Synthesis of enantiopure azetidine 2-carboxylic acids and their incorporation into peptides

Article abstract of DOI:10.1016/S0957-4166(03)00493-2

Enantiopure azetidine 2-carboxylic acids were prepared by hydrolysis of the corresponding 2-cyano azetidines, without ring cleavage of the azetidine or epimerization. The produced amino acids, which are conformationally constrained analogues of phenylalan

Full text of DOI:10.1016/S0957-4166(03)00493-2

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 2407–2412
Synthesis of enantiopure azetidine 2-carboxylic acids and their
incorporation into peptides
Franc¸ois Couty,* Gwilherm Evano and Nicolas Rabasso
SIRCOB, UMR CNRS 8086, Universite´ de Versailles, 45, avenue des Etats-Unis, 78035, Versailles Cedex, France
Received 21 May 2003; accepted 23 June 2003
Abstract—Enantiopure azetidine 2-carboxylic acids were prepared by hydrolysis of the corresponding 2-cyano azetidines, without
ring cleavage of the azetidine or epimerization. The produced amino acids, which are conformationally constrained analogues of
phenylalanine, can be cleanly debenzylated and used for the synthesis of tripeptides. In the course of the synthesis of new
enantiopure 2-cyano azetidines through intramolecular alkylation of a metallated amino nitrile, it was found that the involved
anionic cyclisation can be thermodynamically controlled, thus enhancing its diastereoselectivity.
© 2003 Elsevier Ltd. All rights reserved.
1. Introduction
a-amino ketone, or a b-amino alcohol7 or a a-amino
ester.8 Since a-amino nitriles are also direct precursors
to a-amino acids through the Strecker synthesis,9 we
herein report the preparation of azetidinic 2-carboxylic
acids by hydrolysis of the amino nitrile and the incor-
poration of such amino acids into a simple peptidic
sequence.
Much synthetic effort has been recently devoted to the
preparation of conformationally constrained a-amino
acids.1 In this field, the modification of the conforma-
tion of bioactive peptides by the inclusion of such
synthetic analogues of proteinogenic a-amino acids into
the peptide sequence is an intensive area of research in
biomedical chemistry.2 In this vast topic, the asymmet-
ric synthesis of cyclic constrained a-amino acids in
which the nitrogen atom of the amino moiety is part of
a ring (e.g. constrained imino acids) is of particular
interest since these amino acids, when incorporated in
peptides, are unable to engage a hydrogen bond and
can therefore deeply influence the spatial conformation
of the peptide.1d Moreover, peptides including such
N-alkyl a-amino acids are usually less prone to enzy-
matic proteolytic cleavage. For those reasons, the
asymmetric synthesis of five-membered ring (proline
derivatives)3 and six-membered ring4 (pipecolic acid
derivatives) a-amino acids have been particularly stud-
ied. In contrast, the asymmetric synthesis of 4-mem-
bered ring (azetidinic a-amino acids) has been much
less reported,5 reflecting the difficult access to those
strained heterocycles, especially in enantiomerically
pure form. We have recently reported a straightforward
preparation of 2-cyano azetidines, starting from com-
mercially available enantiopure b-amino alcohols
(Scheme 1).6 In these compounds, the a-amino nitrile
moiety can be conveniently transformed into either an
Scheme 1. Synthesis of enantiopure functionalized azetidines
from N-cyanomethyl b-amino alcohols.
2. Results
2.1. Synthesis of 2-cyano azetidines
Cyano azetidines 3-5 were prepared as previously
described from (1R,2S)-ephedrine 3 and 4 and (R)-
phenylglycinol 5.6 N-Benzyl analogues 9 and 10 were
prepared following a similar procedure depicted in
Scheme 2 from (1R,2S)-N-benzyl norephedrine 6.
* Corresponding author. Tel.: 33(1)39254455; fax: 33(1)39254452;
e-mail: couty@chimie.uvsq.fr
0957-4166/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0957-4166(03)00493-2

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F. Couty et al. / Tetrahedron: Asymmetry 14 (2003) 2407–2412
Cyanomethylation of 6 followed by chlorination gave 8
as a single isomer. Treatment of 8 by LiHMDS resulted
in intramolecular alkylation, to give an inseparable
mixture of N-benzyl-2-cyano azetidines 9 and 10 in
good yield.
Scheme 3. Metallated aminonitrile 11 accounts for the equili-
bration of 10 into 9.
2.2. Synthesis of azetidinic a-amino acids
Hydrolysis of 2-cyano azetidines into amino acids
required very harsh conditions: prolonged heating in
concentrated HCl (35% wt.) was necessary to com-
pletely hydrolise the intermediate amide. Moreover, in
some cases, a co-solvent such as CCl4 (for 5), or AcOH
(for 9+10) had to be used because of the low solubility
of the intermediates in the reaction mixture. Neverthe-
less, under these drastic conditions, neither the ring
opening of the azetidine ring11 nor epimerization was
observed. The amino acids 13 and 14 were isolated with
excellent yield after purification by ion exchange chro-
matography, and hydrochloride 15 was cleanly deben-
zylated prior to purification by ion exchange
chromatography, yielding amino acid 16 that can be
viewed as a conformationally constrained analogue of
phenylalanine.12 Finally, cis or trans-enriched epimeric
mixtures of 9+10 obtained following the above condi-
tions gave the same epimeric mixtures of amino acids
17+18 (Scheme 4).
