Acylium ion, generation

Acyl cations (acylium ions) generated by treating an acyl chloride or acid anhydride with aluminum chloride attack aromatic rings to yield ketones The arene must be at least as reactive as a halobenzene Acyl cations are relatively stable and do not re arrange... 

Ether cleavage can also be effected by reaction with acetic anhydride and Lewis acids such as BF3, FeCl3, and MgBr2.97 Mechanistic investigations point to acylium ions generated from the anhydride and Lewis acid as the reactive electrophile. 

These substrates, studied exclusively by Conia, are a hybrid in that they contain all of the requisite carbon atoms for the ring but still require a coupling reaction. The action of hot poiyphosphoric acid on alkenic esters (77) may begin by cracking the alkyl carbon-oxygen bond to produce an alkene and the parent acid. This new composition is in principle identical to the Dev mixture just described, and likely proceeds similarly by acylium ion generation and electrophilic attack on the in situ generated alkene (equation 42). 

Friedel-Crafts acylation of alkenes (Daizens-Nenitzescu reaction ) with unsaturated acylium ions generated from acid halides and Lewis acids constitutes a general synthesis of divinyl ketones. 

The first report of an intramolecular addition of a vinylsilane came from the Burke laboratory The reaction was us for the preparation of spiro[4.S]decadienones. As shown in Scheme 2, the addition of the vinylsilane to the intermediate acylium ion generated using titanium tetrachloride resulted in the formation of the enone system (3) -> (4). 

Unsaturated acylium ions generated from alkenic acids or anhydrides react with alkenes to produce cy-clopentenones (equation with cycloheptene the major products arise from ring contraction. 

The building up of the furo[3,4-a]pyrrolizine ring system (6) (Table 1) has been achieved by cyclization via 2-aza-Cope rearrangement of the A-acylium ion generated when the hydroxylactam (114) is treated with formic acid <90H(30)50i>. 

In Friedel-Crafts acylations, the electrophile is an acylium ion, generated in the following way ... 

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