Unexpected transformations of 1-aza-1,3,4-trienes in alcoholic potassium hydroxide. New classes of aza dienes

Abstract

Aza dienes [1, 2] and aza trienes [2, 3] are useful and important intermediates in the synthesis of both heterocyclic and open-chain polyfunctionalized compounds. We have discovered reactions of isothiocyanates with carbanions derived from alkenes [4], allenes [5], acetylenes [6], and butadienes [7], which have become an inexhaustible source of various aza diene and aza triene systems [8, 9]. Methyl N-ethyl-2-methoxybuta-2,3-dienimidothioate (I, 1-aza-1,3,4-triene) is a precursor of 1-ethyl-3methoxy-2-methylsulfanyl-1H-pyrrole (II), 5-methoxy-2-methyl-6-methylsulfanyl-2,3-dihydropyridine (III) [8], and 3-methoxy-2-methylsulfanyl-4,5-dihydro-3H-azepine (IV) [10] and is readily available from lithiated methoxyallene and ethyl isothiocyanate (Scheme 1). Using compound I as an example we performed for the first time a simple and convenient synthesis of unusual aza dienes, hemiaminal ethers with rare heteroatom substituents. It was found that treatment of 1-aza-1,3,4-triene I (without isolation from the reaction mixture) with a solution of potassium hydroxide in methanol at room temperature resulted in the formation of previously unknown methyl (Z)-2-methoxy-N-(1-methoxyethyl)but-2-enimidothioate (V) (Scheme 1) as a mixture of syn and anti isomers in ~45% yield (unoptimized). A probable reaction path involves isomerization of 1-aza-1,3,4-triene I into 2-aza-1,3,5-triene VI, addition of methanol at the C=N bond of VI, and subsequent [1,5]-H shift in adduct VII thus formed (Scheme 2). ISSN 1070-4280, Russian Journal of Organic Chemistry, 2011, Vol. 47, No. 1, pp. 146–149. © Pleiades Publishing, Ltd., 2011. Original Russian Text © N.A. Nedolya, O.A. Tarasova, O.G. Volostnykh, A.I. Albanov, B.A. Trofimov, 2011, published in Zhurnal Organicheskoi Khimii, 2011, Vol. 47, No. 1, pp. 146–149.