Oxidative Prins and Prins/FriedelCrafts cyclizations for the stereoselective synthesis of dioxabicycles and hexahydro-1H-benzo[f]isochromenes via the benzylic CH activationElectronic supplementary information (ESI) available: Preparation of starting materials and copies of 1H and 13C NMR spectrum of products.. CCDC reference number 833511. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ob06489d

1-Benzyl ethers of ( E )- and ( Z )-hex-3-en-1,6-diols and hept-3-en-1,7-diols undergo a smooth oxidative cyclization with DDQ in the presence of In(OTf) 3 through a sequential CH bond activation and an intramolecular Prins cyclization to afford the corresponding trans - and cis -fused hexahydro-2 H -furo[3,2- c ]pyrans and octahydropyrano[4,3- b ]pyrans respectively in good yields with an excellent stereoselectivity. Aryl tethered homoallylbenzyl ethers such as benzyl ethers of ( E )- and ( Z )-6-arylhex-3-enyl alcohols undergo a tandem Prins/FriedelCrafts cyclization in the presence of stoichiometric amounts of DDQ and SnCl 4 via the benzylic CH bond activation to furnish the corresponding trans - and cis -fused hexahydro-1 H -benzo[ f ]isochromenes in good yields with complete stereoselectivity. A novel and versatile method for the stereoselective synthesis of bicyclic and tricyclic tetrahydropyran derivatives through a sequential benzylic CH bond activation and an intramolecular Prins cyclization is described.