Photolyses of 1-alkoxy-9-diazoanthrones 12 in benzene induce abstraction of hydrogen from the side chain, followed by cyclization (→ 15→ 16) or disproportionation (→ 17+ 18) of the intervening biradicals 20. In alcohols, reduction of triplet anthronylidenes (314→ 21→ 22) competes with the formation of 20, and intramolecular electron transfer of 20 leads eventually to the acetals 24.
