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Heterocyclic Chemistry
Heterocyclic chemistry is another area of importance in synthetic chemistry. A large number of natural products and target drug compounds contain a heterocyclic core. Synthetic routes toward these compounds are usually quite challenging. Microwave-induced heterocyclic chemistry has been extensively examined with pyrimidine derivatives49,125,130,131,169,238-251, pyrroles252-257, pyridines59,64,68,108,258-274, β-lactams11,111,275-286, indoles287-298, γ-carbolines299, quinolines and quinolones47,96,110,112,113,118,300-303, quinazolines67,304-309, imidazoles133,155-156,258,264,265,310-327, other azole/azoline derivatives57,58,64,71,106,107,109,119,133,327-348, furans349-353, and 1,3-dipolar cycloadducts61-63,155,157,177,355-378.
The Biginelli three-component condensation reaction is a one-pot synthesis to dihydropyrimidines. These heterocyclic systems contribute to enhanced pharmacological efficiency in a variety of biological effects, including antiviral, antitumor, antibacterial, and anti-inflammatory activities. With normal conventional heating, these reactions can take approximately 24 hours for complete transformation with only low to moderate yields. Upon microwave irradiation, the Biginelli reaction was successfully completed in five minutes, with 60-90% yields (Scheme 48).354
Scheme 48
The Paal-Knorr condensation/cyclization reacts 1,4-diketones with primary amines to form N-substituted pyrroles. This synthesis requires at least twelve hours of prolonged thermal heating and added Lewis acids to activate the diketones. With microwaves, transformation occurred in anywhere from 30 seconds to two minutes with very high yields (75-90%) (Scheme 49).255
Scheme 49
Scheme 50
Substituted dihydropyridines are known to be calcium channel blockers and are quite biologically active. They can be synthesized via the one-pot Hantzsch pyridine reaction. In this particular reaction, an aldehyde, two equivalents of a β-ketoester, and ammonium hydroxide are combined in the same reaction vessel. One equivalent of the β-ketoester and the aldehyde undergo an aldol condensation. The other equivalent reacts with the ammonium hydroxide to yield an enamine. The final transformation, as shown in Scheme 50, results in a dihydropyridine derivative. Classical thermal heating takes over 24 hours, whereas these reactions occur in five minutes or less with microwave irradiation.259,260,267
For decades, synthetic and medicinal chemists have been greatly interested in β-lactams. These four membered chiral heterocycles are versatile synthons in natural product synthesis. Additionally, they can easily undergo rearrangements to yield other heterocyclic and acyclic compounds. Bose and coworkers have done extensive research on microwave-induced β-lactam synthesis.11,278,279,282,283,285 Their early work on β-lactams via conductive heating led to extremely low yields. Utilization of microwave heating for five minutes on an α,β-unsaturated acid chloride and a Schiff base (imine) provides β-lactams in 65-70% yield (Scheme 51). Another synthetic strategy to β-lactams utilizes di-azoketones (Scheme 52).275 Upon irradiation with microwaves, the diazoketone compound is transformed into a ketene, which then rapidly cyclizes with the imine to yield a β-lactam. Classical conditions rarely yielded a product, whereas microwave irradiation produced 60-80% yield of β-lactam.
