The double [3 + 2] photocycloaddition reaction.

A remarkable double [3 + 2] photocycloaddition reaction that results in the formation of fenestrane 2 from aromatic acetal 1 is reported. During the formation of 2, four carbon-carbon bonds, five new rings, and seven new stereocenters are created in a one-pot process. The reaction occurs in a sequential manner from the linear meta photocycloadduct 3, while the angular meta photocycloadduct 4 undergoes an alternative fragmentation-translocation photoreaction to afford angular tricycle 6.