Synthesis of a Quaternary N ,N′ -Aminal-Containing A-E Ring System of Palau′amine via an Enamide-Type Overman Rearrangement Reaction: Synthesis of a Quaternary N ,N′ -Aminal-Containing A-E Ring System of Palau′amine via an Enamide-Type Overman Rearrangement Reaction