Diene 2 could be formed from lactone 3 by an aldol
reaction with aldehyde 4 and subsequent oxidation. The spiroquaternary
center could be constructed by Claisen rearrangement
of allyl alcohol 5 obtained by aldol condensation between
bicyclic enone 6 and the corresponding glycolaldehyde
derivative. Further, bicyclic enone 6 could be accessed via the
Diels−Alder reaction of methylacrolein and diene