A Model Route Toward the Synthesis of Conformationally Constrained Polyhydroxylated Dipeptides from Natural Carbohydrates

Article abstract of DOI:10.1055/s-2003-43341

Enantiopure 6,7-diacetoxy-3-t-butoxycarbonylaminoazabicyclo[3.3.0]octan-2- one-8-carboxylic acid 14 (pyrrolizidinone amino acid) was synthesized in 14 steps and 5.8% overall yield from tri-O-benzyl-D-arabinose 5 through the formyl C-imino-sugar 9 as a key

Full text of DOI:10.1055/s-2003-43341

LETTER
2345
A Model Route Toward the Synthesis of Conformationally Constrained
Polyhydroxylated Dipeptides from Natural Carbohydrates
C
onstrained Polyh
l
ydroxy
e
lated
D
ipep
s
tides sandro Dondoni,*a,b Alberto Marra,*a,b Barbara Richichib
a
Dipartimento di Chimica, Laboratorio di Chimica Organica, Università di Ferrara, Via Borsari 46, 44100 Ferrara, Italy
b
Interdisciplinary Center for the Study of Inflammation, Università di Ferrara, Via Borsari 46, 44100 Ferrara, Italy
Fax +39(0532)291167; E-mail: adn@dns.unife.it
Received 6 October 2003
synthetic modification since these functionalities may
serve as anchoring points for side chain moieties implicat-
ed in ligand-receptor interaction.6,7 Actually, it has been
demonstrated that the three dimensional structure and the
stereoelectronic properties of side chain groups of amino
acids are critical for biological activity and for selective
ligand-receptor interaction.7,10b Therefore, diastereomeric
hydroxylated peptidomimetics may feature different con-
formational arrangements required in molecular recogni-
tion. An easy access to a collection of hydroxylated
dipeptidic scaffolds derived from 1 may provide useful
tools to probe and elucidate the structure-activity relation-
ships of these peptide surrogates in different molecular
events in which natural peptides are implicated. More-
over, since molecules containing a specific pharmacoph-
oric peptide sequence have been proved to display a
tumor-homing ability,12 hydroxylated derivatives of pep-
tidomimetics 1 can be used as linkers between the pep-
tide-binding motif coupled to the amine and carboxylate
appendages and specific drugs anchored on the hydroxyl
handles. This application opens opportunities of markedly
improving the delivery and selectivity of drugs12 in sever-
al diseases, such as inflammation and cancer, where inte-
grins are implicated.
Abstract: Enantiopure 6,7-diacetoxy-3-t-butoxycarbonylamino-
azabicyclo[3.3.0]octan-2-one-8-carboxylic acid 14 (pyrrolizidi-
none amino acid) was synthesized in 14 steps and 5.8% overall
yield from tri-O-benzyl-D-arabinose 5 through the formyl C-imino-
sugar 9 as a key intermediate.
Key words: integrins, peptidomimetics, pyrrolizidinone amino
acids, reverse-turn, RGD
The field of peptidomimetics1 has gained an enormous
importance in recent years, particularly with the emer-
gence of conformationally constrained systems that mim-
ic certain structural features and therapeutic effects of
natural peptides.2 A special class of rigidified peptidomi-
metics is constituted of the azabicyclo[X.Y.0]alkanone
amino acids2b,3 1 (Figure 1), i. e. fused bicyclic dipeptides
that simulate the bioactive conformation of the b-turn
sites.4 The stereocontrol3,5 at the chiral carbon backbone
and ring-fusion center, the side chain attachment,6,7b and
the ring size,5,8 are issues, which have been widely ad-
dressed. These molecules have been used not only to
mimic a dipeptide motif2b,4c,6b but also as a scaffold featur-
ing an amine and a carboxylate handles suitable for the
linkage with an important pharmacophoric tripeptide,
namely the Arg–Gly–Asp (RGD) sequence,9 which is
implicated in several biological events including angio-
genesis and osteoporosis.10
A convenient route was envisaged (Scheme 1) to enan-
tiopure 6,7-dihydroxylated azabicyclo[3.3.0]octanone
amino acids 2 (pyrrolizidinone amino acids) from polyhy-
droxylated formyl pyrrolidines 3, so-called formyl C-imi-
nosugars, which in turn have been made readily available
from recent work in our laboratory13 by thiazole-based
aminohomologation of pentofuranoses 4. In this way one
would take advantage of the hydroxyl groups and their
stereochemistry already in place in compound 3 and
would exploit the formyl group for the construction of the
second functionalized pyrrolidine ring.
