Acetonium ions

The subsequent fate of the acetonium ion depends on the molecular environment and reaction conditions. Transient formation of a steroidal acetonium ion as a reaction inter-... 

This ion has been variously named as an "acetoxonium" ion or a "dioxoienimn" ion. We shall refer to it as the "acetonium" ion, expressing both its origin and the delocalisation of its charge over the three bridging atoms. 

Acetonium-ion bridged intermediates are not peculiar to the 3,5-position, although no salts have so far been isolated from other derivatives. Pregnane-i7a,20a,2i-triols, which are normally the minor products from reduction of the corticosteroid dihydroxyacetone side chain, have been prepared from the readily available 17a,20 8,21-triols by solvolysis of... 

On adding water to the acetonium ion (26), however, the bridge is attacked rapidly to give the two monoacetates of the 2/9,3j -diol [ioy,xo8]. This reaction sequence effects m-hy-droxylation from the more hindered side of the olefinic bond, and so gives the as-glycol of configuration opposite to that obtained with osmium tetroxide. Unfortunately, the process does not seem to be applicable to more highly substituted olefins. 

Acetonium ions involving the 5 a, 6 a- and i6a,i7a-positions must be invoked to explain the conversions of 3j8,5ct-diacetoxy-6/S-chlorocholestane into the 3jS,5a,6a-triol 3,6-diacetate [121] and of the i7a-acetoxy-i6ji -bromopregnan-2o-one 14) into the i6a-acetoxy"i7a hydroxypregnan 20-one 15) [122] by the action of silver acetate. Ag+-catalysed halide elimination... 

Substituted ammonium ions derived from nitrogen bases with names ending in -amine receive names formed by changing -amine into -ammonium. When known by a name not ending in -amine, the cation name is formed by adding the ending -ium to the name of the base (eliding the final vowel) e.g., anilinium, hydrazinium, imidazolium, acetonium, dioxanium. 

We may also note that acetonium or benzonium bridged ions are intermediates in the hydroxylation of olefinic bonds with iodine and silver acetate or benzoate (see p. 99). 

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