Synthesis of bicyclic sugar azido acids and their incorporation in cyclic peptides

Article abstract of DOI:10.1039/b006192l

Bicyclic amino acids derived from the natural sugars D-arabinofuranose and D-fructofuranose have been synthesised; their ability to induce a turn conformation in peptides has been exploited in the preparation of a cyclic RGD loop mimetic.

Full text of DOI:10.1039/b006192l

Synthesis of bicyclic sugar azido acids and their incorporation in cyclic
peptides
Francesco Peri, Laura Cipolla, Barbara La Ferla and F. Nicotra*
Department of Biotechnology and Biosciences, University of Milano-Bicocca, Piazza della Scienza 2, I-20126
Milano, Italy. E-mail: francesco.nicotra@unimib.it; Tel +39.02.64483457; Fax +39.02.64483565
Received (in Cambridge, UK) 31st July 2000, Accepted 13th October 2000
First published as an Advance Article on the web
Bicyclic amino acids derived from the natural sugars
D
-
arabinofuranose and -fructofuranose have been synthes-
D
ised; their ability to induce a turn conformation in peptides
has been exploited in the preparation of a cyclic RGD loop
mimetic.
The conformational rigidity of the pyran and furan rings makes
sugar amino acids (SAAs) interesting building blocks in the
induction of precise secondary structures in peptides and in the
construction of peptidomimetics. In this context, b- -glucopyr-
D
anose was used as scaffold in the preparation of the first
nonpeptide somatostatin mimetic;1 in a different biological
context the same sugar moiety was employed to present the
guanidine group crucial for the biological activity of thrombin
inhibitors.2 By varying the mutual positions of the amino and
carboxylic groups in the glucopyranose scaffold, a set of SAAs
was designed and found to be capable of inducing linear or b-
turn conformations when incorporated into Leu-enkephalin
peptide analogues.3 Oligomers of pyranose sugar amino acids4
(‘carbopeptoids’) have been synthesised, and the furanose ring
has also been exploited as a building block for carbopeptoid
assembling.5 Homo-oligomers of tetrahydrofuran amino acids
derived from the arabinofuranose were shown to adopt a novel
repeating b-turn type secondary structure in tetrameric units
stabilized by intramolecular hydrogen bond.6
Scheme 1 Reagents and conditions: i, I2, CH2Cl2, 0 °C, 12 h; ii, separation
of diastereomers by flash chromatography; iii, Bu4NN3, toluene, 60 °C; iv,
Ac2O, CF3COOH, 0 °C; v, MeONa, MeOH; vi, CrO3–H2SO4, acetone–
water.
mode with formation of a cyclic iodoether with debenzylation.7
Obviously, only the b anomer of 4 reacted and this allowed the
easy separation from the a-anomer. The bicyclic iodoether 5
was obtained as a mixture of diastereomers (95% yield based on
the b anomer), the major isomer having the 2A-(S) configuration
(20% d.e.). Compounds R-5 and S-5 were separated by flash
chromatography and their relative configurations at the C-2A
assigned by NOESY analysis.8 The reaction of the iododer-
ivative 5 with tetrabutylammonium azide in toluene to give
azidoderivative 5 (60 °C, 24 h, 87% yield), as well as the
subsequent synthetic steps, were carried out separately on the
two diastereoisomers with similar yields. In order to introduce
the carboxylic function, 5 was regioselectively debenzylated at
the primary hydroxy group by controlled acetolysis (Ac2O,
CF3COOH, 0 °C), followed by saponification of the acetate and
We present here the design and the synthesis of novel azido
acids (1 and 2) with bicyclic structures, derived from
arabinofuranose and -fructofuranose respectively. Given that
D
-
D
the azido group is chemically equivalent to a protected amine,
these compounds can be used as building blocks for solid phase
assembly of peptoids. Our main goal in the design of this type
of SAAs was to lock the furanose ring in a rigid conformation
through the formation of fused rings or spiro-bicycles, thus
mimicking protein reverse turns (1) or constraining a linear
peptide conformation (2). The transformation of the commer-
cially available starting materials into the corresponding
bicyclic azido acids are based on the same synthetic strategy,
once the respective allyl-C-furanosides have been formed
(Schemes 1 and 2).
The 2,3,5-tri-O-benzyl- -arabinofuranose was converted
D
into the anomeric acetate 3, which was allylated by treatment
with allyltrimethylsilane in the presence of catalytic amounts of
BF3–Et2O. The reaction, effected at rt in dry acetonitrile,
afforded 4 as a 1+1 mixture of a and b diastereomers (72%
yield). Iodocyclization (Scheme 1) was then carried out with
iodine in CH2Cl2 at 0 °C. This step is crucial to the formation of
the bicyclic structures of the arabino- and fructo-derived
compounds and consists of the opening of the intermediate
iodonium ion by attack of the g-benzyloxy groups in the 5-exo-
Scheme 2 Reagents and conditions: i, I2, CH2Cl2, 0 °C, 12 h; ii, separation
of diastereomers by flash chromatography; iii, Bu4NN3, toluene, 60 °C; iv,
Ac2O, CF3COOH, 0 °C; v, MeONa, MeOH; vi, CrO3–H2SO4, acetone–
water.
DOI: 10.1039/b006192l
Chem. Commun., 2000, 2303–2304
This journal is © The Royal Society of Chemistry 2000
2303

