Site-selective structural modification of toosendanin enables one-step synthesis of 12-hydroxyamoorastatin: A natural tautomeric antitumor lead with low toxicity.

Natural limonoid triterpenoids toosendanin (1) and 12-hydroxyamoorastatin (5) have been reported to possess broad-spectrum antitumor activity. However, development of antitumor drugs for the treatment of 1 has reached an impasse, due to its severe hepatotoxicity. Notably, compound 5 is a C-12 deacetylated product of 1, but scholars have studied and exploited 5 much less than 1 especially on antitumor activity and toxicity, of which the resource scarcity and structural ambiguity of 5 may be great hindrances. To address these concerns, a site-selective modification of 1 was developed, in which C-12 deacetylation with LiHMDS enabled the one-step synthesis of 5 from 1, solving its insufficient resources. We then revised the 5/7-tautomerization to C-12/C-29-tautomerization of HAR (5) by extensive NMR, LC-MS, DFT-calculations, and X-ray analyses, and determined the relative content of the tautomers in solution phase. Moreover, the biological evaluation of the synthesized derivatives prompted identification of 5 as a natural lead with better antitumor activities in vitro/in vivo. Namely, 5 exhibited antitumor activity through multiple mechanisms, including inhibition of DDR repair, down-regulation of the DNA damage stress-associated transcription factor HSF-1 and its downstream HSPs, promotion of synthetic lethality, disruption of the BAX/BCL2 homeostasis, and elicitation of cellular autophagy. Crucially, 5 achieved 62 % TGI (vs. 17 % for 1) in the JIMT-1 xenograft tumor models without exhibiting hepatotoxicity, providing a solid support to the development of natural limonoid antitumor lead.