Thermodynamic stabilities of different isomers and tautomers of lithium and potassium buta-2,3-dienimidothioates (ambident N,S-centered anionic adducts derived from methoxyallenyl carbanion and methyl isothiocyanate) as real or potential precursors of 3-methoxy-N,N-dimethylthiophen-2-amine were estimated by quantum-chemical calculations in terms of the density functional theory. Gradient channels corresponding to concerted (one-step) and stepwise (two-step) mechanisms of formation of thiophene ring from the most stable conformers and tautomers were localized. The formation of thiophene structure from lithium and potassium buta-2,3-dienimidothioates in two steps via intramolecular cyclization to thiophen-2(5H)-ylidene anion and its isomerization into thienylamide was found to be more probable. The activation barriers in both stepwise and concerted cyclization mechanisms considerably decrease in going from lithium to potassium as counterion. * For communication XVII, see [1].Thiophene and its derivatives are widely used not only in organic synthesis [2] but also in practice, in particular in the manufacture of dyes, plasticizers, plastics, analytical reagents, insecticides, pharmaceuticals, and perfume compositions [3], especially in the preparation of new materials [4]. Some natural thiophene derivatives, e.g., vitamin H (biotin) play an important role in biological processes [5]. Therefore, extension of the series of these compounds and search for rational ways of their synthesis are important problems [6].We were the first to show that acetylene, allene, and butadiene carbanions and heterocumulenes, primarily isothiocyanates and isocyanates, generated in situ are universal building blocks for generally one-pot assembly of most fundamental aza, oxa, and thia heterocycles [7], including thiophehes [7-9]. While developing studies in this field, we found that the yield of thiopene considerably depends not only on the reactant structure and reaction conditions, but also on the nature of counterion. Using as an example the reaction of α-lithiated methoxyallene I with methyl isothiocyanate, leading to ambident anion II′, we obtained experimental proofs for the effect of counterion nature (Li + or K + ) on the yield of 3-methoxy-N,N-dimethylthiophen-2-amine (III) (Schemes 1, 2). In particular, we found that aminothiophene III is formed directly from intermediate II′ in 32% yield (after distillation; Scheme 1, i). It was presumed that the reaction involves intramolecular cyclization of lithium butadienimidothioate II′A (S-centered azatriene anion) with formation of iminothienyl anion IV′ with lithium as counterion and subsequent isomerization of IV′ into lithium thienylamide V′. Treatment of the latter with methyl iodide yields thiophenamine III (Scheme 2).Replacement of lithium cation in intermediate II′ by potassium (via addition of powdered potassium tert-butoxide to the reaction mixture) increases the yield of thiophenamine III to 75% (Scheme 1, ii) [9]. Likewise, the reaction is most likely to occur through