Although the concerted photochemical [2+2] cycloaddition is allowed, the reaction between enones and alkenes is stepwise and involves discrete diradical intermediates.
[2] Subsequent investigations demonstrated the utility of the photochemical [2+2] cycloaddition of enones to alkenes, requiring only "sunlight in California for 6.5 months".
At this point, the enone forms an exciplex with the ground state alkene, eventually giving the triplet diradical.
In their excited state, the polarity of enones is reversed so that the β carbon possesses a partial negative charge.
In the transition state for the first bond formation, the alkene tends to align itself so that the negative end of its dipole points away from the β carbon of the enone.
This energy-wasting process competes with cycloaddition[8] and is evident in reactions that yield mixtures of cis- and trans-fused products.
When the tether between the enone and alkene is two atoms long, bent products predominate due to the rapid formation of five-membered rings.
[13] The Favorskii rearrangement established the carbon skeleton of cubane, and further synthetic manipulations provided the desired unfunctionalized target.