Amino esters, azomethine ylides additions

Microwave-induced 1,3-dipolar cycloadditions involving azomethine ylides have been widely reported in the literature. Bazureau showed that imidates derived from a-amino esters 120, as potential azomethine ylides, undergo 1,3-dipolar cyclo-additions with imino-alcohols 121 in the absence of solvent under microwave irradiation. This reaction leads to polyfunctionalized 4-yliden-2-imidazolin-5-ones 122 (Scheme 9.36) [87]. 

One problem in the anti-selective Michael additions of A-metalated azomethine ylides is ready epimerization after the stereoselective carbon-carbon bond formation. The use of the camphor imines of ot-amino esters should work effectively because camphor is a readily available bulky chiral ketone. With the camphor auxiliary, high asymmetric induction as well as complete inhibition of the undesired epimerization is expected. The lithium enolates derived from the camphor imines of ot-amino esters have been used by McIntosh s group for asymmetric alkylations (106-109). Their Michael additions to some a, p-unsaturated carbonyl compounds have now been examined, but no diastereoselectivity has been observed (108). It is also known that the A-pinanylidene-substituted a-amino esters function as excellent Michael donors in asymmetric Michael additions (110). Lithiation of the camphor... 

The employment of bifunctional urea-tertiary amine 104 or monothiourea 105 catalysts selectively promotes either the Michael addition or cycloaddition process, respectively, [60]. As depicted in Scheme 2.30, the tertiary amine group would activate the in situ formed a-amino esters to produce azomethine ylides A (or enolates), whereas the nitroalkene counterparts would be activated by the thiourea (urea) moiety through a double H bonding interaction (B). 

Cycloaddition reactions have a great fascination for many synthetic chemists, as they can produce molecules with considerable stereo- and regio-specificity. Grigg has published extensively over the last few years on the addition of azomethine ylides to activated double bonds, and previously suggested that these additions were stereospecific. A paper from his group now, however, has modified this view, and the work has shown that the extent of stereo- and regio-control varies with the substituents on the or-amino-ester imine, and on the reactivity of the dipolarophile. 

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