Rhodium-Catalyzed Intramolecular Cyclization of Naphthol- or Phenol-Linked 1,6-Enynes Through the Cleavage and Formation of sp~2 C-O Bonds

摘要

Recently, a number of π-electrophilic transition-metal-catalyzed intramolecular migratory cycloisomerization reactions, which involve sp~3 carbon-heteroatom bond cleavage, have been developed for the synthesis of five-membered hetero-cycles. In these reactions, copper, silver, gold, palladium, and platinum complexes were frequently employed because of their high catalytic activity for π-bond activation. For example, the transition-metal-catalyzed intramolecular cyclization of 2-alkynylphenyl ethers leading to benzofuran derivatives was reported (Scheme 1). In this reaction, the Jt-electrophilic transition-metal catalyst activates the alkyne π-bond, which generates an (alkenyl)metal intermediate bearing an allyl or benzyl cation fragment. This cation fragment shifts to the most nucleophilic β-position relative to the oxygen atom, which affords the benzofuran derivative and regenerates the transition-metal catalyst. Accordingly, this mode of cyclization is limited to the cleavage and formation of sp~3 C-O bonds by the generation of stabilized cation fragments (Scheme 1). Herein, we disclose the unprecedented transition-metal-catalyzed intramolecular cyclization of naphthol- or phenol-linked 1,6-enynes leading to vinyl-naphthofuran or vinylbenzofurans through the cleavage and formation of sp~2 C-O bonds.