# Total Synthesis and Biological Evaluation of 22-Hydroxyacuminatine and the Related Natural Products Norketoyobyrine and Naucleficine

## TL;DR

This paper describes a new method to synthesize complex natural compounds with potential anticancer properties and tests their effectiveness against cancer cells.

## Contribution

A versatile synthetic method for introducing substituents on the E-ring of aromathecin compounds is developed and applied to synthesize natural products.

## Key findings

- The total synthesis of 22-hydroxyacuminatine, norketoyobyrine, and naucleficine was successfully achieved.
- Naucleficine and its intermediate showed moderate antiproliferative activity against HCT-116 cells with IC50 values of 55.58 and 41.40 μM.
- The synthetic method includes key steps like isoquinoline N-oxide formation and C–N bond construction via the Mitsunobu reaction.

## Abstract

Aromathecin compounds—which contain the same indolizine core structure as camptothecin-like compounds—are expected to show anticancer activity. Among them, 22-hydroxyacuminatine—which has a substituent on the E-ring of the pentacyclic scaffold—exhibits topoisomerase 1 inhibitory activity; therefore, the development of efficient methods for its synthesis has been actively pursued. Herein, we report a versatile synthetic methodology for introducing various substituents on the E-ring, leading to the total synthesis of 22-hydroxyacuminatine as a model compound of the aromathecin family. The synthesis comprises the following key steps: the synthesis of an isoquinoline N-oxide via the thermal cyclization of 2-alkynylbenzaldehyde oxime, the subsequent Reissert–Henze-type reaction to yield an isoquinolone, and the construction of the indolizine moiety (CD-ring) through C–N bond formation via the Mitsunobu reaction. Consequently, a pentacyclic benz[6,7]indolizino[1,2-b]quinolin-11(13H)-one framework is obtained. Using this methodology, the total synthesis of the natural products norketoyobyrine and naucleficine and an intermediate of the latter, which are indoloquinolizidine-type alkaloids, was achieved, and their antiproliferative activity against HCT-116 human colon cancer cells and HepG2 human liver cancer cells was assessed. Naucleficine and its intermediate exhibited moderate antiproliferative activity against HCT-116 cells, with IC50 values of 55.58 and 41.40 μM, respectively.