With this scalable synthesis in hand, we next turned to determining the mechanism of action (MoA) of PA . First, we sought to identify sites that could accommodate a "fully functionalized" retrieval tag to facilitate chemical proteomic target identification while not interfering with the biological activity of the parent structure . Here, an evaluation of portimine A analogs with modifications at multiple sites (Fig. ) in Jurkat (human T lymphocyte) and HCC1806 (human breast cancer) cells confirmed that fully-synthetic PB (2) showed significantly less toxicity in both cancer cell lines compared to PA (1) (7), as did epi-portimine A (ePA, 38), the C-5 epimer of PA . Taken together, these results suggests that the anti-proliferative activity of PA (1) is dependent on the stereochemistry of C-5, and that ePA could be employed as an inactive control compound for further studies. We also noted that phenyl-derivative Ph-PA showed similar activity to PA (1) in both cell lines, indicating the terminal vinyl group (C-21/22) is not essential for the observed biological activity. Based on these results, we pursued the assembly of a diazirine-alkyne (DA) containing photoaffinity tag at the C-18 position of PA (1) through Suzuki coupling with the key intermediate (compound 21, for detailed synthesis steps, see SI). Encouragingly, we observed both the epimeric photoaffinity probes PA-DA (35-2) and ePA-DA (35-1) retained identical activities as their parent analogs, suggesting that they could serve as target ID tools (Fig. ).