Synthesis of Dihydrobenzopyrans via Hydride Transfer Cyclization

Significance

Reported is a concise and new synthetic approach to highly substituted dihydro-benzopyrans 2 via hydride transfer cyclization of ortho-vinylaryl alkyl ethers 1 using catalytic or excess of Sc(OTf)₃. The cyclization occurs readily with cyclic and acyclic aliphatic ethers assuredly driven by the formation of a stable oxonium ion. However, with benzyl ether the reaction occurs in low yield and requires increased catalyst loading. In regard to the hydride acceptor component, R^² and R^³ = EWG, the best results are obtained with the diester or diketone substrates 1. Moreover, arene substitution affected the reactivity of the ­reaction, in particular when R⁴ = EDG is in C3 or C4 positions (at C3 the yields of products were poorer than at C4).