Stereoselective Phosphine-Catalyzed Synthesis of Highly Functionalized Diquinanes

Two rings to rule them all: A versatile method has been developed for the room‐temperature synthesis of diquinanes from acyclic precursors, thereby generating two rings, three stereocenters, and a double bond with high selectivity. The products of the double cyclization can be derivatized with excellent diastereoselection into an array of highly functionalized compounds. [reaction image] In 2003, Tomika and co‐workers reported an intriguing PnBu3‐catalyzed diastereoselective cyclization of certain yne‐diones to form bicyclic furanones with two new stereocenters (Figure 1).1 They proposed that conjugate addition of the phosphine to the alkyne is followed by tautomerization, which furnishes zwitterionic enolate A. Next, an intramolecular aldol reaction provides B, and then a second conjugate addition generates bicycle C (the conversion of A into C by a concerted cycloaddition may also be considered). Tautomerization and then elimination of the phosphine affords the bicyclic furanone. The investigation by Tomita et al. focused mainly on symmetrical substrates (R1=-C≡CR), although they did report reactions of two unsymmetrical yne‐diones which cyclized in relatively modest yield (41–50 %).