Synthesis and solution conformation of homo-β-peptides consisting of N-mannofuranosyl-3-ulosonic acids

Article abstract of DOI:10.1021/jo900966b

(Chemical Equation Presented) The synthesis and solution conformation of homo-oligomers of β-aminoacids, β-N-mannofuranosyl-3-ulosonic acids, have been studied by NMR, MD simulation, and circular dichroism. These oligomers feature a spirocyclic disubstitu

Full text of DOI:10.1021/jo900966b

pubs.acs.org/joc
Synthesis and Solution Conformation of Homo-β-peptides Consisting of
N-Mannofuranosyl-3-ulosonic acids
†
Manuel Andreini, Claude Taillefumier,* Franc-oise Chretien, Vincent Thery, and
,‡
†
‡
ꢀ
Yves Chapleur†
†
ꢀ ꢀ ꢀ
Groupe SUCRES, Nancy Universite UMR 7565 Universite Henri Poincare, Nancy 1-CNRS, BP 70239,
F-54506, Nancy-Vandoeuvre, France, and ‡Clermont Universiteꢀ, Universiteꢀ Blaise Pascal, Laboratoire
ꢁ
SEESIB (UMR 6504-CNRS), F-63177 Aubiere cedex, France
claude.taillefumier@univ-bpclermont.fr
Received May 8, 2009
The synthesis and solution conformation of homo-oligomers of β-aminoacids, β-N-mannofuranosyl-
3-ulosonic acids, have been studied by NMR, MD simulation, and circular dichroism. These
oligomers feature a spirocyclic disubstitution and a N,O-acetal functionality at the β-carbon of the
backbone, an unprecedented situation in the realm of β-peptides. Our study shows that tetramer 10
and hexamer 11 adopt a characteristic secondary structure. In the hexamer 11, NMR investigations
coupled with MD simulations suggest the preference for a double C8 turn forming conformation.
Introduction
compared with R-peptides, not only in term of primary
structure but also, and this is the most striking observation,
in term of secondary structure.4 Indeed it has been demon-
strated that linear and cyclic short β-peptidic sequences are
able to fold into well-defined structures such as turn,5
helical,6 and sheet-like conformations.7 In addition they
exhibit a greater stability toward metabolization by micro-
organisms and to protease and peptidase degradations.8 All
of these interesting features make them the most thorou-
ghly studied oligomers as peptidomimetics.4c,9 It is now
recognized that helical β-peptides are able to mimic protein
β-Peptides that are oligomers of β-amino acids have been
the most studied class of foldamers1 since the pioneering
work of Gellman2 and Seebach.3 In β-peptides, the presence
in the backbone of an additional carbon atom between the
amino and the carboxyl groups allows a greater diversity
(1) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173–180.
(2) For the first model study for the folding of β-peptides, see: Dado,
G. P.; Gellman, S. H. J. Am. Chem. Soc. 1994, 116, 1054–62.
(3) For the first synthesis of a β-hexapeptide, see: Seebach, D.; Overhand,
€
M.; Kuhnle, F. N. M.; Martinoni, B.; Oberer, L.; Hommel, U.; Widmer, H.
Helv. Chim. Acta 1996, 79, 913–941.
(4) For reviews on β-peptides folding, see: (a) Seebach, D.; Matthews,
J. L. Chem. Commun. 1997, 2015–2022. (b) DeGrado, W. F.; Schneider, J. P.;
Hamuro, Y. J. Pept. Res. 1999, 54, 206–217. (c) Cheng, R. P.; Gellman, S. H.;
DeGrado, W. F. Chem. Rev. 2001, 101, 3219–3232. (d) Seebach, D.;
Schreiber, J. V.; Arvidsson, P. I.; Frackenpohl, J. Helv. Chim. Acta 2001,
(6) Koert, U. Angew. Chem., Int. Ed. 1997, 36, 1836–1837. Seebach, D.;
€
Wang, D. P.; Gao, Y.; Zhao, Y. L. Acc. Chem. Res. 2008, 41, 1418–1427.
(7) Langenhan, J. M.; Guzei, I. A.; Gellman, S. H. Angew. Chem., Int. Ed.
2003, 42, 2402–2405.
Hook, D. F.; Glattli, A. Biopolymers 2006, 84, 23–37. Wu, Y. D.; Han, W.;
€ €
€
84, 271–279. (e) Martinek, T. A.; Fulop, F. Eur. J. Biochem. 2003, 270, 3657–
(8) (a) Sebach, D.; Abele, S.; Sifferlen, T.; Hanggi, M.; Gruner, S.; Seiler,
3666. (f) Seebach, D.; Beck, A. K.; Bierbaum, D. J. Chem. Biodiversity 2004,
1111–1239. (g) Guichard, G. Pseudo-Peptides in Drug Discovery; Nielsen, P. E.,
Ed.; Wiley-VCH: Weinheim, 2004; pp 33-120.
(5) Chung, Y. J.; Christianson, L. A.; Stanger, H. E.; Powell, D. R.;
Gellman, S. H. J. Am. Chem. Soc. 1998, 120, 10555–10556. DeGrado, W. F.;
Schneider, J. P.; Hamuro, Y. J. Pept. Res. 1999, 54, 206–217. Daura, X.;
Gademann, K.; Schaefer, H.; Jaun, B.; Seebach, D.; Van Gunsteren, W. F. J.
Am. Chem. Soc. 2001, 123, 2393–2404. Langenhan, J. M.; Gellman, S. H.
Org. Lett. 2004, 6, 937–940. Baldauf, C.; Pisabarro, M. T. J. Phys. Chem. B.
2008, 112, 7581–7591.
P. Helv. Chim. Acta 1998, 81, 2218–2243. (b) Frackenpohl, J.; Arvidsson, P.
I.; Schreiber, J. V.; Seebach, D. ChemBioChem 2001, 2, 445–455. (c) Hook,
D. F.; Gessier, F.; Noti, C.; Kast, P.; Seebach, D. ChemBioChem 2004, 5,
691–706. (d) Hook, D. F.; Bindschaedler, P.; Mahajan, Y. R.; Sebesta, R.;
Kast, P.; Seebach, D. Chem. Biodiversity 2005, 2, 591–632.
(9) Seebach, D.; Gardiner, J. Acc. Chem. Res. 2008, 41, 1366–1375.
Arnold, U.; Hinderaker, M. P.; Nilsson, B. L.; Huck, B. R.; Gellman,
S. H.; Raines, R. T. J. Am. Chem. Soc. 2002, 124, 8522–8523. Gelman, M.
A.; Richter, S.; Cao, H.; Umezawa, N.; Gellman, S. H.; Rana, T. M. Org.
Lett. 2003, 5, 3563–3565.
DOI: 10.1021/jo900966b
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Andreini et al.
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secondary structures and thereby can act as protein-protein
interaction inhibitors.8,10 For example, they have shown
promising biological activities as antibacterial agents11 and
somatostatin receptor binding12 or anticancer drugs.13 Self-
association of helical β-peptides toward protein-like assem-
blies is also currently studied,14 and mixed R/β-peptides
designed for specific biological functions have also received
much attention in recent years.10c,15,16
work. Peptides containing sugar amino acid building
blocks17,18 have been used extensively in the area of pepti-
domimetic studies,19 oligosaccharide mimetics,20 and as
structural element to induce secondary structures.21
β-Peptides consisting of sugar β-amino acids have also
received much attention. Oligomers incorporating sugar
β-amino acid residues were found to form helical secondary
structures.22,23 Glycosylated β-peptide24 chains that retain
typical helical secondary structure despite the presence of
highly oxygenated carbohydrate appendages have also been
described.25 Sharma and Kunwar have designed C-linked
carbo-β-peptides in which furanoid sugars are linked to a
β-peptide backbone by the C-4 (sugar numbering). Oligo-
mers derived from C-linked carbo-β3-amino acids with
alternating chirality form both 10/12 and 12/10 right-handed
helices, whereas mixed β-peptides derived from alternating
C-linked carbo-β3-amino acids and β-hGly units adopt left-
handed 10/12 and 12/10 helical structures.26 A more surpris-
ing situation is when the anomeric center is merged into the
β-carbon of a β-amino acid residue. We have previously
shown that such structures, N-glycosyl-3-ulosonic acids, are
readily prepared from exo-glycals.27 They are useful building
blocks for the synthesis of spiro sugar-diazepinediones hetero-
cycles28 and can be incorporated into small mixed peptides.29
We recently described short homo-oligomeric cyclic peptides
constituted of anomeric sugar β3,3-amino acids.30 We now give
Sugar amino acids are sugars with both an amino group
and a carboxyl group attached to the carbohydrate frame-
(10) (a) Kritzer, J. A.; Stephens, O. M.; Guarracino, D. A.; Reznik, S. K.;
Schepartz, A. Bioorg. Med. Chem. 2004, 13, 11–16. (b) Stephens, O. M.; Kim,
S.; Welch, B. D.; Hodsdon, M. E.; Kay, M. S.; Schepartz, A. J. Am. Chem.
