Microwave-assisted group-transfer cyclization of organotellurium compounds

Primary- and secondary-alkyl aryl tellurides, prepared by arenetellurolate ring-opening of epoxides/ O-allylation, were found to undergo rapid (3-10 min) group-transfer cyclization to afford tetrahydrofuran derivatives in 60-74% yield when heated in a microwave cavity at 250 degrees C in ethylene glycol or at 180 degrees C in water. To go to completion, similar transformations had previously required extended photolysis in refluxing benzene containing a substantial amount of hexabutylditin. The only drawback of the microwave-assisted process was the loss in diastereoselectivity which is a consequence of the higher reaction temperature. Substitution in the Te-aryl moiety of the secondary-alkyl aryl tellurides (4-OMe, 4-H, 4-CF(3)) did not affect the outcome of the group-transfer reaction in ethylene glycol. However, at lower temperature, using water as a solvent, the CF(3) derivative failed to react. The microwave-assisted group-transfer cyclization was extended to benzylic but not to primary- and secondary-alkyl phenyl selenides.     

Microwave-assisted rapid decomposition of persulfate

Microwave irradiation has been a promising alternative to conduct several chemical reactions. In this work the microwave effects in potassium persulfate decomposition rate, under controlled conditions of temperature and microwave power, were evaluated. Higher decomposition rate constants were obtained in microwave irradiated reactions in comparison with conventional heated ones. To study the effect of high power microwave irradiation, a pulsed irradiation strategy was developed, in which the samples were repeatedly heated within short intervals of time at high power levels (500 or 1400 W). A great decomposition percentage was achieved in shorter irradiation times, showing the kinetic advantages of microwave-assisted reactions. However, it was found no differences in the reaction yields, even when high power levels were involved, proving that microwave enhancements may arise only from the ability to quickly provide a large amount of energy to the reaction medium.     

Microwave-assisted rapid synthesis of methyl 2,4,5-trimethoxyphenylpropionate, a metabolite of Cordia alliodora

Microwave assisted condensation of asaronaldehyde (2) with malonic acid in piperidine-AcOH provides 2,4,5-trimethoxycinnamic acid (3) in 87% yield within 4 min, which upon further reduction with PdCl2- HCOOH-aq. NaOH gives 3-(2,4,5-trimethoxy)phenyl propionic acid (4) in 88% yield within 3 min. Esterification of 4 with MeOH-H+ gives methyl 2,4,5-trimethoxyphenylpropionate (1), a metabolite of Cordia alliodora, in 94% yield within 3 min (overall 69% yield).     

Microwave-assisted C-H bond activation: a rapid entry into functionalized heterocycles

[reaction: see text] Microwave irradiation strongly accelerates the rhodium-catalyzed intramolecular coupling of a benzimidazole C-H bond to pendant alkenes. The cyclic products were formed in moderate to excellent yields with reaction times less than 20 min. Additionally, the use of microwave irradiation allowed the reactions to be performed without any solvent purification and with minimal precautions to exclude air.     

Microwave-assisted rapid synthesis of novel optically active poly(amide-imide)s containing hydantoins and thiohydantoins in main chain

Rapid and highly efficient synthesis of novel optically active poly(amide-imide)s (PAIs) 6(a-f) was achieved using microwave irradiation. These were made from the polycondensation reactions of 4,4^′-carbonyl-bis(phthaloyl-l-alanine) diacid chloride [N,N^′-(4,4^′-carbonyldiphthaloyl)] bisalanine diacid chloride 5 with six different derivatives of hydantoin and thiohydantoin compounds 4(a-f) in the presence of a small amount of a nonpolar organic medium that acts as a primary microwave absorber. Hydantoin and thiohydantoin derivatives 4(a-e) were synthesis from the reactions between benzil or benzil derivatives 3(a-e) with urea and thiourea. 5,5-Dimethylhydantoin 4f was synthesis from the reactions between acetone cyanohydrin 3f and ammonium carbonate. The polycondensation proceeded rapidly, and was completed within 10 min giving a series of PAIs with an inherent viscosity about 0.25-0.45 dL/g. The resulting PAIs 6(a-f) were obtained in a high yield and were optically active and thermally stable. All of the above compounds were fully characterized by means of Fourier transform infrared spectroscopy, elemental analyses, inherent viscosity (η_inh), solubility tests and specific rotation. Thermal properties of the PAIs 6(a-f) were investigated using thermal gravimetric analysis.     

Microwave-assisted,rapid synthesis of 2-vinylquinolines and evaluation of their antimalarial activity

A rapid and efficient synthesis of 2-vinylquinolines via trifluoromethanesulfonamide-mediated olefination of 2-methylquinoline and aldehyde under microwave irradiation is reported. Biological evaluation of these scaffolds demonstrates that 2-vinylquinolines 3x-3z possess excellent antimalarial activities against chloroquine-resistant Dd2 strain of Plasmodium falciparum (IC₅₀ < 100 nM).     

