Synthesis with 1,2-Oxazines. 3. Reactions of a-Chloro Aldonitrones with Enol Ethers: A Synthetic Route to Medium-Ring Lactones

Cyclic enol ethers can undergo a Ag+induced cycloaddition with a-chloro nitrones. The corresponding polycy-clic adducts were converted to enamoid structures of type 17b via the immonium tetraphenylborate salts. The existence of an intramolecular ketal and the N-alkyl-5,6-dihydro-2H-l,2-oxazine ring as moieties in 17b and 21a-c allowed a thermolysis to the 10-12-membered lactones through cleavage of a central C-C bond in the polycyclic system. Structural effects on the thermolysis have been noted.