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GREENER SYNTHESIS OF HETEROCYCLIC COMPOUNDS USING MICROWAVE IRRADIATION
Citation:
VARMA, R. S. GREENER SYNTHESIS OF HETEROCYCLIC COMPOUNDS USING MICROWAVE IRRADIATION. Presented at 21st International Congress for Heterocyclic Chemistry, Sydney, AUSTRALIA, July 15 - 20, 2007.
Impact/Purpose:
To inform the public.
Description:
An introduction of our interest in the microwave-assisted greener synthesis of a variety of heterocyclic compounds will be presented. It involves microwave (MW) exposure of neat reactants (undiluted) catalyzed by the surfaces of recyclable mineral supports, such as alumina, silica, clay, or their ‘doped’ versions, for the rapid one-pot assembly of heterocyclic compounds, such as flavonoids, related benzopyrans and quinolone derivatives [1]. The strategy to assemble oxygen- and nitrogen-heterocycles from in situ generated reactive intermediates via enamines [2] or using hypervalent iodine reagents [3] will be described. Examples of multicomponent reactions that can be adapted for rapid parallel synthesis include solventless synthesis of dihydropyrimidine -2(1H)-ones (Biginelli reaction) [4], imidazo[1,2-a]annulated pyridines, pyrazines, and pyrimidines (Ugi reaction) [5]. Organic synthesis in aqueous media [6] is rapidly gaining importance in view of the fact that the use of many toxic and volatile organic solvents, particularly halogenated hydrocarbons, contributes to pollution. Water is relatively benign [7] and has been utilized in combination with MW irradiation for the N-alkylation of amines by alkyl halides in the presence of aqueous NaOH [8]. This nucleophilic substitution chemistry can be manipulated using microwaves to generate cyclic amines via double N-alkylation of primary amines by dihalides or tosylates [9]. The protocol circumvents running multi-step reactions to assemble N-aryl azacycloalkanes, isoindoles, etc. and avoids the use of expensive metal catalysts in building aryl C-N bonds; functional groups such as carbonyl, ester, hydroxyl, etc. remain unaffected under these mild reaction conditions. The approach is also extended to hydrazines thus providing access to dihydropyrazoles (Scheme) [9].