Photoinduced molecular transformations. 100. Formation of furocoumarins and furochromones via a .beta.-scission of cyclobutanoxyl radicals generated from [2 + 2] photoadducts from 4-hydroxycoumarin and acyclic and cyclic alkenes. X-ray crystal structures of benzocyclopentacyclobutapyranone, cyclopentafurobenzopyranone, and benzocyclobutapyranone derivatives