Rh-catalyzed isomerization and intramolecular redox reaction of alkynyl ethers affording dihydropyrans and ketoolefins

Abstract

When arylmethyl 4-(sulfonyl)-3-butynyl ether and a catalytic amount of Rh(2)(tfa)(4) (tfa = CF(3)CO(2)-) were heated in toluene, 2-aryl-3-(sulfonyl)-5,6-dihydro-2H-pyran was produced in good yield. Ring closure proceeded in a highly regioselective manner, and no isomeric five-membered product was detected. Alternatively, 2-(5-sulfonyl-4-pentynyl)tetrahydrofuran lacking a benzyl substituent afforded a different product, 9-sulfonyl-1-hydroxy-8-nonen-4-one, upon reaction with the same rhodium catalyst at reflux in wet toluene. Both reactions most likely involve cleavage of the C-H bond alpha to ether in the substrate and subsequent intramolecular transfer of the hydrogen atom to sulfonylacetylene, which is supported by experiments using deuterium-labeled starting materials.