m
HO2C
N
n
O
NH2
1
Figure 1 Azabicyclo[X.Y.0]alkanone amino acids
HO
OH
HO
OH
RO
OR
4
7
6
3
Our interest in this field was stimulated by the growing
demand of efficient and versatile methodologies for the
stereocontrolled introduction of side chain functionalities
in 1. In particular, the functionalization with hydroxyl
groups11 of the carboxyl bearing ring should allow further
2
8
5
5
OH
HO2C
6
CHO
N
O
N
R
4
3
OH
OR
2
4
3
O
NH2
2
Scheme 1
SYNLETT 2003, No. 15, pp 2345–2348
0
1
.1
2
.2
0
0
3
Advanced online publication: 21.11.2003
DOI: 10.1055/s-2003-43341; Art ID: G26703ST
© Georg Thieme Verlag Stuttgart · New York

2346
A. Dondoni et al.
LETTER
CbzHN
10
P(O)(OMe)2
CO2Me
Aiming to obtain the orthogonally protected polyhydrox-
ylated formyl pyrrolidine 9 starting from the known13 thi-
azole-masked precursor 6, the selective removal of the 5-
O-benzyl group was carried out by acetolysis and the re-
sulting free hydroxyl group of 714 was protected with p-
methoxyphenol (PMP) via Mitsunobu reaction to give 814
(Scheme 2). Application of silver-based thiazole-to-
formyl unmasking protocol15 (N-methylation, reduction,
hydrolysis) to 8 afforded the key intermediate 914 in 84%
yield.
BnO
OBn
NHCbz
DBU
81%
CH
C
N
Bn
PMPO
CO2Me
11
HO
OH
1. Et3N, MeOH
2. Ac2O, Py
1. (Boc)2O, DMAP
2. H2, Pd(OH)2
NHBoc
54%
85%
CO2Me
N
H
BnO
OBn
BnO
OBn
BnO
OH
OBn
1. H2SO4, Ac2O
PMPO
4 steps
ref. 13
2. MeONa, MeOH
12
N
65%
OH
N
Bn
O
OBn
AcO
OAc
S
AcO
OAc
1. (NH4)2Ce(NO3)6
2. CrO3, H2SO4
6
5
H
H
74%
N
HO2C
N
OBn
BnO
OBn
PMPO
p-MeOC6H4OH
PPh3, DIAD
O
NHBoc
O
NHBoc
N
N
80%
13
N
Bn
N
14
PMPO
Bn
S
S
Scheme 3
7
8
1. MeOTf
2. NaBH4
between these protons. The selective removal of the
PMP group of 13 by treatment with cerium ammonium ni-
trate (CAN) led to the corresponding primary alcohol,
which when submitted to the Jones oxidation furnished
the target 6,7-diacetoxy pyrrolizidinone amino acid 14
(Scheme 3) that was fully characterized as methyl ester.17
The orthogonal protection of the functional groups of 14
should allow a variety of synthetic elaborations. Hence
the use of this constrained dipeptide or its O-functional-
ized derivatives18 as substrates for the synthesis of cyclo-
peptides by insertion of pharmacophoric peptides such as
RGD (Arg–Gly–Asp) or LDT (Leu–Asp–Thr)19 now
becomes of interest.
BnO
OBn
3. AgNO3, H2O
84%
CHO
N
Bn
PMPO
9
Scheme 2
The aldehyde 9 was allowed to react with the commercial-
ly available phosphonate 1016 and 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU) as a base in a Horner–
Emmons olefination reaction to give 1117 as a single iso-
mer in 81% yield whose E or Z configuration was not de-
termined (Scheme 3).
In conclusion, an efficient route for the synthesis of a new
class of conformationally constrained peptido-mimetics
has been developed. This approach should be amenable to
the preparation of a collection of polyhydroxylated azabi-
cycloalkanone amino acids of type 1 where molecular di-
versity is achieved by both stereochemical and ring size
variations. Studies are underway in our laboratory using a
variety of polyhydroxylated formyl N-heterocycles
(formyl C-iminosugars), which are accessible through the
thiazole-based aminohomologation technique.13
Subsequent Boc-protection at the enamino nitrogen
atom,8b hydrogenation of the crude intermediate over
Pd(OH)2 removed O- and N-benzyl protective groups and
reduced the carbon-carbon double bond leading to the
iminosugar a-amino acid 12 as a mixture of diastereoiso-
mers. Intramolecular amide formation under basic condi-
tions (Et3N) at 60 °C and acetylation transformed 12 into
a mixture of pyrrolizidinone 1317 and its C-3 epimer. Each
of these bicyclic compounds was isolated in a pure form
by medium pressure column chromatography in 54% and
24% yield, respectively. The configuration at the newly
formed stereocenter, viz. the C-3, of the major product 13
was assigned by NOE difference experiments. Upon irra-
diation of the H-5 proton a significant NOE with H-3 and
H-7 was observed, thus indicating a cis-relationship
Acknowledgment
We gratefully acknowledge MIUR (COFIN 2002) for financial sup-
port and Interdisciplinary Center for the Study of Inflammation
(Ferrara, Italy) for a fellowship to B. R.
