Photochemistry of 2,2‐Dimethyl‐2 H ‐Furo[3,4‐ b ]Pyran‐4,7(3 H , 5 H )‐Dione

Irradiation (λ > 340 nm) of the title compound 2a in t ‐BuOH affords 3,3,10,10‐tetramethyl‐2,8‐dioxatricyclo‐[4.3.2.0 1,6 ]undecane‐5,9‐dione (3) via photoextrusion of 2‐methylpropene and subsequent photocycloaddition of 2a to the alkene. The same regioisomer 3 is formed selectively and in much higher yields on irradiating 2a in the presence of excess 2‐methylpropene. Irradiation of 3 (λ = 300 nm) in t ‐BuOH in the presence of the same alkene gives a 6:1 mixture of spirooxetanes 7a and 7b but not α‐cleavage products. In 2‐propanol, 2a is photoreduced to a 2:1 mixture of diastereoisomeric hydrodimers 9 .