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The reaction mechanisms for the cyclizations of N-methyl-2-(2-chloropyridin-3-yloxy)acetamide to 1-methylpyrido[3,2-b][1,4]oxazin-2-one and 1 -methyl-pyrido[2,3-b][1,4]oxazin-2-one were investigated using ab initio Hartree-Fock, second-order Moller-Plesset perturbation, single point coupled cluster with both single and double substitution, and density functional theory methods. The 5-membered spiro intermediate (2) is optimized from the cyclization of the acyclic reactants through the proton-transfer reaction, and this intermediate proceeds continuously to the 6-membered intermediate through either a stepwise or a concerted reaction. In the stepwise reaction, an N-bridge-type intermediate as a stable structure is optimized, whereas, in the concerted reaction, the O-bridge-type intermediate is not optimized.

Characteristics of the Intermediates in the Cyclization Reactions of Heterocycle-fused[1,4]oxazine Derivatives: Stepwise versus Concerted