Dear Krystyna,
Several years ago we have synthesized a set of heptapeptides with
N-methyl amino acids in different points of peptide chain using
BOC/Bzl chemistry. The coupling conditions were standard in accordance
with the literature - BOC-AA + DIC/HOBt, 3 eq in DMF. For the on-line
control of condensation progress the solution of bromophenol blue in DMF
seems to be very useful (or the addition of its DMF solution to the
suspension
of sample in DCM). We havenít observed any diketopiperazine formation in
our case (MBHA resin, ...Lys-N-Me-D-Leu, or ...N-Me-Lys-D-Leu), but have
seen the great difference in coupling efficiency depending on the position
of N-methyl amino acid (and its nature). Sometimes we should repeat
condensation four times. Sure, I mean the coupling of next amino acid (as
for the addition of N-methyl one - there is no any difficulties). Another
interesting thing we have found was the unusual cleavage of peptide bond in
one peptide, containing N-methyl amino acid in the course of TFMSA cleavage
and deprotection.
Hope it may be of some help.
With best regards,
Sergey Burov
Group of peptide and polymer
microspheres synthesis, IMC RAS
199004, Bolshoi pr. 31, St.-Petersburg, Russia
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