Synlett 2003, No. 15, 2345–2348 © Thieme Stuttgart · New York

LETTER
Constrained Polyhydroxylated Dipeptides
2347
13.7 Hz, 2 H, PhCH2N), 3.63 (dd, J5,6 = 4.5 Hz, J6,OH = 6.3
Hz, 2 H, 2 × H-6), 3.34 (dt, 1 H, H-5), 2.65 (t, 1 H, OH).
Compound 8. [a]D = +35.5 (c 0.5, CHCl3). 1H NMR (400
MHz, CDCl3): d = 7.68 (d, J = 3.3 Hz, 1 H, Th), 7.38–7.17,
7.00–6.96 (2 × m, 16 H, 3 × Ph, Th), 6.81–6.72 (m, 4 H,
MeOPh), 4.69, 4.54 (2 × d, J = 11.8 Hz, 2 H, PhCH2O), 4.46
(d, J2,3 = 2.5 Hz, 1 H, H-2), 4.37, 4.31 (2 × d, J = 11.9 Hz, 2
H, PhCH2O), 4.29 (dd, J3,4 = 2.3 Hz, 1 H, H-3), 4.17 (dd, J5,6a
= 7.1 Hz, J6a,6b = 9.0 Hz, 1 H, H-6a), 4.12 (dd, J4,5 = 5.6 Hz,
1 H, H-4), 4.08, 3.99 (2 × d, J = 13.6 Hz, 2 H, PhCH2N), 3.92
(dd, J5,6b = 5.1 Hz, 1 H, H-6b), 3.77 (s, 3 H, Me), 3.67 (ddd,
1 H, H-5). Compound 9. [a]D = +2.5 (c 0.6, CHCl3). 1H NMR
(400 MHz, CDCl3): d = 9.30 (d, J = 1.3 Hz, 1 H, CHO),
7.40–7.20, 7.16–7.11 (2 × m, 15 H, 3 × Ph), 6.84 (s, 4 H,
MeOPh), 4.59, 4.51 (2 × d, J = 11.8 Hz, 2 H, PhCH2O), 4.44,
4.28 (2 × d, J = 11.7 Hz, 2 H, PhCH2O), 4.29 (dd, J5,6a = 7.7
Hz, J6a,6b = 9.3 Hz, 1 H, H-6a), 4.27, 3.76 (2 × d, J = 13.2 Hz,
2 H, PhCH2N), 4.11 (dd, J2,3 = 1.2 Hz, J3,4 = 1.5 Hz, 1 H, H-
3), 4.10 (dd, J4,5 = 4.3 Hz, 1 H, H-4), 4.09 (dd, J5,6b = 5.3 Hz,
1 H, H-6b), 3.78 (s, 3 H, Me), 3.68 (ddd, 1 H, H-5), 3.38 (dd,
1 H, H-2). 13C NMR (100 MHz, CDCl3): d = 204.7 (C-1),
153.9, 153.0, 115.4, 114.6 (MeOPh), 138.8, 137.5, 137.4,
129.1–127.5 (3 × Ph), 84.2 (C-3), 80.0 (C-4), 76.2 (C-2),
71.8 (PhCH2O), 71.4 (PhCH2O), 67.2 (C-6), 66.2 (C-5), 60.7
(PhCH2N), 55.8 (MeO).
References
(1) (a) Olson, G. L.; Bolin, D. R.; Bonner, M. P.; Bös, M.; Cook,
C. M.; Fry, D. C.; Graves, B. J.; Hatada, M.; Hill, D. E.;
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(8) (a) Gosselin, F.; Lubell, W. D. J. Org. Chem. 1998, 63,
7463. (b) Angiolini, M.; Araneo, S.; Belvisi, L.; Cesarotti,
E.; Checchia, A.; Crippa, L.; Manzoni, L.; Scolastico, C.
Eur. J. Org. Chem. 2000, 2571. (c) Gosselin, F.; Lubell, W.
D. J. Org. Chem. 2000, 65, 2163.
(9) (a) Haubner, R.; Schmitt, W.; Hölzemann, G.; Goodman, S.