Jones oxidation. Compound 1 was obtained in 48% yield over
three steps. The spiro-azido acid 2 was prepared in a similar
way, as shown in Scheme 2, the iodocyclization of 6 affording
stereoselectively R-7 as the major product9 (d.e. 33%). The
iododerivative was reacted with tetrabutylammonium azide in
toluene (60 °C, 24 h) then submitted to acetolysis affording R-
10,10 and finally converted into azido acid R-2. The overall
yields for the transformation of 3 into 1 and of 6 into 2 were
respectively 40% and 35%, S-111 and R-2 being obtained as the
major isomers.
NMR and molecular dynamics indicate that both arabino- and
fructo-derived bicycles adopt rigid, spatially well-defined
conformations. In particular the sugar-amino acids derived from
precursors S-1 and R-1 are rigid turn mimetics, both molecules
can be incorporated in peptide sequences and used to replace i
+ 1 and i + 2 residues of a protein b-turn.
of HBTU, HO-Bt and DIPEA and finally totally deprotected
with TFA–TIS–water (95+2.5+2.5), affording 915 in 63% yield
over the last two steps.
The high efficiency of the synthesis of compounds 1 and 2 is
likely to offer opportunities for the preparation of a wide range
of carbohydrate-derived bicyclic amino acids with improved
conformational rigidity to be used as templates, inducing
precise secondary structure conformations in peptides. Detailed
conformational analysis of these compounds was carried out by
NMR and molecular dynamics and will be reported else-
where.
We gratefully acknowledge the contribution of Carola
Cassani to the experimental work.
Notes and references
Moreover, these conformationally constrained sugar-amino
acids may be incorporated as rigid templates into cyclic
peptides in order to lock a bioactive conformation. Several
templates have been included in cyclic peptides containing the
RGD loop with the aim of obtaining new inhibitors of the
adhesive interaction between the avb3 and the aIIbb3-type
integrins and their ligands.12 As example, cyclic peptide 9 was
synthesised on solid phase (Scheme 3) by the well-established
Fmoc protocol. The dipeptide Arg(Pmc)-Gly was assembled
onto Sasrin resin with a loading of 0.47 mmol g21, then
compound S-1 was coupled in the presence of HBTU,13 HO-Bt
and DIPEA. The N-terminal azide reduction to the amine and
the coupling with Fmoc-Asp(tBu) was effected one-pot in the
presence of DIC, Bu3P, HO-Bt in dry DMF–toluene (2+1) at rt
for 24 h.14 This reaction was found to be very efficient and high
yielding (as assessed from the loading value calculated after the
Fmoc deprotection of the aspartate), in sharp contrast with the
inefficiency of other well-established methods we employed to
reduce this azide. The tetrapeptide was then cleaved from the
resin (1% TFA in CH2Cl2 with immediate neutralisation of the
effluents with pyridine) maintaining all protecting groups,
purified by flash chromatography on silica gel, cyclised in a
dilute solution of DMF (peptide conc. 0.5 mM) in the presence
1 K. C. Nicolau, J. M. Salvino, K. Raynor, S. Pietranico, T. Reisine, R. M.
