Synthesis of various 3-substituted 1,2,4-oxadiazole-containing chiral β3- and α-amino acids from Fmoc-protected aspartic acid

Article abstract of DOI:10.1021/jo0345953

Various 3-substituted chiral 1,2,4-oxadiazole-containing Fmoc-β ³- and α-amino acids were synthesized from FmoC-(L or D)-Asp(OtBu)-OH and Fmoc-L-Asp-OtBu, respectively, in three steps (i.e., condensation of an aspartyl derivative with different

Full text of DOI:10.1021/jo0345953

Syn th esis of Va r iou s 3-Su bstitu ted 1,2,4-Oxa d ia zole-Con ta in in g
Ch ir a l â3- a n d r-Am in o Acid s fr om F m oc-P r otected Asp a r tic Acid
Abdallah Hamze´, J ean-Franc¸ois Hernandez,* Pierre Fulcrand, and J ean Martinez
Laboratoire des Aminoacides Peptides et Prote´ines, CNRS UMR 5810, Universite´s Montpellier I et II,
Faculte´ de Pharmacie, 15 avenue Charles Flahault, BP 14491, 34093 Montpellier ce´dex 5, France
hernandez@colombes.pharma.univ-montp1.fr
Received May 7, 2003
Various 3-substituted chiral 1,2,4-oxadiazole-containing Fmoc-â3- and -R-amino acids were
synthesized from Fmoc-(L or D)-Asp(OtBu)-OH and Fmoc-L-Asp-OtBu, respectively, in three steps
(i.e., condensation of an aspartyl derivative with differentially substituted amidoximes, formation
of the 1,2,4-oxadiazole, and cleavage of the tert-butyl ester). These compounds represent new series
of nonnatural amino acids, which could be used in combinatorial synthesis. A simple protocol has
been developed to generate the 1,2,4-oxadiazole ring. Indeed, common methods resulted in cleavage
of the Fmoc group or required long reaction times. We found that sodium acetate in refluxing ethanol/
water (86 °C) was a convenient and efficient catalyst to promote conversion of Fmoc-amino acyl
amidoximes to 1,2,4-oxadiazoles, and this procedure proved to be compatible with Fmoc protection.
It is shown that these compounds can be prepared without significant loss of enantiomerical purity.
Furthermore, the alkaline conditions used to cleave the Fmoc protecting group from these compounds
did not induce epimerization of their chiral center.
SCHEME 1 a
In tr od u ction
In an effort to develop new chiral nonproteinogenic R-
and â-amino acids that could be incorporated into biologi-
cally relevant peptides, as well as into protease inhibitor
structures, we focused on the synthesis of heterocycle-
containing amino acids. In the case of peptides, new
properties could be expected from such compounds and
remain to be investigated. The use of heterocycles as
scaffolds to optimally place substituents in protease
pockets is an important strategy for the search of pro-
tease inhibitors with improved pharmacokinetic proper-
ties. Heterocycle-containing amino acids would facilitate
incorporation of the scaffold while offering several pos-
sibilities of variation.
One such heterocycle is the well-known 1,2,4-oxadia-
zole (Scheme 1).1 It has been frequently used as ester or
amide bioisostere2 and was found to help in the design
of compounds with improved physicochemical properties
and bioavailability.3 One interesting feature is the pos-
sible participation in hydrogen bonding (as acceptor) with
a target (the active site of a protease, for instance) as
observed for compounds containing 1,3,4-oxadiazole.4
Furthermore, usual synthetic methods (see below) allow
a
1,2,4-Oxadiazole is substituted in 5 by the acyl-derived moiety
(R′) and in 3 by the amidoxime substituent (R).
preparation of heterocycles with substituents in positions
3 and 5 (Scheme 1). Several papers have reported the
use of 1,2,4-oxadiazole in peptide mimetics, including the
design of amino acyl-Gly dipeptidomimetics,5 signal
transduction inhibitors,6 or cell adhesion inhibitors.7
1,2,4-Oxadiazole-containing amino acids are prepared
from amino diacid compounds (e.g., aspartyl and glutamyl
derivatives). Several examples of aspartyl-derived 1,2,4-
(4) Odagaki, Y.; Ohmoto, K.; Matsuoka, S.; Hamanaka, N.; Nakai,
H.; Toda, M.; Katsuya, Y. Bioorg. Med. Chem. 2001, 9, 647-651.
(5) (a) Borg, S.; Estenne-Bouhtou, G.; Luthman, K.; Cso¨regh, I.;
Hesselink, W.; Hacksell, U. J . Org. Chem. 1995, 60, 3112-3120. (b)
Borg, S.; Vollinga, R. C.; Labarre, M.; Payza, K.; Terenius, L.; Luthman,
K. J . Med. Chem. 1999, 42, 4331-4342.
(1) (a) Behr, L. C. In The Chemistry of Heterocyclic Compounds;
Weissberger, A., Ed.; J ohn Wiley: New York, 1962; Vol. 17, pp 245-
262. (b) Clapp, L. B. In Advances in Heterocyclic Chemistry; Katritzky,
A. R., Ed.; Academic Press: New York, 1976; Vol. 20, pp 65-116. (c)
Hemming, K. J . Chem. Res., Synop. 2001, 209-216; 601-620.
