When you said Tyr in position 3 did you mean position 3 of the peptide as
opposed to tyrosine with the OH on position 3 of the side chain ring?
If there are no good attachment points for on-resin cyclization, you could
synthesize the peptide, detach from the support, and cyclize in solution.
Although our work concerned head-to-tail cyclization, our paper may be a
useful starting point.
McMurray, J.S., and Lewis, C.A. The synthesis of cyclic peptides using
Fmoc solid-phase chemistry and the linkage agent
4-hydroxymethyl-3-methoxyphenoxybutyric acid. Tetrahedron Letters,
34:8059-8062, 1993.
First synthesize the linear precursor on one of the hyper-acid labile
resins such as Sasrin, 2-chlorotrityl- or resin derivatized with e.g.
4-hydroxymethyl-3-methoxyphenoxybutyric acid. Use no protection on the Tyr
side chain. If it is at all possible, use a Na-Boc amino acid at the
N-terminus. Cleave with weak acid such as 1% TFA in DCM which will leave
the side chain protection intact. Aftyer cleavage and rotovapping off the
solvent, we generally precipitate the peptide in H2O, filter and dry the
peptide to a powder.
At this stage dissolve the peptide in DMF to 1 mM and cyclize. Use a
reagent capable of ester formation such as HBTU or carbonyldiimidazole (the
rest of the group can help you out here). You can monitor cyclization by
HPLC.
When complete, scavenge ions (e.g. BF4- PF6-, DIPEAH+) with a mixed bed ion
exchange resin and ca 20% H2O, then filter and remove solvent.
Alternatively, remove DMF, disolve in EtOAc and do acid-base extractions.
Treat with TFA and purify. Use the minimum of nucleophilic scavengers fot
the reason discussed next.
If you left an Fmoc group on the N-terminus, be careful in removing it.
Your cyclic peptide is a phenyl ester which is a somewhat activated ester,
and may possibly be cleaved with piperidine to give a C-terminal
piperamide. Look for this as a +85 amu peak on the mass spec.
I hope this helps.
Good luck
John S. McMurray
The University of Texas M. D. Anderson Cancer Center
Department of Neuro-Oncology, Box 316
1515 Holcombe Blvd.
Houston, TX 77030
Ph: 713-745-3763
Fax: 713-745-1183