Scheme 2. Azetidinic substrates choosen for their transforma-
tion into a-amino acids. Reagents and conditions: (a)
BrCH2CN, K2CO3, CH3CN, 97%; (b) SOCl2, CH2Cl2, 77%;
(c) 1.2 equiv. LiHMDS, −78°C to −50°C, 1 h: 10/9: 60/40,
78%; 2 equiv. LiHMDS, −78°C to −30°C, 2 h, 10/9: 30/70; 2
equiv. LiHMDS, −78°C to −30°C, 5 h, 10/9: 8/92, 84%.
In the course of the synthesis of these new 2-cyano
azetidines, we have brought to light a very important
feature concerning the anionic cyclisation. The
diastereoselectivity of this ring closure was found to be
highly dependent on the amount of base used and also
on the duration of the experiment (Scheme 2). Pro-
tracted experiments at low temperatures with an excess
of base give trans-9 with 84% d.e. while the cis-isomer
10 is predominantly produced when the reaction time is
shortened. These results suggest that thermodynamic
control is operative in this reaction, leading to the more
stable trans-isomer.10 This equilibration is slow and
proceeds through the metallated amino nitrile 11: it
should be noted that this anion is only produced in low
concentrations at −30°C, since a quench with CD3OD
at this temperature afforded a mixture of 9 and 10 with
less than 10% of deuterium incorporation at C-2. How-
ever this is sufficient for the equilibration to proceed.
The occurrence of such a metallated azetidine was best
seen using azetidines 3 and 4 (mixture of isomers at
C-2) as the substrates. In these compounds, bearing a
N-methyl instead of a N-benzyl group, the acidic pro-
ton is sterically less hindered and complete metallation
occurs readily with LDA at −90°C. This anion can then
be trapped with methyl iodide, to give 12 with no
selectivity. It should be mentioned that this anion
proved to be particularly reactive, leading to degrada-
tion products if the alkylating agent was not added in
the reaction mixture before the base (Scheme 3).
Scheme 4. Hydrolysis of azetidinic amino nitriles. Reagents
and conditions: (a) 35% HCl, 50°C, 72 h, then DOWEX
(50X8-200), 97% (13), 97% (14); (b) 35% HCl, CCl4, 80°C, 7
days (quant.); (c) H2, Pd/C, then DOWEX (50X8-200), 78%;
(d) 35% HCl, AcOH, 100°C, 72 h.
This crude epimeric mixture of amino acids was then
coupled with (L)-Ala-OMe to give the diastereoiso-
meric dipeptides 19 and 20, which were easily separated
by flash chromatography. Debenzylation of these com-
pounds, followed by coupling to (L)-N-Boc-Ala, gave
tripeptides 21 and 22 in modest to good yield (Scheme
5). These tripeptides were shown by NMR to exist as a
roughly 1/1 mixture of rotamers in CDCl3. Recording
Having in hand a set of enantiopure 2-cyano azetidines,
we next focused on the conditions for their hydrolysis
into amino acids.

F. Couty et al. / Tetrahedron: Asymmetry 14 (2003) 2407–2412
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3.1. (4S,5R)-[Benzyl-(2-hydroxy-1-methyl-2-phenyl-
ethyl)-amino]-acetonitrile 7
To a solution of N-benzyl norephedrine 6 (3.0 g, 12.6
mmol) in acetonitrile (120 mL) was added potassium
carbonate (2.6 g, 18.9 mmol) and bromoacetonitrile
(2.6 mL, 37.8 mmol). After 12 h of reflux, the mixture
was concentrated under reduced pressure and the
residue was taken up in water and ether. Usual workup
followed by flash chromatography (E/CyH 1/1) gave
title compound as an oil (3.42 g, 97%). Rf: 0.52 (E/
1
CyH: 1/1); [h]2D0=−8.8 (c 1, CHCl3); H NMR: 1.19 (d,
J=6.8 Hz, 3H, Me), 2.80 (bs, 1H, OH), 3.09 (quadd,
J=4.0 and 6.8 Hz, 1H, CHMe), 3.51 (s, 2H, CH2CN),
3.91 (AB syst, J=13.4 Hz, 2H, CH2Ph), 5.01 (d, J=2.6
Hz, 1H, PhCH), 7.25–7.43 (m, 10H, Ar); 13C NMR:
10.6 (CH3), 55.6 (CH2), 38.1 (CH2), 62.7 (CH), 74.0
(CH), 116.8 (CN) 125.9, 127.4, 127.8, 128.3, 128.8,
128.9 (CHAr), 137.0, 142.0 (CqAr); Anal. Calc. for
C18H20N2O: C: 77.11; H: 7.19; N: 9.99. Found: C:
76.84; H: 7.32; N: 9.56.
