An Ammonia-Triggered Stereocontrolled Conversion of α γ-Lactone to the Central Tetrahydropyran of Pederin, Psymberin, and Onnamides D-F

The onnamides, pederin, and psymberin have each attracted attention because of their potent biological activity and interesting structural features. A short (eight steps) and efficient stereocontrolled route to the central tetrahydropyran ring in these natural products has been developed from (S)-malic acid. The key sequence involves the conversion of a ?-lactone to a tetrahydropyran structure triggered by the addition of ammonia.