(2) (a) Diana, G. D.; Volkots, D. L.; Nitz, J . T.; Bailey, R. T.; Long,
M. A.; Vescio, N.; Aldous, S.; Peveur, D. C.; Dutko, F. J . J . Med. Chem.
1994, 37, 2421-2436. (b) Andersen, K. E.; J orgensen, A. S.; Braestrup,
C. Eur. J . Med. Chem. 1994, 29, 393-399. (c) Andersen, K. E.; Lundt,
B. F.; J oergensen, A. S.; Braestrup, C. Eur. J . Med. Chem. 1996, 31,
417-425.
(6) (a) Buchanan, J . L.; Vu, C. B.; Merry, T. J .; Corpuz, E. G.;
Pradeepan, S. G.; Mani, U. N.; Yang, M.; Plake, H. R.; Varkhedkar,
V. M.; Lynch, B. A.; MacNeil, I. A.; Loiacono, K. A.; Tiong, C. L.; Holt,
D. A. Bioorg. Med. Chem. Lett. 1999, 9, 2359-2364. (b) Vu, C. B.;
Corpuz, E. G.; Merry, T. J .; Pradeepan, S. G.; Bartlett, C.; Bohacek,
R. S.; Botfield, M. C.; Eyermann, C. J .; Lynch, B. A.; MacNeil, I. A.;
Ram, M. K.; van Schravendijk, M. R.; Violette, S.; Sawyer, T. K. J .
Med. Chem. 1999, 42, 4088-4098. (c) Buchanan, J .; Bohacek, R.; Vu,
C. B.; Luke, G. P.; Ariad Pharmaceuticals, WO 99/47529 A1, 23 Sept.
1999.
(7) Durette, P. L.; Hagmann, W. K.; Kopka, I. E.; MacCoss, M.;
Merck & Co., WO 00/71572 A1, 30 Nov. 2000.
(3) Naka, T.; Kubo, K. Curr. Pharm. Des. 1999, 5, 453-472.
10.1021/jo0345953 CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/21/2003
7316
J . Org. Chem. 2003, 68, 7316-7321

Fmoc-Protected 1,2,4-Oxadiazole-Containing â3- and R-Amino Acids
oxadiazole compounds can be found in the literature, with
in addition to references already mentioned,5a,7 the
synthesis of phenolic amino acids,8 alanine analogues,9
or the synthesis of combinatorial libraries of oxadiazole
compounds on solid support.10
1,2,4-Oxadiazoles are most commonly synthesized in
solution and on solid support10-13 from amidoximes and
carboxylic acid derivatives in two steps (Scheme 1).
During the first step, the amidoxime prepared by the
addition of hydroxylamine to a nitrile compound is
O-acylated by an activated carboxylic acid derivative.
Preactivation or in situ activation protocols have been
reported, including the use of acid chloride,11 acid fluo-
ride,12 symmetrical anhydride,3b,5,14 active ester,6 ester
with sodium ethoxide,13 CDI,15,16 acyl-palladium com-
plex,17 UNCA,18 carbodiimides (DCC,9,15 DIC,10a and
EDC6,15,19), BOP-Cl,15 and TBTU.20 The heterocycle is
subsequently formed by intramolecular cyclodehydration.
This can be performed either after isolation of the
O-acylated amidoxime precursor or immediately follow-
ing its formation in a one-pot reaction. Numerous condi-
tions of dehydration have been reported, mostly involving
heating (above 100 °C) in solvents such as DMF,16,20
diglyme,10,15 pyridine,5,6 in the presence or not5,6,9,10,15,20
of additives such as EDC,7 CDI,16 or Burgess reagent.19
Cyclization could also proceed at room temperature when
a strong basic reagent (NaOEt13 or TBAF11,21) is present.
All of these approaches generally require long reaction
times.
Most amino acid derived 1,2,4-oxadiazoles are synthe-
sized by coupling the R-carboxylic group of an R-amino
acid to amidoximes, leading to heterocycles in which the
amino acyl derived moiety is attached at the C-5 position
(Scheme 1). It is also possible to obtain the regioisomer
with attachment at the C-3 position when the R-carboxyl
group is first converted into nitrile, the amino acyl
derived amidoxime precursor.22 These derivatives are
usually obtained from Boc-protected amino acids. One
group reported the use of Fmoc-amino acids in the
synthesis of oxadiazole libraries.10 Cyclization is per-
F IGURE 1. Structure of Fmoc-protected 1,2,4-oxadiazole-
containing â3- (S enantiomer, compounds 4a -g; R enantiomer,
compounds 7a -f) and R- (S enantiomer, compounds 10a -d )
amino acids.
formed on a solid support by heating in a neutral solvent
without the need of a dehydrating agent, conditions that
are compatible with Fmoc protection. However, reaction
time is rather long (6 h) and no yield has been precisely
presented.
This paper describes the preparation of chiral 1,2,4-
oxadiazole-containing Fmoc-â3- and -R-amino acids from
Fmoc-Asp protected on its â- and R-carboxylic group,
respectively. Fmoc is a protecting group more suitable
than Boc for the use of these derivatives in combinatorial
synthesis.
