1,
4,
4a,
5,
10,
3,
react with acetylacetone and acetoacetate esters upon heating in ethanol to give derivatives of a novel heterocyclic system, hexahydro-l,3,4thiadiazolo[2.3-alquinoxalino[2,3-dlpyrrole. Diaddition reactions of bffunctional nucleophiles to 1,4.diazinium cations I, which lead to the formation of cyclic adducts If, appear to be reversible processes. Their equilibria can be shifted to the right, as expected, in the presence of any type of base (such as di-and triethylamine, sodium ethoxide, etc.), and the resulting hydrogenated condensed 1,4.diazine derivatives can then be isolated in high yields [2,3].
Compounds of type II are relatively stable in the crystalline state, but addition of equimolar amounts of mineral acids to their solutions results in complete dissociation of the cyclic adducts H to the 1,4-diazinium cation precursors and the corresponding dinucleophiles [2, 3]. In the presence of catalytic amounts of acids, or upon heating in protic solvents, the cycliz~on products dissociam to a small degree, which provides a basis for different types of isomcrization reactions or other reactions, leading to changes in the regio-and stereoorientation of the annelated ring, and even to changes in the actual nature of the aunelated ring [2,3]. The direction or pathway of the interconveasion reactions of condensed tetrahydropyrazines upon treatment with catalytic amounts of acids is determined by the relative bond strengths in the corresponding hydrogenated heterocycle systems. Thus, according to mass spectrometric data [4], annelation of N,S-bifunctional reagents to 1,4-diazines results in the formation of less stable rings than reactions with C,N-, N,N-, and other nucleophiles. At the same ~ne, thiazolo[4,6-b]-and thiazino [5,6-b]annelated tetrahydroquinoxalines are particularly susceptible to acid-catalyzed regioisomerization and other types of reactions [5,6].
In the present paper we have investigated the reactions of 1,3,4-thiadi~no [5,6-b]quinoxaline HI upon treatment with CHacids such as acetylacetone and acetoacetate esters IVa-c. Considering the possibility of dissociation of thiadiazinoquinoxaline III to quinoxalinium cation V and thiobenzhydrazide VI, we would expect the most likely reaction products to include furo[2, 3-b]quinoxaline derivatives VII, from cation V reacung with dicarbonyl compounds IV [7], or pyridazino [3,4-b]quinoxalines VIII, in the case that hydrazone IX, formed from thiobenzhydrazide VI and ~-dicarbonyl compounds, can react as a 1,4-N,Cdinucleophile in a subsequent cyc~tion reaction [8].
*For Communication 28, see [I].