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Chapter 1
Synthesis of functionalized pyrimidines
Microwave assisted synthesis
In general, the standard procedure for the Biginelli condensation involves one
pot condensation of the three building blocks in a solvent such as ethanol using a
strongly acidic catalyst that is hydrochloric acid. One major drawback of this
procedure, apart from the long reaction time involving reflux temperature, is the
moderate yields frequently observed when using more complex building blocks.
Microwave irradiation (MW) has become accepted tool in organic synthesis, because
the rate enhancement, higher yields and often, improved selectivity with respect to
conventional reaction conditions. 45 The publication by Dandia A. et al. 46 described
microwave-enhanced solution-phase Biginelli reactions employing ethyl acetoacetate,
thiourea and a wide variety of aromatic aldehydes as building blocks (Figure-20).
Upon irradiation of the individual reaction mixtures (ethanol, catalytic HCl) in an
open glass beaker inside the cavity of a domestic microwave oven the reaction times
were reduced from 2–24 hours of conventional heating 80 °C, reflux to 3–11 minutes
under microwave activation (ca. 200 –300 W). At the same time the yields of DHPMs
obtained were distinctly improved compared to those reported earlier using
conventional conditions.
Figure-20
In recent years, solvent free reactions using either organic or inorganic solid
supports have received more attention. 47 There are several advantages to perform
synthesis in dry media: (i) short reaction times, (ii) increased safety, (iii) economic
advantages due to the absence of solvent. In addition, solvent free MW processes are
also clean and efficient. Gopalakrishnan M. and co-workers have reported Biginelli
reaction under microwave irradiation in solvent-free conditions using activated fly ash
as catalyst, an industrial waste (pollutant) is an efficient and novel catalyst for some
selected organic reactions in solvent free conditions under microwave irradiation. 48
Department of Chemistry, Saurashtra University, Rajkot-360005 11