Irradiation of &methyl spiro [2.5] oct-7-en-4-one through its singlet excited
state resulted in the formation of 4-methyl spirO [2.5] oct-4-en -6-one. We have
used the cyclopropyl moiety in the molecule as a probe to study the operating
mechanism in the photochemistry of ?, ?-unsaturated compounds. The results
showed that this photoisomerization probably occurs through a concerted