Microwave assisted synthesis of N-arylheterocyclic substituted-4-aminoquinazoline derivatives

A simple, efficient, and general method has been developed for the synthesis of various N-aryl heterocylic substituted-4-aminoquinazoline compounds from 4-chloro- quinazoline and aryl heterocyclic amines under microwave irradiation using 2-propanol as solvent. The advantages of the use of microwave irradiation in relation to the classical method were demonstrated.


Introduction
As part our ongoing research program on heterocyclic compounds which may serve as leads for designing novel antitumor agents, we were particularly interested in 4-substituted quinazolines [1][2].We considered the well known activity of the quinazoline nucleus in chemotherapy, where many of its substituted derivatives are effective antitumor agents [3][4][5][6].Furthermore, more recent data has reported that a broad class of quinazolines also act as potent and highly selective inhibitors of epidermal growth factor receptor (EGFR) or epidermal growth factor receptor tyrosine kinase (EGFR-TK) [7][8][9]; members of this class are expected to have great therapeutic potential in the treatment of malignant and nonmalignant epithelial diseases [10][11].In view of these facts, and in order to study the influence of 4-position substituted on antitumor activity, we have now prepared a series of new 4-aryl heterocyclic aminoquinazolines containing diverse heterocyclic moieties in the hope of discovering more active ATP site inhibitors.In the classical synthesis of these compounds [12][13], a mixture of 4-chloroquinazoline and aniline are refluxed at 80°C for 12 h. in 2-propanol.This method, however, involves long reaction times and complex handling and gives low yields of products.We have recently developed a new synthesis method for the target compounds using microwave irradiation in 2-propanol (Scheme 1).The new method requires short reaction times, is very easy and mild and environmentally friendly.To the best of our knowledge, this is the first report on the synthesis of new quinazoline compounds containing heterocycle moieties using microwave irradiation.Scheme 1. Synthesis of N-arylheterocyclic substituted-4-aminoquinazoline derivatives.

Results and Discussion
The reaction results with or without microwave irradiation are shown in Table 1.It can be seen that the presence of microwave irradiation both accelerated the reactions and gave higher yields.The reaction time for synthesis of compounds 5a~5g was shortened from 12 h.to only 20 min.with the one-step microwave-assisted procedure.
In order to optimize the reaction parameters, we selected compound 5b for further study under different conditions.These results are shown in Table 2. Without microwave irradiation (Table 1) compound 5b could be obtained in 37.3% after 12 h.When the reaction was carried out under microwave irradiation at 80°C for 10 min.the yield of 5b was increased to 91.0%, and increased further to 96.5% when the reaction time was extended to 20 min.(Table 2, entries 1 and 2).However, no further improvement of the yield was noted when the reaction time was prolonged to 30 min.(Table 2, entry 3) and the yield even decreased a little, a fact we attribute to the formation of byproducts.Consequently, we chose 20 min.as the optimum reaction time (Table 2, entry 2).As for the effect of the microwave power, it could be seen that when it was increased from 40 to 60, 80 and 100W, the yields of 5b were 75.9%, 96.5%, 98.8%, and 97.0%, respectively (Table 2, entries 2 and 4-6).Hence, it's better for the reaction to be carried out at 60W or higher power settings than at 40W.No improvement was observed under irradiation when the microwave power varied from 80 to 100W (Table 2, entries 5 and 6).When the reaction temperature was increased from 30°C to 50°C, 70°C or 80°C, 5b was obtained in 79.9 %, 84.0 %, 90.0 % and 96.5 % yields respectively (Table 2, entry 2 and entries 7-9).a Yields of isolated products.Each reaction was repeated three times and the result was averaged.

Conclusions
In summary, the present new method of the formation of N-arylheterocyclic substituted-4-aminoquinazoline derivatives 5a~5g under microwave irradiation offers several advantages: faster reaction rates and high yields, while the classical method of formation of N-arylheterocyclic substituted-4-aminoquinazoline derivatives involves long reaction times (12 h).All compounds 5a~5g were fully characterized by spectroscopic methods.
Foundation for Science and Technology Excellent Talent in Guizhou Province, China.

Synthesis of N-arylheterocyclic substituted-4-aminoquinazolines 5a-g (microwave method):
A mixture of 4-chloroquinazoline (3.0 mmol) and aryl heterocyclic amine (3.0 mmol) in 2-propanol (30 mL) was stirred for three min., then the mixture was irradiated in the microwave oven at 60W for 20 min.Upon completion of the reaction, as monitored by TLC, the solvent was removed under reduced pressure and the residue was washed with water, filtered off and purified by silica gel column chromatography (petroleum ether-ethyl acetate, 5:1 v:v) to give the title compounds.

Table 1
Yields a and reaction conditions used for the microwave assisted synthesis of 5a-5g.
a Yields of isolated products.b Reaction conditions: i-PrOH, reflux under MW (60W power).c Reaction conditions: i-PrOH, reflux temperature.

Table 2 .
Different conditions used for the microwave assisted synthesis of 5b.