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Chapter – 4 Studies on different types of reactions…..
used while differently substituted β-keto esters have also been used for the
synthesis of diversified dihydropyrimidine derivatives via Biginelli reaction.
Use of 4-hydroxycoumarin in the synthesis of dihydropyrimidine
synthesis is not much reported. The tautomeric form (1) of 4-hydroxycoumarin
acts as a cyclic β-keto ester (Fig. 4.27) and condenses with
aldehydes in the presence of urea / thiourea under acidic conditions and gives
rise to the expected coumarin fused pyrimidines.
Fig. 4.27
Brambhatt et. al. 305 reported synthetic method for the fused
benzopyranopyrimidine derivatives. They reported the poor yielded synthetic
procedure with the longer time period.
Microwave assisted green chemical approach was applied to the
synthesis of benzopyranopyrimidines by Kidwai et. al. 306 This method not
only gave better yield with reduced reaction time but also eliminated the need
of an external acid due to the usage of acidic solid support. Since the reaction
takes place under acidic conditions, the effect of three different acidic
inorganic support viz. acidic alumina, montmorillonite K10 clay and silica gel
was explored. The same reaction was also carried out in solution phase under
MW. A conventional method with basic alumina in oil bath maintained at 110–
120°C was also attempted, but the reaction took from 4 to 6 h to give the
required product in about 39–60 % yield. This clearly indicates that the
coupling of microwaves with the solid supported reagent is more than simple
thermal effects. Furthermore They 307 eliminated the use of inorganic solid
support and condensed all the three components i.e. 4-hydroxycoumarin,
aromatic aldehydes and urea / thiourea, neat under microwave irradiation.
Department of Chemistry, Saurashtra University, Rajkot – 360 005 215