Soc. 2005, 127, 13126–13127. (c) Sadowsky, J. D.; Fairlie, W. D.; Hadley, E.
B.; Lee, H. S.; Umezawa, N.; Nikolovska-Coleska, Z.; Wang, S.; Huang, D.
C. S.; Tomita, Y.; Gellman, S. H. J. Am. Chem. Soc. 2007, 129, 139–154.
(11) For antibacterial properties see, for example: Liu, D.; DeGrado, W.
F. J. Am. Chem. Soc. 2001, 123, 7553–7559. Arvidsson, P. I.; Frackenpohl, J.;
Ryder, N. S.; Liechty, B.; Petersen, F.; Zimmermann, H.; Camenisch, G. P.;
Woessner, R.; Seebach, D. ChemBioChem 2001, 2, 771–773. Porter, E. A.;
Weisblum, B.; Gellman, S. H. J. Am. Chem. Soc. 2002, 124, 7324–7330.
Epand, R. F.; Raguse, T. L.; Gellman, S. H.; Epand, R. M. Biochemistry
2004, 43, 9527–9535. Schmitt, M. A.; Weisblum, B.; Gellman, S. H. J. Am.
Chem. Soc. 2007, 129, 417–428. Mowery, B. P.; Lee, S. E.; Kissounko, D. A.;
Epand, R. F.; Epand, R. M.; Weisblum, B.; Stahl, S. S.; Gellman, S. H. J. Am.
Chem. Soc. 2007, 129, 15474–15476.
(12) Gademann, K.; Ernst, M.; Hoyer, D.; Seebach, D. Angew. Chem.,
Int. Ed. 1999, 38, 1223–1226. Gademann, K.; Ernst, M.; Seebach, D.; Hoyer,
D. Helv. Chim. Acta 2000, 83, 16–33. Gademann, K.; Kimmerlin, T.; Hoyer,
D.; Seebach, D. J. Med. Chem. 2001, 44, 2460–2468.
(13) Sun, J. J.; Zhou, X. D.; Liu, Y. K.; Tang, Z. Y.; Sun, R. X.; Zhao, Y.;
Uemura, T. J. Cancer Res. Clin. Oncol. 2000, 126, 595–600. Gademann, K.;
Seebach, D. Helv. Chim. Acta 2001, 84, 2924–2937.
(21) For examples, see: Long, D. D.; Smith, M. D.; Marquess, D. G.;
Claridge, T. D. W.; Fleet, G. W. J. Tetrahedron Lett. 1998, 39, 9293–9296.
Claridge, T. D. W.; Long, D. D.; Baker, C. M.; Odell, B.; Grant, G. H.;
Edwards, A. A.; Tranter, G. E.; Fleet, G. W. J.; Smith, M. D. J. Org. Chem.
2005, 70, 2082–2090. Jockusch, R. A.; Talbot, F. O.; Rogers, P. S.; Simone,
M. I.; Fleet, G. W. J.; Simons, J. P. J. Am. Chem. Soc. 2006, 128, 16771–
16777.
(22) Gruner, S. A. W.; Truffault, V.; Voll, G.; Locardi, E.; Stockle, M.;
Kessler, H. Chem.;Eur. J. 2002, 8, 4365–4376. Chandrasekhar, S.; Reddy,
M. S.; Jagadeesh, B.; Prabhakar, A.; Rao, M. H. V. R.; Jagannadh, B. J. Am.
Chem. Soc. 2004, 126, 13586–13587.
(23) For carbohydrate-derived β-peptides based upon an oxetane, see:
Claridge, T. D. W.; Goodman, J. M.; Moreno, A.; Angus, D.; Barker, S. F.;
Taillefumier, C.; Watterson, M. P.; Fleet, G. W. J. Tetrahedron Lett. 2001,
42, 4251. Barker, S. F.; Angus, D.; Taillefumier, C.; Probert, M. R.; Watkin,
D. J.; Watterson, M. P.; Claridge, T. D. W.; Hungerford, N. L.; Fleet, G. W.
J. Tetrahedron Lett. 2001, 42, 4247.
(24) (a) Palomo, C.; Oiarbide, M.; Landa, A.; Gonzalez-Rego, M. C.;
Garcia, J. M.; Gonzalez, A.; Odriozola, J. M.; Martin-Pastor, M.; Linden, A.
J. Am. Chem. Soc. 2002, 124, 8637–8643. (b) Sharma, G. V. M.; Reddy, V. G.;
Chander, A. S.; Reddy, K. R. Tetrahedron: Asymmetry 2002, 13, 21–24. (c)
Dondoni, A.; Massi, A.; Sabbatini, S.; Bertolasi, V. Tetrahedron Lett. 2004,
45, 2381–2384.
(25) Norgren, A. S.; Arvidsson, P. I. Org. Biomol. Chem. 2005, 3, 1359–
1361. Simpson, G. L.; Gordon, A. H.; Lindsay, D. M.; Promsawan, N.;
Crump, M. P.; Mulholland, K.; Hayter, B. R.; Gallagher, T. J. Am. Chem.
Soc. 2006, 128, 10638–10639. Norgren, A. S.; Arvidsson, P. I. J. Org. Chem.
2008, 73, 5272–5278. Inaba, Y.; Kawakami, T.; Aimoto, S.; Ikegami, T.;
Takeuchi, T.; Nakazawa, T.; Yano, S.; Mikata, Y. Carbohydr. Res. 2009,
344, 613–626.
(26) (a) Sharma, G. V. M.; Reddy, K. R.; Krishna, P. R.; Sankar, A; R.;
Narsimulu, K.; Kumar, S. K.; Jayaprakash, P.; Jagannadh, B.; Kunwar, A.
C. J. Am. Chem. Soc. 2003, 125, 13670–13671. (b) Sharma, G. V. M.; Reddy, K.
R.; Krishna, P. R.; , A. R.; Jayaprakash, P.; Jagannadh, B.; Kunwar, A. C. Angew.
Chem. 2004, 116, 4051-4055; Angew. Chem., Int. Ed. 2004, 43, 3961-3965.
Sharma, G. V. M.; Subash, V.; Reddy, N. Y.; Narsimulu, K.; Ravi, R.; Jadhav, V. B.;
Murthy, U. S. N.; Kishore, K. H.; Kunwar, A. C. Org. Biomol. Chem. 2008, 6,
4142–4156. Sharma, G. V. M.; Rao, K. S.; Ravi, R.; Narsimulu, K.; Nagendar, P.;
Chandramouli, C.; Kumar, S. K.; Kunwar, A. C. Chem. Asian. J. 2009, 4, 181–
193.
(14) Raguse, T. L.; Lai, J. R.; LePlae, P. R.; Gellman, S. H. Org. Lett.
