Ultrasound-assisted synthesis of pyrimidines and their fused derivatives: A review

The pyrimidine scaffold is present in many bioactive drugs; therefore, efficient synthetic routes that provide shorter reaction times, higher yields, and site-selective reactions are constantly being sought. Ultrasound (US) irradiation has emerged as an alternative energy source in the synthesis of these heterocyclic scaffolds, and over the last ten years there has been a significant increase in the number of publications mentioning US in either the construction or derivatization of the pyrimidine core. This review presents a detailed summary (with 140 references) of the effects of US (synergic or not) on the construction and derivatization of the pyrimidine core through classical reactions (e.g., multicomponent, cyclocondensation, cycloaddition, and alkylation reactions). The main points that were taken into consideration are as follows: chemo- and regioselectivity issues, and the results of conventional heating methods compared to US and mechanistic insights that are also presented and discussed for key reactions.

NHC-catalyzed β-specific addition of N-based nucleophiles to allenoates

N-heterocyclic carbene (NHC) catalyzed reactions of nitrogenous heterocycles or trifluoromethylated acylhydrazone with allenoates gave regiospecific β-adducts. DFT calculations rationalized the origination of regio- and E/Z selectivities.

One-Pot Metal-Free Synthesis of 3-CF3-1,3-Oxazinopyridines by Reaction of Pyridines with CF3CO-Acetylenes

The reaction of pyridines with trifluoroacetylated acetylenes was investigated. It was found that the reaction of various pyridines with two molecules of CF₃CO-acetylenes proceeds under mild metal-free conditions. As a result, efficient stereoselective synthesis of 3-arylethynyl-3-trifluoromethyl-1,3-oxazinopyridines was elaborated. Target heterocycles can be prepared in up to quantitative yields.

Electrophilic halogenation of hydrazones of CF3-ynones. Regioselective synthesis of 4-halo-substituted 3-CF3-pyrazoles

An efficient approach for the synthesis of 4-halo-3-CF3-pyrazoles was elaborated using the cyclization of hydrazones of CF3-ynones induced by electrophilic halogenation. This method can be used for the regioselective synthesis of 3-CF3-pyrazoles substituted at the 4-position with any halogen (I, Br or Cl). The high (up to quantitative) yields, mild conditions and simplicity of the reaction protocol are the advantages of the proposed method. It was demonstrated that cross-coupling with 4-iodo-3-CF3-pyrazoles can be used for the synthesis of a broad family of tetrasubstituted pyrazoles. Moreover, it was found that this method is suitable for the synthesis of 4-substituted derivatives of Celebrex and SC-560.