Scheme 5. Peptide chemistry with azetidinic a-amino acids.
Reagents and conditions: (a) HOBt, 1-[3-(dimethyl-
amino)propyl]-3-ethylcarbodiimide, HCl, Et3N, (L)-Ala-
OMe, HCl, CH2Cl2, 58% overall from a crude 92/8 mixture of
9/10, 19, 72% from stereoisomerically pure cis-18 20; (b) (i)
TMSCl/MeOH, (ii) H2, Pd/C then HOBt, 1-[3-(dimethyl-
amino)propyl]-3-ethylcarbodiimide, HCl, Et3N, (L)-N-Boc-
Ala, CH2Cl2, 32% overall 21, 78% overall 22.
3.2. (4S,5R)-[Benzyl-(2-chloro-1-methyl-2-phenyl-ethyl)-
amino]-acetonitrile 8
the spectra of 22 in DMSO-d6 modified the ratio of
rotamers (3/7) and at higher temperature (355 K) some
of the signals began to fuse indicating that the tempera-
ture of coalescence was near.
To a solution of 7 (1.02 g, 3.65 mmol) in CH2Cl2 (22
mL) was added thionyl chloride (530 ml, 7.3 mmol).
After 1 h of reflux, the mixture was neutralized by the
addition of a saturated aqueous solution of NaHCO3.
Usual workup, followed by flash chromatography (E/
CyH: 1/9) gave 8 as a pale yellow solid (842 mg, 77%).
Rf: 0.70 (E/CyH: 1/1); Mp: 100°C; [h]2D0=−32.4 (c 0.4,
In summary, we have described a straightforward syn-
thesis of azetidinic a-amino acids from 2-cyano azetidi-
nes. These amino acids are constrained analogues of
phenylalanine and can be conveniently incorporated
into a peptidic sequence. Furthermore, we have also
demonstrated that the anionic cyclisation leading to
2-cyano azetidines can be thermodynamically con-
trolled, thus increasing the diastereoselectivity of the
process.
1
CHCl3); H NMR: 1.48 (d, J=6.6 Hz, 3H, Me), 3.35
(ABsyst, J=17.7 Hz, 2H, CH2CN), 3.46 (quint, J=6.6
Hz, 1H, CHMe), 3.73 (d, 1H, J=13.4 Hz, CHHPh),
3.87 (d, 1H, J=13.4 Hz, CHHPh), 4.95 (d, J=7.6 Hz,
1H, PhCH), 7.00–7.05 (m, 2H, Ar), 7.22–7.28 (m, 3H,
Ar), 7.33–7.39 (m, 5H, Ar); 13C NMR: 12.1 (CH3), 39.0
(CH2), 54.4 (CH2), 63.7 (CH), 66.3 (CH), 116.8 (CN)
127.7, 127.9, 128.4, 128.7, 128.8 (CHAr), 137.0, 141.1
(CqAr); Anal. Calc. for C18H19N2Cl: C: 72.35; H: 6.41;
N: 9.37. Found: C: 72.51; H: 6.45; N: 9.12.
3. Experimental section
1
General comments. H and 13C spectra (CDCl3 solution
3.3. (2R,3S,4S)-1-Benzyl-4-methyl-3-phenyl-azetidine-2-
carbonitrile 9 and (2S,3S,4S)-1-Benzyl-4-methyl-3-
phenyl-azetidine-2-carbonitrile 10
unless otherwise stated) were recorded on a Bruker AC
200 or 300 spectrometer at 200, 300 (1H), 50.3 and 75.5
(13C) MHz; chemical shifts are reported in ppm from
TMS. Optical rotations were determined with a Perkin
Elmer 241 instrument. All reactions were carried out
under argon. Column chromatography was performed
on silica gel 230–400 mesh by using various mixtures of
diethyl ether (E), ethyl acetate (AcOEt), cyclohexane
(CyH) and petroleum ether (PE). TLCs were run on
Merck Kieselgel 60F254 plates. Melting points are
uncorrected. THF was distilled from sodium/benzophe-
none ketyl. The mention of ‘usual workup’ means: (i)
decantation of the organic layer; (ii) extraction of the
aqueous layer with ether; (iii) washing the combined
organic layers with brine and drying of the combined
organic phases over MgSO4; (iv) solvent evaporation
under reduced pressure. Compositions of stereoisomeric
mixtures were determined by NMR analysis on crude
products before any purification.