2001, 3, 3963–3966. Qiu, J. X.; Petersson, E. J.; Matthews, E. E.; Schepartz,
A. J. Am. Chem. Soc. 2006, 128, 11338–11339. Horne, W. S.; Price, J. L.;
Keck, J. L.; Gellman, S. H. J. Am. Chem. Soc. 2007, 129, 4178–4180. Price,
J. L.; Horne, W. S.; Gellman, S. H. J. Am. Chem. Soc. 2007, 129, 6376–6377.
Petersson, E. J.; Schepartz, A. J. Am. Chem. Soc. 2008, 130, 821–823.
(15) For a review, see: Horne, W. S.; Gellman, S. H. Acc. Chem. Res.
2008, 41, 1399–1408.
(16) Gung, B. W.; Zou, D.; Miyahara, Y. Tetrahedron 2000, 56, 9739–
9746. Price, J. L.; Horne, W. S.; Gellman, S. H. J. Am. Chem. Soc. 2007, 129,
6376–6377. Choi, S. H.; Guzei, I. A.; Gellman, S. H. J. Am. Chem. Soc. 2007,
129, 13780–13781. Choi, S. H.; Guzei, I. A.; Spencer, L. C.; Gellman, S. H.
J. Am. Chem. Soc. 2008, 130, 6544–6550.
(17) For reviews on sugar amino acids, see: (a) Lohof, E.; Burkhart, F.;
Born, M. A.; Planker, E.; Kessler, H. In Advances in Amino Acid Mimetics and
Peptidomimetics; JAI Press: Stamford, CT. 1999; Vol. 2, pp 263-292. (b)
Dondoni, A.; Marra, A. Chem. Rev. 2000, 100, 4395–4421. (c) Gruner, S. A.
W.; Locardi, E.; Lohof, E.; Kessler, H. Chem. Rev. 2002, 102, 491–514. (d)
Schweizer, F. Angew. Chem., Int. Ed. 2002, 41, 230–253. Jensen, K. J.; Brask, J.
Biopolymers 2005, 80, 747–761. Chakraborty, T. K.; Srinivasu, P.; Tapadar, S.;
Mohan, B. K. Glycoconjugate J. 2005, 22, 83–93. Risseeuw, M. D. P.; Overhand,
M.; Fleet, G. W. J.; Simone, M. I. Tetrahedron: Asymmetry 2007, 18, 2001–
2010.
(18) Fuchs, E.-F.; Lehmann, J. Chem. Ber. 1975, 108, 2254–2260. Nico-
laou, K. C.; Florke, H.; Egan, M. G.; Barth, T.; Estevez, V. A. Tetrahedron
Lett. 1995, 36, 1775–1778.
(19) For leading references, see: Chakraborty, T. K.; Jayaprakash, S.;
Diwan, P. V.; Nagaraj, R.; Jampani, S. R. B.; Kunwar, A. C. J. Am. Chem.
Soc. 1998, 120, 12962–12963. Chakraborty, T. K.; Ghosh, S.; Jayaprakash,
S.; Sharma, J. A. R. P.; Ravikanth, V.; Diwan, P. V.; Nagaraj, R.; Kunwar,
A. C. J. Org. Chem. 2000, 65, 6441–6457. Chakraborty, T. K.; Ghosh, S.;
Jayaprakash, S. Curr. Med. Chem. 2002, 9, 421–435. Chakraborty, T. K.;
Ramakrishna Reddy, V.; Sudhakar, G.; Uday Kumar, S.; Jagadeshwar
Reddy, T.; Kiran Kumar, S.; Kunwar, A. C.; Mathur, A.; Sharma, R.;
Gupta, N.; Prasad, S. Tetrahedron 2004, 60, 8329–8339. Prasad, S.; Mathur,
A.; Jaggi, M.; Sharma, R.; Gupta, N.; Reddy, V. R.; Sudhakar, G.; Kumar,
S. U.; Kumar, S. K.; Kunwar, A. C.; Chakraborty, T. K. J. Pept. Res. 2005,
66, 75–84. Chakraborty, T. K.; Roy, S.; Kumar, S. K.; Kunwar, A. C.
Tetrahedron Lett. 2005, 46, 3065–3070. Chakraborty, T. K.; Kumar, S. U.;
Mohan, B. K.; Sarma, G. D.; Kiran, M. U.; Jagadeesh, B. Tetrahedron Lett.
2007, 48, 6945–6950.
(27) Lakhrissi, M.; Chapleur, Y. Angew. Chem., Int. Ed. 1996, 35, 750–
752. Taillefumier, C.; Chapleur, Y. Chem. Rev. 2004, 104, 263–292.
(28) Taillefumier, C.; Thielges, S.; Chapleur, Y. Tetrahedron 2004, 60,
2213–2224.
(29) Taillefumier, C.; Lakhrissi, Y.; Lakhrissi, M.; Chapleur, Y. Tetra-
hedron: Asymmetry 2002, 13, 1707–1711.
(20) For leading references, see: Wessel, H. P.; Mitchell, C.; Lobato, C.
M.; Schmid, G. Angew. Chem., Int. Ed. 1996, 34, 2712–2713. Suhara, Y.;
Yamaguchi, Y.; Collins, B.; Schnaar, R. L.; Yanagishita, M.; Hildreth, J. E.
K.; Shimada, I.; Ichikawa, Y. Biorg. Med. Chem. 2002, 10, 1999–2013.
Sicherl, F; Wittmann, V. Angew. Chem., Int. Ed. 2005, 44, 2096–2099.
(30) Andreini, M.; Taillefumier, C.; Fernette, B.; Chapleur, Y. Lett. Org.
Chem. 2008, 5, 360–364.
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SCHEME 1. Synthesis of Building Blocks for Oligomerization
a full account of the synthesis and of the conformational
preference of their linear precursors. Relatively little work
concerns the conformational behavior of homo-oligomers
consisting of β,β-disubstituted β-amino acids.31 β2,2- or
β3,3-amino acids do not adopt the most common secondary
structures described in the β-peptide field.8a Oligomers 7-11 do
not resemble other β-peptides previously reported in the litera-
ture. They are geminally disubstituted by two sides of a cycle at
the β-carbon. This situation has been described at the R-carbon;
the X-ray crystal analysis of a trimer consisting of 1-(amino-
methyl)-cyclohexane-carboxylic residues features 10-mem-
bered hydrogen-bonded rings,8a while C-8 pseudocycles32,33
are observed with oligomers bearing cyclopropylidene rings.34
Geminally disubstituted β-amino acids in which the β-carbon is
embedded in a cycloalkane ring have been used as turn inducers
in short peptides35 but have not been incorporated in β-peptide
chains. To the best of our knowledge homo-oligomers 7-11
are the first ones featuring a spirocyclic substitution at the
β-carbon. Moreover, the β-carbon of the peptide chain
corresponds to the anomeric carbon of a sugar ring, and as a
consequence this carbon features a N,O-acetal functionnality.36
saponification of the ester function of 2 in order to obtain the
free carboxylic partner for the oligomerization study. Sapo-
nification of this compound resulted in a decarboxylation
process whatever the reaction conditions used and gave
ketose 4. We reasoned that this reaction would be avoided
by deactivation of the nucleophilic properties of the anome-
ric nitrogen. We therefore decided to protect the free amino
group of 3 as a carbamate. A benzyloxycarbonyl protecting
group, orthogonal to the sugar acetonides, was chosen and
readily installed by reaction with benzyloxycarbonyl chlor-
ide (CbzCl) and NaHCO3 in methanol. The resulting
N-protected β-amino ester 5 was isolated as a single β
anomer in accordance with our previous observation.29
Hydrolysis of the ester was then performed with K2CO3 in
aqueous methanol and yielded the expected carboxylic acid
building block 6 quantitatively.
