Microwaves condensation

Thirumurugan and Perumal [16] effectively synthesized bisaryl-3-pyridinyl-l,2, 4-triazine derivative (i) following the microwave condensation of cinnamils with pyridine carboxtrisamidrazone in methanol. The triazine derivatives were obtained in good yields (85-66%) and showed good fluorescent properties when photochemical analysis was carried out. 

Villemin D, Alloumn AB (1991) Dry reaction under microwave condensation of sulfones... 

Using the microwave Just decreased the reaction time to 3-30minutes. The dudes in the article used a household, 500W Brazilian microwave (Yikesl). They cut a whole in the top of the microwave to allow the condenser apparatus to pass through the oven. They then killed themselves most likely. But not before they were able to scratch down this procedure as they slowly burned to death ... 

GENERAL PROCEDURE The Alcoholic alkaline solution is prepared by prolonged stirring of 8.8g (or 4.4g) of KOH pellets in 30mL of alcohol. The alkaline solution is placed in a round-bottom flask provided with a reflux condenser (microwave or conventional systems). Then 4.0g of Safrole (or eugenol) is added and the solution heated."... 

Sulfur bromides are but poorly characterized and there are few reliable data on them. SBr2 probably does not exist at room temperature but has been claimed as a matrix-isolated product when a mixture of S2Cl2/SCl2 Br2 Ar in the ratio 1 1 150 is passed through an 80-W microwave discharge and the product condensed on a Csl... 

The Pictet-Spengler reaction has been carried out on various solid support materials " and with microwave irradiation activation.Diverse structural analogues of (-)-Saframycin A have been prepared by carrying out the Pictet-Spengler isoquinoline synthesis on substrates attached to a polystyrene support. Amine 20 was condensed with aldehyde 21 followed by cyclization to give predominantly the cis isomer tetrahydroisoquinoline 22 which was further elaborated to (-)-Saframycin A analogues. 

Microwaves, electromagnetic spectrum and. 419 Mincralocorticoid, 1083 Minor groove (DNA), 1104-1105 Mitomycin C, structure of, 970 Mixed aldol reaction, 885-886 requirements for. 885-886 Mixed Claisen condensation reaction, 890-891... 

Like oxalic acid, oxalic esters and o-phenyldiamines give 2,3(l//,477)-quinox-alinediones that bear substituents according to those on the substrate such condensations appear to be assisted substantially by microwave irradiation. 

The Ugi reaction is the four-component condensation of an amine, aldehyde or ketone, carboxylic acid and isocyanide to give an o -acylamino amide [22-24], Although this process has the potential to introduce considerable diversity, the products themselves are not heterocycles but through appropriate choice of substrates, latent functionality in one of the precursors can intercept either an intermediate or further derivatize the acylamino amide Ugi product through post-modification. Thus variants of the Ugi reaction have been investigated under microwave-assisted conditions for the synthesis of diverse heterocyclic libraries [16,19-24],... 

The microwave acceleration of Ugi condensations on a sohd support have been utilized in the synthesis of an 18-membered targeted hbrary of a-acylamino amides [60], Irradiation of the four components, immobilizing the amine on TentaGel S RAM, for 3-5 min in a single-mode microwave synthesizer gave the products in moderate to excellent yields and high purity (Scheme 7). 

Microwave and fluorous technologies have been combined in the solution phase parallel synthesis of 3-aminoimidazo[l,2-a]pyridines and -pyrazines [63]. The three-component condensation of a perfluorooctane-sulfonyl (Rfs = CgFiy) substituted benzaldehyde by microwave irradiation in a single-mode instrument at 150 °C for 10 min in CH2CI2 - MeOH in the presence of Sc(OTf)3 gave the imidazo-annulated heterocycles that could be purified by fluorous solid phase extraction (Scheme 9). Subsequent Pd-catalyzed cross-coupling reactions of the fluorous sulfonates with arylboronic acids or thiols gave biaryls or aryl sulfides, respectively, albeit it in relatively low yields. 

Linking the ketone and carboxylic acid components together in an Ugi reaction facilitates the synthesis of pyrrolidinones amenable to library design. The three-component condensation of levulinic acid 30, an amine and isocyanide proceeds under microwave irradiation to give lactams 31 [65]. The optimum conditions were established by a design of experiments approach, varying the equivalents of amine, concentration, imine pre-formation time, microwave reaction time and reaction temperature, yielding lactams 31 at 100 °C in poor to excellent yield, after only 30 min compared to 48 h under ambient conditions (Scheme 11). 

Fewer procedures have been explored recently for the synthesis of simple six-membered heterocycles by microwave-assisted MCRs. Libraries of 3,5,6-trisubstituted 2-pyridones have been prepared by the rapid solution phase three-component condensation of CH-acidic carbonyl compounds 44, NJ -dimethylformamide dimethyl acetal 45 and methylene active nitriles 47 imder microwave irradiation [77]. In this one-pot, two-step process for the synthesis of simple pyridones, initial condensation between 44 and 45 under solvent-free conditions was facilitated in 5 -10 min at either ambient temperature or 100 ° C by microwave irradiation, depending upon the CH-acidic carbonyl compound 44 used, to give enamine intermediate 46 (Scheme 19). Addition of the nitrile 47 and catalytic piperidine, and irradiation at 100 °C for 5 min, gave a library of 2-pyridones 48 in reasonable overall yield and high individual purities. 

