Catalytic Enantioselective Synthesis of Guvacine Derivatives through [4 + 2] Annulations of Imines with α-Methylallenoates.

P-Chiral [2.2.1] bicyclic phosphines (HypPhos catalysts) have been applied to reactions between α-alkylallenoates and imines, producing guvacine derivatives. These HypPhos catalysts were assembled from trans-4-hydroxyproline, with the modular nature of the synthesis allowing variations of the exocyclic P and N substituents. Among them, exo-( p-anisyl)-HypPhos was most efficacious for [4 + 2] annulations between ethyl α-methylallenoate and imines. Through this method, ( R)-aplexone was identified as being responsible for the decrease in the cellular levels of cholesterol.