With the necessary building blocks 3 and 6 in hand, we
turned to the synthesis of dimer 7. The crude building blocks 3
and 6 were coupled using O-(7-azabenzotriazol-1-yl)-N,N,N0,
N0-tetramethyluronium hexafluorophosphate salt37 (HATU)
and diisopropylethylamine (DIPEA) in DMF giving a single
glycosyl-β-dipeptide 7 in 70% yield after silica gel chromatog-
raphy as an easily handled foam. The configuration at the
anomeric center of each unit in 7 was established unambigu-
ously by long-range NOE correlation (N-HfH-4, ulosonic
numbering), and we therefore showed that starting from a R/β
mixture of amines 3, only the anomer having the two C3-NH
links trans to the 4,5-dioxolane (ulosonic acid numbering) was
obtained. Dipeptide 7 served as a common intermediate to
prepare the two requisite units 8 and 9 for the tetramer
synthesis. A portion of 7 was converted to the free carboxylic
dipeptide unit 9 using the above-described procedure for
saponification of the monomer 5. The remainder of 7 was
converted in the unmasked dipeptide amine 8 by hydrogeno-
lysis. Finally, coupling of 8 and 9 using HATU and DIPEA in
DMF yielded the tetramer 10 in 47% overall yield (three steps).
Iteration of the process (saponification of the tetramer 10 and
coupling with 8) gave the hexamer 11 in 50% overall yield.
The solution conformation of these β3,3-peptides was
investigated using NMR and CD spectroscopies and mole-
cular dynamic simulations.
Results and Discussion
Synthesis. The synthesis of the key building blocks for
oligomerization is given in Scheme 1. N-Glycosyl-amino
ester 2 was prepared by aza-Michael addition of benzyl-
amine29 to the exo-glycal (manno configuration) 1.27 Hydro-
genolysis of the N-benzyl group lead to an R/β-mixture of
N-glycosyl amine 3.
However, starting from this N-glycosyl-amino ester mix-
ture, amidation furnished a single anomer in which the
anomeric nitrogen is anti to the 4,5-fused acetonide
(ulosonic numbering). We next turned our attention to the
(31) Heinonen, P.; Virta, P.; Lonnberg, H. Tetrahedron 1999, 55, 7613–
7624. Abele, S.; Seebach, D. Eur. J. Org. Chem. 2000, 1–15.
€
(32) Gademann, K.; Hane, A.; Rueping, M.; Jaun, B.; Seebach, D.
Angew. Chem., Int. Ed. 2003, 42, 1534–1537.
(33) Baquero, E. E.; James, W. H. III; Choi, S. H.; Gellman, S. H.; Zwier,
T. S. J. Am. Chem. Soc. 2008, 130, 4795–4807. Baquero, E. E.; James, W. H.
III; Choi, S. H.; Gellman, S. H.; Zwier, T. S. J. Am. Chem. Soc. 2008, 130,
4784–4794.
(34) Abele, S.; Seiler, P.; Seebach, D. Helv. Chim. Acta 1999, 82, 1559–
1571.
(35) Palomo, C.; Oiarbide, M.; Bindi, S. J. Org. Chem. 1998, 63, 2469–
2474. Mollica, A.; Paglialunga Paradisi, M.; Torino, D.; Spisani, S.; Lucente,
G. Amino Acids 2006, 30, 453–459.
(36) For the synthesis of N-glycosyl-amino acid building blocks via a
Ritter reaction, see: (a) Schweizer, F.; Lohse, A.; Otter, A.; Hindsgaul, O.
Synlett 2001, 1434–1436. (b) Penner, M.; Taylor, D.; Desautels, D.; Marat,
K.; Schweizer, F. Synlett 2005, 212–216. (c) Penner, M.; Schweizer, F.
Carbohydr. Res. 2007, 342, 7–15.
NMR Spectroscopy. Spectra were recorded at 400 MHz in
deuterated chloroform and/or benzene-d6. The structure of
the oligomers 7, 10, and 11 was unambiguously established.
In particular the stereochemistry of the anomeric linkages
was systematically confirmed using NOE correlations.
(37) Carpino, L. A. J. Am. Chem. Soc. 1993, 115, 4397.
J. Org. Chem. Vol. 74, No. 20, 2009 7653

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FIGURE 1. Solvent titration plot for the amide NH of tetramer 10 (a) and hexamer 11 (b) showing the variation of chemical shift with % of
DMSO-d6 to C6D6 at 298 K.
SCHEME 2. Oligomerization Reactionsa
aTo facilitate the discussion, the residues in tetramer 10 and hexamer 11 are identified alphabetically from the N- to C-terminus. Letters that identify the
residues are drawn inside of each sugar ring.
Proton assignments for each residue were established from a
combination of 2D (COSY45, HMQC, TOCSY) techniques,
and the sequential assignment of the four (compound 10) and
six (compound 11) residues was established using through-
bond long-range 1H-13C correlations (HMBC).
into a benzene solution showed distinct amide environments
for hexamer 11 (Figure 1b). The Δδ ppm values exhibited by
the NH groups were 0.60, 0.51, 0.13, 0.21, 0.48, and 1.10 for
NHA, NHB, NHC, NHD, NHE, and NHF, respectively,
showing that NHF was the most shifted one and thus the
most exposed to the solvent. The internal NHC and NHD
appeared to be more involved in intramolecular H-bonds.
DMSO-d6 titration of tetramer 10 was also performed (Δδ
NH ppm 0.38, 0.27, 0.34, and 1.10 for NHA, NHB, NHC, and
NHD). Again, the NH group at the N-terminus was the most
exposed to the solvent (Figure 1a).
Despite the fact that both tetramer 10 and hexamer 11 are
homo-β-peptides, consisting of up to six identical residues,
signal dispersion was observed throughout their H NMR
1
(CDCl3 and C6D6) spectra, suggesting the occurrence of a
conformational preference. Well-dispersed chemical shifts of
the amide resonances (from 7.48 to 8.21 ppm for compound
10 and from 7.58 to 8.51 ppm for compound 11), particularly
in d6-benzene, allowed the assignment of all the NH signals.
This dispersion of the amide protons was also indicative of a
hydrogen-bonding structure in solution. DMSO-d6 titration
Usually in β-peptides, the NH-CH(β) coupling constants
for individual residues allow to establish the relative disposi-
tion of the corresponding H-atoms and the torsion angles
found can be used as restraints, in addition to NOEs, in
7654 J. Org. Chem. Vol. 74, No. 20, 2009

Andreini et al.
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ensemble. The resulting ten most stable structures for 11
have been extracted and minimized with constraints.
The superposition of the 10 lowest energy conformers is
shown in Figure 3. Three structures are in the first half of the
MD simulation and the other seven are distributed through-
out the second half. The N- and C-terminus, which do not
participate in hydrogen bonding, display important fraying
while the backbone of the central residues C and D is more
well-defined. Depending on the conformations, NHE adopts
two well distinct orientations. One of them allows the
NHE-COB hydrogen bond to be formed (12-membered
ring), but the latter is not corroborated by DMSO NMR
titration. Conformers displaying this hydrogen bond appear
in the second half of the simulation (see for example structure
B in Figure 4 obtained after 119 700 steps), they are char-
acterized by a central turn that brings residues B and E close
˚
together and by backbone lengths ranging from 12.4 to 9.8 A
(Figure 3, b). Whatever the conformation, NHB is never
involved in H-bonding while NMR data (Δδ ΝHB = 0.51
ppm, DMSO-d6 titration) could suggest that it is weakly H-
bonded. A reasonable interpretation is that a weak H-bond
could be established occasionally with the highly crowded
and oxygenated sugar portions of the oligomer.39 This kind
of hydrogen bond is observed in some of the 10 lowest energy
conformations (see Supporting Information). The more
˚
FIGURE 2. Solution infrared spectra (2 mM in CHCl3) of mono-
mer 5 (A), dimer 7 (B), and hexamer 11 (C).
extended structure at ca. 18 A length (Figure 4a) is actually
the one in best agreement with NMR titration. This con-
formation shows two consecutive eight-membered ring hy-
drogen bonds, NHC-COA and NHD-COB.
molecular dynamics simulations. In contrast, the absence of
CH(β) in β,β-disubstituted β-amino acids and their oligo-
mers makes them more difficult to analyze from a conforma-
tional point of view. Due to important overlapping in the
1H NMR spectrum of 11, only few cross-peaks could be
considered unambiguously from the 2D ROESY experi-
ments. All the cross-peaks identified except one
(NHAfH4c) were NHfH4 NOEs, involving the NHs and
H4 of neighboring residues (NHAfH4B, NHBfH4C,
NHEfH4F, NHBfH4A, NHDfH4C).
