Synthesis of 2H-Benzopyran via Electrophilic Cyclization

Significance

A general synthesis of benzopyrans from propargylic aryl ethers triggered by electrophilic cyclization using inexpensive electrophiles such as I₂, ICl and PhSeBr is reported. The reactions proceed under mild conditions tolerating a small range of substituents in both coupling partners. Starting propargylic ethers are conveniently available from phenols; the product benzopyrans incorporate a handle, R⁵ = I for further chemistry. The cyclization undoubtedly proceeds by the formation of iodonium or selenonium intermediates followed by electrophilic attack from the electron-rich aromatic ring. Interestingly, the cyclization occurs in a 6-endo-dig mode.