Enium ions

Coupling to both termini of the 2-methyl-2-propcnyl residue occurs when the complex 5, formed from the iodo compound 4 and stiver tetratluoroborate, is allowed to react with an excess of aldehyde, giving rise to diastcreomerically pure 4-fluoro-2,4,6-trisubstituted tetrahy-dropyrans 617. The details of the reaction and, as well, the origin of stereoselectivity is unclear at present. It would not be surprising if the reaction is restricted to those aldehydes leading in the first step to homoallylic alcohols capable of forming mesomerically highly stabilized carb-enium ions on solvolysis. 

The use of the Lewis acid BiBr3 gave the best results, with the formation of an ox-enium ion 2-306 as intermediate. 

Sterically hindered phenol (24) was elec-trochemically oxidized to afford a phenox-enium ion (25) that reacted with O- and A-nucleophiles to yield a cyclohexadienyl-protected amino acid (26) (Scheme 9) [64]. 

Rodrigues,]. A. R. Abramovitch, R. A. de Sousa,]. D. F. Leiva, G. C. Diastereoselective Synthesis of Cularine Alkaloids via Enium Ions and an Easy Entry to Isoquinolines by Aza-Wittig Electrocyclic Ring Closure.]. Org. Chan. 2004, 69, 2920-2928. 

Enium Ions of Other Group 16 Elements. Furukawaetal.316,320 have obtained enium ions of the heavier chalcogen elements stabilized by intramolecular complexation with dimethylamino groups (322). Resonances of the benzylic and methyl protons in the 1H NMR spectrum of cation 322a are shifted downfield... 

Elimination of water is particularly favored from p-chlorophenyF and (p-trifluoromethyl)phenyl selenides, whereas elimination of seleninic acid is mainly observed from methylseleno derivatives. Here, two different effects work in the same direction. The arylseleno group, more than the methylseleno group, makes the a-hydrogen acidic, and has at the same time a lower tendency to stabilize a p-caib-enium ion via the formation of a seleniranium ion. 

Strong Lewis acids abstract alkoxide groups from amide acetals or lactam acetals to give the corresponding iminium salts, e.g. (96 Scheme 10). Alkoxide transfer from amide acetals to less stable carb-enium ions is also a route from amide acetals to iminium compounds (97). 

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