Chemistry of four‐membered cyclic nitrones. Reaction with a non‐nucleophilic base: Stereoselective ring opening of in situ generated 1,2‐dihydroazetes and structure elucidation of the resulting α,β‐unsaturated oximes by X‐ray analysis

The four‐membered cyclic nitrones (2,3‐dihydroazete 1‐oxides) 1a‐g isomerize to the corresponding oximes 3 and 4 on treatment with potassium tert ‐butoxide. In N,N ‐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 6 H ‐1,2‐oxazin‐6‐ones 5c‐e by reaction in acetic acid. The oxime 4a has the ( E,E ) stereochemistry and the O ‐benzoyl derivative of 3b (6) the ( Z,E ) stereochemistry, as shown by X‐ray analysis. In solution, NOE difference spectroscopy indicates the ( Z,E ) and ( E,E ) configuration for 3a and 4a , respectively. The α,β‐unsaturated ketone ( Z )‐ 8 was prepared by reaction of ( Z,E )‐ 3b with bispyridine silver permanganate. Reaction of ( Z )‐ 8 with hydroxylamine in pyridine yields ( Z,E )‐ 3b and the 6 H ‐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‐1,2‐dihydroazete. In this electrocyclic reaction, a strong preference for outward rotation of the hydroxy group is observed.