MW-Assisted Syntheses

A particularly useful application of MW-assisted synthesis at elevated pressure has been in the preparation of radiopharmaceuticals containing isotopes with short half-lives, such as C-ll (half-life 20 min) and F-18 (half-life 110 min) [25-27]. Clearly, these compounds have to be synthesized very rapidly in order to give products with high radiochemical yield. For example, [1-11C] tyrosine 12 was synthesized using the two step Bucher-Strecker method by the reaction of p-hydroxyphenylacetaldehyde bisulfite adduct 11 with K11CN and (NH4)2C03 followed by hydrolysis with aqueous NaOH (Scheme 4.7)... 

Desyl ethers are key intermediates in the synthesis of biologically active furanopy-rones. MW-assisted synthesis of these has been performed in an open vessel under PTC solvent-free conditions (Eq. 19) by alkylation of 7-hydroxy-4-methyl coumarin with desyl chloride [30],... 

The kinetics of the acid-catalyzed esterification reaction of 2,4,6-trimethylbenzoic acid in i-PrOH under microwave irradiation have been investigated [84], A simple and practical technique for MW-assisted synthesis of esters has been reported wherein the reactions are conducted either on solid mineral supports or by using a phase transfer catalyst (PTC) in the absence of organic solvents [85], The esterification of enols with acetic anhydride and iodine has also been recorded [86],... 

Scheme 6.45 MW-assisted synthesis of S-heterocycle-containing liquid crystalline compounds.

The Gewald synthesis can be performed on a solid support. Treatment of resin-bound nitrile 559 with carbonyl compounds and sulfur gives resin-bound aminothiophenes 560. Acylation and removal of the resin provide thiophenes 561 (Scheme 88) <2001TL7181>. The same resin-bound nitrile 559 reacts under MW irradiation to give 560 <2004S3055>. MW-assisted synthesis of 2-A -acylthiophenes 563 from 562 on a solid support has also been reported <2003SL63>. 

The solvent-free MW-assisted synthesis of 2-amino-substituted isoflav-3-enes has been reported it ean be aehieved in one pot via the in situ generation of enamines and their subsequent reaetions with salicylaldehydes (Scheme 8.22). This environmentally friendly procedure does not require azeotropic removal of water using large excess of aromatic hydrocarbon solvents for the generation of enamines. 

This MW-assisted synthesis of azides, thiocyanates and sulfones proved to be a useful alternative that avoids the use of environmentally detrimental volatile chlorinated hydrocarbons. Various functional groups such as ester, carboxylic acid, carbonyl and hydroxyl were unalTected under the mild reaction conditions employed. ... 

MW-assisted synthesis of acridine and quinazolines was performed on thin layer chromatographic plates that were readily amenable to parallel synthesis of their libraries (05JHC703). 

Condensation of urea with carbonyl compounds A rapid and efficient MW-assisted synthesis of hydantoins and thiohydantoins was described by Muccioli et al. [117]. The most straightforward conditions for synthesis of phenytoin are the base-catalyzed condensation using benzil and urea, known as the Biltz synthesis (Eq. 33). MW activation of the Biltz synthesis of phenytoin improved both yield and reaction time. The first step consists in MW activation of the reaction of benzil with (thio)urea the second includes the conversion of the resulting 2-(thio)hydantoin to hydantoin using hydrogen peroxide. When reactions were performed at the same temperature under both reaction conditions, yields were by far better under the action of MW and emphasized the evident specific MW effects. These are perfectly expected when one considers the polar TS involved in the first step (nucleophilic addition of neutral NH2 group on carbonyl moiety). 

MW-assisted synthesis proceeded more rapidly than with conventional heating, and reaction time was reduced to 30 min with the yield of approximately 67-71% of the high-molecular-weight fraction (FP MeOH), which is more important here. Under conventional conditions this fraction was afforded in ca 10-12% yield within 30 min. Under conventional conditions similar yields of the polyethers... 

