Irradiation of methyl 9-anthroate through a Uranium filter gave the 1,
4-10',9' and 1,4-9',10' cyclodimers as well as the 9,10-10',9' cyclodi
mer; this is the first example of intermolecular photodimerization inv
olving the 1, 4, 9', and 10' positions of meso-substituted anthracenes
. These dissymmetrical cyclodimers were found to be thermally more lab
ile than the 9,10-10',9' cyclodimer.