Synthesis, structural aspects and cytotoxicity of the natural cyclopeptides yunnanins A, C and phakellistatins 1, 10

Yunnanins A and C, two cyclic heptapeptides occurring in the roots of Stellaria yunnanensis, and phakellistatins 1 and 10, a hepta- and an octacyclopeptide first isolated from marine sponges of the genus Phakellia, were efficiently synthesized using a combination of solid and solution-phase techniques. Structural analysis on the synthetic members of the yunnanin series showed that the synthetic sample of yunnanin A exhibited a configurational pattern at the Pro peptide linkages identical to the natural product (trans-Pro³, trans-Pro⁵), while yunnanin C was obtained as a complex mixture of geometric/conformational isomers; the major isomer (trans-Pro³) was indistinguishable from the natural cyclopeptide and co-occurred along with lower amounts of a mixture (1:1 ratio) of two different rotamers, both displaying cis geometry at the Pro³ linkage. In the phakellistatin series, the synthetic phakellistatin 1 (determined as cis-Pro¹, cis-Pro³, cis-Pro⁵) was identical to the natural one, while two different isomeric products of phakellistatin 10 could be obtained: a major one (trans-Pro¹, trans-Pro⁴, trans-Pro⁶) showing spectral properties superimposable with the natural metabolite, and a minor geometric isomer of the natural cyclopeptide. Interestingly, the synthetic cyclopeptides, although found to be chemically identical with their natural counterparts, did not display the same biological properties (in vitro cytotoxicity against a panel of cancer cell lines), leaving presently open the question whether or not the potent bioactivity reported in the literature should really be attributed to these natural cyclic peptides.