L.; Jonczyk, A.; Kessler, H. J. Am. Chem. Soc. 1996, 118,
7881. (b) Belvisi, L.; Bernardi, A.; Checchia, A.; Manzoni,
L.; Potenza, D.; Scolastico, C.; Castorina, M.; Cupelli, A.;
Giannini, G.; Carminati, P.; Pisano, C. Org. Lett. 2001, 3,
1001. (c) Marinelli, L.; Lavecchia, A.; Gottschalk, K.-E.;
Novellino, E.; Kessler, H. J. Med. Chem. 2003, 46, 4393.
(10) (a) Haubner, R.; Finsinger, D.; Kessler, H. Angew. Chem.
Int. Ed. 1997, 36, 1374. (b) Gottschalk, K.-E.; Kessler, H.
Angew. Chem. Int. Ed. 2002, 41, 3767. (c) Hynes, R. O.
Nature Med. 2002, 8, 918.
(15) Dondoni, A.; Marra, A.; Scherrmann, M.-C.; Bertolasi, V.
Chem.–Eur. J. 2001, 7, 1371.
(16) Schmidt, U.; Lieberknecht, A.; Wild, J. Synthesis 1984, 53.
(17) Compound 11. [a]D = –8.7 (c 0.8, CHCl3). 1H NMR (400
MHz, CDCl3): d = 7.34–7.18 (m, 20 H, 4 × Ph), 7.08 (br s, 1
H, NH), 6.81–6.72 (m, 4 H, MeOPh), 6.21 (d, J = 8.2 Hz, 1
H, CH=), 5.09, 5.01 (2 × d, J = 12.3 Hz, 2 H, PhCH2OCO),
4.51, 4.46 (2 × d, J = 11.8 Hz, 2 H, PhCH2O), 4.45 (s, 2 H,
PhCH2O), 4.12 (dd, J5,6a = 7.2 Hz, J6a,6b = 9.4 Hz, 1 H, H-6a),
4.04 (dd, J3,4 = 3.0 Hz, J4,5 = 5.8 Hz, 1 H, H-4), 3.92 (dd,
J5,6b = 4.7 Hz, 1 H, H-6b), 3.89, 3.80 (2 × d, J = 13.6 Hz,
2 H, PhCH2N), 3.89 (dd, J2,3 = 4.8 Hz, 1 H, H-3), 3.78, 3.71
(2 × s, 6 H, 2 × Me), 3.56 (dd, 1 H, H-2), 3.44 (ddd, 1 H, H-
5). Compound 13. [a]D = –45.3 (c 0.4, CHCl3). 1H NMR
(400 MHz, CDCl3): d = 6.86-6.79 (m, 4 H, MeOPh), 5.50
(dd, J6,7 = J7,8 = 7.7 Hz, 1 H, H-7), 5.19 (dd, J5,6 = 7.3 Hz, 1
H, H-6), 4.94 (br d, J3,NH = 5.5 Hz, 1 H, NH), 4.75 (dd,
J8,9a = 3.9, J9a,9b = 10.0 Hz, 1 H, H-9a), 4.44 (dd, J3,4a = 6.9
Hz, J3,4b = 12.1 Hz, 1 H, H-3), 4.28 (ddd, J8,9b = 1.0 Hz, 1 H,
H-8), 3.87 (dd, 1 H, H-9b), 3.76 (s, 3 H, Me), 3.64 (dd,
J4a,5 = 5.3 Hz, J4b,5 = 10.1 Hz, 1 H,, H-5), 2.96 (ddd,
J4a,4b = 11.8 Hz, 1 H, H-4a), 2.10, 2.02 (2 × s, 6 H, 2 × Ac),
1.96 (ddd, 1 H, H-4b), 1.41 (s, 9 H, t-Bu). Compound epi-13.
[a]D = –34.9 (c 0.7, CHCl3). 1H NMR (400 MHz, CDCl3):
d = 6.85–6.78 (m, 4 H, MeOPh), 5.52 (dd, J6,7 = 7.8 Hz,
J7,8 = 7.6 Hz, 1 H, H-7), 5.12 (dd, J5,6 = 7.6 Hz, 1 H, H-6),
5.04 (br s, 1 H, NH), 4.69 (dd, J8,9a = 4.6 Hz, J9a,9b = 10.2 Hz,
1 H, H-9a), 4.28 (ddd, J8,9b = 1.6 Hz, 1 H, H-8), 4.05–3.97
(m, 2 H, H-3, H-5), 3.94 (dd, 1 H, H-9b), 3.75 (s, 3 H, Me),
2.48 (ddd, J3,4a = 7.6 Hz, J4a,4b = 13.6, J4a,5 = 9.5 Hz, 1 H, H-
4a), 2.34 (ddd, J3,4b = 2.2 Hz, J4b,5 = 6.8 Hz, 1 H, H-4b), 2.10,
1.99 (2 × s, 6 H, 2 Ac), 1.42 (s, 9 H, t-Bu). Compound 14
Methyl Ester. [a]D =
(11) Another class of conformationally constrained
polyhydroxylated dipeptides has been recently described,
see: Tremmel, P.; Brand, J.; Knapp, V.; Geyer, A. Eur. J.
