We present the results of our study towards the synthesis of 2,3-disubstituted 7-azaindoles by cyclization of 2-pyridylhydrazones of aliphatic and alkylaromatic ketones under the conditions of Fischer reaction, catalyzed by polyphosphoric acid.

The acylation of pyrrolo [1,2-a]pyrazines with acetic anhydride and the acid chlorides of various carboxylic acids has been studied. It has been shown that pyrrole [1,2-a]pyrazines are selectively acylated at the α-position of the pyrrole ring when it is free. Products of the condensation of 1-methylsubstituted pyrrolo [1,2-a]pyrazines have been obtained for the first time in the process of acetylation.In a continuation of a study of the behavior of the pyrrolo[1,2-a]pyrazine system under electrophilic substitution conditions, we have investigated the acylation of alkyl-, aryl-, and hetaryl-substituted pyrrolo-[1,2-a]pyrazines. It was established previously that acetylation of 3,4-dihydropyrrolo[1,2-a]pyrazines with acetic anhydride (in the presence of magnesium perchlorate as a Lewis acid) proceeded selectively at the pyrazine atom N (2) and formed N-acyl-substituted derivatives in 25-87% yield [1].In contrast to the hydrogenated system, acylation of aromatic pyrrolo[1,2-a]pyrazines occurs at the free α-position of the pyrrole ring. For example boiling the pyrrolo[1,2-a]pyrazines 1a-c with acetic anhydride in the presence of tin(IV) chloride or magnesium perchlorate proceeded selectively to give the monosubstituted products 2-4, but the yields of the acyl derivatives did not exceed 14%. N N R N N R Me O (MeCO) 2 O SnCl 4 Mg(ClO 4 ) 2 or 1a-c 2-4 1a, 2 R = Me; 1b, 3 R = Et; 1c, 4 R = Ph Under analogous conditions 1-propylpyrrolo[1,2-a]pyrazine (1d) reacted with the formation of a mixture of two products of acylation in low overall yield (10%) -6-acetyl-1-propylpyrrolo[1,2-a]pyrazine (5) and 6,8-diacetyl-1-propylpyrrolo[1,2-a]pyrazine (6). N N Me N N Me Me O N N Me Me O O Me (MeCO) 2 O SnCl 4 1d 5 6 + __________________________________________________________________________________________

A novel method for the direct synthesis of 5-arylidene-2-thiohydantoins from thioureas and aromatic aldehydes in the presence of base in ethanol was developed. Application of this efficient method allowed preparing thiohydantoins, which are difficult to synthesis by traditional methods.

A simple synthesis of some hard-to-reach heterocycles containing 2,3-disubstituted 5-bromo-1H-pyrrolo[2,3-b]pyridine framework by Fisсher indole cyclization in polyphosphoric acid has been developed. A particularly valuable feature of this synthetic route is the possibility to build a 5-bromo-7-azaindole scaffold with alkyl or aryl substituents at positions 2 and 3 from available starting materials.