Resu lts a n d Discu ssion
Two series of 1,2,4-oxadiazole-containing amino acids
have been synthesized from Fmoc-protected aspartyl
derivatives (Figure 1). The condensation of differentially
substituted amidoximes to the R- or â-carboxylic group
of an aspartyl residue followed in both cases by the
formation of the heterocyclic ring and the deprotection
of the remaining carboxylic group led to the Fmoc-
protected 1,2,4-oxadiazole-containing â3- or R-amino ac-
ids, respectively.23 Physical data of the synthesized Fmoc-
amino acid derivatives are presented in Table 1.
Syn th esis of 1,2,4-Oxa d ia zole-Con ta in in g F m oc-
â3-Am in o Acid s. 1,2,4-Oxadiazole compounds 4a -g and
7a -f were synthesized in three steps from an aspartyl
derivative (Scheme 2).24 The R-carboxylic group of Fmoc-
L-Asp(OtBu)-OH (or Fmoc-D-Asp(OtBu)-OH) was first
condensed to variously substituted amidoximes, which
have been prepared using classical procedures, i.e.,
treatment of nitrile compounds with hydroxylamine in
refluxing ethanol.25
(8) Moussebois, C.; Heremans, J . F.; Merenyi, R.; Rennerts, W. Helv.
Chim. Acta 1977, 60, 237-242.
(9) De Melo, S. J .; Sobral, A. D.; de Lima Lopes, H.; Srivastava, R.
M. J . Braz. Chem. Soc. 1998, 9, 465-468.
(10) (a) He´bert, N.; Hannah, A. L.; Sutton, S. C. Tetrahedron Lett.
1999, 40, 8547-8550. (b) He´bert, N.; Hannah, A. L.; Trega Biosciences,
WO 00/25768 A1, 11 May 2000.
(11) Rice, K.; Nuss, J . M. Bioorg. Med. Chem. Lett. 2001, 11, 753-
755.
(12) Sams, C.; Lau, J . Tetrahedron Lett. 1999, 40, 9359-9362.
(13) Bedford, C. D.; Howd, R. A.; Dailey, O. D.; Miller, A.; Nolen,
H. W.; Kenley, R. A.; Kern, J . R.; Winterle, J . S. J . Med. Chem. 1986,
29, 2174-2183.
(14) Liang, G.-B.; Qian, X. Bioorg. Med. Chem. Lett. 1999, 9, 2101-
2104.
(15) Liang, G.-B.; Feng, D. D. Tetrahedron Lett. 1996, 37, 6627-
6630.
(16) Deegan, T. L.; Nitz, T. J .; Cebzanov, D.; Pufko, D. E.; Porco, J .
A., J r. Bioorg. Med. Chem. Lett. 1999, 9, 209-212.
(17) Young, J . R.; DeVita, R. J . Tetrahedron Lett. 1998, 39, 3931-
3934.
(18) Huck, J . Ph.D. Thesis, University of Montpellier II, France,
2002.
(19) Rudolph, J .; Theis, H.; Hanke, R.; Endermann, R.; J ohannsen,
L.; Geschke, F.-U. J . Med. Chem. 2001, 44, 619-626.
(20) Poulain, R. F.; Tartar, A. L.; De´prez, B. P. Tetrahedron Lett.
2001, 42, 1495-1498.
(21) Gangloff, A. R.; Litvak, J .; Shelton, E. J .; Sperandio, D.; Wang,
V. R.; Rice, K. D. Tetrahedron Lett. 2001, 42, 1441-1443.
(22) Sollner, M.; Levacic, S.; Pecar, S. In Peptides 1996; Ramage,
R., Epton, R., Eds.; Mayflower Scientific: U.K., 1998; pp 809-810.
Several coupling conditions have been examined for
O-acylation of acetamidoxime (1a ). We first used HBTU
(23) The amino acyl derived moiety is attached at the 5-position of
the heterocycle while the diversity originating from the amidoximes
is held by the 3-position.
(24) Similar â3-amino acids could also be prepared from â-HomoAsp
(â-amino-glutaric acid). In this case, the symmetry of this derivative
allows the preparation of both enantiomers following homologation of
an L-aspartyl derivative used as a single precursor. See: Hamze´, A.;
Hernandez, J .-F.; Martinez, J . Tetrahedron Lett. 2003, 44, 6079-6082.
(25) (a) Eloy, F. R. L. Chem. Rev. 1962, 62, 155-183. (b) Weller, H.
N.; Poss, M. A.; E. R. Squibb & Sons, US 5,236,916, 17 Aug. 1993.
J . Org. Chem, Vol. 68, No. 19, 2003 7317

Hamze´ et al.
