Synthetic and biological studies of phaeosphaerides.

The signal transducer and activator of transcription 3 (STAT3) has been validated as a suitable target for cancer therapy. Recent evidence by our group and others has shown that phaeosphaerides act as inhibitors of the STAT3 pathway. An efficient synthetic sequence to phaeosphaeride 1a has been previously disclosed. In this work, the first total synthesis of (±)-phaeosphaeride B (1d) and the unnatural phaeosphaeride 1b is reported. Additionally, the biological activities of 1a and 1b were investigated. (6S,7S,8S)-1a and (6R,7S,8S)-1b inhibited granulocyte colony-stimulating factor (GCSF)-stimulated phosphorylation of STAT1, STAT3, and STAT5 and IL-6-stimulated nuclear translocation of STAT3 alpha. In an SPR-based assay, (6S,7S,8S)-1a and (6R,7S,8S)-1b showed minimal ability to inhibit binding of STAT3 to its immobilized phosphotyrosylpeptide ligand (IC50 > 100 μM). Thus, (6S,7S,8S)-1a and (6R,7S,8S)-1b are likely upstream inhibitors of a kinase in the STAT signaling pathway and do not act through the inhibition of STAT3 dimerization by the blocking of the SH2 binding domain.