The synthesis of pyrido[2,3-d]pyrimidin-7(8H)-ones has also been achieved by a microwave-assisted MCR [87-89] that is based on the Victory reaction of 6-oxotetrahydropyridine-3-carbonitrile 57, obtained by reaction of an Q ,/3-unsaturated ester 56 and malonitrile 47 (Z = CN). The one-pot cyclo condensation of 56, amidines 58 and methylene active nitriles 47, either malonitrile or ethyl cyanoacetate, at 100 °C for benzamidine or 140 °C for reactions with guanidine, in methanol in the presence of a catalytic amount of sodium methoxide gave 4-oxo-60 or 4-aminopyridopyrimidines 59, respectively, in only 10 min in a single-mode microwave reactor [87,88]... 

The one-pot synthesis of thiazolo[3,4-a]benzimidazoles has been reported using a microwave-assisted condensation-cyclization (see Scheme 17) of a substituted 1,2-diamine, substituted benzaldehyde and mercaptoacetic acid [74]. Heating the mixture at reflux for 12 min using a single-mode microwave reactor for the most part gave the fused benzimidazoles in improved yield and dramatically shorter times, when compared to classical conditions of heating at reflux in benzene for 24-48 h (Scheme 29). 

With a similar approach, Dave and Shah have developed a simple, fast, solvent-free and high-yielding, variant of the Gould-Jacob type synthesis of thieno[3,2-e]pyrimido[l,2-c]pyrimidines. In this example the conventional condensation between 4-aminothieno-2,3-dyrimidines and diethyl ethoxymethylenemalonate via acyclic intermediates (usually performed in 5-6 h) was compared with a solvent-free single-step microwave procedure (7-10 min) applying a multimode microwave oven (BPL 700T, Mumbai, India) [7], Scheme 6. 

Thiazolines (2,3-dihydrothiazoles) were also prepared under microwave irradiation. Hamelin and coworkers have described the alumina-supported solvent-free synthesis of various 4-iminothiazolines by condensation of disymmetric thioureas and a-chloro ketone (Scheme 10). The experiments... 

One of the first published microwave-assisted synthesis of benzothiazoles is the condensation of a dinucleophile such as 2-aminothiophenol, with an ortho-ester (neat) in the presence of KSF clay in a mono-mode microwave reactor operating at 60 W under a nitrogene atmosphere [ 12] (Scheme 12). Traditional heating (oil bath, toluene as solvent and KSF clay) gave the expected products in similar yields compared to the microwave experiments but more than 12 h were required for completion. Solvent-free microwave-assisted syntheses of benzothiazoles was also described by attack of the dinucleophiles cited above on benzaldehydes and benzaldoximines [13] (Scheme 12). This methodology was performed in a dedicated monomode microwave reactor... 

Following a similar strategy, trifluoro acetyl ketene diethyl acetal was successively condensed with 2-aminothiophenol in the presence of toluene in a multimode microwave oven (8 min at 980 W) to give the 2-(l,l,l-trifluoro-acetonyl)benzothiazole ring in an excellent yield (93%) [14] (Scheme 13). hi this work, the temperature reached during reaction was not controlled reducing the reproducibility of the process. 

Condensation of 2-aminothiophenol with the /3-chlorocinnamaldehyde in the presence ofp-toluene sulfonic acid (PISA) gave good yield of benzothia-zole (Scheme 14). The mechanism suggested in this work is beUeved to proceed via a nucleophilic attack of the sulfur atom in an addition-ehmination sequence followed by a spontaneous cyclization and ejection of acetaldehyde [15]. These investigations were performed in a domestic microwave reactor and need 1.5 min for completion (65% yield). Here again, oil bath heating seems to be inferior, providing a maximum conversion of 53% after... 

Fig. 33 Microwave-assisted fluorous Ugi condensations. Reagents and conditions a MeOH, MW 100°C, 10-20 min b TFA-THF, MW 100 °C, 10-20 min. R = Ph, furyl, 3-Me-pyridil, i-Bu, MeSC2H4, PhC2H4 R = t-Bu, cylohexyl. Bn or Bu, m-xylU...

Oxetanes are present in several biologically active natural compounds as, for example, the taxol ring skeleton. An interesting method used to obtain this particular ring is the thermal [2 -i- 2] cycloaddition reaction. Longchar and co-workers reported a novel [2-1-2] cycloaddition of /1-formil enamides 5, often used in other cycloaddition and condensation processes, with acetylenic dienophiles 6 under microwave irradiation (in a domestic oven) to afford ox-etenes 7 in 80% yields [29]. This reaction was directed towards the synthesis of D-ring annelated heterosteroids (Scheme 2). 

Another series of pyrroles, structurally related to amino acids, was obtained in a microwave-assisted solvent-free condensation of a-amino acid methyl esters with chloroenones, which provided the four-carbon unit of the pyrrole. The reaction was carried out by mixing the reagents on silica gel and irradiating for 2-6 min inside a multimode microwave cavity (Scheme 7). The authors reported higher yields and cleaner products when microwaves were used instead of conventional heating [34],... 

An interesting family of polycyclic pyrroles was described in 2005 using again the synthetic sequence of a Stetter reaction for the preparation of the starting 1,4 diketones followed by a microwave-assisted Paal-Knorr condensation [35]. For example, cyclopentenone 23 (obtained in a Pauson-Khand cyclization) reacted imder Stetter reaction conditions to give the amino ketone 25 (Scheme 8). The microwave-assisted Paal-Knorr cyclization of 25 with different amines gave a small collection of tricychc pyrrole 2-carbox-amides. 

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