Microwave irradiation fused heterocycles

Sequential Suzuki-Miyaura cross-couplings and Cadogan cyclizations were developed under microwave dielectric heating conditions to access a variety of 2-substituted carbazoles and other fused heterocyclic systems (Scheme 13). The use of microwave irradiation not only minimized the proto-... 

Lindsley and co-workers developed a general procedure towards the collection of diverse heterocyclic scaffolds from common 1,2-diketone intermediates 96. Substituted quinoxalines 97, fused pyrazolo [ 4,5-g ] quinoxalines 98 and imidazolo[3,4-g]quinoxalines 99 as well as pyrido[2,3-fo]pyrazines 100 and Ihicno[3,4-fo Ipyrazincs 101 have been prepared in excellent yields [132] (Scheme 54), employing optimized reaction conditions (microwave heating of equimolar mixtures of 1,2-diketone 96 and diamine components at 160 °C for 5 min in 9 1 MeOH - AcOH). The use of microwave irradiation resulted in reduced reaction times (5 min vs. 2-12 hours), improved yields as well as the suppressed formation of polymeric species a characteristic of traditional... 

The hetero-Diels-Alder reaction is amongst the most efficient processes for the synthesis of six-membered heterocyclic ring systems. Solvent-free conditions have been used to improve reactions of heterodienophiles and heterodynes with low reactivities. Cado et al. (1997) have described the hetero-Diels-Alder reaction of ethyl lH-perimidine-2-acetate as heterocyclic ketene aminal with ethyl propiolate nnder solvent-free conditions with focused microwave irradiation. The new fused perimi-dines (23) were obtained in good yields (67-98%). 

This account provides a summary of a chapter on Microwave Irradiation for Accelerating Organic Reactions in Advances in Heterocyclic Chemistry, volume 90, Part II Six, Seven-membered, Spiro and Fused Heterocycles by E. S. H. El Ashry, A. A. Kassem and E. Ramadan. Microwave irradiation (MWI) has been used extensively in organic synthesis. Application of MWI leads to many advantages, like the use of non-corrosive and inexpensive reagents, in addition to the eco-friendly "green chemistry" economical and environmental impacts. 

Keywords Microwave irradiation, green chemistry, organic synthesis, six and seven-membered heterocycles, spiro and fused heterocycles... 

General reviews Tierney, J. R. and Lidstrom, R. (Eds), Microwave-Assisted Synthesis , Blackwell, 2005 Microwave irradiation for accelerating organic reactions. Rart I Three-, four- and five-membered heterocycles , El Ashry, E. S. H., Ramadan, E., Kassem, A. A. and Hagar, M., A v. Het-erocycl. Chem., 2005, 88,1 Rart II Six-, seven-membered, spiro, and fused heterocycles , El Ashry, E. S. H., Ramadan, E. and Kassem, A. K,Adv. Heterocycl. Chem., 2006, 90, 1. 

A variety of fused 3-aminoimidazoles has been synthesized by a microwave-assisted Ugi three-component coupling reaction catalyzed by scandium triflate in methanol as solvent. The reactions of heterocyclic amidines with aldehydes and isocyanides were performed in 33 to 93% yields within only 10 min of microwave irradiation using a simple one-stage procedure [119]. 

Reaction of strong CH-acidic such as Meldrum s acid or barbituric acid derivatives, with aldehydes and urea was studied by Shaabani et al. this led to an efficient sol-vent-free synthesis of spiro-fused heterocycles by use of microwave irradiation [158]. 

Microwave Irradiation for Accelerating Organic Reactions - Part II Six-, Seven-Membered, Spiro, and Fused Heterocycles... 

El Ashry, E. S. H., Kassem, A. A., Ramadan, E., Microwave Irradiation for Accelerating Organic Reactions - Part II Six-, Seven-Membered, Spiro, and Fused Heterocycles, 90, 1. 

Volume 90 of Advances in Heterocyclic Chemistry contains four chapters and the indexes. The book commences with Part II of Applications of Microwave Irradiation for Accelerating Reactions in Heterocycles. This continues Part I, which was published in Volume 88 of Advances in Heterocyclic Chemistry in 2005. Whereas Part I dealt with 3-, 4- and 5-membered heterocycles, the present Part II deals with 6-and 7-membered and spiro and fused heterocycles. Parts I and II have both been contributed by E. S. H. El Ashry, A. A. Kassem and E. Ramadan of Alexandria University, Egypt, and make available the large amount of recent work in this expanding area. 

Keywords Five-membered heterocycles Fused heterocycles Heterocycles Microwave irradiation Six-membered heterocycles... 

IL-supported one-pot multicomponent reaction for the construction of bi-heterocyclic-fused pyrrolo[l,2-a]benz-imidazoles 114 under microwave irradiation were reported by Sxm and coworkers (Scheme 12) [47]. The IL-anchored diamines 97 were coupled with cyanoacetic acid followed by cyclodehydration to generate key intermediate 2-sub-stituted benzimidazoles 113. IL-supported 113 reacted with aldehyde and isocyanide through Knoevenagel condensation followed by [4 +1] cycloaddition in one-pot affording pyrrolo[l,2-fl]benzimidazole 114. Remarkably, this is the first time applied IL support and isocyanide-based multicomponent react for the synthesis of fused tricyclic heterocycles under microwave irradiation. 

In an effort to synthesize fused 7/5 heterocyclic ring system to obtain boosted bioactivity, Gupta et al. [102] condensed two heterocyclic systems, triazole and triazepine, into a single framework by the cyclocondensation of 5-aryl-3,4-diamino-l,2,4-triazoles with p-chlorocinnamaldehydes in the presence of catalytic amount of p-TsOH in DMF xmder microwave irradiation (also conventional oil bath heating) to afford 3,6-diaryl-5H-[l,2,4] triazolo[4,3-fc]-l,2,4-triazepines (103). Both the heterocyclic motifs (triazepine and triazole) are well-known core units of several drugs and bioactive compoxmds (Scheme 34). 

Almost simultaneously, Hua and co-workers developed a versatile and straightforward route to construct multisubstituted isoquinolines and relative fused pyridine heterocycles (60) by using readily availableketones (59) and alkynes (Scheme 7.41) [110]. The reaction involves condensation of aryl ketones and hydroxylamine, rhodium(III)-catalyzed C-H bond activation of the in situ generated aryl ketone oximes, and cycUzation with internal alkynes. This reaction proceeds under external-oxidant-free and moderately mild conditions. Later, a similar one-pot multi-component process promoted by a Rh(III) catalyst that generates substituted isoquinolines under microwave irradiation conditions was uncovered by Jun [111]. 

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