Heteroatom substitution alkyllithiums

B. Cyclization Reaction of Heteroatom-substituted Olefinic Alkyllithiums. 334... 

Reaction of nucleophiles with the polarized N=C bond of azines proceeds via dearomatization and formation of the corresponding 1,2-adduct. With alkyllithiums, for example, it is possible to isolate the dihydro products by careful neutralization of the reaction mixtures these are, in general, rather unstable, however, and can easily be reoxidized to the fully aromatic compounds (Scheme 4). The dihydro adducts formed in these direct nucleophilic addition reactions can also be utilized for the introduction of substituent groups /3 to the heteroatom. Thus, reaction of (35) with one of a number of electrophiles, followed by oxidation of the intermediate dihydro product, constitutes a simple and, in many cases, effective method for the introduction of substituent groups at both the 2- and 5-positions of the pyridine ring (Scheme 4). Use of LAH in this sequence, of course, results in the formation of 3-substituted pyridines. 

Alkyllithium compounds metallate dibenzofuran, dibenzothiophene and IV-alkylcarbazoles (in increasing order of difficulty) to form compounds of type (297) substitution occurs ortho to the heteroatom as expected from benzene chemistry. 

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