1.3- Dipoles azomethine ylids

The ambivalence of an unsubstituted azomethine ylid is generally accepted as a fundamental property of 1,3-dipoles, though it had never been shown by any experimental work until reports affording clear evidence were published.411-413 Both employed the silyl procedure, and precursors in which one carbon a- to nitrogen was distinguished from the other. 

Finally, enantiomerically pure sulfinimines have also been used as precursors of chiral imidazolidines by 1,3-dipolar cycloaddition with azomethine ylids [181]. Reactions of different arylsulfinimines 245 with dipoles 246 are highly stereoselective, mainly affording diastereoisomer 247 (absolute configuration unequivocally established by X-ray studies), which was readily transformed into vicinal diamine 248 (Scheme 111). 

In contrast, other reactions are endo selective, as in the cycloaddition 6.354 of an azomethine ylid to dimethyl maleate giving largely (80 20) the endo adduct endo-6.355.869,870 Thus the stereoselectivity depends in a not always predictable way upon the dipole, the dipolarophile and their substituents, in contrast to Diels-Alder reactions, which more often than not obey the endo rule. It is advisable, when planning a synthesis, to look up close analogies before relying upon the exo or endo stereoselectivity of a 1,3-dipolar cycloaddition. 

Azomethine ylids are useful dipoles in the synthesis of pyrroline-containing compounds. Table 11.22 indicated electron-deficient alkenes and simple alkenes react fastest (in that order), but the orbital coefficients... 

Figure D.7 illustrates the application of the rules to delocalized systems A8a-1, A8b-1, A8c-1, A9a-1, A9b-1, and A9c-1. In particular, 1,3-dipoles are prominent members of the tricentric cases. Azomethine ylids, azomethine imines, nitrones, carbonyl ylids (e.g. A8c), carbonyl imines, carbonyl oxides fit prototype A8 bent nitrile ylids, may also be treated as A8. The linear 1,3-dipoles are treated as a triply-bonded systems with two mutually orthogonal sets of paired faces.

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