TABLE 1. P h ysica l Da ta of 1,2,4-Oxa d ia zole-Con ta in in g
Am in o Acid s
protecting group. Heating the acyl acetamidoxime 2a in
DMF at 110 °C, conditions that have been used success-
fully in the case of Boc-protected amino acyl amidoximes
derivatives,20 did not allow recovery of the desired
protected oxadiazole 3a . Instead, we isolated a compound
a
compound
[R]D
mp (°C)b
tR (min)c
yield (%)d
4a
7a
4b
7b
4c
-33
+33
-49
+49
-35
+34
-36
+36
-27
+27
-40
+40
-32
-18
-14
-14
-13
117-119
128-130
180-182
186-188
179-181
182-184
172-174
190-192
79-81
2.85
2.74
3.43
3.45
3.28
3.25
2.42
2.46
3.18
3.16
3.04
3.10
3.28
2.81
3.46
3.29
2.42
71
73
71
84
42
69
25
50
59
59
54
76
55
35
38
56
20
1
that was identified as dibenzofulvene by H NMR spec-
troscopy,26,27 showing that cleavage of the Fmoc group
had occurred. This cleavage was probably promoted by
dimethylamine, which might be produced from thermal
decomposition of DMF. Another method allowing the
synthesis of 1,2,4-oxadiazoles from Boc-amino acids uses
refluxing pyridine.5 In the case of Fmoc-protected deriva-
tives these conditions cannot be applied, and we observed
extensive cleavage of the protecting group together with
the appearance of dibenzofulvene. One possible explana-
tion is the potential presence of amino contaminants in
commercially available pyridine. The Fmoc group is very
but not entirely resistant to pyridine. Traces of diben-
zofulvene have been detected in pyridine solutions of
Fmoc derivatives after 10 h at room temperature.28
Pyridine itself might also induce Fmoc cleavage at high
temperature. The catalytic use of a strong base such as
TBAF at room temperature has also been documented
for the conversion of Boc-protected amino acyl ami-
doximes.18 However, the use of similar conditions in our
study (0.1 equiv of TBAF in THF for 48 h) produced a
heterogeneous reaction mixture where the presence of
2a could be evidenced. Exposure to higher quantities of
TBAF would not be relevant as this strong base can easily
remove the Fmoc group.29 In addition, Borg et al.
observed that TBAF induced racemization of the R-carbon
of a Boc-Phe-derived 1,2,4-oxadiazole.5b Finally, treat-
ment with a tertiary amine (TEA, 1 equiv) at room
temperature left the starting material unchanged.
7c
4d
7d
4e
7e
4f
112-114
171-173
172-174
153-155
168-170
nde
7f
4g
10a
10b
10c
10d
nde
176-178
a
b
t ) 20 °C, c ) 0.01, MeOH. Melting points were measured
using a Kofler apparatus and are uncorrected. c Reverse-phase
HPLC analyses were run on a Chromolith SpeedRod C18 column
(0.46 cm × 5 cm); gradient 0-100% CH3CN/H2O/0.1% TFA over 5
min, 3 mL/min flow rate; the slight differences in retention times
observed between the enantiomers (4a /7a to 4f/7f) are not
d
significant. Overall yield calculated from Fmoc-protected aspartyl
derivative. e Not determined, hygroscopic powder.
SCHEME 2. Syn th esis of
1,2,4-Oxa d ia zole-Con ta in in g F m oc-â3-Am in o Acid s
fr om F m oc-L-Asp (OtBu )-OHa ,b
Other described methods were thought to be Fmoc-
incompatible or required long reaction times. Therefore,
we searched for suitable conditions to achieve both Fmoc
integrity and fast conversion. Sodium acetate appeared
to us as a possible candidate, since we already noted its
use in a dehydration reaction such as the formation of
semicarbazone from aldehyde and semicarbazide.30 In
fact, cyclic dehydration of acyl amidoxime 2a to oxadia-
zole 3a in the presence of sodium acetate (1-1.1 equiv
in ethanol/water at 86 °C) was found to proceed cleanly
with good yield (75% after crystallization). Monitoring
by reverse-phase HPLC showed that 2a was totally
consumed within 2 h, whereas less than 50% of 2a was
converted into 3a in the absence of sodium acetate during
the same period (Figure 2). This result showed that
sodium acetate dramatically increased the dehydration
rate. The completely protected oxadiazole compounds
3a -g and 6a -f were generated under these suitably mild
conditions in good yields (50-84%), suggesting the
general use of this method. Thus, it is shown that this
procedure is compatible with the widely used protecting
a
Reagents and conditions: (i) DIC, HOBt, DCM; (ii) CH3COONa,
EtOH/H2O, 86 °C, 2 h; (iii) TFA/DCM (50:50). bThe same synthetic
pathway starting from Fmoc-D-Asp(OtBu)-OH, was used for acyl
amidoximes 5a -f, fully protected oxadiazoles 6a -f, and final
compounds 7a -f.
and the even more reactive HBTU/HOBt pair (1 equiv
of each), which are very efficient coupling reagents and
represent conditions close to those utilized successfully
by Poulain et al.20 (i.e., TBTU/HOBt) for coupling of
amidoximes to carboxylic acid derivatives. These condi-
tions led to the expected O-acylated compounds albeit
with significant contamination. The widely used EDC/
HOBt6,15,19 gave similar results. However, the O-acylated
amidoxime derivative 2a , characterized by mass spec-
trometry and 1H NMR analyses was the sole product
when DIC or DIC/HOBt was used. The latter was
preferred over DIC alone to limit the possible deactiva-
tion into N-acylurea and the loss of enantiomeric purity
during coupling (see below) and was used for the syn-
thesis of all compounds.
