Synthesis of Optically Active Vasicinone Based on Intramolecular Aza-Wittig Reaction and Asymmetric Oxidation1

Both optical isomers of a quinazoline alkaloid, vasicinone, were synthesized by two different methods. The first method used (3S)-3-hydroxy-γ-lactam as a chiral synthon, which was, after O-TBDMS protection, o-azidobenzoylated followed by treatment with tri-n-butylphosphine to afford (S)-(−)-vasicinone via the tandem Staudinger/intramolecualr aza-Wittig reaction. The second method utilized asymmetric oxygenation of deoxyvasicinone with (1S)-(+)- or (1R)-(−)-(10-camphorsulfonyl)oxaziridine (the Davis reagent), respectively. The aza-enolate anion of deoxyvasicinone was treated with (S)-(+)-reagent to afford (R)-(+)-vasicinone in 71% ee, while the reaction with (R)-(−)-reagent gave (S)-(−)-vasicinone in 62% ee. The optical purity was analyzed by HPLC on specially modified cellulose as a stationary phase. These results provided a facile method to prepare both optical isomers of vasicinone and confirmed the recently reversed stereochemistry of natural (−)-vasicinone.