Ketiminium salts

SCHEME 5.7 Komatsu s ketiminium salt-mediated epoxidation. 

Recently, two other groups have shown that exocyclic iminium salts can be useful mediators in asymmetric epoxidation. Komatsu has developed a system based on ketiminium salts [14], prepared through the condensation of aliphatic cyclic amines with ketones. A chiral variant was also produced, derived from prolinol and cyclohexanone, which gave 70% yield and 39% ee for cinnamyl alcohol (Scheme 5.7). 

S. Minakata, A. Takemiya, K. Nakamura, 1. Ryu, M. Komatsu, Epoxidation of olefins mediated by aliphatic ketiminium salts, Synlett 12 (2000) 1810. 

The mechanism of this reaction is illustrated in order to show the low-energy iminium structure of the cT-complex. 2-Acylpyrrole is only formed after the ketiminium salt is hydrolysed with water. 

Systematic studies have shown that intramolecular [2+2] cycloadditions of unsaturated ketenes or ketiminium salts (278) constitute a simple and reasonably general approach to cyclobutanones (279), which are useful as precursors to butyrolactones following Baeyer-Villiger oxidation. The potential of this method is clearly shown by the efficient formation of the bicyclo[5.2.0]nonane (280) and of the... 

Ketiminium salts have been shown to participate readily in [2-r 2]-cyclo-additions with olefins [35]. Ghosez reported a classic in this area, involving the use of prolinol-derived ketiminium species (Scheme 18.28) [122]. Treatment of a-chloroenamine 172 with ZnCl2 facilitated chloride abstraction under mild conditions to yield an intermediate ketiminium salt 173. The reactive cumulene participated in a diastereoselective cycloaddition with cyclo-pentene to afford the chiral ketone 174 (> 97 % ee), after hydrolytic removal of the auxiliary. These diastereoselective [2 2]-cycloadditions of Ghosez provide a rare example of a chiral auxiliaiy-based approach to chiral cyclobutanones. 

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