2-Alkoxypropenals react with 2-aminoethanethiol to give tautomeric mixtures of the corresponding 2-(1-alkoxyvinyl)-1,3-thiazolidines and 2-(2-alkoxy-2-propenylideneamino)ethanethiols in quantitative yield. The state of the ring-chain tautomeric equilibrium depends on the polarity and acidity of the medium and initial reactant ratio, and it does not change on raising the temperature and is only slightly affected by microwave irradiation.Functionalized thiols, such as 2-sulfanylethanol [1] and 1,2-and 1,3-dithiols [2], are known to add to 2-alkoxypropenals in neutral medium according to the Markownikoff rule to give methylglyoxal O,S-acetals. However, in the presence of bases (K 2 CO 3 , Et 3 N) the addition of thiols to 2-alkoxypropenals occurs at the 1,4-positions with formation of 3-alkylsulfanyl-2-alkoxypropanals in 75-100% yield [3]. Primary amines react with 2-ethoxypropenal to afford Schiff bases [4]; as noted in [5], in the reaction of acrolein with nucleosides, the primary amino groups of cytosine and thymine can add to the 1,4-position. The above data suggest that nucleophilic attack on 2-alkoxypropenal can be directed at the double C=C bond or aldehyde group, depending on the nucleophile nature, catalyst, and reaction medium.The goal of the present work was to elucidate regioand chemoselectivity in the addition of a difunctional nucleophile, 2-aminoethanethiol (cysteamine [6]), to 2-alkoxypropenals, depending on the reaction conditions (solvent nature, polarity and acidity of the medium, temperature, microwave activation) and to study ring-chain tautomerism of the resulting Schiff bases and thiazolidines. A considerable interest in the chemistry [7] and pharmacology of thiazolidine derivatives [8,9] stimulates studies on the synthesis of new compounds of this series. 2-Alkyl-1,3-thiazolidines are especially important, for they exhibit radioprotective and antimutagenic activity [6]. Furthermore, thiazolidine ring is a structural fragment of a number of natural compounds and biologically active substances [10]; therefore, studies on the dynamics of ring-chain equilibrium of thiazolidine derivatives are important for understanding biochemical processes [11].The reactions of 2-alkoxypropenals Ia and Ib with 2-aminoethanethiol (II) were expected to follow pathway a (Scheme 1) which involves 1,2-addition of the amino group. Elimination of water from intermediate A could give rise to vinylthiazolidine III or Schiff base IV. Isomers III and IV are interconvertible; it is known that even in neutral aqueous medium thiazolidine ring exists in equilibrium with the open-chain iminothiol structure [12] or initial reagents (from which it was synthesized) [13]. An alternative initial stage is sometimes considered to be attack on the carbonyl carbon atom by the SH group of 2-aminoethanethiol with formation of monothio semiacetals (pathway b) [14]. This reaction sequence was observed in reactions of α,β-unsaturated ketones with β-amino thiols [15]. By analogy with the data of [3], one more reaction pathway may be 1,4-addi...