(26) 1H NMR (CDCl3) δ 7.67 (d, 2H), 7.63 (d, J ) 6.6 Hz, 2H), 7.37-
7.28 (ddd, J ) 1.2 Hz, 2H), 7.25-7.20 (ddd, J ) 1.2 Hz, 2H), 6.01 (s,
2 olefinic H).
(27) (a) Neuenschwander, M.; Vo¨geli, R.; Fahrni, H.-P.; Lehmann,
H.; Ruder, J .-P. Helv. Chim. Acta 1977, 60, 1073-1086. (b) Cho, B. P.
Tetrahedron Lett. 1995, 36, 2403-2406.
(28) Carpino, L. A. J . Org. Chem. 1980, 45, 4250-4252.
(29) Ueki, M.; Amemiya, M. Tetrahedron Lett. 1987, 28, 6617-6620.
(30) McConnell, R. M.; York, J . L.; Frizzell, D.; Ezell, C. J . Med.
Chem. 1993, 36, 1084-1089.
The O-acyl amidoximes were isolated before subse-
quently being cyclized. Several cyclization conditions
were applied to check their compatibility with the Fmoc
7318 J . Org. Chem., Vol. 68, No. 19, 2003

Fmoc-Protected 1,2,4-Oxadiazole-Containing â3- and R-Amino Acids
subsequent conversion of the resulting acyl amidoximes
8 to the oxadiazoles 9 required longer reaction times (4-5
h) compared to the cyclization involving the R-carboxyl
group. The mechanism of ring closure implies an initial
attack of the amidine group on the carbonyl of the acyl
group. The observed lower reactivity could be explained
by the lower electrophilic character of the â-carboxylic
group compared to the R-one. Despite this difference, the
reaction went to completion, leading to similar yields of
oxadiazoles (43-82%).
The final deprotection step using TFA in DCM gener-
ated Fmoc-R-amino acids 10a -d . Lower overall yields
were obtained in the R-amino acid series compared to the
â3-amino acid series (Table 1). This result is mainly the
consequence of a less efficient condensation of the as-
partyl â-carboxylic group with amidoximes.
Ster eoch em ica l Asp ects. Optical purity is an es-
sential characteristic of amino acids. Synthesis from
enantiopure aspartyl derivatives should lead to oxadia-
zole compounds without any significant epimerization.
Epimerization might occur during the activation of the
R-carboxylic group of aspartic acid derivatives and/or
condensation with amidoximes31 as already mentioned5a
and depends on the activation procedure. In addition,
once the 1,2,4-oxadiazole ring is formed, the amino acyl
derived chiral RCH, adjacent to the heterocycle, might
be subject to racemization under acidic or alkaline
conditions (see below). To check the preservation of chiral
integrity during the synthesis, compound 4a was coupled
to L-Phe-OMe. Reverse-phase HPLC analysis of the
resulting dipeptide showed that no significant epimer-
ization occurred during the synthesis of 4a as more than
97% of the expected (S,S) diastereoisomer was recovered.
In this context, it is of interest to note that optical
rotation of the S (4a -f) and R (7a -f) derivatives have,
as expected, opposite signs, and for an identical R
substituent, very close absolute values (Table 1).
F IGURE 2. Reverse-phase HPLC monitoring of the conver-
sion of compound 2a to the oxadiazole 3a , in the presence (A)
or the absence (B) of sodium acetate (HPLC conditions are
indicated in footnote c of Table 1).
SCHEME 3. Syn th esis of
1,2,4-Oxa d ia zole-Con ta in in g F m oc-r-Am in o Acid s
fr om F m oc-L-Asp -OtBu a
It was reported that the deprotection of Boc-protected
1,2,4-oxadiazole-derived dipeptidomimetics using TFA,
following their incorporation on solid support, resulted
in epimerization.5b No mechanism was proposed to
explain this surprising finding. Furthermore, as men-
tioned above, the same group of investigators reported
that racemization could be induced by TBAF.5b This could
be prevented using adapted conditions, but it points out
the lability of the amino acyl derived R-proton when it is
adjacent to the heterocycle. The electron-attracting prop-
erties of the oxadiazoles, which have been exploited in
the design of R-keto-oxadiazole inhibitors of serine pro-
teases,32 might be responsible for the observed epimer-
izations. This phenomenon was also observed for R-keto
carbonyl inhibitors of proteases where the chiral center
(RCH) adjacent to the electrophilic carbonyl group is
subject to racemization under alkaline conditions.33
a
Reagents and conditions: (i) DIC, HOBt, DCM; (ii) CH3COONa,
EtOH/H2O, 86 °C, 4-5 h; (iii) TFA/DCM (50:50).
groups, Fmoc, t-Bu ester, Boc (results not shown), Z
(compounds 3e, 6e), benzyl and allyl ester (results not
shown), as well as with functionalities such as amides
(compounds 3d , 6d ).