These C8 conformations are also found individually
among some of the 10 lowest energy structures and are
believed to occur in the tetramer 10. It is worth mention-
ing that eight-membered hydrogen-bonded rings between
N-H (i) and CdO (i - 2) have been observed in a cyclic
homo-β-peptide prepared from tetramer 10.30 Consecutive
C8 pseudocycles have been observed in the field of β-peptides
when a hydroxy group is present in the 2-position of each
amino acid residue,32 in the crystal structures of a dimer, a
trimer, and a tetramer consisting of 1-(aminomethyl)cyclo-
propanecarbonyl moieties34, and in a dimer and trimer of a
family of oxanorbornene β-peptides.40 Eight-membered hy-
drogen-bonded rings were also found in oligomers composed
of R-aminooxy acids41 and in aza-β3-peptides.42
Modeling of H-8 and H-12 Helices. NMR titration and
MD simulations tend to show that the secondary structure of
hexamer 11 is mainly governed by two hydrogen bonds
between CO and NH of amides in the backbone, namely,
the NHC-COA and NHD-COB. Nevertheless it seemed of
interest to evaluate whether helices described in the β-peptide
field could be conceivable in our case. To answer this
question, the construction of several models of helices for
the hexamer 11, by fixing the torsional angles in the back-
bone was performed. H-8 (φ -69°, ψ -64, θ 120°),43 H-12
Solution IR spectrum of compound 11 in CHCl3 (Figure 2)
was also used to differentiate hydrogen-bonded from non-
hydrogen-bonded amides and as such gave further evidence
of the presence of hydrogen-bonded conformations.
The solution IR spectrum of the monomeric unit 5
(Figure 2A) exhibited only one sharp band at 3404 cm-1
corresponding to the carbamate N-H stretch. Shoulders at
3417 and 3423 (Figure 2B and C, respectively) were assigned
to the N-H carbamate. Dimer 7 (Figure 2B) showed only
one amide environment at 3359 cm-1 consistent with a
non-hydrogen-bonded amide, whereas the hexamer 11
(Figure 2C) showed a broad signal with two N-H stretches
at 3352 and 3288 cm-1 consistent with non-hydrogen-
bonded and hydrogen-bonded amides.34
MD Simulation.38 The previously obtained set of NOEs
was used to restrain the MD simulation by fixing a set of
˚
distances ranging from 2 to 5 A. The simulation system was
generated by placing the hexamer 11 in a cubic solvent box
˚
(39/38/42 A) containing 512 molecules of benzene. After one
first minimization over the entire system, MD simulation
was performed at 600° K during 200 000 step integrations of
1 fs. The system was equilibrated for 5000 fs in the NVT
(39) Chakraborty, T. K.; Jayaprakash, S.; Srinivasu, P.; Madhavendra,
S. S.; Ravi Sankar, A.; Kunwar, A. C. Tetrahedron 2002, 58, 2853–2859.
(40) Doerksen, R. J.; Chen, B.; Yuan, J.; Winkler, J. D.; Klein, M. L.
Chem. Commun. 2003, 2534–2535.
(41) Yang, D.; Qu, J.; Li, B.; Ng, F.-F.; Wang, X.-C.; Cheung, K.-K.;
Wang, D.-P.; Wu, Y.-D. J. Am. Chem. Soc. 1999, 121, 589–590.
€
(42) Salaun, A.; Potel, M.; Roisnel, T.; Gall, P.; Le Grel, P. J. Org. Chem.
2005, 70, 6499–6502.
€ €
(43) Fulop, F.; Martinek, T. A.; Tot, G. K. Chem. Soc. Rev. 2006, 35, 323–
334.
ꢀ
(38) Sybyl package; Tripos Inc.: 1699 South Hanley Rd., St. Louis, MO
63144-2917.
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Andreini et al.
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FIGURE 3. (A) Structure of β-hexapeptide 11. Bundles of the 10 lowest energy conformers incorporating NOE-derived distances constraints
(first structure obtained after 39 900 steps). Carbon atoms are in black, nitrogens in blue, and oxygens in red. Side chains and hydrogens atoms
˚
(except amide NH) omitted for clarity. (B) Variation of the length (A) of 11 during MD simulation.
FIGURE 4. Stick view of two representative structures among the
10 lowest energy conformations obtained from constrained MD
simulations. (A) Structure that most satisfies the DMSO NMR
titration, this structure shows two consecutive eight-membered ring
hydrogen bonds, NHC-COA and NHD-COB, and was obtained
after 39 900 steps. (B) One example of structure displaying the 12-
membered ring hydrogen bond NHE-COB in addition to the eight-
FIGURE 5. 3D structures of H-8 (a, b) and H-12 (c, d) helices.
membered ring hydrogen bond NHC-COA (119 700 steps). The
Dashed lines correspond to hydrogen bonding. The models were
generated with the SYBYL package and energetically minimized
˚
total number of integration steps is equal to 200 000. The length (A)
of the structures are calculated from the C(CO) at the N-terminus to
the C(CO) at the terminal carboxylic moiety. Dashed lines corre-
spond to hydrogen bonding.
€
using the Tripos Force Field with Gasteiger-Huckel charges
(distance-dependent dielectric constant 5) up to a root-mean-square
˚
(rms) gradient of 0.05 kcal/mol A (Powell Algorithm). In (a) and (c)
3
all carbohydrate moieties have been omitted for clarity, carbons are
shown in black, nitrogens in blue, and oxygens in red. For models
(b) and (d) only the dioxolane moieties at the non-reducing end of
the sugars have been omitted, and the same code of color has been
used for the peptidic backbone while the sugar moieties are repre-
sented in different colors.
(φ -87°, ψ -109°, θ 92°),44 and H-14 (φ -148°, ψ -138°,
θ 61°)44 helices were chosen, the first two because eight- and
12-membered hydrogen bonds have been found along the MD
simulation, and the H-14 because the latter is known to be
impossible for β3,3-peptides and as such represent a comparison
model. Figure 5 shows the resulting models for H-8 and H-12
after minimization. As expected the H-14 secondary structure do
not withstand the minimization process and is completely
destabilized (not shown). H-8 and H-12 helices that were not
found in our MD simulation do not satisfy all of the NOE
constraints (see Supporting Information).
Circular Dichroism Spectroscopy. Circular dichroism
(CD) spectroscopy is a useful method to probe the secondary
structure of synthetic oligomers. For β-peptides for which
structural information from NMR or RX are known, CD
fingerprints have been correlated to typical secondary struc-
tures such as 314, 2.512, 12/10 helices, and hairpin turns. CD
spectra for dimer 7, tetramer 10 and hexamer 11 were
recorded in methanol and acetonitrile at 25 °C and at an
amide bond concentration of 100 μM, for example, 33 μM
for the tetramer that has three amide bonds in the backbone.
Monomer 5, which has no amide chromophore, was also
analyzed to ensure that it does not exhibit any significant CD
signal in the far UV (Figure 6). In acetonitrile, tetramer 10
and hexamer 11 display a minimum at ca. 229 nm, a max-
imum at ca. 200 nm and a zero-crossing at ca. 213 nm. A very
similar pattern is observed in MeOH: a minimum at 228 nm,
a maximum at 199 nm, and a zero-crossing at 212 nm. First
of all, whatever the solvent, CD spectra do seem to indicate
€
(44) Baldauf, C.; Gunther, R.; Hofmann, H.-J. Angew. Chem., Int. Ed.
2004, 43, 1594–1597.
7656 J. Org. Chem. Vol. 74, No. 20, 2009

Andreini et al.
