I ment that IMHO pyroglutamate (Fmoc-pGluOAll) formation via intramolecular
urethane nitrogen acylation is the most probable reaction pathway when you
strongly activate Glu side chain. And I'm afraid that it can't be helped.
That's why I offered you an ideal solution based on using Trt-chloride
resin.
I'm almost sure that quality of your 2,6-dichlorobenzyl chloride is OK and
that DMAP method
or using Bop or PyBop will result in the same pyroglutamic acid derivative
predominantly.
If there's no time to wait for Trt-chloride resin delivery you may venture
converting the Wang resin with an excess of Ph3P-CBr4 into bromide and
attach Fmoc-Glu(OH)OAll to the latter as cesium salt.
Regards,
Igor
Laboratory of Peptide Chemistry
Branch of Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry
Pushchino, Moscow Region
142292, Russian Federation
=================================================================
19.10.98
Dear Igor,
I just forgot to ask you a question:
what do you mean with Fmoc-pGlu-OAll exactly?
I tried to carry out the attachment of the 1st aa, but it wasn't
successful. I am having a second try, maybe it's the 2,6
dichlorobenzyl chloride I have been given, but I checked it out on
TLC and it seems OK.
If things don't work out this time, I must resort to the DMAP method
or use a Bop or PyBop derivative.
Thanks
Luca
----------------------------------------------------------------------------
------
16.10.98 (2)
Dear Luca,
If you will succeed in attachment of your derivative to Wang resin by the
mixed anhydride method mentioned in your mail - just don't forget to carry
out capping after coupling of second residue as well. Note that 1-10 %
chain loss due to pGluOAll cleavage is almost inevitable.
I wish you success in your first SPPS!
Igor
----------------------------------------------------------------------------
------
16.10.98
Dear Igor,
thanks for your piece of advice.
As a matter of fact, I had to go on with my synthesis because I only
have 2 weeks left to carry it out. So, after talking with some people
here who work on SPPS, I opted for attachment of the FmocGlu OAll
with the Sieber method (used by Novabiochem, too) which employs 2,6
dichlorobenzoyl chloride and pyridine. There should be no dipeptide
formation or enantiomerization.
Now I'm going to test spectrophotometrically the resin substitution
after treatment with Pip/NMP.
Hope things have worked out, it's the first time I do a SPPS, always
worked in solution before.
I will let you know
Luca
----------------------------------------------------------------------------
-------------
16.10.98 (1)
Dear Luca,
You will surely find all relevant procedures in Stewart & Young or
Sheppard-Atherton's Practical Approach.
But I would like to warn you about serious possibility of Fmoc-pGluOAll
formation during strong activation of Glu side chain COOH (in the presence
of DMAP - in particular).
So check resin substitution (by UV-quantitation of Fmoc-Piperidine) on
aliquote before capping!
In your particular case I would prefer to perform side chain anchoring via
nucleophilic substitution using trityl chloride resin and DIEA salt of your
FmocGluOAll (only 10% excess over resin is needed). This type of anchoring
will also minimize loss of the pGluOAll in the course of second amino acid
coupling.
If you would like to try the latter approach you could purchase high
quality trityl chlotide resin at reasonable price from
PepChem Goldammer&Clausen,
Im Winkelrain 73, D-72076 Tuebingen
Tel +49 7071 600384 Fax +49 7071 600393
mobil +49 171 8477888
E-Mail: nils.clausen@uni-tuebingen.de
They will provide the reliable anchoring procedure as well.
(I am not working for PepChem !)
Best wishes,
Igor Rodionov
==================================================
Luca Corte message of 12.10.98
Hi there!
I'm about to set up a SPPS with Wang Resin. The first aa to link is Fmoc
GluOAll. Can anybody suggest me a procedure to perform this reaction
and a capping protocol to"turn off" the resin functional groups
which won't react with FmocGluOAll?
Thanks a lot
Luca
Luca Corte
Biostrands
Area Science Park
Padriciano 99
34201 Trieste
e-mail: Luca.Corte@com.area.trieste.it