½aꢁD ¼ ꢀ103 (c 1.12, hexane). H NMR d 0.9 (t and d,
6H), 1.1–1.5 (m, 16H), 1.6–1.75 (m, 4H), 2.1–2.35 (m,
2H), 2.5 (br t, 1H); 13C NMR d 14.2, 14.5, 19.6, 23.1,
24.9, 28.8, 30.5, 31.0, 31.9, 32.3, 38.1, 39.9, 65.9, 67.5.
22
1
Compound 4: 13% yield; ½aꢁD ¼ ꢀ87:5 (c 0.83, MeOH); H
NMR d 0.9 (m, 6H), 1.1–1.5 (m, 16H), 1.6–1.7 (m, 16H),
2.4 (m, 2H), 3.25 (br t, 1H); 13C NMR d 14.2, 14.5, 19.3,
22.9, 24.7, 25.0, 25.1, 26.4, 29.4, 30.9, 32.4, 35.9, 56.7,
58.6, 59.1. Anal. Calcd for C15H29N: C, 80.62; H, 13.11;
N, 6.27. Found: C, 80.93; H, 13.58; N, 6.52.
4.4. General procedure for preparation of 11
A solution of alcohols 10 (0.46 mmol) was stirred with
pyridinium dichromate (0.26 g, 0.66 mmol) in CH2Cl2
(1 mL) for 24 h at 25 °C. The mixture was diluted with a
mixture of ether–pentane (1:1) and then filtered over Celite,
after which it was concentrated to give final products 11,
which were purified by flash chromatography.
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1.1–1.4 (m, 6H), 1.5–1.7 (m, 4H), 1.8–2.0 (m, 4H), 2.3–
2.5 (m, 2H), 3.8 (m, 1H), 4.0 (m, 1H), 5.14 (m, 2H), 6.0–
6.2 (m, 1H), 6.6–6.8 (m, 1H), 7.3 (m, 5H); 13C NMR d
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57.1, 57.5, 58.8, 59.4, 67.4, 67.5, 128.7, 129.2, 132.9,
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