Photodimerization of dimethyl 3-dehydrogibberellenate and subsequent photoepimerization and cleavage reactions

Irradiation (λ > 300 nm) of dimethyl 3 - dehydrogibberellenate 1, as a heteroannular dienone, in t-BuOH (10^?2 M) for 2 hr led to cyclobutane-type photodimer A in 85% yield. However, irradiation of 1 under the same conditions but for 3 hr gave not only photodimer A but also two other photodimers B and C which were found to be the products of photoepimerization reaction of A. The stereochemistry of A, B and C was fully elucidated on the basis of physico-chemical techniques. At lower concentration (10^?4 M) of A in t-BuOH, irradiation with λ 254nm and λ 313 nm resulted in β-cleavage to 1.