Org. Chem. 2003, 878.
(12) Arap, W.; Pasqualini, R.; Ruoslahti, E. Science 1998, 279,
377.
(13) Compound 6 was prepared in gram scale quantities by
stereoselective addition of 2-lithiothiazole to the nitrone
derived from D-arabinose 5 followed by dehydroxylation of
the resulting open-chain hydroxylamine and cyclization by
intramolecular nitrogen-carbon bond formation via SN2
process. See: (a) Dondoni, A.; Perrone, D. Tetrahedron Lett.
1999, 40, 9375. (b) Dondoni, A.; Giovannini, P.; Perrone, D.
J. Org. Chem. 2002, 62, 7203.
–63.4 (c 0.3, CHCl3). 1H NMR (400 MHz, CDCl3): d = 5.61
(dd, J6,7 = 9.3 Hz, J7,8 = 8.2 Hz, 1 H, H-7), 5.10 (dd, J5,6 = 8.4
Hz, 1 H, H-6), 5.08 (br d, J3,NH = 7.0 Hz, 1 H, NH), 4.58 (d,
1 H, H-8), 4.53 (ddd, J3,4a = 6.5 Hz, J3,4b = 12.0 Hz, 1 H, H-
3), 3.78 (s, 3 H, Me), 3.69 (ddd, J4a,5 = 5.4 Hz, J4b,5 = 9.5 Hz,
1 H, H-5), 3.02 (ddd, J4a,4b = 12.5 Hz, 1 H, H-4a), 2.16 (ddd,
1 H, H-4b), 2.09, 2.06 (2 × s, 6 H, 2 × Ac), 1.43 (s, 9 H, t-
Bu). 13C NMR (100 MHz, CDCl3): d = 173.4 (Me3COCO),
173.3 (C-2), 170.4, 169.6 (CH3CO), 167.8 (CO2Me), 80.1
(14) Compound 7. [a]D = +22.2 (c 1.2, CHCl3). 1H NMR (300
MHz, CDCl3): d = 7.74 (d, J = 3.3 Hz, 1 H, Th), 7.32–7.14
(m, 16 H, 3 × Ph, Th), 4.68, 4.50 (2 × d, 2 H, J = 11.7 Hz,
PhCH2O), 4.58, 4.46 (2 × d, J = 11.7 Hz, 2 H, PhCH2O), 4.44
(d, J2,3 = 4.8 Hz, 1 H, H-2), 4.27 (dd, J3,4 = 4.5 Hz, 1 H, H-
3), 4.16 (dd, J4,5 = 6.6 Hz, 1 H, H-4), 4.01, 3.82 (2 × d, J =
Synlett 2003, No. 15, 2345–2348 © Thieme Stuttgart · New York

2348
A. Dondoni et al.
LETTER
a4b7 integrins, is responsible for the lymphocytes
recruitment to inflamed colon. Cyclic peptidomimetics
containing the LDT motif are inhibitors of this recognition
process and therefore may lead to a new, organ specific
treatment of inflammatory diseases. (a) Gottschling, D.;
Boer, J.; Schuster, A.; Holzmann, B.; Kessler, H. Angew.
Chem. Int. Ed. 2002, 41, 3007. (b) Gottschling, D.; Boer, J.;
Marinelli, L.; Voll, G.; Haupt, M.; Schuster, A.; Holzmann,
B.; Kessler, H. ChemBioChem 2002, 575.
(Me3C), 75.7 (C-7), 75.3 (C-6), 57.7 (C-5), 56.5 (C-8), 54.3
(C-3), 52.8 (MeO), 38.8 (C-4), 28.3 (Me3C), 20.6 and 20.3
(CH3CO).
(18) Unnatural hetero-bifunctional ligands bearing the sialyl
Lewis oligosaccharide and the RGD peptide sequence have
been recently prepared, see: Matsuda, M.; Nishimura, S.-I.;
Nakajima, F.; Nishimura, T. J. Med. Chem. 2001, 44, 715.
(19) The LDT-mediated binding between the mucosal addressin
cell adhesion molecule-1 (MAdCAM-1) and its receptor, the
Synlett 2003, No. 15, 2345–2348 © Thieme Stuttgart · New York