Cyclic Tetrapeptide Natural Product Asperterrestide A: Total Synthesis and Structural Revision

We have studied for the total synthesis of asperterrestide A, which is a cyclic tetrapeptide natural product reported to be composed of D-alanine, anthranilic acid,(2R,3S)-3-hydroxy-N-methylphenylalanine, and D-isoleucine or D-allo-isoleucine. We initially performed the total synthesis of the reported structure of asperterrestide A via solid-phase peptide synthesis and macrocyclization in solution. However, the spectral data of the synthetic(9R,25R)-derivative did not match those of the natural product. Then, we considered the true structure should be the (9S,25S)-stereoisomer because all NOE correlations predicted from its three-dimensional structures obtained by theoretical calculations were consistent with those observed in the natural product. Thus, the(9S,25S)-stereoisomer was synthesized through solution-phase synthesis because N-acylation of a polymer-supported(2S,3S)-3-hydroxy-N-methylphenylalanine derivative resulted in low yield. The(9S,25S)-stereoisomer was successfully synthesized and was found to be identical to the natural product. Therefore, the stereochemical configuration of asperterrestide A was corrected from(9R,25R)to(9S,25S). We also tested whether the DFT calculation of carbon chemical shifts can discriminate stereoisomers of cyclic tetrapeptides. After conformational analysis of four possible stereoisomers of asperterrestide A, Boltzmann-weighted carbon chemical shifts based on DFT calculations using Spartan'20 revealed a DP4 accuracy of 96%, which is sufficient to determine the stereochemical configuration among the stereoisomers. In addition, three 9-deoxy derivatives were synthesized and the cell growth inhibitory activity of the five synthetic compounds against human cancer cells(U937, MOLT-4, and A549)was evaluated. The results showed that the(9S,25S)-stereoisomer is as potent as asperterrestide A, whereas the(9R,25R)derivative is not. The(9S,25S)-3-deoxy analogs were found to be equipotent to the natural product although the activity of the(9S,25R)-deoxy analog was lost.