Lithiation, aromatic compounds mechanism

To start the discussion, we will try to explain the experimental data on the basis of the anionic cyclization mechanism proposed in Scheme 28.1. First, we will consider the lithiation step. Following the proposed mechanism, lithiation of 7V-ben-zoyl oxazolidine 4 should take place at the a position to the amide nitrogen. However, the deuterium-labeling experiments indicate that lithiation of 4 occurs exclusively in the ortho position(s) of the aromatic ring. The first step of the reaction should be then the formation of lithiated compound 13 which must be transformed in some way, into a-lithiated compound 14 in order to cyclize (Scheme 28.6). 

The mechanism of the dearomatization-cyclization of aromatic amides takes place by initial lithiation at the ortho position of the aromatic ring. This species is in equilibrium with the more stable a-amide lithiated compound, which forms the final product by means of a 6n electrocyclic ring closure. 

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