Synthesis of oligosubstituted bullvalones: shapeshifting molecules under basic conditions.

Oligosubstituted bullvalones were synthesized in eight steps from 2,6-cycloheptadienone via a unique Lewis acid catalyzed intramolecular cyclopropanation of a stabilized sulfur ylide, leading directly to the tetracyclic cage structure. Upon exposure to base, the substituted bullvalones tautomerized to a hydroxybullvalene, which underwent dynamic interconversion into numerous, structurally distinct bullvalones. This system represents a "self-contained" dynamic combinatorial library in which the dynamic nature can be modulated by external reagents.