Total synthesis and conformational studies of ceratospongamide, a bioactive cyclic heptapeptide from marine origin

The first total synthesis of cis,cis-ceratospongamide (cyclo[l-Pro-l-Ile-Me-oxazoline-l-Phe-l-Pro-thiazole-l-Phe-]) was accomplished and confirmed by X-ray crystal analysis. The heating of cis,cis-ceratospongamide in DMSO converted it not to the trans,trans isomer but to the trans,trans-[d-allo-Ile]-ceratospongamide, which was confirmed by total synthesis. Its solution conformation was constructed by the dynamic simulated annealing method using ROE cross peaks, revealing a rounded and flat ring structure which is in contrast with the slim and tight structure of cis,cis isomer. The results shows that the trans,trans-[d-allo-Ile] isomer is the main thermal product of cis,cis-ceratospongamide.