A Facile Phenol-Driven Intramolecular Diastereoselective Thermal/Base-Catalyzed Dipolar [2 + 2] Annulation Reactions: An Easy Access to Complex Bioactive Natural and Unnatural Benzopyran Congeners

The complex bioactive natural and unnatural benzopyran congeners have been synthesized using one-/two-step approaches in very good yields from the reactions of two different dihydroxyphthalides, natural
resorcyclic acid derivative, and trihydroxybenzophenone with citral and/or farnesal, via the phenol-driven
intramolecular diastereoselective thermal/base-catalyzed dipolar [2 + 2] cycloaddition reactions and three
different thermal intramolecular cyclization reactions. The effects of the nature and the position of phenolic
groups in the starting materials on the course of these cycloaddition reactions have also been described.
Depending upon the absence or presence of intramolecular hydrogen bonding of the phenolic group with
the carbonyl moiety in the starting materials, these phenol-driven intramolecular thermal/base-catalyzed
dipolar [2 + 2] cycloaddition reactions either furnished the kinetically controlled products or directly
formed the thermodynamically controlled rearranged products, respectively.