To a solution of chloride 8 (1 mmol) in THF (15 mL)
was added dropwise at −78°C a solution of LiHMDS (1
M solution in THF, 1.2 to 3 equiv.). The mixture was
stirred for the time and temperature specified in Scheme
2 and then quenched by the addition of an aqueous,
saturated solution of NH4Cl. Usual workup, followed
by flash chromatography (E/CyH: 1/9) gave mixtures
(see text) of cyanoazetidines 9 and 10 as oils. Com-
1
pound trans-9: Rf: 0.57 (E/CyH: 3/7); H NMR: 1.21
(d, 3H, J=5.7 Hz, Me), 3.32–3.45 (m, 1H, H4), 3.51 (t,
1H, J=8.3 Hz, H3), 3.68 (d, 1H, J=8.3 Hz, H2) 3.80
(AB syst., 2H, J=12.7 Hz, CH2Ph), 7.14–7.45 (m, 10H,
Ar); 13C NMR: 20.9 (CH3), 49.6 (CH), 55.3 (CH2),
60.8, 66.8 (CH), 119.2 (CN), 120.7, 127.5, 127.8, 128.5,
128.8, 129.1, 129.4 (CHAr), 135.9, 137.6 (CqAr). Com-
1
pound cis-10: Rf: 0.62 (E/CyH: 3/7); H NMR: 1.26 (d,

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F. Couty et al. / Tetrahedron: Asymmetry 14 (2003) 2407–2412
3H, J=5.0 Hz, Me), 3.64 (t, 1H, J=8.3 Hz, H3),
3.80–3.94 (m, 1H, H4), 3.87 (AB syst., 2H, J=13.2 Hz,
CH2Ph), 4.56 (dd, J=0.9 and 7.5 Hz, H2), 7.17–7.48
(m, 10H, Ar); 13C NMR: 20.1 (CH3), 47.0 (CH), 56.4
(CH2), 57.8, 65.4 (CH), 116.2 (CN), 127.0, 127.5, 127.8,
127.9, 128.5, 128.8, 129.1, 129.4 (CHAr), 135.7, 137.0
(CqAr); Anal. Calc. for C18H18N2: C: 82.41; H: 6.92; N:
10.68. Found: C: 81.93; H: 7.03; N: 10.72.
cyanoazetidine 4 (289 mg, 1.55 mmol), amino acid 14
was obtained as a colorless glass (309 mg, 97%). Mp:
1
100–103°C; [h]D20=+64 (c 1.4, H2O); H NMR (D2O):
1.57 (d, J=6.5 Hz, 3H, Me), 2.991 (s, 3H, NMe), 3.70
(t, J=9.7 Hz, 1H, H3), 4.24 (dq, J=9.7 and 6.5 Hz, 1H,
H4), 4.60 (d, J=9.7 Hz, 1H, H2), 7.28–7.41 (m, 5H,
Ar); 13C NMR (D2O): 16.4 (CH3), 39.5 (CH), 48.4
(CH), 69.4 (CH), 71.2 (CH), 127.5, 128.7, 129.5
(CHAr), 136.4 (CqAr), 172.4 (CꢀO); Anal. Calc. for
C12H15NO2: C: 70.22; H: 7.37; N: 6.82. Found: C:
70.32; H: 7.49; N: 6.75
3.4. (2RS,3S,4S)-1,2,4-trimethyl-3-phenyl-azetidine-2-
carbonitrile 12
To a solution of 7/3 epimeric mixture of 3 and 4 (194
mg, 1.04 mmol) and methyl iodide (100 mL, 1.6 mmol)
in THF (10 mL) cooled to −90°C, was added a freshly
prepared solution of LDA (0.6 M in THF, 3.5 mL, 2.6
mmol). The mixture was stirred at −90°C for 10 min,
gradually warmed to −60°C, and quenched at this
temperature by the addition of aqueous NH4Cl. Usual
workup, followed by flash chromatography (AcOEt/
EP: 8/92) gave 12 as a 41/58 mixture of isomers. Minor
isomer: Rf: 0.74 (E/PE: 7/3); [h]2D0=−76 (c 1.9, CHCl3)
1H NMR: 1.01 (s, 3H, Me), 1.28 (d, 3H, J=6.2 Hz,
Me), 2.26 (s, 3H, Me), 3.52 (dq, J=8.0 and 6.2 Hz, 1H,
H4), 3.42 (d, 1H, J=8.0, H3), 7.11–7.31 (m, 5H, Ar);
13C NMR: 14.9, 19.2, 33.9 (CH3), 53 (CH), 59.7 (Cq),
61.7 (CH), 122.7 (CN), 127.7, 128.1, 128.8 (CHAr),
135.0 (CqAr). Major isomer: Rf: 0.60 (E/PE: 7/3);
[h]2D0=−48 (c 1.3, CHCl3) 1H NMR: 1.19 (d, 3H, J=6.0
Hz, Me), 1.54 (s, 3H, Me), 2.35 (s, 3H, Me), 3.07 (d,
1H, J=9.0, H3), 3.52 (dq, J=9.0 and 6.0 Hz, 1H, H4),
7.14–7.31 (m, 5H, Ar); 13C NMR: 19.7, 25.2, 36.8
(CH3), 55.9, 63.3 (CH), 67.8 (Cq), 118.0 (CN), 127.8,
128.0, 128.7 (CHAr), 135.4 (CqAr).