Scheme 51
Scheme 52
The Fisher indole cyclization is a one-pot reaction to substituted indolic compounds. This powerful synthetic reaction utilizes an arylhydrazone and an aldehyde or ketone. After a [3,3]-sigmatropic rearrangement, elimination of ammonia yields an indole. Microwave irradiation greatly accelerates (385-fold rate enhancement) this rearrangement, resulting in successful completion of the reaction in less than 30 seconds (Scheme 53).294-296
Scheme 53
The Graebe-Ullmann synthesis, which yields γ-carbolines, is another one-pot reaction. This two-step procedure firsts reacts benzotriazole with a 4-chloropyridine followed by polyphosphoric acid addition to produce thermolytic ring closure. Traditionally, both steps require extremely high temperatures for reaction to occur, and the yields are very dependent on temperature control. With microwave heating, the first step occurs in ten minutes (160 W), plus an additional five minutes (or until nitrogen evolution ceases), after the addition of polyphosphoric acid (Scheme 54).299
Scheme 54
Quinolines and quinazolines are hetero-bicyclic ring systems containing one and two nitrogen atoms, respectively. These compounds and their derivatives have been shown significant interest by medicinal chemists because of the numerous natural products and potential drug compounds that contain their heterocyclic core. The classic Skraup quinoline synthesis requires large amounts of sulfuric acid and high temperatures, which can be quite a violent combination, and usually does not produce satisfactory yields. With microwave irradiation, it has been reported that an aniline, an alkyl vinyl ketone, and indium(III) chloride (catalyst) on silica gel provide 4-alkylquinolines in 80-90% yield (Scheme 55).300 Quinazolines can be synthesized from an N-arylimino dithiazole derivative and sodium hydride in refluxing ethanol. Reaction times were greatly reduced from 40 hours to two hours with microwave energy (Scheme 56).304
Scheme 55
Scheme 56
Azole derivatives — which include imidazoles, oxazoles, thiazoles, and tri/tetraazoles — are five-membered rings containing at least two heteroatoms, one being nitrogen. Once again, these compounds contain very important heterocyclic cores that are common in drugs and natural products. Benzimidazoles, -oxazoles, and -thiazoles can be easily synthesized from o-arylenediamines or other o-arylene heteronucleophiles and a substituted acetylketene diethyl acetal with microwave irradiation (Scheme 57).327 No reaction occurred with thermal heat.
Scheme 57
Another route to substituted imidazoles can be accomplished by condensation of a 1,2-dicarbonyl compound with an aldehyde and ammonia. Classically, this reaction requires four hours of intense heating in acetic acid. With microwave irradiation, imidazoles were produced in 20 minutes (Scheme 58).316
Scheme 58
Triazoles and tetrazoles contain, as their prefixes indicate, three and four nitrogen atoms, respectively. Treatment of arylnitriles with hydrazine dichloride and hydrazine hydrate in ethylene glycol provides substituted 1,2,4-triazoles. Conventional methods require 45-60 minutes of intense heating and provide moderate yields. Microwave irradiation produced the triazole products in about five minutes (Scheme 59).345 Tetrazoles can also be synthesized from nitriles, these via palladium-catalyzed cyanation of organo bromides with zinc cyanide. Subsequent addition of sodium azide and ammonium chloride yield tetrazoles. Conductive heating usually takes from seven hours to four days, while microwaves yielded product in 15 minutes (Scheme 60).348
Scheme 59
Scheme 60
Furans also contain a common heterocyclic core that is seen in many natural products and drug compounds. Diepoxides, in the presence of sodium iodide, rearrange to form substituted furan ethers. Using thermal heat, this reaction proceeds in five hours with only a 43% yield. With microwave irradiation, rearrangement only took five minutes with an 88% product yield (Scheme 61).352 Additionally, ferrocenyl substituted acrylaldehydes react with a β-hydroxy (or thiol) ester to yield furans (thiophenes) in two minutes under microwave heating (Scheme 62).353 Conventional methods require 24 hours of reflux.
Scheme 61
Scheme 62
A 1,3,4-oxadiazole synthesis, as shown in Scheme 63, was executed in two steps.354 The second stage of this reaction was performed, first, with conventional heating methods at 150 °C for 90 minutes. In a separate reaction using microwave energy, the second stage was completed at the same temperature, but it only took five to ten minutes. This reaction was 100-fold faster, even though the measured bulk temperatures were the same.
Scheme 63
Another organic transformation that benefits from the use of microwave irradiation is a 1,3-dipolar reaction.23,61-63,155,157,177,355-378 These types of reactions are cycloadditions, but they are being discussed in this section because they form heterocyclic moieties. One of the main reactants, which are generated in situ, is a 1,3-dipole. 1,3-Dipoles are heteroatom molecules that contain both a positive and a negative charge. Due to ionic conduction, these ionic species will directly interact with the microwave energy being applied. They readily react with a dipolarophile, which is usually an electrophilic alkene or alkyne, to form heterocyclic ring systems. Using conventional methods, these cycloadditions can take 1-2 days. In Scheme 64, nitrile oxides (X = O) and nitrile imines (X = NPh) are irradiated with microwaves to yield different heterocyclic compounds in only 30 minutes.355
Scheme 64
Instruments
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