A general synthesis of dihydrobenzofurans by intramolecular conjugate addition

ortho Iodophenoxy crotonates 3e–7e cyclize to dihydrobenzo[b]furans 1 and 9–12 when subjected to lithium–iodine exchange at −100 °C. Naphtho[2,1-b]furan 13 can be similarly obtained from α-iodonaphthoxy crotonate 8e. Methylation of this reaction mixture insitu provides a single diastereomer 14; its relative configuration is assigned by 1 H nuclear magnetic resonance, and its formation rationalized by steric arguments. Diastereoselective deuteration is also observed in this case.