however, this has no bearing on the utility of the reaction since
peptides are fully assembled prior to Dmab deprotection (see
ESI†). In addition, no side products relating to epimerisation
or aspartimide formation were formed under these optimised
reaction conditions, even after prolonged treatments (see ESI†).
We have observed that the nature of the amino acid C-terminal
to the site of derivatisation has a dramatic effect on the efficiency
of solid phase reactions of aspartic acid and glutamic acid side
chains. As such, our next goal was to investigate the potential
scope of this improved solid phase aspartylation protocol, via
the incorporation of alternative amino acids C-terminal to the
Dmab-protected aspartic acid residue. Resin bound tripeptide 3
served as the starting point for the synthesis of three resin bound
peptides (Scheme 3). Peptides bearing penultimate glycine, proline
and valine residues were synthesised by SPPS. Glycine was incor-
porated in its Dmb-protected form (again to prevent aspartimide
formation). The inability of proline to form aspartimides, and the
sterically hindered nature of the valine side chain, meant that the
backbone amides were not protected with Dmb groups in these
cases. Treatment of the resin bound pentapeptides with hydrazine,
followed by 5 mM NaOH, gave the corresponding free acids 4,
7 and 8, which were subsequently reacted with 5. Side chain
deprotection and cleavage from the resin using an acidic cocktail
gave the desired glycopeptides 6, 9 and 10 in high yields (quant.,
95% and 96%, respectively) as determined by LC-MS analysis.
Isolation of these glycopeptides by preparative HPLC provided 6,
9 and 10 in moderate to high yields.
Scheme 4 Solid phase synthesis of N-linked glycopeptide 11.
strates the orthogonal nature of the deprotection/aspartylation
conditions to a large range of protecting groups commonly
employed in solid-phase chemistry including Boc, tBu ethers, tBu
esters and trityl (Trt) groups and illustrates the utility of the solid
phase aspartylation method for the synthesis of more complex
glycopeptides.
In summary, improved conditions for the removal of Dmab
esters from the side chain of aspartic acid residues on the solid
phase have been developed and implemented in a novel solid-
phase strategy for the simple, rapid and efficient construction of
N-linked glycopeptides in a convergent manner. Applications of
this procedure are ongoing in this laboratory for the synthesis
of N-linked glycopeptide fragments which can be used for the
ligatory assembly of therapeutic glycopeptides and glycoproteins.
In addition, it is anticipated that the Dmab deprotection method
will find application in a host of synthetic endeavours, including
the synthesis of cyclic peptides.
Acknowledgements
We would like to thank Dr. Kelvin Picker for his assistance with
HPLC and LC-MS analysis.
Notes and references
1 W. C. Chan, B. W. Bycroft, D. J. Evans and P. D. White, J. Chem. Soc.,
Chem. Commun., 1995, 2209–2210.
2 T. Johnson, M. Liley, T. J. Cheeseright and F. Begum, J. Chem. Soc.,
Perkin Trans. 1, 2000, 2811–2820.
Scheme 3 Solid phase synthesis of N-linked glycopeptides 6, 9 and 10.
3 M. Valldosera, M. Monso, C. Xavier, P. Raposinho, J. D. G. Correia, I.
Santos and P. Gomes, Int. J. Pept. Res. Ther., 2008, 14, 273–281.
4 T. Berthelot, M. Goncalves, G. Lain, K. Estieu-Gionnet and G. Deleris,
Tetrahedron, 2006, 62, 1124–1130.
5 M. Goncalves, K. Estieu-Gionnet, G. Lain, M. Bayle, N. Betz and G.
Deleris, Tetrahedron, 2005, 61, 7789–7795.
6 J. P. Malkinson, M. Zloh, M. Kadom, R. Errington, P. J. Smith and M.
Searcey, Org. Lett., 2003, 5, 5051–5054.
7 M. Cudic, J. D. Wade and L. Otvos, Tetrahedron Lett., 2000, 41, 4527–
4531.
Finally, to demonstrate the applicability of our solid-phase
aspartylation method for the preparation of longer glycopeptides
bearing an internal glycan, we embarked on the synthesis of
11 (Scheme 4). Synthesis of fully protected peptide 12 was
achieved from 3 using standard Fmoc-SPPS followed by Dmab-
deprotection of the internal aspartic acid residue (see ESI†). Solid
phase aspartylation of 5 and acidolytic deprotection and cleavage
from the resin gave the desired glycopeptide 11 in 61% isolated
yield based on the Fmoc loading of 3. This example clearly demon-
8 J. Ruczynski, B. Lewandowska, P. Mucha and P. Rekowski, J. Pept.
Sci., 2008, 14, 335–341.
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