A simple and practical technique for MW-assisted synthesis of esters has been reported wherein the reactions are conducted either on solid mineral supports or by using a phase-transfer catalyst (PTC) in the absence of organic solvents [95],... 

Scheme 8.71. MW-assisted synthesis of 3-aryl-4-hydroxyquinolin-2(l H)-ones.

Scheme 8.75. MW-assisted synthesis of pyrido fused-ring systems.

Another interesting MW-assisted MCR for construction of the DHP ring as part of a more complex bicyclic scaffold was reported by Quiroga et al. They realized the MW-assisted synthesis of both pyrazolopyridines 7 and pyrido[2,3-d]pyrimidines 8 under solvent-free conditions (Scheme 17.8) [38]. 

In another MW-assisted synthesis of pyridones, a ketone, dimethylformamide di-methylacetal 16, and a methylene active nitrile are combined (Scheme 17.12) [44]. 

The estrogenic properties displayed by isoflav-3-ene derivatives have attracted the attention of medical chemists. Varma et al. have discovered a facile and general method for the MW-assisted synthesis of isoflav-3-enes substituted with basic moieties at the 2-position (Scheme 2.2-45) [134]. This convergent one-pot approach exploits the in situ generated enamine derivatives that are subsequently reacted with o-hydroxyaldehydes in the same pot... 

The rapid MW-assisted synthesis of bridgehead nitrogen heterocycdes has heen achieved under solvent-free conditions as exemplified by the synthesis of pyrimi-dino[l,6-a]benzimidazoles [135] and 2,3-dihydroimidazo[l,2-c]pyrimidines [136] from N-acylimidates and activated 2-benzimidazoles or imidazoline ketene ami-nals respectively (Scheme 2.2-46). 

An improved preparation of indolizinoquinolines using MW-assisted synthesis involves the Friedlander reaction of o-aminobenzaldehydes or imines with tetrahydroindolizinediones to form a quinoline (02SL2077). 

MW-assisted synthesis has been carried out using standard organic solverrts rmder open-vessel conditions. If solvents are heated by MWI at atmospheric pressrrre in an open vessel, the boiling point of the solvent typically hmits the reaction temperature that can be achieved. In order to, nonetheless, achieve high reaction rates, high-boiling MW-absorbing solvents have been frequently used in an open-vessel... 

Raghvendra et al. have synthesized a simple, convenient MW-assisted synthesis of 2-hydiazino-[l,3,4]thiadiazepino[7,6-Z ]quinolines (39a-h) in the presence of / -TsOH catalyst from the reaction of snbstitnted 2-chloro-3-fomtyl-qninoline (1) and caihi-dimide (37) in shorter time with good yield (Scheme 6.16) [55]. 

Nadaraj et al. carried out MW-assisted synthesis of 4-methyl-2-hydroxy-quinolines (46) and 2-methyl-4-hydroxyquiniolines (47) from aniline and ethyl acetoacetate (Scheme 6.19) [58],... 

Direct MW irradiation of solid reactants has been used to prepare solids. MW can also be used as an aging source in hydrothermal synthesis (see below). MW heating can also be combined with other synthesis procedures, for instance, with sol-gel procedures to prepare solid oxide fuel cells such as Lai >,Sr >,Fe03+5 and Lai Sr Fe eCoi j,03+5 [6], or in a MW-induced autocombustion to produce LaMnOs [7]. The application of MW to synthesize perovskites has been recently reviewed [8]. MW-assisted synthesis of YBa2Cu307 e can be achieved by... 

The high-temperature solution procedure was recently modified by the introduction of microwave-assisted (MW) polycondensation. This technique now widely used in organic chemistry, employed to promote chemical reactions in extremely fast and sometimes unconventional ways. The MW-assisted synthesis of PAs was performed to promote the condensation of aromatic diacids and diamines under Yamazaki conditions. The conventional heating system, i.e., temperature control oil bath, is replaced by the MW system, which... 

MW-Assisted Synthesis of Bisubstituted Yttrium Carnet Nanoparticles... 

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