Efficient Niementowski synthesis of novel 1 , 3,10,12-tetra-substituted-8 H -pyrido[2',3':4,5]pyrimido[6,1-b ]quinazolin-8-ones

Novel 1,3,10,12-tetrasubstituted-8 H -pyrido[2',3':4,5]pyrimido[6,1-b ]quinazolin-8-ones were synthesised by a Niementowski reaction involving condensation of substituted anthranilic acids with a 5,7-disubstituted-3 H -pyrido[2,3-d ]pyrimidin-4-ones.


Introduction
Quinazolines and condensed pyrimidines show a wide spectrum of biological activities and have been exhaustively reviewed.Pyrido [2,3-d] pyrimidines are considered to be bioisosteres of quinazolines.The concept of bioisosterism has been exploited by medicinal chemists as an approach to the drug design.This has lead to the synthesis of various types of condensed pyrimidines, which show a wide range of biological activities.A rapid progress in the work on fused quinazolinones and pyridopyrimidines has given rise to a number of compounds exhibiting potent pharmacological actions like adenosine kinase inhibitory activity [1][2][3] , EGFr and C-erbB-2 inhibitory acitivity 4 , antibacterials 5 , and phosphodiesterase 5 inhibitory activity [6][7][8][9] .Examples of natural nitrogen heterocycles are Rutaecarpine which possesses intrinsic diuretic, uterotonic and hypertensive 10 , and Luotonine A3, which exhibits antitumor activity 11 .These natural compounds contain a quinazolinone moiety fused with indolopyrido and pyrroloquinoline ring systems, respectively (Figure 1).In search of new fused heterocyclic compounds with potential pharmaceutical value and in association with our work on the application of microwaves in organic chemistry 12 , we planned to prepare novel tetracyclic 1,3,10,12tetrasubstituted-8H-pyrido[2',3':4,5]-pyrimido [6,1-b]quinazolin-8-ones 4, from substituted anthranilic acids 3 and pyrido [2,3-d]pyrimidines 2 which constitutes the Niementowski condensation 20 .

Figure 1
Microwave-assisted reactions are now well recognized and have gained popularity as indicated by the large number of papers currently published on this topic [13][14][15][16] The beneficial effects of microwave irradiation are finding an increased role specially in process chemistry and in cases where usual methods require forcing conditions or prolonged reaction times.Microwaves have also shown an advantage when products may decompose under prolonged reaction conditions.The possibilities offered by microwave-assisted reactions are particularly attractive for multi-step synthesis 17 and drug discovery process 18,19 where purification are highly desirable.In this paper the one-step synthesis of the novel pyrido [2',3':4,5]pyrimido [6,1b]quinazoline ring was realized under microwave irradiation aimed at developing an original and environmentally friendly procedure.

Results and Discussion
The first step in this reaction involved the synthesis of pyrido [2,3-d]pyrimidin-4-ones 2 which took place by reacting 2-amino-4,6-disubstitutednicotinonitriles 1 with formic acid.The second step of the route involves the reaction anthranilic acids 3 and pyrido [2,3-d]pyrimidines 2 to give the products 4 via the Niementowski condensation 20 .The latter reaction is either carried out in microwave and by classical heating using polyphosphoric acid, or by the direct fusion method.It is assumed that formation of products 4 requires an intramolecular acyl substitution between the pyrimidine nitrogen and the carboxylic acid group of the intermediate carboxylic acid (Scheme 1).A comparative study of the three methods used for synthesizing compounds 4, conventional heating (Method A), microwave heating (Method B) and direct fusion method (Method C) showed that the microwave method gave overall cleaner and higher yielding reactions (Table 1).To our knowledge, all compounds synthesised by this reaction are novel.The IR (KBr) spectra of 4 showed characteristic C=O stretching vibrations in the region 1785-1650cm -1 .The C=C and C=N ring stretching vibrations appeared in the area around 1620 and 1570-1520 cm -1 .The IR bands due to NH and COOH vibrations were not present in any of the spectrum of the compound 4 which ruled out the possibility of any remaining uncyclised material.
The direct cyclization to compounds 4 is also supported by 1 HNMR spectral data of these compounds since no resonance due to the NH or OH protons appeared in any of the spectra.

Experimental Section
General Procedures.The microwave assisted synthesis were performed in scientific microwave oven RAGA's microwave oven.Melting points were determined in open capillaries using a Thermonik C-PMB-2 precision melting point and boiling point apparatus, (Mumbai, India) and are uncorrected.The purity of the compound was routinely checked by TLC using silica gel-G and the spots were exposed in iodine vapour.IR spectra were recorded using KBr pellets on a Shimadzu 1600 Spectrophotometer from Shimadzu International Incorporation, (Vmax cm -1 ), 1 H NMR spectra on Bruker Avance 400 Spectrometer (Bruker AG, Fallanden, Switzerland) at 400 MHz using TMS as internal standard (Solvent CDCl 3 and chemical shift in δ ppm) and mass spectra (EI-MS) were recorded on a Jeol SX-102 spectrometer (Jeol Ltd.Tokyo,Japan).Elemental analyses were carried out at Heraeus Carlo Erba 1180 CHN analyzer (from Heraeus Instrument GmbH, Hanau, Germany).All the chemicals were purchased from Aldrich Company Ltd.Dorset (UK).

Synthesis of 8H-pyrido[2',3':4,5]pyrimido[6,1-b]quinazolin-8-ones (4a-u): General procedures Method A (classical)
A mixture of the appropriate compound 2a-g (0.61 mmole) and anthranilic acid 3a-c (3.4 mmole) in polyphosphoric acid (5 ml) was stirred and heated at 160 o C for 4 hours.The reaction mixture was cooled, slowly added to ice-water mixture and neutralized with aqueous ammonia after which a solid precipitated.The solid was filtered, washed with water, dried and recrystallised from ethanol to yield the corresponding title compound 4a-u.

Method B (microwave)
A mixture of the appropriate compound 2a-g (0.61 mmole) and anthranilic acid 3a-c (3.4 mmole) in polyphosphoric acid (5ml) was introduced in Pyrex tube.The Pyrex tube was irradiated for 10 min (power output 140W).The reaction tube was cooled, slowly added to icewater mixture and neutralized with aqueous ammonia when a solid precipitated.The solid was filtered, washed with water, dried and recrystallised from ethanol to yield the corresponding title compound 4a-u.

Method C (direct fusion method)
In a round bottom flask, a mixture of the appropriate compound 2a-g (0.61 mmole) and anthranilic acid 3a-c (3.4 mmole) were ground thoroughly and the mixture was heated above its melting point (+5ºC).The reaction mixture was kept in molten state for 5-10 minutes and then was cooled gradually.The residue obtained on cooling was triturated with petroleum ether, filter and recrystallised from ethanol to yield the corresponding title compound 4a-u.

Table 1 .
Physical data of compound 4a-u