Jeremic T, Linden A, Heimgartner H. New 21- and 24-atom Aib-containing cyclopeptides.
J Pept Sci 2008;
14:1051-61. [PMID:
18523966 DOI:
10.1002/psc.1041]
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Abstract
The synthesis of two Aib-containing cyclic octapeptides, cyclo(Leu-Aib-Phe-Aib-Gly-Aib-Phe-Aib) (1) and cyclo(Gly-Aib-Aib-Val-Aib-Leu-Aib-Phe) (2), and a cyclic heptapeptide cyclo(Gly-Aib-Val-Aib-Leu-Aib-Phe) (3), is described. The linear precursors of 1-3 were prepared using solution-phase techniques, and the cyclisation was also accomplished in solution. Among the coupling reagents examined in the final macrolactamisation step, PyAOP, HATU and DEPC/DEPBT efficiently yielded cyclised products. However, the success of the cyclisation was found to be dependent on the coupling reagent used. The two octapeptides 1 and 2 were obtained in much better yields (up to 63%) than the cycloheptapeptide 3 (30-37%). In addition, crystal-state conformational analysis of 2 was performed by X-ray crystallography. Six intramolecular hydrogen bonds stabilise the structure characterised by two consecutive type II'/I beta-turns, two consecutive type II/III' beta-turns, one gamma-turn, and one inverse gamma-turn.
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