Natural product protulactone A: Total synthesis from D-galactose, X-ray analysis and biological evaluation

Synthesis of protulactone A (PLA, 1) and twelve of its analogues have been achieved starting from d-galactose. PLA was isolated in the crystalline state, and its crystal structure was determined utilizing X-ray crystallography, which confirmed the assumed stereochemistry at all stereocenters. All tested compounds displayed antiproliferative activity against a panel of tumour cell lines, and all of them were non-cytotoxic toward the normal cells (MRC-5). Natural product PLA (1) was the most active against the K562 and MCF-7 cell lines (IC₅₀ 6.52 and 2.20 μM, respectively). Some of the synthesized derivatives showed very potent cytotoxicity, especially analogues 11, 13 and 15 (IC₅₀ 1.08-1.14 μM against MCF-7), and 9 and 14 (IC₅₀ 1.29 and 1.64 μM against K562). SAR analysis indicated important structural motifs for antiproliferative activity. Unfortunately, PLA (1), its C-7 epimer (2) and demethylated analogue (3) did not display a significant antimicrobial activity (two Gram-positive and two Gram-negative bacteria and one fungal strain) and they also cannot affect the ability to modulate bacterial communication.