Diastereocontrol in [4+4]-photocycloadditions of pyran-2-ones: effect of ring substituents and chiral ketal

4-Mesyloxypyran-2-ones joined to furan by a three-carbon linker undergo intramolecular-crossed [4+4]-photocycloaddition with high or complete selectivity for the exo cycloadcluct. When a C-2-symmetric ketal was present on the tether adjacent to the pyranone ring, moderate levels of asymmetric induction were obtained. (C) 2009 Elsevier Ltd. All rights reserved.