3.7. (2R,3R)-(1-Benzyl-3-phenyl-azetidine-2-yl)-carbox-
ylic acid 15,HCl
Cyano azetidine 5 (289 mg, 1.15 mmol) was dissolved
into a biphasic mixture of aqueous HCl (35% wt., 20
mL) and carbon tetrachloride (20 mL). The solution
was then heated with vigorous stirring at 80°C for 7
days and concentrated under reduced pressure. The
residue was triturated in diethyl ether, and the obtained
white solid (370 mg) engaged without further purifica-
tion in the debenzylation step. Mp: 185–190°C (dec);
1
[h]2D0=+57 (c 0.6, DMSO); H NMR (DMSO-d6): 4.04–
4.12 (m, 3H, H3,4), 4.52 (s, 2H, CH2Ph), 5.30 (d, J=9.3
Hz, 1H, H2), 7.35–7.65 (m, 10H, Ar); 13C NMR
(DMSO-d6): 38.7 (CH), 55.1 (CH2), 56.5 (CH2), 69.2
(CH), 70.3 (CH), 127.7, 127.9, 128.6, 128.8, 129.5
(CHAr), 130.8, 136.7 (CqAr), 167.7 (CꢀO).
3.8. (2R,3R)-(3-Phenyl-azetidine-2-yl)-carboxylic acid
16
A suspension of hydrochloride 15 (237 mg, 0.78 mmol)
and Pd/C (10% wt., 200 mg) in MeOH (20 mL) and
water (20 mL) was vigorously stirred under an atmo-
sphere of hydrogen for 6 h. The suspension was then
filtrated over Celite, and the filtrate concentrated under
reduced pressure. The residual solid was purified on an
ion-exchange resin (Dowex 50X8-200; 6 g). Elution
with water until neutrality, followed by 1% aqueous
NH4OH, gave fractions of the title compound that were
then concentrated under reduced pressure. Drying
under high vacuum (0.5 mmHg/70°C for 48 h) gave
amino acid 16 as a white solid (106 mg, 78%). Mp:
175–185°C (dec); [h]2D0=−118 (c 2, H2O); 1H NMR
(D2O): 4.07–4.21 (m, 3H, H3,4), 4.88 (d, J=7.4 Hz, 1H,
H2), 7.33–7.44 (m, 5H, Ar); 13C NMR (D2O): 38.7
(CH), 48.8 (CH2), 65.8 (CH), 42.0 (CH), 127.4, 128.6,
129.5 (CHAr), 138.3 (CqAr), 173.3 (C=0). HRMS–
FAB (m/z): [M+H+] calcd for C10H12NO2, 178.0868;
found, 178.0863.
3.5. (2R,3S,4S)-(1,4-Dimethyl-3-phenyl-azetidin-2-yl)-
carboxylic acid 13
Compound 3 (381 mg, 2.04 mmol) was melted and then
dissolved into an aqueous solution of HCl (35% wt., 35
mL) with vigorous stirring. The solution was then
heated at 50°C for 72 h, and evaporated to dryness
under reduced pressure. The residual white solid was
then dissolved into a small amount of distilled water,
and purified on an ion-exchange resin (Dowex 50X8-
200; 15 g). Elution with water until neutrality, followed
by 1% aqueous NH4OH, gave fractions of the title
compound that were concentrated under reduced pres-
sure. Drying under high vacuum (0.5 mmHg/70°C for
48 h) gave amino acid 13 as a colorless glass (407 mg,
1
97%). Mp: 94–96°C; [h]2D0=−61 (c 1.4, H2O); H NMR
(D2O): 1.51 (d, J=7.0 Hz, 3H, Me), 2.81 (s, 3H, NMe),
4.01 (dd, J=10.5 and 7.7 Hz, 1H, H3), 4.67–4.79 (m,
2H, H2,4), 7.28–7.41 (m, 5H, Ar); 13C NMR (D2O): 13.7
(CH3), 33.4 (CH), 44.9 (CH), 67.4 (CH), 70.3 (CH),
127.7, 128.1, 128.6 (CHAr), 134.9 (CqAr), 170.0 (C=
O); Anal. Calc. for C12H15NO2: C: 70.22; H: 7.37; N:
6.82. Found: C: 70.13; H: 7.31; N: 6.85.
3.9. (2R,3S,4S)-1-Benzyl-4-methyl-3-phenyl-azetidine-2-
carboxylic acid 17, HCl and (2S,3S,4S)-1-benzyl-4-
methyl-3-phenyl-azetidine-2-carboxylic acid 18, HCl
To a solution of epimeric mixtures of cyanoazetidines 9
and 10 (0.58 mmol) in glacial acetic acid (5 mL) was
added concentrated HCl (35% wt., 10 mL). The mixture
was heated under vigorous stirring for 72 h at 100°C
and then concentrated under reduced pressure. Starting
3.6. (2S,3S,4S)-(1,4-Dimethyl-3-phenyl-azetidin-2-yl)-
carboxylic acid 14
Following the above procedure and starting from

F. Couty et al. / Tetrahedron: Asymmetry 14 (2003) 2407–2412
2411
with epimeric mixtures in which the cis-compound 10
was the major isomer (60/40 mixture), a solid residue
was obtained. Washing this solid with a 1/1 mixture of
ether/acetone gave pure cis-10. Starting with trans-
cyano azetidine 9 (92/8 mixture) an oil was obtained
and engaged without further purification in the next
step. Trans-17 (dechlorhydrated by treatment with a
phosphate buffer, followed by extraction in CH2Cl2);
1H NMR (200 MHz, CDCl3): 1.39 (d, 3H, J=5.7 Hz,
Me), 3.65–3.90 (m, 1H, H4), 3.83 (t, 1H, J=8.8 Hz, H3),
4.04 (d, 1H, J=13.2 Hz, CHHPh), 4.16 (d, 1H, J=8.8
Hz, H2), 4.47 (d, 1H, J=13.2 Hz, CHHPh), 7.12–7.61
(m, 10H, Ar), 8.39 (bs, 1H, COOH); 13C NMR: 18.0
(CH3), 47.2 (CH2), 57.9, 64.8, 68.9 (CH), 126.9, 128.7,
129.0, 130.9 (CHAr), 138.4 (CqAr), 171.2 (CO); MS
(IC, CH4): m/z 282 (MH+, 100), 236 (13), 134 (10), 91
(8); HRMS (IC, CH4): [M+H+] calcd for C18H20NO2
282.1494; found 282.1491. Cis-18, HCl: Mp 270–290°C;
11H, Ar, NH); 13C NMR: 15.7, 18.7 (Me) 46.9, 47.8
(CH), 52.3 (Me), 54.2 (CH2), 60.4, 68.5 (CH), 126.9,
127.4, 128.1, 128.6, 128.7, 128.9 (CHAr), 138.3, 138.4
(CqAr), 169.5, 173.0 (CO); Anal. Calc. for C22H26N2O3:
C: 72.11; H: 7.15; N: 7.64. Found: C: 71.06; H: 7.24; N:
7.56.
3.11. Coupling with (L)-Boc-Ala-OH: general
procedure
To a solution of dipeptide 19 or 20 (0.3 mmol) in
MeOH (8 mL) was added TMSCl (0.6 mmol), followed
by Pd/C (10% wt., 40 mg) after 5 min. The suspension
was vigorously stirred under an atmosphere of hydro-
gen for 24 h, filtrated over Celite, and concentrated
under reduced pressure. Crude debenzylated dipeptides
were used without purification for the next step. To a
solution of dipeptide (0.36 mmol) in dichloromethane
were added 2-hydroxybenzotriazole (0.72 mmol), (L)-
Boc-Ala-OH (0.43 mmol), 1-(3-dimethyl-aminopropyl)-
3-ethyl carbodiimide (0.54 mmol) and triethylamine
(1.08 mmol). After 12 h of stirring at rt, the addition of
water was followed by usual workup (CH2Cl2) and
purification by flash chromatography (AcOEt/CyH: 6/4
for 21 and AcOEt for 22) and gave tripeptides 21 or 22.
Tripeptide 21: (yield: 32%); Rf=0.68 (AcOEt); [h]2D5=
−47 (c 0.6, CHCl3); 1H NMR (300 MHz, 263 K,
CDCl3): Mixture of two rotamers: 1.29 (d, 3H, J=7.2
Hz, Me), 1.30 (d, 3H, J=7.2 Hz, Me), 1.38 (d, 3H,
J=7.2 Hz, Me), 1.39 (s, 9H, Boc), 1.42 (d, 3H, J=7.2
Hz, Me), 1.45 (s, 9H, Boc), 1.59 (d, 3H, J=6.2 Hz, Me),
1.60 (d, 3H, J=6.2 Hz, Me), 3.35 (t, 1H, J=5.3 Hz,
CHPh), 3.68 (t, 1H, J=6.0, CHPh), 3.69 (s, 3H, OMe),
3.73 (s, 3H, OMe), 4.17 (quint, 1H, J=6.9 Hz, CHMe),
