Stereoselective synthesis of cis and trans four-membered cyclic nitrones

Article abstract of DOI:10.1002/recl.19881070202

Nitroalkenes 3-5 react with ynamines (1-aminoacetylenes) 6 to yield four-membered cyclic nitrones (2,3-dihydroazete 1-oxides) 7-13.The nitroalkenes 3 and 5c give the cis four-membered cyclic nitrones 7-11, whereas 4 and 5b yield the trans four-membered cyclic nitrones 12-13 upon reaction with 6.Nitroalkene 4i reacts with 6c to give a 1:1 mixture of the cis and trans four-membered cyclic nitrones 9g and 13i.The trans stereochemistry of trans-N,N-diethyl-2,3-dihydro-3-(2-methoxynaphthalenyl)-2-methyl-4-phenyl-2-azetecarboxamide 1-oxide (13k) was elucidated by means of X-ray analysis.Only from the reaction of 1-nitrocyclopentene (5a) with 6g, the initially formed (4+2) cycloadduct, the nitronic ester 17 has been isolated.The thermal ring contraction of 17 yields 3a,4,5,6-tetrahydro-N,N-dimethyl-3-phenyl-3H-cyclopentisoxazole-3-carboxamide (19), the structure of which was established by X-ray analysis.The trans four-membered cyclic nitrones are thermally relatively stable compared with the cis nitrones.The mechanism of the stereoselective formation of the nitrones is related to the conformation of the (4+2) cycloadduct 16, which could be correlated with Chem-X and MNDO calculations.