Pyridine continued

The scope and efficiency of [4+2] cycloaddition reactions used for the synthesis of pyridines continue to improve. Recently, the collection of dienes participating in aza-Diels Alder reactions has expanded to include 3-phosphinyl-l-aza-l,3-butadienes, 3-azatrienes, and l,3-bis(trimethylsiloxy)buta-l, 3-dienes (1,3-bis silyl enol ethers), which form phosphorylated, vinyl-substituted, and 2-(arylsulfonyl)-4-hydroxypyridines, respectively <06T1095 06T7661 06S2551>. In addition, efforts to improve the synthetic efficiency have been notable, as illustrated with the use of microwave technology. As shown below, a synthesis of highly functionalized pyridine 14 from 3-siloxy-l-aza-1,3-butadiene 15 (conveniently prepared from p-keto oxime 16) and electron-deficient acetylenes utilizes microwave irradiation to reduce reaction times and improve yields <06T5454>. 

Cycloadditions continue to be a powerful way to build pyridines. Indeed, a recent review describes the Diels—Alder reaction of azadienes to form pyridines either by the traditional thermocyclic manner or when catalyzed by transition metals (140rganic Chemistry FrontierslOlO). In addition to cycloaddition approaches, the other main route to pyridines continues to be cyclocondensations. 

Determination Weigh the specified amount of the sample into the titration flask of the apparatus and dissolve in 50 ml of analytical-reagent grade pyridine. Continue as described above for the standardisation beginning with Insert the electrodes. . and ending with. . at the point of inflection of the curve. ... 

Among other condensed thiazole ring systems, imidazo[2,l-6]thiazoles (tetramisole is a member of this class), thiazolo-pyrimidines, and thiazolo-pyridines continue to attract the most interest. Attention is concentrated on synthesis and biological activity. Unexpectedly, dihydrothiazolo[3,2-a]pyridinium bromide is readily brominated when the pyridine ring has a 3-nitro-substituent but not when it has a 3-amino-substituent. 

---