4.27 (quint, 1H, J=6.9 Hz, CHMe), 4.51 (quint, 1H,
J=7.2 Hz, CHMe), 4.54 (quint, 1H, J=6.0 Hz,
CHMe), 4.60–4.84 (m, 2H, CHMe), 4.63 (d, 1H, J=5.6
Hz, H2), 4.75 (d, 1H, J=6.6 Hz, H2), 5.02 (d, 1H,
J=5.6 Hz, NH), 5.20 (d, 1H, J=8.5 Hz, NH), 7.24–
7.40 (m, 10H, Ar), 7.98 (d, 1H, J=6.9 Hz, NH), 8.78 (d,
1H, J=8.5 Hz, NH); 13C NMR: 18.2, 19.1, 22.5 (broad,
CH3), 28.4 (CH3), 45.1, 46.2 (broad, CH), 48.2, 52.5
(CH3), 64.5, 66.2 (broad, CH), 80.3 (Cq), 127.2, 127.6,
129.0 (CHAr), 139.5 (CqAr), 155.5 (broad, NCOO),
169.8, 173.0, 178.7 (broad, CO). Tripeptide 22: (yield:
78%); Rf=0.48 (AcOEt); [h]2D5=−4.2 (c 0.3, CHCl3); 1H
NMR (300 MHz, 298 K, CDCl3): Mixture of two
rotamers: 0.68 (d, 3H, J=7.0 Hz, Me), 1.04 (d, 3H,
J=7.3 Hz, Me), 1.28 (d, 3H, J=6.8 Hz, Me), 1.38 (d,
3H, J=6.8 Hz, Me), 1.45 (s, 9H, Boc), 1.47 (s, 9H,
Boc), 1.60 (d, 3H, J=5.9 Hz, Me), 1.62 (d, 3H, J=5.6
Hz, Me), 3.61–3.71 (m, 2H, CHPh), 3.68 (s, 3H, OMe),
3.69 (s, 3H, OMe), 3.89 (quint, 1H, J=7.3 Hz, CHMe),
4.24 (quint, 2H, J=7.0 Hz, CHMe), 4.37 (quint, 1H,
J=7.2 Hz, CHMe), 4.78 (quint, 1H, J=5.9 Hz,
CHMe), 5.00 (d, 1H, J=9.8, H2), 5.02 (d, 1H, J=8.2
Hz, NH), 5.09 (quint, 1H, J=6.2, CHMe), 5.27 (d, 1H,
J=6.5 Hz, NH), 5.54 (d, 1H, J=9.4 Hz, NH), 5.94 (d,
1H, J=6.8 Hz, NH), 7.01 (d, 1H, J=6.3 Hz, NH),
7.20–7.37 (m, 10H, Ar); 13C NMR: 16.8, 17.5, 17.9, 18.0
(CH3), 20.0, 22.1, 28.3, 46.0, 46.6, 46.8, 47.1, 47.5, 47.7
(CH), 52.1, 52.3 (CH3), 61.7, 62.9, 64.2, 65.9 (CH), 79.5,
79.8 (Cq), 127.6, 127.7, 128.2, 128.4, 128.6, 128.7
1
[h]2D0=+5.5 (c 0.3, H2O); H NMR (200 MHz, D2O+
NaOD): 0.84 (d, 3H, J=5.9 Hz, Me), 3.25 (t, 3H,
J=8.2 Hz, H3), 3.58 (d, 1H, J=12.5 Hz, CHHPh), 3.71
(d, 1H, J=12.4 Hz, CHHPh), 3.92 (d, 1H, J=9.2 Hz,
H2), 4.02 (quint, 1H, J=6.6 Hz, H4), 6.92–7.13 (m,
10H, Ar); 13C NMR: 21.3 (CH3), 48.9 (CH2), 56.3, 65.8,
72.1 (CH), 129.3, 129.9, 130.1, 131.1, 131.2, 132.1
(CHAr), 140.9, 141.7 (CqAr), 180.2 (CO); MS (IC,
CH4): m/z 282 (MH+, 40), 281 (100), 236 (18), 154 (22),
135(25), 124 (15), 106 (20), 91 (6); HRMS (IC, CH4):
[M+H+] calcd for C18H20NO2 282.1494; found 282.1492.
3.10. Coupling with (L)-Ala-OMe: general procedure
To a solution of crude N-Bn azetidinic amino acid, HCl
17 or 18 (0.094 mmol) in dichloromethane (2 mL) was
added 2-hydroxybenzotriazole (0.19 mmol), (L)-Ala-
OMe, HCl (0.14 mmol), 1-(3-dimethyl-aminopropyl)-3-
ethyl carbodiimide (0.11 mmol) and triethylamine (0.47
mmol). After 12 h of stirring at rt, the reaction was
quenched by the addition of water. Usual workup and
purification by flash chromatography (AcOEt/CyH: 4/
6) gave dipeptides 19 or 20. Dipeptide 19: 58% (starting
from a 92/8 crude epimeric mixture of 17/18); Rf=0.58
(AcOEt/CyH=1/1); [h]2D5=−2.4 (c 0.8, CHCl3); 1H
NMR (200 MHz, CDCl3): 1.14 (d, 3H, J=5.9 Hz, Me),
1.45 (d, 3H, J=7.3 Hz, Me), 3.09 (t, 1H, J=7.9 Hz,
CHPh), 3.39 (quadd, 1H, J=5.9 and 7.9 Hz, CHMe),
3.65 (d, 1H, J=12.5 Hz, CHHPh), 3.69 (d, 1H, J=7.9
Hz, H2), 3.77 (s, 3H, OMe), 3.82 (d, 1H, J=12.3,
CHHPh), 4.52 (quint, 1H, J=7.5, NHCHMe), 7.18–
7.43 (m, 10H, Ar), 7.88 (d, 1H, J=7.5 Hz, NH); 13C
NMR: 18.8, 22.1 (Me) 47.6, 49.9 (CH), 52.5 (Me), 61.5
(CH2), 65.9, 69.8 (CH),126.9, 127.1, 127.6, 128.6, 128.7,
129.7 (CHAr), 137.4, 140.2 (CqAr), 172.1, 173.3 (CO).
