I2–PPh3 mediated spiroannulation of unsaturated β-dicarbonyl compounds. The first synthesis of (±)-negundoin A

An efficient and stereoselective spiroannulation of unsaturated enols is reported. Unsaturated β-dicarbonyl compounds undergo cyclization by reaction with catalytic I2–PPh3, affording the corresponding spiro enol ether derivatives, with complete regio- and stereoselectivity, under mild conditions. Utilizing this new methodology, the first total synthesis of the anti-inflammatory diterpene negundoin A and a naturally occurring trypanocidal aldehyde is reported.