S. Zahariev et al. / Tetrahedron Letters 47 (2006) 4121–4124
4123
mized and isomerized products that makes product puri-
fication difficult and often impossible.
The use of 2-nitrobenzylamine for bromine displace-
ment in Strategy C is a new method for the preparation
of backbone-caged peptides (Table 2, entry 11–12). It
can also be applied for the synthesis of cyclic peptides
and—combined with orthogonal side chain protected
aspartic acid—for the synthesis of Asp/Asn-derivatives,
for example, glycopeptides15 or side- to side-chain cyclic
peptides.
Table 1 shows the data of preformed building blocks
prepared for Strategy A (1–9) and for Strategy B (10).
Table 2 summarizes the characteristics of VKDGYI as
prepared with the three strategies described above. In
general, the yield and purity of the products reach,
and even exceed, the level obtained with Hmb protec-
tion. We also noted that the removal of the Dmb pro-
tecting group from the dipeptide building blocks by
TFA is ꢀ30% faster than that of Hmb (for more details
see Supplementary data). In addition, ESI-MS showed
that the peptides were free of Asu and piperidide.
Supplementary data
Supplementary data associated with this article can be
Finally, we prepared the above mentioned HA2-TAT
peptideà using Strategies A and C. The yield/purity of
the products were comparable with those obtained with
the Hmb-protected dipeptide building block, and both
were found to be free of Asu/piperidides. Both the
preformed building blocks (Strategies A and B) and
the sub-monomeric approach (Strategy C) introduce
an open chain, proline-like structure, which can disrupt
unwanted H-bonds during the synthesis of difficult
peptides. Derivative 10 (Strategy B) is useful for the
synthesis of Xaa-Gly sequences (including Asn/Asp-
Gly) and potentially allows the incorporation of substi-
tuted N-benzylglycine at any point of the synthesis of
‘difficult’ peptides. The ‘sub-monomeric route’ (Strategy
C) is simple and flexible (different amines can be used for
halogen-displacement), does not require separate or
additional synthetic steps and, at the same time, is more
efficient and ꢀ30 times more cost effective than the use
of premade building blocks. With this procedure, the
yields of aryl substituted N-benzylglycine were nearly
quantitative. The new backbone protecting groups used
here were in many respects superior to the commercial
reagents (shelf stable, easily TFA- or photo-cleavable,
orthogonal) and applicable for the synthesis of both
peptide acids and peptide amides.
References and notes
1. Barany, G.; Merrifield, R. B. In The Peptides: Analysis,
Synthesis and Biology; Gross, E., Meienhofer, J., Eds.;
Academic Press: New York, 1980; Vol. 2, pp 190–208, and
references cited therein; Radkiewicz, J. L.; Zipse, H.;
Clarke, S.; Houk, K. N. J. Am. Chem. Soc. 2001, 123,
3499–3506; Clarke, S. Ageing Res. Rev. 2003, 2, 263–285;
Robinson, N. E.; Robinson, A. B. Molecular Clocks:
Deamidation of Asparaginyl and Glutaminyl Residues in
Peptides and Proteins; Althouse Press: Cave Junction, OR,
2004; pp 1–443 and references cited therein.
2. Lauer, J. L.; Fields, C. G.; Fields, G. B. Lett. Pept. Sci.
1995, 1, 197–205, and references cited therein.
3. Wade, J. D.; Mathieu, M. N.; Macris, M.; Tregear, G. W.
Lett. Pept. Sci. 2000, 7107–7112.
4. Karlstrom, A.; Unden, A. Tetrahedron Lett. 1996, 37,
4243–4246.
5. Mergler, M.; Dick, F. J. Pept. Sci. 2005, 11, 650–657, and
references cited therein.
6. Alsina, J.; Kates, S. A.; Barany, G.; Albericio, F. Methods
Mol. Biol. 2005, 298, 195–208, and references cited therein;
Hong, Z.; Wang Yali, W.; Voelter, W. Tetrahedron Lett.
1995, 36, 8767–8770.
7. Quibell, M.; Owen, D.; Packmann, L. C.; Johnson, T.
J. Chem. Soc., Chem. Commun. 1994, 20, 2343–2344; Ede,
N. J.; Ang, K. H.; James, I. W.; Bray, A. M. Tetrahedron
Lett. 1996, 37, 9097–9100; Zeng, W.; Regamey, P. O.;
Rose, K.; Wang, Y.; Bayer, E. J. Pept. Res. 1997, 49, 273–
279, and references cited therein.
8. Offer, J.; Johnson, T.; Quibell, M. Tetrahedron Lett. 1997,
38, 9047–9050; Howe, J.; Quibell, M.; Johnson, T.
Tetrahedron Lett. 2000, 41, 3997–4001; Miranda, L. P.;
Meutermans, W. D.; Smythe, M. L.; Alewood, P. F. J.
Org. Chem. 2000, 65, 5460–5468, and references cited
therein; Meutermans, W. D.; Bourne, G. T.; Golding, S.
W.; Horton, D. A.; Campitelli, M. R.; Craik, D.; Scanlon,
M.; Smythe, M. L. Org. Lett. 2003, 5, 2711–2714.
à Automated peptide synthesis of HA21-20-HIV-TAT48-57
:
First synthesis of peptide H-GLFGAIAGFIENGWEG-
MIDGGRKKRRQRRR-OH containing the sequences of hemagglu-
tinin HA21-20 and HIV-TAT48-57 was assembled on TentaGel S Trt-
Arg(Pbf)-resin (90 lm, substitution 0.2 mmol gÀ1, 0.05 mmol scales,
Fluka) on automated peptide synthesizer SP3 (Protein Technology),
using standard coupling/deprotection cycle.
The second synthesis was carried out in an identical manner, except
that the commercially available building block Fmoc-Asp (OtBu)-
(Hmb)Gly-OH was used to introduce the DG-sequence.
The third synthesis was carried out in an identical manner, except
that the synthesis of H-(Dmb)Gly20-peptide-Ile-resin was prepared by
a two step sub-monomer solid-phase synthesis, using Dmb-NH2 for
bromine displacement11 (see above and the text). Double coupling
was performed from Met17 to N-terminal Gly1 including.
The DCM washed peptide resins were cleaved/deprotected with
TFA/TIPS/H2O (95/2.5/2.5, v/v/v) for 2.5 h, and the peptides were
isolated in the usual manner. The crude peptides were characterized
by RPHPLC (10–55% MeCN in 60 min, 1.5 mL minÀ1) and LC/ESI-
MS (for more details see Supplementary data), as described.11
´
9. Nicolas, E.; Pujades, M.; Bacardit, J.; Giralt, E.; Alberi-
cio, F. Tetrahedron Lett. 1997, 38, 2317–2320.
10. Sampson, W. R.; Patsiouras, H.; Ede, N. J. J. Pept. Sci.
1999, 5, 403–409, and references cited therein.
11. Zahariev, S.; Guarnaccia, C.; Zanuttin, F.; Pintar, A.;
Esposito, G.; Maravic, G.; Krust, B.; Hovanessian, A. G.;
Pongor, S. J. Pept. Sci. 2005, 11, 17–28, and references
cited therein.
12. Mergler, M.; Dick, F.; Sax, B.; Weiler, P.; Vorherr, T.
J. Pept. Sci. 2003, 9, 36–46.
13. Tamm, L. K. Biochim. Biophys. Acta 2003, 1614, 14–
23.