JOCArticle
peaks at 215 and 200 nm.45 The presence of the additional
signal at 215 nm makes the general shape different and as a
consequence do not allow any future comparison. The
polyproline I helix and oligomers of (S)-nipecotic acids also
feature CD signals above 225 nm, until 232 nm in aliphatic
alcohol,s46 but their whole spectra are once again different
from those that characterize our oligomers.
Actually the characteristics of our curves can be compared
to values (max ∼195 nm, min ∼225, zero-crossing 212-222)
reported by Yang et al. for turns and helices consisting of
R-aminoxy acids that display eight-membered hydrogen
bonds.41 At the moment, in the field of β-peptides, no CD
pattern is associated to C8 pseudocycle secondary structure.
The combination of NMR and simulation also suggest that
eight-membered hydrogen bonds are present, but for the
moment comparison of our data with that of oligomers
consisting of R-aminoxy acids remains hazardous since R-
aminoxy acids may display specific CD patterns.
Conclusion
We have synthesized and studied the solution conforma-
tion of short homo-oligomers build from N-mannofurano-
syl-3-ulosonic acids. In these β-peptides, the amino group of
each β-amino acid is part of a N,O-acetal, an unprecedented
situation in the field of β-peptides. To the best of our
knowledge homo-oligomers from β-amino acids with a cyclic
disubstitution at the β-carbon also represents a non-docu-
mented topological situation. The lack of CH(β) made the
NMR analysis less informative than that of more traditional
β-peptide chains; nevertheless, NMR and IR gave evidence
of the existence of a defined solution conformation stabilized
by H-bonding. Constrained MD simulations, in addition to
the DMSO NMR titration suggest the presence of two
consecutives C8 turns. The CD curves could be reminiscent
to that observed with helices consisting of R-aminoxy acids
that display eight-membered hydrogen bonds, even though
the peptidic chains are different. The propensity of this novel
family of β-peptide oligomers for local folding, involving
eight-membered ring H-bonding, should be confirmed with
N-glycosyl-3-ulosonic acid building blocks of different con-
figurations in their protected and deprotected forms to probe
the conformational preference in aqueous environments.
FIGURE 6. CD spectra of oligomers 7, 10, and 11 in acetonitrile
(A) and in methanol (B). All spectra were recorded at room
temperature and have been normalized as amide bond concentra-
tion.
that the tetramer and hexamer adopt the same regular
conformation. The Cotton effect does not increase on going
from the tetramer to the hexamer. This could suggest that the
tetramer is long enough to adopt a local conformation that is
also observed for the hexamer. The spectra of the dimer 7 are
somewhat different in methanol and acetonitrile. In acetoni-
trile, the dimer does not display a positive peak around
200 nm, and in both solvents an additional trough is ob-
served at ca. 270 nm. The benzyloxycarbonyl group should
account for this CD signal, and it would suggest that this
group is in a very different environment in the dimer as
compared to the tetramer and the hexamer. Nevertheless, the
dimer also exhibits the negative Cotton effect around
225-229 nm, suggesting that it is conformationally preorga-
nized in a manner similar to that of longer oligomers.
The absorption patterns found in methanol and acetoni-
trile are distinct from those that have been correlated to
secondary structures in the β-peptide field. For example,
both H-14 and H-12 helices display a maximum around
200 nm, the CD spectra of oligomers 10 and 11 also feature a
maximum at ca. 200 nm, but the wavelengths of the minima,
around 228 nm, are red-shifted in comparison to typical
values for H-14 (ca. 215 nm) or H-12 (ca. 220 nm) helices
derived from R-L-amino acids. In the field of β-peptides, one
can find CD fingerprints featuring red-shifted signal above
225 nm. For example β3,3-oligomers from Seebach’s group
show CD pattern with a weak trough around 230 nm and two
Experimental Section
Compounds 127a and 229 have been previously reported.
Methyl 3-Amino-2,3-dideoxy-4,5:7,8-bis-O-isopropylidene-D-
manno-3-octulofuranosonate 3. The synthesis of compound 3
has been previously reported.28 Oil, Rf 0.34 (Hex/EtOAc 1:2);
[R]20 -5.3 (c 0.3; CHCl3); νmax (thin film) 3416 (NH2), 1734
D
(CdO) cm-1; 1H NMR (250 MHz, CDCl3) δ 4.86-4.77 (m, 1H,
H4), 4.51-4.46 (m, 1H, H5), 4.38-4.31 (m, 1H, H7), 4.14-3.90
0
(m, 2H, H6, H8 ), 3.71, 3.70 (2s, 3H, CO2CH3), 3.58 (dd, 1H, H8,
J
J
7,8=3.0, Jgem=7.0 Hz), 2.91 (d, Jgem=17.0 Hz, H2), 2.66 (d,
gem=17.0 Hz, H2), 2.63 (br s, H2), 2.25 (br s, 2H, NH2), 1.52,
1.46, 1.44, 1.34, 1.31 (5s, 12H, 4 ꢀ C(CH3)2); 13C NMR (62.9
MHz, CDCl3) δ 171.9, 170.2 (CdO), 112.9, 109.5, 109.3 (2 ꢀ
C(CH3)2), 92.9, 92.8 (C3), 81.0, 80.6, 78.9, 78.4, 77.5, 76.4, 73.5,
70.2 (4C, C4, C5, C6, C7), 67.5, 67.2 (C8), 52.2, 51.9 (CO2CH3),
41.8, 40.6 (C2), 27.2, 26.4, 26.2, 25.6, 25.4, 24.9 (4 ꢀ CH3).
€
(45) Seebach, D.; Sifferlen, T.; Mathieu, P. A.; Hane, A. M.; Krell, C. M.;
Bierbaum, D. J.; Abele, S. Helv. Chim. Acta 2000, 83, 2849–2864.
(46) Huck, B. R.; Langenhan, J. M.; Gellman, S. H. Org. Lett. 1999, 1,
1717–1720.
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Methyl 2,3-Dideoxy-4,5:7,8-bis-O-isopropylidene-3-(benzylo-
xycarbonyl)-amino-r-D-manno-3-octulofuranosonate 5. To a
cold solution of the free amine 3 (3.83 g, 11.56 mmol) in MeOH
(60 mL) were added successively, in one portion, solid Na2CO3
(3.9 g, 46 mmol, 4 equiv) and dropwise benzyl chloroformate (2
mL, 14 mmol, 1.2 equiv). The reaction mixture was stirred 1 h at
room temperature, after which it was concentrated under re-
duced pressure. The residue was taken up by CH2Cl2 (30 mL)
and washed with water (3 ꢀ 10 mL). The organic layer was dried
over MgSO4, filtered, and concentrated, giving the crude residue
which was purified by column chromatography on silica
(EtOAc in hexane, 20%) to afford compound 5 (4.0 g, 8.67
mmol, 75%) as an oil. [R]25D -8.3 (c 0.4, CHCl3); νmax (thin film)
film) 1720 (CdO, ester), 1685 (CdO, amide I) cm-1; 1H NMR
(400 MHz, CDCl3) δ 7.38-7.32 (m, 5H, aromatics), 7.15 (s, 1H,
NHB), 6.63 (s, 1H, NHA), 5.25 (d, 1H, H4A), 5.15 (d, 1H, H4B),
5.13 (d, 1H, -CH2Ph), 5.05 (d, 1H, Jgem=12.3 Hz, -CH2Ph),
4.96 (dd, 1H, J4A-5A = 5.9 Hz, H5A), 4.88 (dd, 1H, J4B-5B=5.9
Hz, J5B-6B=3.8 Hz, H5B), 4.33-4.40 (m, 2H, H7A, H7B), 4.22
(dd, 1H, J5A-6A=3.8 Hz, J6A-7A=7.5 Hz, H6A), 4.13 (dd, 1H,
J6B-7B=6.6 Hz, H6B), 4.04-4.09 (m, 2H, H8A, H8B), 3.93-3.99
0
0
(m, 2H, H8 A, H8 B), 3.64 (s, 3H, OMe), 3.04 (d, 1H, Jgem=15.9
0
Hz, H2B), 2.90-2.98 (m, 2H, H2 B, H2A), 2.77 (d, 1H, Jgem=14.7
0
Hz, H2 A), 1.54, 1.48, 1.45, 1.42, 1.37, 1.31 (8s, 24H, 8 ꢀ CH3);
13C NMR (100.6 MHz, CDCl3) δ 171.7 (CO2Me), 170.3 (2 ꢀ
CdO(NH)), 155.7 (CdO(Cbz group)), 136.5 (Cipso), 128.4-128.9
(4 ꢀ CAr), 113.3 (2 ꢀ Cacetal 4-5), 109.4, 109.6 (Cacetal 7-8), 93.5
(C3A), 93.1 (C3B), 85.1 (C4A), 84.8 (C4B), 81.8 (C6A), 81.5 (C6B),
81.1 (2 ꢀ C5), 73.8 (C7B), 73.4 (C7A), 67.2 (-CH2Ph), 67.0 (C8A),
66.8 (C8B), 52.3 (OMe), 42.6 (C2A), 37.8 (C2B), 24.6, 24.7, 25.5,
25.7, 26.2, 26.3, 27.1, 27.3 (8 ꢀ CH3); m/z (ESþ) 765 (M þ Hþ,
100%). Anal. Calcd for C37H52N2O15 (764.8): C, 58.11; H, 6.85;
N, 3.66. Found: C, 58.06; H, 6.93; N, 3.59.
