Acylimine/acyliminium precursors

Scheme 1 Carbonylamino sulfones as acylimine/acyliminium precursors...

Many of these reactions occur with formation of V-acyliminium ions 2 as intermediates (equation 1), which give the enamides after elimination of the electrofuge E+ from / -carbon atom, or the V-acylimines by removal of R4 group from nitrogen atom. In that way, the V-acyliminium ions 2 appear not only as the intermediates in acid-catalyzed conversions of enamides3 but also as direct precursors of the latter. In other words, the chemistry of enamides and their V-acylimine tautomers is closely connected with V-acyliminium chemistry, a topic which has been reviewed comprehens-ively3,5-7,27-31. 

The a-amidoalkylation reaction, which involves the addition of carbon nucleophiles (primarily aromatic rings, alkenes, cyanides, isocyanides, alkynes, organometallic and active methylene-containing compounds) to substituted amides (89) where X is a leaving group (Scheme 17), has been extensively re-viewed. A variety of organometallic condensations have since been reported which extend the scope of this reaction. The reactive intermediates in these reactions are considered to be N-acylimines (88) or N-acyliminium salts (SKI). Direct displacement of the a-haloalkylamide precursors (89 X = halogen) cannot in certain cases be discounted. 

Since the seminal paper of Speckamp and Hiemstra [390], N-acyliminium ion species emerged as being among the most versatile of intermediates in the creation of new carbon-carbon bonds. The broad spectrum of reactivity of N-acylimin-ium ions toward different carbon nucleophiles, as well as their ability to participate as both dienophiles and electron-deficient heterodienes in cycloaddition reactions, has made them attractive potential solid-phase intermediates, allowing the generation of chemical diversity starting from a common precursor. 

A remarkable observation is the cyclization of biscarbamate (194) to y-lactone (195) mediated by methanesulfonic acid (equation 133). No trace of the expected 8-lactone is formed, presumably because of the unfavorable ester geometry in the transition state required for six-membered ring formation. Such problems with 8-lactone formation have been observed also in Diels-Alder chemistry. A nice example related to A-acyliminium chemistry is the thermal hetero Diels-Alder reaction of acylimine precursor (196) to bicyclic -y-lactone (197) in good yield. The corresponding intramolecular cycloaddition of (198) fails. Finally, Lewis acid mediated cyclization of allylsilane (199) is unsuccessful, although lactonization of (200) proceeds very well (equation 135, cf. equation 119). ... 

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