Penta-substituted pyridine synthesis

Boger initially developed the aforementioned methodology to construct penta-substituted pyridine 216 in the formal total synthesis of antitumor antibiotic streptonigrin 217. " The requisite 1,2,4-triazene 214 was generated via a Diels-Alder/retro-Diels-Alder sequence between nitrogen-containing dienophile 212 and 1,2,4,5-tartazine 213. Exposure of enamine 214 to 215 resulted in the formation of 216. Similar chemistry was also applied in the synthesis of the structurally related lavendamycin 217. ... 

Liebeskind reported a copper-catalyzed Chan-Lam C-N crosscoupling methodology for JV-imination of boronic acids by using oxime O-carboxylates 291 as iminating agents and either Cu(I)-thiophene-2-carboxylate (CuTC) or Cu(OAc)2 as the catalyst under nonbasic and nonoxidizing conditions.Subsequently, the A-alkenylated a,3-unsaturated ketoxime 0-pentafluorobenzoates 293, were preeursors in a cascade reaction for the one-pot synthesis of tri-, tetra-, and penta-substituted pyridines 295 in moderate to excellent isolated yields. 

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