Oxidant-free, three-component synthesis of 7-amino-6-benzo[]chromen-6-ones under green conditions.

An oxidant-free three-component synthesis of biologically significant 7-amino-6-benzo[]chromen-6-ones was established involving a Sc(OTf) catalyzed three-component reaction between primary amines, β-ketoesters and 2-hydroxychalcones under green conditions. In this strategy, both the B and C rings of 6-benzo[]chromen-6-ones were constructed simultaneously starting from acyclic precursors by generating four new bonds including two C-C, one C-N and one C-O in a single synthetic operation. The mechanism of this sequential cascade process involves the initial formation of a β-enaminone intermediate followed by Michael addition with 2-hydroxychalcone, intramolecular cyclization, dehydration, lactonization and aromatization steps. Unlike the related literature approaches, this reaction delivered the products without the addition of any external oxidants to achieve the key aromatization step.