Abstract The role of structure in the reaction between enamines and nitroolefins has been evaluated. A synthetic method is presented for preparing substituted cyclic nitronic esters incorporating a 6-membered ring. Reaction of cyclopentanone, cyclohexanone and cycloheptanone enamines derived from pyrrolidine and morpholine with 2-nitro-1-phenylpropene, 2-nitro-1-phenyl-1-butene and α-nitrostilbene in hexane solvent at 0–25° led to 3-methyl-, 3-ethyl-, or 3-phenyl-4-phenyl-5,6-polymethylene-5,6-dihydro-4H-1,2-oxazine 2-oxides 9a-j (31–94% yield). Spectral data support the assigned structures and establish stereochemistry. The reaction is limited to α-substituted nitroolefins and enamines derived from cycloalkaxones having fewer than eight ring members. Enamines derived from aldehydes and acyclic ketones failed to produce nitronic esters. Alkaline hydrolysis of nitronic esters 9a-j by treatment with aqueous ethanolic potassium hydroxide, followed by acidification with acetic arid, leads to products whose structure is determined by the nature of substituents and ring size of the 5,6-polymethylene group. Esters 9d and 9e derived from cyclohexanone gave 8a-hydroxy-3-methyl-4-phenyl-4a,5,6,7,8,8a-hexahydro-4 H -1,2-benzoxazine 2-oxide ( 4 ), found to be identical with the product of Michael addition of cyclohexanone to 2-nitro-1-phenylpropene. Ester 9h derived from cycloheptanone gave the Nef product, 2-(1-phenyl-acetonyl)cycloheptanone ( 19 ). Esters 9g, j having 3,4-diphenyl substituents produced 3,4,5-triphenylisoxazole. The results of the present investigation provide additional examples of nucleophilic addition of nitronate oxygen to carbon-hetero atom double bonds; addition occurs intramolecularly on carbon of an iminium ion in the zwitterion intermediate 6 .