Readily available 5-acyl-4-hydroxy-3,6-dihydro-2H-1,3-thiazine-2,6-diones with primary alkyland arylamines in mild conditions (boiling in propan-2-ol) to give Schiff bases. In more rigid conditions (boiling in DMF), the reaction is accompanied by COS liberation and provides 1-substituted 6-alkyluracils. This previously unknown reaction possesses a considerable synthetic potential and can be considered as a new, general, and regioselective synthetic approach to 1-substituted 6-alkyluracils.Over the past years 1,3-thiazines and their hydroxy derivatives have attracted increasing attention, which, in particular, is due to the fact that they can be converted into derivatives of malonic acid, azines, and azoles under the action of nucleophiles [234]. 2,4,6-Trioxo-1,3-thiazines have so far been hardly accessible, and, therefore, little has been known on their reactions with N-, O-, and S-nucleophiles, except for those involving the initial member of this series of compounds [5]. Recently we developed a facile and convenient synthesis of 5-acyl-4-hydroxy-1,3-thiazine-2,6-diones from malonic acid, potassium thiocyanate, and carboxylic acid anhydrides in a corresponding acid medium [6,7]. Here we present data on reaction of these compounds with certain N-nucleophiles.It was found that the reactions of thiazine Ia with primary alkyl-or arylamines in propan-2-ol results in exclusive formation, even in the presence of a considerable excess of the amine and under prolonged boiling, of Schiff bases IIa3IIs (Table 1). The products are stable, well-crystallized substances readily soluble in dioxane and acetone.Thus, the reactions of 5-acylthiazines Ia3Ic with primary amines under these conditions involve no thiazine ring cleavage and give rise to ketimines IIa3IIs.Under more rigid conditions, quite unexpected events occur: Boiling of thiazine Ia in excess aniline gives 6-methyl-1-phenyluracil (IIIe) in high yield. The same product was obtained by boiling of an equimolar mixture of thiazine Ia with aniline in DMF.We decided to find out if this previously unknown ÄÄÄÄÄÄÄÄÄÄÄÄ 1 For communication CXXI, see [1]. Scheme 1. 8 A = ?9 = l r j O O H R 1 O O S NH 7776 R 2 3NH 2 8 A = ?9 = l r j N O R 1