Microwave-assisted synthesis utilizing supported reagents: a rapid and efficient acylation procedure

[reaction: see text] The application of microwave heating to a polymer-assisted solution-phase (PASP) synthesis technique has been utilized to develop a rapid and efficient protocol for the solution-phase synthesis of amides from either amine or carboxylic acid cores.     

Microwave-assisted polymer-supported acylation of 15-membered azalides—a rapid and efficient procedure

The application of microwave heating to a polymer-assisted solution phase synthesis technique has been used to develop a rapid and efficient protocol for the solution phase synthesis of amides from unprotected polyfunctional 15-membered azalides.     

Microwave-assisted,facile, rapid and solvent-free one pot two-component synthesis of some special acylals

A facile,rapid and solvent-free method for the conversion of acids and dihalomethane to the corresponding methylene diesters(acylals) using microwave as activators or assistor,is reported.This method is particularly powerful for the diesterification of carboxylic acids,which afford methylene diesters in good to excellent yields(up to >99%).When the intermediate is trapped,a "double successive S_N2 reactions" mechanism is proved.     

Microwave-assisted ring-closing metathesis of diallylamines: a rapid synthesis of pyrrole and pyrroline derivatives

The ring-closing metathesis (RCM) reactions of diallylamines occurred under controlled microwave irradiation. In addition to the high reaction rate, the key features of the reaction are that it can be carried out without deactivation of the substrates and without the use of Lewis acids.     

Microwave-assisted rapid extraction of red dye from Caesalpinia sappan heartwood

Since ancient times Caesalpinia sappan heartwood dye has been well-known for its medicinal and dyeing properties. Isolation of the red dye using both conventional and newly developed microwave method was carried out. The conventional heating of 2 h provided 0.656 +/- 0.049 g of the dye and by microwave heating at 540 W for 20 min, the yield obtained was 0.747 +/- 0.047 g. Both the dyes were found to be the same as evidenced by UV, TLC and HPTLC studies. Antioxidant activity of the dyes was also carried out using DPPH and nitric oxide methods to confirm the similarity in their biological activity. The procedure developed can be used for the fast extraction of the red dye of C. sappan without affecting the nature of the product.     

Microwave-assisted synthesis utilizing supported reagents: a rapid and versatile synthesis of 1,5-diarylpyrazoles

The application of microwave heating to a silica-assisted solution-phase synthesis technique has been utilized to develop a rapid and efficient two-step protocol for the preparation of pyrazoles from aryl methyl ketone and aryl hydrazine monomers.     

Microwave-assisted rapid electrophilic fluorination of 1,3-dicarbonyl derivatives with Selectfluor

A microwave-assisted fluorination method for 1,3-dicarbonyl compounds using Selectfluor^® has been developed. 2-Monofluorinated products can be obtained in high yield in neutral reaction conditions with addition of 1 eq. of Selectfluor^®. Treatment of 1,3-dicarbonyls with 3 eq. of Selectfluor^® in the presence of tetrabutylammonium hydroxide (TBAH) as the base results in the formation of 2,2-difluorinated derivatives only.     

Microwave-assisted synthesis of dinucleoside analogues containing a thiazolidin-4-one linkage via one-pot tandem Staudinger/aza-Wittig/cyclization

Dinucleosides containing a thiazolidin-4-one linkage were prepared by one-pot tandem Staudinger/aza-Wittig/intermolecular cyclization under microwave irradiation and their structures were confirmed. Preliminary examination of HIV-RT inhibition showed that the dinucleosides containing (R)-thiazolidin-4-one linkage are significantly more active than those containing (S)-thiazolidin-4-one linkage.     

Microwave-assisted rapid and efficient synthesis of C-alkyl imidazoisoquinolinone derivatives

The synthesis of a set of 10-benzyl-2,3-dihydroimidazo[1,2-b]isoquinolin-5(1H)-one and 5-oxo-imidazo[1,2-b]isoquinolin-10-yl)-N-phenylacetamide derivatives was achieved by exposing the corresponding alkylating agent and imidazoisoquinolinone to microwave irradiation and traditional oil bath heating in the presence of K₂CO₃ and DMAP. The microwave technique as well as DMAP as base accelerated the alkylation reaction for 2-6 min giving 79-88% yields.     

Microwave-assisted sequential amide bond formation and intramolecular amidation: a rapid entry to functionalized oxindoles

A general method has been developed for the synthesis of N-substituted oxindoles. The two-step process involves initial microwave-assisted amide bond formation between 2-halo-arylacetic acids and various alkylamines and anilines, followed by a palladium-catalyzed intramolecular amidation under aqueous conditions. In the case of alkylamines, the procedure can be carried out as a one-pot process without isolation of the intermediate amide. [structure: see text]     

Structure of veprisone, a constituent of Vepris bilocularis

Veprisone a neutral crystalline constituent of Vepris bilocularis has been shown to be methyl epi-isoobacunoate having structure I.