From 1,2,4-triazines and tributyl(ethynyl)tin to stannylated bi- and terpyridines: The cycloaddition pathway

Recently, we reported on cycloadditions between electron-deficient heterodi enes and tributyl(ethynyl)tin, which provide a new pathway to stannylated p yridazines and, in one special case, pyridines. In order to broaden the syn thetic scope of these reactions, we have developed hetero [4+2] cycloadditi on reactions between a number of tailor-made 1,2,4-triazines 5-9 (acting as heterodienes), and tributyl(ethynyl)tin (acting as dienophile). The desire d 1,2,4-triazines are readily available, in moderate to very good yields, b y the condensation reactions of appropiate carbamidrazones and glyoxals. Th ese cycloadditions open up a novel route to regiospecifically stannylated 2 ,2'-bi- and 2,2',6',2 "-terpyridines 1-4, 11 in good yields. The stannanes 1-4, 11 are versatile synthetic intermediates, and with this strategy vario us substituents can be incorporated directly by substitution of the stannyl group, as was shown for halogens and carbon electrophiles under Stille con ditions.