Dear Enrico:
For review on diketopiperazines see:
Ovchinnikov, Yu.A.; Ivanov, V.T. In: The Proteins. (Neurath, H.; Hill,
R.L. Eds.), Academic Press: New York - London, Vol.5; 1982; 459-474.
We used the procedure below which is convenient but note that prolonged
heating cause racemization of DKPs. Procedure is based on: Suzuki, K.;
Sasaki, Y.; Endo, N.; Mihara, Y. Chem. Pharm. Bull. 1981, 29(1), 233-7 &
Ueda, T.; Saito, M.; Kato, T.; Izumia, N. Bull. Chem. Soc. Japan 1983,
56(2), 568-72.
DIKETOPIPERAZINES (DKPs)
Dipeptide trifluoroacetates TFA*H-Xaa1-Xaa2-OR (R=Me, Et, Bzl) were
dissolved in 0,1 M AcOH-s-BuOH (750 ml), and NMM (5,5 ml, 50 mmol) was
added. The resulting weakly acidic solution was refluxed for total 9
hrs. Usually, diketopiperazines began to crystallize out from the
solution on cooling already after 2 hour of heating. The reaction
mixture was cooled after each 2-3 hours of heating and precipitated DKP
was removed by filtration. Each crop of cyclic dipeptide was collected
on a filter separately, washed with methanol and ether and then
recrystallized from s-BuOH. Total yields of DKPs were in the range
50-80%.
Good luck.
Igor Rodionov
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