Efficient synthesis of the azaspirocyclic core structure of halichlorine and pinnaic acid by intramolecular acylnitroso ene reaction.

[reaction: see text] A new efficient strategy for the construction of the 6-azaspiro[4.5]decane ring system was developed using intramolecular ene reaction of the acylnitroso compound. The spirocyclic ene product obtained as a single diastereomer was subsequently subjected to highly stereoselective ethynylation, leading to the azaspirodecane core of halichlorine and the pinnaic acids.