Three-Component Tandem Knoevenagel/Hetero Diels−Alder Reactions − Total Synthesis of (±)-Preethulia Coumarin

Starting from 4-hydroxy-5-methylcoumarin (7), (±)-preethulia coumarin (3) was synthesized in six steps and in 4% overall yield. The key synthetic procedure was a new type of Lewis acid catalysed, three-component Knoevenagel/hetero Diels−Alder reaction, which employs α-dicarbonyl compounds to generate chromandiones, and vinyl ethers to trap them. Some unexpected rearrangements were observed during the manipulation of various acetalic Diels−Alder adducts, resulting in the serendipitous assembly of bridgehead and fused tetracyclic coumarin assemblies.