Addition of benzyl ethers to alkynes: a metal-free synthesis of 1-isochromenes

Bromotrimethylsilane (TMSBr)-promoted intramolecular cyclization of ( o -arylethynyl)benzyl ethers to form 1 H -isochromenes at room temperature is reported. Further studies indicated that vinyl carbocations are the reaction intermediates which are stabilized by the conjugated aryl groups. Thus, O -addition of benzyl ethers/tetrahydropyrans to alkynes was achieved under metal-free, acidic conditions. These reaction conditions were compatible with an alkynyl Prins reaction; therefore, 1 H -isochromenes were produced directly from alkynyl benzaldehydes and alkynyl alcohols using a one-pot procedure. The synthesis of 1 H -isochromenes was achieved by addition of benzyl ethers to alkynes, activated with bromotrimethylsilane (TMSBr).