Dipeptide 20: 72% (starting from isomerically pure
cis-18); Rf=0.65 (AcOEt/CyH=8/2); [h]2D5=−70.2 (c
0.5, CHCl3); Mp 138–140°C; 1H NMR (200 MHz,
CDCl3): 0.95 (d, 3H, J=7.0 Hz, Me), 1.42 (d, 3H,
J=6.6 Hz, Me), 3.45 (dd, 1H, J=3.1 and 9.2 Hz,
CHPh), 3.68 (s, 3H, OMe), 3.69 (d, 1H, J=13.2 Hz,
CHHPh), 3.92 (quint, 1H, J=7.0 Hz, CHMe), 3.94 (d,
1H, J=13.6 Hz, CHHPh), 4.02–4.11 (m, 1H,
NHCHMe), 4.30 (d, 1H, J=9.2 Hz, H2) 7.18–7.43 (m,

2412
F. Couty et al. / Tetrahedron: Asymmetry 14 (2003) 2407–2412
(CHAr), 135.1, 135.9 (CqAr), 1554, 155.6 (NCOO),
166.1, 166.9, 172.7, 173.0, 173.4, 174.0 (CO); Anal.
Calc. for C23H33N3O6: C: 61.73; H: 7.43; N: 9.39.
Found: C: 61.68; H: 7.55; N: 9.24.
Lett. 1997, 38, 89–92. For a recent example, see:
Karoyan, P.; Quancard, J.; Vaissermann, J.; Chassaing,
G. J. Org. Chem. 2003, 68, 2256–2265.
4. Couty, F. Amino Acids 1999, 16, 397–420.
5. (a) Kozikowski, A. P.; Tu¨ckmantel, W.; Reynolds, I. J.;
Wrobleski, J. T. J. Med. Chem. 1990, 33, 1561–1571; (b)
Kozikowski, A. P.; Liao, Y.; Tu¨ckmantel, W.; Wang, S.;
Pshenichkin, S.; Surin, A.; Thomsen, C.; Wrobleski, J. T.
J. Bioorg. Med. Chem. Lett. 1996, 6, 2559–2564; (c)
Hanessian, S.; Fu, J. M.; Chiara, J.-L.; Di Fabio, R.
Tetrahedron Lett. 1993, 34, 4157–4160; (d) Hanessian, S.;
Bernstein, N.; Yang, R.-Y.; Maguire, R. Bioorg. Med.
Chem. Lett. 1999, 9, 1437–1442; (e) Carlin-Sinclair, A.;
Couty, F.; Rabasso, N. Synlett 2003, 726–729.
6. Agami, C.; Couty, F.; Evano, G. Tetrahedron: Asymme-
try 2002, 297–302.
7. Couty, F.; Prim, D. Tetrahedron: Asymmetry 2002, 13,
2619–2624.
8. Couty, F.; Durrat, F.; Prim, D. Tetrahedron Lett. 2003,
44, 5209–5213.
9. Strecker, A. Liebigs. Ann. Chem. 1850, 75, 27.
10. Similar equilibration to more stable 2,3-trans azetidines
has already been noticed with azetidinic amino ketones.
See Ref. 7.
Acknowledgements
CNRS is acknowledged for its generous financial sup-
port.
References
1. (a) Stammer, C. H. Tetrahedron 1990, 46, 2231–2254; (b)
Giannis, A.; Kolter, T. Angew. Chem., Int. Ed. Engl.
1993, 32, 1244–1246; (c) Hanessian, S.; McNaughton-
Smith, G.; Lombart, H-G.; Lubell, W. D. Tetrahedron
1997, 53, 12789–12854; (d) Gibson, S. E.; Guillo, N.;
Tozer, M. Tetrahedron 1999, 55, 585–615; (e) Park, K-H.;
Kurth, M. J. Tetrahedron 2002, 58, 8629–8659.
2. (a) Gibson, S. E.; Guillo, N.; Kalindjian, S. B.; Tozer, M.
J. Bioorg. Med. Chem. Lett. 1997, 7, 1289–12920; (b)
Thompson, P. E.; Steer, D. L.; Aguilar, M. I.; Hearn, M.
T. W. Bioorg. Med. Chem. Lett. 1998, 8, 2699–2704; (c)
Saha, B.; Nandy, J. P.; Shukla, S.; Siddiqui, I.; Iqbal, J.
J. Org. Chem. 2002, 67, 7858–7860.
11. Fowden, L. Nature 1955, 176, 347–349.
12. For the synthesis of racemic 16, see: Blythin, D. J.;
Green, M. J.; Lauzon, M. J. R.; Shue, H.-J. J. Org.
Chem. 1994, 59, 6098–6100. For an asymmetric synthesis
of the cis-diastereoisomer of 16, see Ref. 5d.
3. Lorthiois, E.; Marek, I.; Normant, J.-F. Tetrahedron