The final removal of the t-Bu ester protecting group
from oxadiazoles 3a -g and 6a -f afforded the chiral
Fmoc-â3-amino acids 4a-g and 7a-f, respectively (Scheme
2). Satisfying overall yields (approximately 50-80%
yields) were obtained for all compounds except for those
with R ) -CH2CONH2 (compounds 4d , 7d , 10d . Table
1). The latter result was partly explained by the difficulty
to obtain pure synthetic amidoxime 1d .
Syn th esis of 1,2,4-Oxa d ia zole-Con ta in in g F m oc-
r-Am in o Acid s. Chiral 1,2,4-oxadiazole derivatives
10a -d were synthesized in three steps as described
above starting from Fmoc-L-Asp-OtBu (Scheme 3).
Various amidoximes (1a -d ) were condensed to the
â-carboxylic group of the aspartyl moiety. Here, the
(31) Acylation by the â-carboxylic group of Asp does not induce
epimerization.
(32) (a) Gyorkos, A.; Spruce, L. W. WO 98/24806. (b) Ohmoto, K.;
Yamamoto, T.; Horiuchi, T.; Imanishi, H.; Odagaki, Y.; Kawabata, K.;
Sekioka, T.; Hirota, Y.; Matsuoka, S.; Nakai, H.; Toda, M. J . Med.
Chem. 2000, 43, 4927-4929.
(33) (a) Harbeson, S. L.; Abelleira, S. M.; Akiyama, A.; Barrett, R.,
III; Carroll, R. M.; Straub, J . A.; Tkacz, J . N.; Wu, C.; Musso, G. F. J .
Med. Chem. 1994, 37, 2918-2929. (b) Brady, S. F.; Sisko, J . T.;
Stauffer, K. J .; Colton, C. D.; Qiu, H.; Lewis, S. D.; Ng, A. S.; Shafer,
J . A.; Bogusky, M. J .; Veber, D. F.; Nutt, R. F. Bioorg. Med. Chem.
1995, 3, 1063-1078.
J . Org. Chem, Vol. 68, No. 19, 2003 7319

Hamze´ et al.
1,1-Dim et h ylet h yl-2-(S)-[[(9H -flu or en -9-ylm et h oxy)-
ca r b on yl]a m in o]-4-[[(1-a m in o-et h yl)im in o]oxy]-4-oxo-
bu ta n oa te (8a , R ) CH3). Compound 8a was prepared from
Fmoc-L-Asp-OtBu (0.5 g, 1.22 mmol) and N-hydroxyethanimi-
damide 1a (0.1 g, 1.34 mmol). Purification by column chro-
matography (EtOAc/hex, 7:3) afforded 0.34 g (61%) of pure
product as a colorless oil: Rf 0.26 (EtOAc/hex, 7:3); m/z (ES+)
468.2 (M + H+), 412.1 (M-tBu + H+), 935.3 (2M + H+); HPLC
Therefore, we checked if removal of the Fmoc protecting
group using a 20% piperidine solution in DMF could also
induce epimerization. Using standard solid-phase peptide
synthesis protocols, 4a has been attached to a solid
support, deprotected, and coupled with Fmoc-L-Phe-OH.
The Fmoc was removed, and the pseudodipeptide was
cleaved from the resin. Reverse-phase HPLC analysis of
the resulting free compound showed almost the sole
presence (>95%) of the expected (S,S) and no (S,R)
diastereoisomer, indicating that no significant epimer-
ization occurred during Fmoc deprotection.
1
tR 3.05 min; H NMR (CDCl3) δ 7.78 (d, J ) 7.5 Hz, 2H), 7.6
(d, J ) 7.2 Hz, 2H), 7.41 (t, J ) 7.2 Hz, 2H), 7.32 (t, J ) 7.4
Hz, 2H), 5.78 (d, J ) 8.3 Hz, 1H), 4.65 (m, 1H), 4.4 (m, 2H),
4.3 (t, J ) 7.1 Hz, 1H), 3.1 (m, 2H), 1.97 (s, 3H), 1.50 (s, 9H).
(B) Gen er a l P r oced u r e for P r ep a r a tion of 1,2,4-Oxa -
d ia zoles (Com p ou n d s 3a -g, 6a -f, 9a -d ). Sodium acetate
(1 or 1.1 equiv) dissolved in water was added to a stirred
solution of the Fmoc-aminoacyl amidoxime (1 equiv) in ethanol,
and the resulting mixture was heated at 86 °C for 2 or 5 h.
After this period, the solution was allowed to cool to room
temperature. When crystallization occurred, the solid was
collected by filtration. If not, the solvents were removed under
reduced pressure, and the residue was partitioned between
ethyl acetate and water. The aqueous layer was then extracted,
and the subsequent organic layer was dried (MgSO4) and
filtered. Evaporating the solvent under vacuum afforded a
crude product, which was generally purified by column chro-
matography.
