Cycloaddition Reactions of 1-Lithio-1,3-dienes with Aromatic Nitriles Affording Multiply Substituted Pyridines, Pyrroles, and Linear Butadienylimines
2006-10-27T00:00:00Z (GMT) by
Fully or partially substituted 1-iodo- or 1-bromo-1,3-dienes could be readily lithiated using <i>t</i>-BuLi or <i>n</i>-BuLi to afford their corresponding 1-lithio-1,3-diene derivatives in quantitative yields. When these in situ generated lithium reagents were treated with organonitriles, depending on the substitution patterns of the butadienyl skeletons, substituted pyridines, pyrroles, and/or linear butadienyl imines were formed in good to excellent yields via <i>N</i>-lithioketimine intermediates. In the cases of 1,2,3,4-tetrasubstituted and 2,3-disubstituted 1-lithio-1,3-dienes, pyridine derivatives or linear butadienyl imines were generally formed depending on the reaction temperatures. When 1,2,3,4-tetrasubstituted 4-halo-1-lithio-1,3-dienes and 1,2-disubstituted 1-lithio-1,3-dienes were treated with organonitriles, pyrrole derivatives or linear butadienyl imines were obtained. Competition between 5-exo and 6-endo cyclization was found to be responsible for the formation of either pyrroles or pyridines. Selective elimination of RLi from the lithiated cyclic N-containing intermediates was observed. The order of elimination was found to be LiCl > Me<sub>3</sub>SiLi > LiH.
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