Reaction of the 2-(l-alkenyl)-substituted four-membered cyclic nitrones ether). 'H-NMR 6: 8.85 (d, in, J = 4.9 HZ, H-31, 8. 18 (d, lH, J = 4.9 Hz, H-4), 7.7-6.7 (m, 9H,

Abstract. 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 fourmembered cyclic nitrones 7-11, whereas 4 and 5b yield the truns four-membered cyclic nitrones 12-13 upon reaction with 6. Nitroalkene 4 i reacts with 6c to give a 1: 1 mixture of the cis and tram 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-~etecarboxamide 1 -oxide (13k) was elucidated by means of X-ray analysis. Only from the reaction of I-nitrocyclopentene (5a) with 6c, 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-cyclopent [c]isoxazole-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.

Abstract. The four-membered cyclic nitrones (2,3-dihydroazete I-oxides) la-g isomerize to the corresponding oximes 3 and 4 on treatment with potassium tert-butoxide. In NAN-dimethylformamide at room temperature, oxime 4 is formed preferentially; in tetrahydrofuran, both the oximes 3 and 4 are formed. The oximes 3c-e are converted into the corresponding 6H-1,2-oxazin-6-ones 5c-e by reaction in acetic acid. The oxime 4a has the (E$) stereochemistry and the 0-benzoyl derivative of 3b ( 6 ) the (ZJ) stereochemistry, as shown by X-ray analysis. In solution, NOE difference spectroscopy indicates the ( Z J ) and ( E J ) configuration for 3a and 4a, respectively. The u,f!-unsaturated ketone (2)-8 was prepared by reaction of (Z$)-3b with bispyridine silver permanganate. Reaction of (2)-8 with hydroxylamine in pyridine yields (Z,E)-3b and the 6H-1,2-oxazin-6-one 5b, which is formed by rapid cyclization of (Z,Z)-3b.The conversion of nitrones 1 into the corresponding oximes 3 and 4 is explained in terms of an electrocyclic ring opening of an intermediate N-hydroxy-l,2-dihydroazete. In this electrocyclic reaction, a strong preference for outward rotation of the hydroxy group is observed.

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The four‐membered Cyclic nitrones (I), which are known or prepared according to known methods, isomerize to the corresponding oximes (II) and (III) on treatment with KOtBu.