Recently, we reported on cycloadditions between electron-deficient heterodienes and tributyl(ethynyl)tin, which provide a new pathway to stannylated pyridazines and, in one special case, pyridines. In order to broaden the synthetic scope of these reactions, we have developed hetero [4+2] cycloaddition reactions between a number of tailor-made 1,2,4-triazines 5-9 (acting as heterodienes), and tributyl(ethynyl)tin (acting as dienophile). The desired 1,2,4-triazines are readily available, in moderate to very good yields, by the condensation reactions of appropiate carbamidrazones and glyoxals. These 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 various substituents can be incorporated directly by substitution of the stannyl group, as was shown for halogens and carbon electrophiles under Stille conditions.

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Full details on the reactivity of the title compound as 4 pi component in inverse-type Diels-Alder reactions, including kinetic data, are reported. Donor-substituted alkynes, alkenes, donor-substituted and unsubstituted cycloalkenes, ketene acetals and aminals, as well as several cyclic enol ethers were used as dienophiles in these investigations. A. number of 4-mono- and 4,5-disubstituted pyridazines can easily be obtained by this method.

Cycloadditions of 3-aryl-1,2,4,5-tetrazines 1a-o with ethynyltributyltin 5 occur with high regioselectivity to yield 3-aryl-5-tributylstannyl-pyridazines 9–23 in 71–95% yield. Moreover, the stannanes obtained could be utilized as starting materials for the preparation of various 3,5-di-substituted pyridazines as shown for 3-phenyl-5-tributylstannyl-pyridazine 9b. Stille cross-couplings with aryl halides yield 3,5-diaryl-pyridazines 26–29, whereas exposure to elemental halogens furnished 3-phenyl-5-halogeno-pyridazines 24 and 25, which can be used as starting compounds for further functionalizations like Heck reaction.

Ethynyltributyltin undergoes inverse type [4+2] cycloadditions with various 1,3,4-oxadiazin-6-ones to afford two regioisomeric stannylated α-pyrones, which can be separated by flash column chromatography. Exposure of the stannanes to elemental halogens in chloroform or THF as solvent yields the corresponding halogeno pyrones. A general exception of this reaction type is also described. Destannylation with dry hydrogen chloride furnishes 3,6-disubstituted α-pyrones.

Ethynyltributyltin 2 cycloadds to 5-phenyl-1,2,4-triazine-3-carboxyclic acid methyl ester 1 to furnish mainly 4-tributylstannyl-6-phenyl-pyridine-2-carboxyclic acid methyl ester 3a besides a small amount of the 3-tributylstannyl isomer 3b. The tributyltin substituent of pyridine 3a can be replaced by Pd-catalyzed acylation and arylation, by halogenation and protonation to yield functionalized pyridines 4 – 7, which are not available by direct cycloaddition of the corresponding alkynes.

MR3-substituted alkynes 2 and 3 (M = Si, Ge, Sn; R = alkyl) show high reactivity in inverse-type Diels-Alder reactions with the π-electron-deficient 1,2,4,5-tetrazine 1 in strict contrast to the corresponding carbon compounds. Kinetic data prove the huge accelerating effect of the trialkyltin substituent, offering a simple access to new heteroaromatic organotin derivatives, which can be easily transformed by standard methods of organotin chemistry.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.