1731 (CdO, ester), 3344 (NH) cm-1 1H NMR (400 MHz,
;
CDCl3) δ 7.32-7.37 (m, 5H, Ar), 6.10 (s, 1H, NH), 5.13 (d,
1H, H4), 5,07 (br s, 2H, -CH2Ph), 4.95 (dd, 1H, J4-5=5.8 Hz,
J5-6=3.6 Hz, H5), 4.35 (m, 1H, H7), 4.15 (dd, 1H, J6-7=7.3 Hz,
H6), 4.05 (dd, 1H, J7-8 =5.8 Hz, H8 ), 3.97 (dd, 1H, J7-8=5.1
0
0
0
Hz, Jgem=8.8 Hz, H8), 3,6 (s, 3H, OMe), 3,05 (d, 1H, H2 ), 2.85
(d, 1H, Jgem=16.0 Hz, H2), 1.48, 1.42, 1.35, 1.32 (4s, 12H, 4 ꢀ
CH3); 13C NMR (100.6 MHz, CDCl3) δ 170.8 (CO2Me), 155.1
(CdO(Cbz group)), 135.9 (Cipso), 128.0-128.5 (4 ꢀ CAr), 109.0,
112.8 (2 ꢀ C(CH3)2), 92.4 (C3), 85.0 (C4), 80.9 (C5 and C6), 73.3
(C7), 66.7 (-CH2Ph), 66.6 (C8), 51.8 (CO2CH3), 38.1 (C2), 26.7,
25.7, 25.3, 24,3 (4 ꢀ CH3). ESI-HRMS: calcd for C23H32O9N [M
þ H]þ 466.2077, found 466.2097.
Tetramer 10. To a solution of compound 7 (190 mg, 0.25
mmol) in methanol (5 mL) were added successively potassium
carbonate (190 mg, 0.25 mmol, 2 equiv) and water (0.2 mL).
After reaction completion, the reaction mixture was neutralized
by adding Amberlite IR-120(Hþ) resin, filtered, and concen-
trated in vacuo to give the crude dimer acid 9.
To a solution of compound 7 (930 mg, 1.21 mmol) in EtOAc
(20 mL) was added 10% Pd/C (93 mg), and the mixture was
stirred overnight under hydrogen at atmospheric pressure. The
catalyst was then removed by filtration through a short pad of
Celite, and the filter cake was washed with EtOAc. The filtrate
and the washings were combined and concentrated in vacuo to
give the crude dimer amine 8.
The crude acid 9 (200 mg, 0.23 mmol) and the crude amine
8 (175 mg, 0.23 mmol, 1.0 equiv) were dissolved in dry DMF
(5 mL) at room temperature under an argon atmosphere. HATU
(131 mg, 0.35 mmol, 1.5 equiv) was added in one portion followed
by dropwise addition of DIPEA (57 μL, 0.35 mmol, 1.5 equiv). The
resulting solution was stirred for 15 h at room temperature. The
reaction mixture was then concentrated under reduced pressure,
diluted with CH2Cl2 (5 mL), and washed successively with satu-
rated aq NaHCO3 (3ꢀ 5mL), brine(5mL), andwater (5mL). The
organic layer was dried over MgSO4 and concentrated under
reduced pressure. The resulting mixture was purified by column
chromatography on silica gel (EtOAc/hexane, 2:3) to yield the
tetramer 10 as a glassy white solid (150 mg, 0.11 mmol, 47%).
Rf 0.64 (ethyl acetate/hexane 7:3). Mp 113 °C; [R]25D þ3.6 (c 0.6,
CHCl3); νmax (thin film) 3367 (NH) cm-1; m/z (ESþ) 1363 [(M þ
H)þ, 21%], 1385 [(M þ Na)þ; 100%]. Anal. Calcd for C65H94-
N4O27 (1363.4): C, 57.26; H, 6.95; N, 4.11. Found: C, 57.39; H,
6.87; N, 4.10. 1H NMR (400 MHz, C6D6) δ7.29-7.22 (m, 5H, Ar),
5.30, 5.07 (2 ꢀ d, 2H, Jgem=12.4 Hz, -CH2Ph), 3.45 (s, 3H, OMe),
1.60, 1.59, 1.52 (s), 1.48 (s), 1.46 (s), 1.45, 1.44, 1.40 (s), 1.24 (s), 1.23
(s), 1.22, 1.17(s) (48H, 16 ꢀ CH3).
2,3-Dideoxy-4,5:7,8-bis-O-isopropylidene-3-(benzyloxycarbo-
nyl)-amino-r-D-manno-3-octulofuranosonic Acid 6. The ester
compound 5 was dissolved in MeOH (10 mL/mmol), and
potassium carbonate (1.1 equiv) and water (1 mL/mmol) were
added. After completion, the reaction mixture was neutralized
by adding Amberlite resin (IR 120), filtered, and concentrated in
vacuo to get a crude free carboxylic acid, which was used directly
in the next step. Compound 6 (730 mg, 1.61 mmol) was
synthezised from 5 (750 mg, 1.61 mmol) in a quantitative yield,
following the general procedure of saponification described
above, and was used directly in the next step. Rf 0-0.2 (Hex/
EtOAc 1:1); 1H NMR (400 MHz, d6-DMSO) δ 12.2 (s, 1H, OH),
8.0 (s, 1H, NH), 7.5-7.3 (m, 5H, Ar), 5.06 (d, 1H, J=12.7 Hz, H-
benz), 5.02 (d, 1H, H-benz), 4.90 (d, 1H, J4,5=5.8 Hz, H4), 4.74
0
(dd, 1H, J5,6=3.5 Hz, H5), 4.20 (m, 1H, H7), 3.97 (dd, 1H, J8,8
8 Hz, J7,8=6.5 Hz, H8), 3.84 (dd, 1H, J6,7=7.2; H6), 3.74 (dd,
=
0
0
0
1H, J7,8 =5.3 Hz, H8 ), 3.25 (d, 1H, J2,2 =16.6 Hz, H2), 2.7 (d,
0
1H, H2 ), 1.5-1.3 (4s, 12H, CH3); 13C NMR (100.6 MHz, d6-
DMSO) δ 171.3 (CO2H), 155.3 (CdO), 137.7 (C-ipso),
129.2-128.4 (C Ar), 112.3, 108.9 (C(CH3)2), 92.9 (C3), 85.0
(C4), 81.0 (C5), 80.0 (C6), 73.3 (C7); 67.0 (C8), 66.0 (CH2Ph),
27.4, 26.5, 25.9, 25.2 (4 ꢀ CH3).
