Diastereoselective synthesis of 1-aza-spiro[5.5]undecane skeleton

The synthesis of various bicyclic enamines (5), bearing a side chain with a multiple C-C bond or a furan ring, was performed from racemic pipecolic ac id through sulfone (Ic). Cyclization of these derivatives (5), via the corr esponding in situ generated N-acyliminium ions, was studied in various acid ic conditions to afford, in some cases, spiro intermediates (2) which posse ss the backbone of the histrionicotoxin family of alkaloids.