1,1-Dim et h ylet h yl-3-(S)-[[(9H -flu or en -9-ylm et h oxy)-
ca r bon yl]a m in o]-3-[3-m eth yl-1,2,4-oxa d ia zol-5-yl]-p r op i-
on a te (3a , R ) CH3). Heating a mixture of 2a (1.9 g, 4.07
mmol) in ethanol (20 mL) and sodium acetate (0.608 g, 4.47
mmol) in water (3 mL) for 2 h afforded 3a , which crystallized
upon cooling as white crystals (1.34 g, 75%): mp 122-124 °C;
m/z (ES+) 450.28 (M + H+), 394.00 (M-tBu + H+), 899.30 (2M
+ H+), 921.26 (2M + Na+); HPLC tR 3.71 min; 1H NMR (CDCl3)
δ 7.80 (d, J ) 7.2 Hz, 2H), 7.61 (m, 2H), 7.42 (t, J ) 7.4 Hz,
2H), 7.32 (m, 2H), 6.2 (d, J ) 8.3 Hz, 1H), 5.38 (m, 1H), 4.42
(m, 2H), 4.28 (t, J ) 7.1 Hz, 1H), 3.1 (dd, 1H), 2.9 (dd, 1H),
2.4 (s, 3H), 1.4 (s, 9H).
Finally, we have to point out that the side chain
orientation of the obtained â3-amino acids is opposite to
that of the starting aspartyl derivatives. Indeed, the
R-carboxylic group of Asp is the precursor of the oxadia-
zole side chain, while the â-carboxylic group becomes part
of the main peptide chain. Thus, if a topology similar to
that of L-amino acid is desired, the precursor must be a
D-aspartyl derivative.
Con clu sion
New series of â3- and R-amino acids containing differ-
ent 3-substituted 1,2,4-oxadiazole heterocycles have been
synthesized. They represent an original addition to the
list of available nonnatural amino acids. Their prepara-
tion as NR-Fmoc-protected derivatives and retention of
chirality during all steps allow their use in peptide and
peptidomimetic syntheses, as well as in solution- and
solid-phase combinatorial syntheses. Although a limited
number of different and representative substituents are
presented here, it is obvious that a much larger diversity
can be easily achieved. The use of sodium acetate in
refluxing ethanol/water during the key cyclic dehydration
step proved to be convenient and Fmoc-compatible and
enables a short, clean, and inexpensive reaction. These
mild conditions represent a very interesting alternative
for the synthesis of 1,2,4-oxadiazoles.
1,1-Dim et h ylet h yl-2-(S)-[[(9H -flu or en -9-ylm et h oxy)-
ca r bon yl]a m in o]-3-[3-m eth yl-1,2,4-oxa d ia zol-5-yl]-p r op i-
on a te (9a , R ) CH3). Heating a mixture of 8a (0.31 g, 0.664
mmol) in ethanol (10 mL) and sodium acetate (0.091 g, 0.664
mmol) in water (1.5 mL) for 5 h afforded crude 9a . Purification
by column chromatography (EtOAc/hex, 2:8) yielded 0.185 g
(62%) of the desired product as a colorless oil: Rf 0.42 (EtOAc/
hex, 2:8); m/z (ES+) 450.2 (M + H+), 471.7 (M + Na+), 393.9
(M-tBu + H+), 899.4 (2M + H+), 921.3 (2M + Na+); HPLC tR
Exp er im en ta l Section
(A) Gen er a l P r oced u r e for P r ep a r a tion of O-Am in o
Acyl Am id oxim es (Com p ou n d s 2a -g, 5a -f, 8a -d ). To a
solution of Fmoc-amino acid (1 equiv) and amidoxime 1 (1.2
equiv) in DCM/DMF (9:1) (15 mL/g) were added HOBt (1.2
equiv) and DIC (1.2 equiv) at -10 °C. The reaction was stirred
for 20 min at this temperature and then for 1.5 h at room
temperature. The solvents were concentrated in vacuo, and
the residue was dissolved in ethyl acetate. The organic layer
was washed with NaHCO3 (twice), H2O, KHSO4 0.5 M (twice),
and brine and dried over MgSO4. Evaporation to dryness
afforded O-amino acyl amidoxime, which was used without
further purification in the next step, unless otherwise indi-
cated.
1
3.56 min; H NMR (CDCl3) δ 7.68 (d, J ) 7.5 Hz, 2H), 7.5 (d,
J ) 7.5 Hz, 2H), 7.32 (t, J ) 7.4 Hz, 2H), 7.25 (t, J ) 7.4 Hz,
2H), 5.8 (d, J ) 7.7 Hz, 1H), 4.7 (m, 1H), 4.35 (m, 2H), 4.21 (t,
J ) 7.1 Hz, 1H), 3.4 (dd, 1H), 3.3 (dd, J ) 5.0 Hz, 21 Hz, 1H),
2.3 (s, 3H), 1.35 (s, 9H).
(C) Gen er a l P r oced u r e for Rem ova l of ter t-Bu tyl Ester
(Com p ou n d s 4a -g, 7a -f, 10a -d ). The tert-butyl-protected
compounds were dissolved in TFA/DCM (50:50, 20-30 mL/g
of compound), and the mixtures were stirred for 2 h at room
temperature. After this period, the solvents were removed
under reduced pressure, and the resulting residues were taken
up in hexane/diethyl ether. The products precipitated and were
collected by filtration.
