Cyclopentenone 2 + 2 cycloreversion

Furthermore this same central bond in enone + alkene photocycloadducts can be cleaved by thermally induced cycloreversion (Sch. 17). The rearrangement of tricycle 63 - obtained from 3,5,5-trimethylcyclohex-2-enone (64) and methyl cyclobut-l-ene-l-carboxylate - to give the /ra .v-decalin 65 proceeds via a sequence wherein the cycloreversion is followed by a transannular ene-reaction [71]. The photocycloisomer 66 of cyclopentenone 67 rearranges further to 68 containing a 5,8,5-sesquiterpene core [72]. 

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