Synthesis of pyridinylketones, and their cyclic derivatives produced from 6-methyl-2-thiouracil

Abstract

A possibility to obtain pyrimidines, containing oxoalkyl moiety in 2 position of the ring from the available 6-methyl-2-thiouracil was shown.The known approach to pyrimidines containing keto group in the 2 position are limited to the use of 2-alkyl-and 2-benzylpyrimidines as the starting compounds. Thus, the nitrosation of 2-benzylpyrimidine followed by the hydrolysis of the formed oxime affords 2-benzoylpyrimidines [1, 2]. The formation of 1,4-dimethyl-2-phenacylpyrimidin-6-one via the benzylation of active methyl group of 1,2,4-trimethylpyrimidin-6-one with benzoic acid ester in an alkaline medium as well as the reaction of 1,4-dimethyl-6-oxopyrimidine-2-carbonitrile with acetophenone sodium salt was reported in [3].A method based on the desulfurization of the alkylation product of 6-methyl-2-thiouracil I by the action of α-bromoketones was suggested as promising for the synthesis of the discussed compounds. The reaction of uracil I with 2-bromodimedone in alkaline medium was found to give 5,5-dimethyl-2-(6-methyl-4-oxo-3,4-dihydropyrimidin-2(1H)-ylidene)cyclohexane-1,3-dione II.As a rule, similar transformations leading to the formation of a carbon-carbon bond owing to the sulfur release from α-azomethine-β-oxoalkyl sulfides require rigid conditions, for example, prolonged heating and the use of trivalent phosphorus compounds that promote desulfurization [4-6].2-Phenacylthiopyrimidines III, the products of thiouracil I S-alkylation with phenacyl bromide [7], were found to be much more stable. Refluxing compounds III in dioxane, pyridine, acetic acid, and dimethylformamide did not result in the desulfurization products.The main component of the product mixture obtained after short-term heating of pyrymidines III to 180-200°C is uracil I. Also the corresponding 2-phenacylpyrimidines IVa (12%) and IVb (9%) and pyrrol [1,2-a]pyrymidine derivatives Va (21%) and Vb (16%) were isolated. Desulfurization of 2-phenacylthiopyrimidine IIIa was carried under the continuous reflux of compound IIIa in DMF in the presence of triphenylphosphine to give 2-phenacylpyrimidine IVa in a high yield.The absence of the methylene signals in the 1 H NMR spectrum of pyrimidines IVа, IVb and the appearance of the singlet signals of amine and methine protons indicate the enamine structure of these compounds (see the table).In addition, the doubling of the signals of pyrimidine ring and protons of the substituents indicates the presence of pyrimidines IV in a solution as Z,Eisomers mixture (1:1).Compound IVa reacts with phenacyl bromides in an alkaline medium involving the exocyclic methine group. In contrast to the initial enaminone IVa, the obtained diketones VIa and VIb are in a keto-imine tautomeric form. According to the 1 H NMR data (see the table), they contain a CH-CH 2 fragment with a chiral methine carbon atom and nonequivalent methylene protons.

Reaction of 6‐Methyl‐2‐Thiouracil and 6‐Phenyl‐2‐Thiouracil with Chloro‐β‐Dicarbonyl and Bromo‐β‐Dicarbonyl Compounds and Their Nitrile Analogs

“…The present work aimed to search conditions for the CC bond formation in position “2” of the pyrimidine ring by Eschenmoser reaction of “sulfide contraction” that in generally results in sulfur elimination from α‐azomethine‐β‐oxoalkylsulfides . This method has found application in synthesis of some natural substances and heterocyclic compounds, including preparation of 2‐(2‐oxo‐2‐arylethyliden)‐1,3‐dihydropyrimidin‐4(1H)‐ones from 2‐phenacylthiouracils . The required condition for the reaction of “sulfide contraction” is the ability of starting compounds to form the episulfide intermediate, whose desulfurization is facilitated by the high temperature in the presence of base or trivalent phosphorous compounds .…”