3-(S)-[[(9H -F lu or en -9-ylm et h oxy)ca r b on yl]a m in o]-3-
[3-m eth yl-1,2,4-oxa d ia zol-5-yl]-p r op ion ic Acid (4a , R )
CH3). Compound 3a (0.20 g, 0.44 mmol) was converted to 4a
as described in Method C to yield a white powder (0.174 g,
100%): mp 117-119 °C; [R]D ) -33 (c 0.01, MeOH); m/z (ES+)
393.94 (M + H+), 787.11 (2M + H+), 809.25 (2M + Na+); HPLC
tR 2.85 min; 1H NMR (CDCl3) δ 7.5 (d, J ) 7.4 Hz, 2H), 7.4 (d,
J ) 7.1 Hz, 2H), 7.18 (t, J ) 7.3 Hz, 2H), 7.1 (t, J ) 7.3 Hz,
2H), 5.82 (d, J ) 9.0 Hz, 1H), 5.2 (m, 1H), 4.2 (m, 2H), 4.1 (t,
J ) 7.0 Hz, 1H), 3.0 (dd, 1H), 2.88 (dd, 1H), 2.1 (s, 3H); 13C
NMR (DMSO-d6) δ 179.4, 171.7, 167.8, 156.4, 144.6, 144.5,
141.6, 128.5, 127.9, 126.0, 121.0, 66.8, 47.4, 45.7, 37.6, 12.0.
1,1-Dim et h ylet h yl-3-(S)-[[(9H -flu or en -9-ylm et h oxy)-
ca r b on yl]a m in o]-4-[[(1-a m in o-et h yl)im in o]oxy]-4-oxo-
bu ta n oa te (2a , R ) CH3). Compound 2a was prepared from
Fmoc-L-Asp(OtBu)-OH (0.7 g, 1.7 mmol), and N-hydroxyetha-
nimidamide 1a (0.151 g, 2.04 mmol). Purification by column
chromatography (EtOAc/hex, 8:2) afforded 0.75 g (95%) of 2a
as a white solid: Rf 0.38 (EtOAc/hex, 7:3); m/z (ES+) 468.21
(M + H+), 411.94 (M-tBu + H+), 935.34 (2M + H+); HPLC tR
1
3.2 min; H NMR (CDCl3) δ 7.8 (d, J ) 7.5 Hz, 2H), 7.6 (d, J
) 7.4 Hz, 2H), 7.41 (t, J ) 7.4 Hz, 2H), 7.31 (t, J ) 7.2 Hz,
2H), 5.90 (d, J ) 8.4 Hz, 1H), 4.75 (m, 1H), 4.4 (m, 2H), 4.21
(t, J ) 7.1 Hz, 1H), 3.0 (dd, J ) 5.0, 16.6 Hz, 1H), 2.8 (dd, J )
5.5, 16.6 Hz, 1H), 2.0 (s, 3H), 1.48 (s, 9H).
7320 J . Org. Chem., Vol. 68, No. 19, 2003

Fmoc-Protected 1,2,4-Oxadiazole-Containing â3- and R-Amino Acids
2-(S)-[[(9H -F lu or en -9-ylm et h oxy)ca r b on yl]a m in o]-3-
[3-m eth yl-1,2,4-oxa d ia zol-5-yl]-p r op ion ic Acid (10a , R )
CH3). Compound 9a (0.175 g, 0.38 mmol) was converted to
10a as described in Method C to yield a white powder (0.142
g, 93%): mp 168-170 °C; [R]D ) -18 (c 0.01, MeOH); m/z
(ES+) 394.1 (M + H+), 416.2 (M + Na+), 787.1 (2M + H+),
809.4 (2M + Na+); HPLC tR 2.81 min; 1H NMR (CDCl3) δ 7.68
(d, J ) 7.5 Hz, 2H), 7.5 (d, J ) 7.3 Hz, 2H), 7.32 (t, J ) 7.4
Hz, 2H), 7.25 (t, J ) 7.4 Hz, 2H), 5.88 (d, J ) 7.1 Hz, 1H),
4.75 (m, 1H), 4.35 (m, 2H), 4.2 (t, J ) 6.8 Hz, 1H), 3.48 (m,
2H), 2.32 (s, 3H); 13C NMR (DMSO-d6) δ 177.5, 172.5, 167.6,
156.7, 144.6, 144.5, 141.6, 128.5, 127.9, 126.1, 126.0, 121.0,
66.7, 52.3, 47.4, 29.1, 12.0.
J ean-Alain Fehrentz for helpful discussion, and Mr.
Michae¨l Buchert for stylistic revision. We thank the
Lebanese University for the award of a research fellow-
ship to A.H.
Su p p or tin g In for m a tion Ava ila ble: (1) General experi-
mental procedures; experimental procedures for synthesis of
amidoximes 1a -g; synthetic details and analytical data for
all other compounds. (2) Copies of 1H NMR spectra of final
compounds 4a -g, 7a -f, and 10a -d . (3) Copies of 13C NMR
spectra of final compounds 4a -g, 7a -f, and 10a -d . This
material is available free of charge via the Internet at
http://pubs.acs.org.
Ack n ow led gm en t. We are grateful to Mr. Pierre
Sanchez for performing mass spectrometry analyses, Dr.
J O0345953
J . Org. Chem, Vol. 68, No. 19, 2003 7321