Nitrone, Geometrical isomerization

Mechanistically, the nitrone cycloaddition is a [4jt -T 2jt]-type concerted process similar to the Diels-Alder reaction [lb]. One of the problems in nitrone cycloaddition compared to the Diels-Alder reaction is the possibiUty of geometrical isomerization of the nitrone moiety during the cycloaddition reaction. In particular, C-alkoxycarbonyl nitrones 1 having electron-withdrawing ester groups are known to exist as equilibrium mixtures of ( )-l and (Z)-l in solution even at room temperature (Eq. 6.2) [2]. 

We considered that the nitrone moiety of 1 would efficiently conjugate with the ester group, and hence the nitrogen-carbon double bond of 1 would have a partial single-bond namre, which would facilitate geometrical isomerization (Eq. 6.12). 

Iwano et al. reported the stereoselective isomerization of geometrically constrained oxaziridines to the corresponding nitrones by photosensitized electron transfer <1995CL67>. For example, irradiation of oxaziridine 10 in 4-methanol in the presence of 9,10-dicyanoanthracene (DCA) gave (i4)-nitrone 11 in 97% yield. 

For such compounds several possible isomeric and tautomeric forms can be considered. These include the (E) and (Z) geometrical isomers (a and b), as well as the tautomers nitrone (c), 5-hydroxy-isoxazolidine (d), and oxaziridine (e). Examination of the and C NMR spectra... 

---