Freidinger and R. Hirschmann, Pept. Chem. Struct. Biol. Proc. Am.
Chem. Symp. 11th, 1990; R. Hirschmann, K. C. Nicolau, S. Pietranico,
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Barbosa, J. Liu, W. Yao, S. Rohrer and A. B. Smith III, J. Med. Chem.,
1998, 41, 1382.
2 H. P. Wessel, D. Banner, K. Gubernator, K. Hilpert, K. Müller and T.
Tschopp, Angew. Chem., Int. Ed. Engl., 1997, 36, 751.
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4 E. F. Fuchs and J. Lehmann, Chem. Ber., 1975, 108, 2254; E. F. Fuchs
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Lehmann, J. Carbohydr. Res., 1976, 49, 267; K. C. Nicolau, H. M.
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8 A consistent NOESY crosspeak (400 MHz, CDCl3) was observed
between H-2A and H-3 protons for the diastereoisomer R-5, that was
absent for S-5.
9 A sequential NOESY correlation allowed the unambiguous attribution
of the configuration at C-2A: for diastereoisomer R-5 H-3/H-1Ab and H-
2A/H-1Aa NOESY crosspeaks were observed thus indicating that H-3 and
H-2A point towards different spatial regions. For diastereoisomer S-8, H-
3/H-1Ab and H-2A/H-1Ab NOESY crosspeaks were observed.
10 Selected data for R-9: MALDI-TOF MS: m/z 467.9 (M), 491.3 (M +
Na), 507.1 (M + K); dH (300 MHz, CDCl3) 1.76 (1H, dd, J 12.9, 9.8, H-
1Aa), 2.04 (3H, s, CH3CO), 2.18 (1H, dd, J 12.9, 6.0, H-1Ab), 3.19 (1H,
dd, J 12.9, 4.9, H-3Aa) 3.47 (1H, dd, J 12.9, 4.0, H-3Ab), 3.52–3.64 (1H,
m, H-5), 3.82 (1H, d, J 9.8, H-1a), 3.87–3.92 (1H, m, H-4), 3.94 (1H, d,
J 1.4, H-3), 4.08–4.20 (2H, m, H-6), 4.25 (1H, d, J 9.8, H-1b), 4.28–4.35
(1H, m, H-2A), 4.42–4.60 (4H, m, CH2-Ph) 7.20–7.40 (10H, m,
Harom).
11 Selected data for S-1: MALDI-TOF MS: m/z 319.9 (M + H); 342.4 (M
+ Na), 358.4 (M + K); dH (300 MHz, CDCl3) 2.09 (1H, m, H-1Ab), 2.31
(1H, ddd, J 14.2, 6.1, 8.3, H-1Aa), 3.30 (1H, dd, J 12.8, 3.8, H-3Ab), 3.55
(1H, dd, J 12.8, 7.3, H-3Aa), 4.13 (1H, m, H-2A), 4.35 (1H, bd, J 3.3, H-2),
4.42 (1H, bd, J 1.7, H-3), 4.61 (1H, bd, J 1.8, H-4), 4.66 (2H, AB
system, CH2-Ph), 4.90 (1H, m, H-1), 7.3–7.4 (5H, m, Harom).
12 M. Aumalley, M. Gurrah, G. Müller, J. Calvete, R. Timpl and H.
Kessler, FEBS Lett., 1991, 291, 50; G. Müller, M. Gurrath, H. Kessler
and R. Timpl, Angew. Chem., 1992, 31, 326; R. Haubner, R. Gratias, B.
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13 Abbreviations: DIC, N,NA-diisopropylcarbodiimide; DIPEA, diisopro-
pylethylamine; HBTU, 2-(1H-benzotriazole-1-yl-)-1,1,3,3-tetramethy-
luronium hexafluorophosphate; HOBt, N-hydroxybenzotriazole; Pmc,
2,2,5,7,8-pentamethylchroman-6-sulfonyl; TIS, triisopropylsilane.
14 Z. Tang and J. C. Pelletier, Tetrahedron Lett., 1998, 39, 4773.
15 Selected data for 9: MALDI-TOF MS: m/z 603.3 (M), 626.6 (M + Na),
642.1 (M + K).
Scheme 3 Reagents and conditions: i, S-1, HBTU, HOBt, DIPEA, DMF, rt,
4 h; ii, Bu3P, DIC, Fmoc-Asp(OtBu), rt, 24 h; iii, piperidine 20% in DMF;
iv, 1% TFA in DCM, neutralisation of the effluent with pyridine; v, 0.5 mM
peptide in DMF, HBTU, HOBt, DIPEA, 12 h, rt; vi, 95% TFA, 2.5% TIS,
2.5% H2O, 10 h, rt.
2304
Chem. Commun., 2000, 2303–2304