Photolysis of cyclobutanones in the presence of acetic acid produced 2-acetoxy-5-alkoxytetrahydrofurans regiospecifically and with retention of configuration at the migrating α-position. The acetoxy
group was replaced by nucleophiles such as allylsilane, trimethylsilyl azide, diethylaluminum
cyanide, and silylated thymine in the presence of Lewis acids.