Dimer 7. A solution of compound 5 (750 mg, 1.61 mmol) was
dissolved in MeOH (16 mL), and potassium carbonate (245 mg,
3.2 mmol, 2 equiv) and water (2.2 mL) were added. After
completion, the reaction mixture was neutralized by adding
Amberlite IR-120(Hþ) resin, filtered, and concentrated in vacuo
to give the crude carboxylic acid 6.
Amine 3 (460 mg, 1.4 mmol) and the carboxylic acid 6 (710
mg, 1.4 mmol, 1.0 equiv) were dissolved in dry DMF (120 mL) at
room temperature under an argon atmosphere. HATU (0.800 g,
2.1 mmol, 1.5 equiv) was added in one portion followed by
dropwise addition of DIPEA (0.23 mL, 2.1 mmol, 1.5 equiv).
The resulting solution was stirred for 15 h at room temperature.
The reaction mixture was then concentrated under reduced
pressure, diluted with CH2Cl2 (50 mL), and washed successively
with saturated aq NaHCO3 (3 ꢀ 20 mL), brine (20 mL), and
water (20 mL). The organic layer was dried over MgSO4 and
concentrated under reduced pressure. The resulting mixture was
purified by column chromatography on silica (EtOAc in hexane,
30%) to yield the dimer 7 as a glassy white solid (0.810 g, 1.06
mmol, 70%). Mp 78 °C; [R]25D -0.9 (c 0.9, CHCl3); νmax (thin
ring
A
B
C
D
NH
H2/H2
H4
H5
H6
7.81
3.23, 3.23
5.40
4.92-4.96
4.60-4.65
4.66-4.70
4.41-4.46
8.21
3.11, 3.39
5.53
4.92-4.96
4.53-4.58
4.60-4.65
4.46-4.50
8.14
3.04, 3.47
5.52
4.66, 4.70
4.41-4.46
4.60-4.65
4.25-4.31
7.48
0
3.26, 3.38
5.45
4.83
4.41-4.46
4.53-4.58
4.21, 4.25-4.31
H7
H8/H8
0
13C NMR (100.6 MHz, C6D6) δ 172.9 (CO2Me), 171.4, 171.1,
170.8, (3 ꢀ CdO(NH)), 156.1 (CdOCbz), 137.5 (Cipso), 128.7
7658 J. Org. Chem. Vol. 74, No. 20, 2009

Andreini et al.
JOCArticle
(4 ꢀ CAr), 66.9 (-CH2Ph), 52.0 (OMe), 113.0, 112.7, 112.5,
112.4, 109.1, 109.0, 108.8 (8 ꢀ C(CH3)2), 94.5, 93.9, 93.8, 93.7
(4 ꢀ C3),85.6, 85.2, 84.8, 84.7 (4 ꢀ C4), 82.3, 82.2 (4 ꢀ C6), 81.7,
81.5, 81.0 (4 ꢀ C5), 74.2, 73.9, 73.6 (4 ꢀ C7), 67.6, 67.5, 67.0, 66.6
(4 ꢀ C8), 42.2, 41.9, 40.8, 38.5 (4 ꢀ C2), 27.4, 27.3, 27.2,
26.9, 26.2, 26.0, 25.9, 25.7, 25.6, 25.4, 24.5, 24.4, 24.35, 24.3
(16ꢀ CH3).
(Cipso), 128.8, 128.6, 128.5 (5 ꢀ CAr), 113.0, 112.9, 112.7,
112.6, 112.5, 109.2, 109.1, 109.0, 108.9, 108.8 (12 ꢀ C(CH3)2),
94.5, 94.3, 94.2, 94.0, 93.9, 93.6 (6 ꢀ C3), 85.6, 85.3, 85.1, 84.9,
84.6 (6 ꢀ C4), 82.3, 82.2, 82.1, 82.0 (6 ꢀ C6), 81.9-81.1 (6 ꢀ C5),
74.2, 74.1, 74.0, 73.9, 73.7 (6 ꢀ C7), 67.6-66.6 (6 ꢀ C8), 66.9
(CH2Ph), 52.0 (OMe), 42.3, 41.9, 41.3, 41.2, 41.1, 38.6 (6 ꢀ C2)
27.4, 27.3, 27.2, 26.9, 26.2, 26.1, 25.9, 25.8, 25.6, 25.5, 25.3, 24.6,
24.5, 24.4, 24.3 (24 ꢀ CH3).
Hexamer 11. To a solution of compound 10 (300 mg, 0.20
mmol) in methanol (5 mL) were added potassium carbonate
(360 mg, 0.40 mmol, 2 equiv) and water (0.4 mL). After reaction
completion, the mixture was neutralized by adding Amberlite
IR-120(Hþ) resin, filtered, and concentrated in vacuo to give the
crude acid.
ring
A
B
C
D
E
F
NH 8.22
0
7.79
H2/H2 3.27, 3.30 3.19, 3.44 3.16, 3.43 3.08, 3.29 3.04, 3.39 3.25, 3.32
8.51
8.43
8.17
7.58
The crude acid (270 mg, 0.20 mmol) and the crude amine 8 (80
mg, 0.14 mmol, 0.7 equiv) were dissolved in dry DMF (5 mL) at
room temperature under an argon atmosphere. HATU (112 mg,
0.30 mmol, 1.5 equiv) was added in one portion followed by
dropwise addition of DIPEA (49 μL, 0.30 mmol, 1.5 equiv). The
resulting solution was stirred for 15 h at room temperature. The
reaction mixture was then concentrated under reduced pressure,
diluted with CH2Cl2 (5 mL), and washed successively with
saturated aq NaHCO3 (3 ꢀ 5 mL), brine (5 mL), and water (5
mL). The organic layer was dried over MgSO4 and concentrated
under reduced pressure. The resulting mixture was purified by
column chromatography on silica gel (EtOAc/hexane, 1:1) to
yield the hexamer 11 (140 mg, 0.071 mmol, 50%). Rf 0.55 (ethyl
acetate/hexane 7:3), [R]25D þ3.2 (c 0.6, CHCl3); νmax (thin film)
3350 (NH), 1735 (CdO ester), 1674, 1668, 1662, 1652 (CdO
amides) cm-1; m/z (ESþ) 1984 [(M þ Na)þ; 100%]. Anal. Calcd
for C93H136N6O39 (1960.8): C, 56.93; H, 6.99; N, 4.28. Found: C,
56.91; H, 7.02; N, 3.93. 1H NMR (400 MHz, C6D6) δ 7.32-7.19
(m, 5H, Ar), 5.33, 5.15 (2 ꢀ d, 2H, Jgem=12.4 Hz, -CH2Ph),
3.47 (s, 3H, OMe), 1.17-1.60 (m, 72H, 24 ꢀ CH3).
H4
H5
H6
H7
5.51
5.41
5.60
5.57
5.62
5.47
4.86-4.39 4.94-4.96 4.86-4.90 5.05-5.03 4.94-4.96 4.86-4.90
4.49-4.50 4.55-4.57 4.55-4.57 4.55-4.57 4.51-4.54 4.55-4.57
4.60-4.69
4.19-4.52
0
H8/H8
Acknowledgment. The authors gratefully acknowledge
Dr. Alison A. Edwards from the Medway School of Phar-
macy at the Universities of Kent and Greenwich at Medway
for helpful discussions about CD analysis.
Supporting Information Available: General experimental,
results of constrained molecular dynamic simulation including
the 10 lowest energy structures, selected distances measured
over the course of the simulation, the view of a H-14 helix
model and measurement of distances in minimized H-8 and
H-12 helices, H and 13C NMR spectra of compounds. This
1
13C NMR (100.6 MHz, C6D6) δ 172.8 (CO2Me), 171.7, 171.4,
171.2, 170.9, 170.8 (5 ꢀ CdO(NH)), 156.2 (CdOCbz), 137.7
material is available free of charge via the Internet at http://
pubs.acs.org.
J. Org. Chem. Vol. 74, No. 20, 2009 7659