Scheme 1. Synthesis of pyrido[2,3-
d]pyrimidine-2,4-diamine (
4) by Kisliuk et al.
[20].
Kisliuk et al. also developed another strategy to synthesize pyrido[2,3-
d] pyrimidine-2,4-diamines as compound
9 (
Scheme 2,
Table 1, entry 2). Starting from 2,4,6-triaminopyrimidine (
5) with the sodium salt of nitromalonaldehyde, they obtained in a single step the 2,4-diamino-6-nitropyrido [2,3-
d]pyrimidine (
7) which was then reduced to its corresponding 6-amino analogue using Raney Ni in DMF. The reductive amination with various aldehydes (ArCHO, in this case 3,4,5-trimethoxybenzaldehyde) provided the desired product
8. In the last step,
8 was
N-methylated by treatment with formaldehyde in the presence of sodium cyanoborohydride
[15] (
Scheme 2). An analog compound (
Table 1, entry 3) was obtained following the same synthetic pathway (
Scheme 2) using 3,5-dimethoxybenzaldehyde.
Scheme 2. Synthesis of pyrido[2,3-
d]pyrimidine-2,4-diamine (
9) by Kisliuk et al.
[15].
In 2008, Queener et al. synthesized
12 starting from 2,4-diamino-6-nitroquinazoline
7 which underwent reduction with hydrogen and Raney nickel at 30-35 psi, providing the desired 2,4,6-triaminoquinazoline (
10) (
Scheme 3). Then, as described above, the 2,5-dimethoxybenzaldehyde ArCHO was added to generate the N9-H precursor
11. The following step was a reductive N9-alkylation using sodium cyanoborohydride which afforded the final compound
[14]. Queener et al. conducted a biological evaluation of this compound
12 (
Scheme 3,
Table 1, entry 4) as a lipophilic inhibitor of dihydrofolate reductase.
Scheme 3. Synthesis of
N6-[(2,5-dimethoxyphenyl)methyl]-
N6-methylpyrido[2,3-
d]pyrimidine-2,4,6-triamine (
12) by Queener et al.
[14].
Piritrexim (PTX) (
Scheme 4 and
Scheme 5,
Table 1, entry 5) is a synthetic antifolate first synthesized by Grivsky, Sigel et al.
[21] with anti-parasitic, anti-psoriatic and anti-tumor properties. Piritrexim inhibited dihydrofolate reductase (DHFR) and also showed good antitumor effects on the carcinosarcoma in rats. An advantage of this compound compared to some analogues is that it does not have effects as an inhibitor of histamine metabolism, reducing the potential risk of side reactions on metabolism. Its degree of lipophilicity, i.e., the affinity of this drug for a lipid environment, allows it to diffuse easily into the cells. The various therapeutical activities listed for piritrexim are on melanoma and urothelial cancer, and promising results in head and neck cancer were already obtained in combination with other molecules
[16].
Scheme 4. Synthesis of 6-[(2,5-dimethoxyphenyl)methyl]-5-methylpyrido[2,3-
d]pyrimidine-2,4-diamine (
18) by Grivsky, Sigel et al.
[21].
Scheme 5. Synthesis of 6-[(2,5-dimethoxyphenyl)methyl]-5-methylpyrido[2,3-
d]pyrimidine-2-amine (
28) by Chan and Rosowsky
[17].
2.2. Pyrido[3,4-d]pyrimidine
This class of pyridopyrimidine is mainly referenced with kinase activity. The first example mentioned herein is Tarloxotinib (
194 in
Scheme 6). It is being studied in the clinical trial NCT03743350 (NSCLC exon 20 or HER2 activating mutation)
[22]. This molecule is a kinase inhibitor targeting all members of the HER family, with a novel mechanism of action. It is a hypoxia-activated prodrug that releases an active metabolite irreversibly targeting the kinase. The goal is to inhibit only HER kinases in tumor cells. Tarloxotinib is a Pan-HER kinase inhibitor.
Scheme 6. [(2
E)-3-({4-[(3-bromo-4-chlorophenyl)amino]pyrido[3,4-
d]pyrimidin-6-yl}carbamoyl)prop-2-en-1-yl]dimethyl[(1-methyl-4-nitro-1
H-imidazol-5-yl)methyl]azanium (
194)
[23].
Carlin et al.
[23] patented in 2015 the preparation of 4-anilinopyrido[3,4-d]pyrimidine prodrugs (
Scheme 6,
Table 1, entry 19) as kinase inhibitors useful for cancer treatment. The procedure is described in
Scheme 6 with classical synthetic methodologies affording the expected compound
194 in twelve steps.
The second example is the BOS172722 derivative (
200 in
Scheme 7,
Table 1, entry 20). This compound, in combination with paclitaxel, was tested in vivo for the treatment of triple hormone receptor-negative breast cancer demonstrating a promising synergy. This selective monopolar spindle 1 (Mps1) kinase inhibitor has been identified as a potential anti-cancer agent because it is involved in the division of cancer cells. This is, therefore, an attractive target for cancer therapy
[24][25]. It has the dual specificity protein kinase TTK as the target.
Scheme 7.
N
8-(2,2-dimethylpropyl)-
N
2-[2-ethoxy-4-(4-methyl-4
H
-1,2,4-triazol-3-yl)phenyl]-6-methylpyrido[3,4-
d
]pyrimidine-2,8-diamine (
200
), BOS172722
.
2.3. Pyrido[4,3-d]pyrimidine
Trametinib (
209 in
Scheme 8,
Table 1, entry 21) is a kinase inhibitor used for specific types of melanoma. This compound, associated with other molecules such as Dabrafenib (Tafilnar) and/or Mekinist (trametinib), has been approved by the FDA in particular for the treatment of degenerative thyroid cancer (ATC)
[26][27].
Scheme 8. Synthesis of
N-(3-(3-cyclopropyl-5-[(2-fluoro-4-iodophenyl)amino]-6,8-dimethyl-2,4,7-trioxo-1
H,2
H,3
H,4
H,6
H,7
H-pyrido[4,3-
d]pyrimidin-1-yl)phenyl)acetamid, Trametinib (
209)
[28].
2.4. Pyrido[3,2-d]pyrimidine
Seletalisib (
229 in
Scheme 9,
Table 1, entry 22) is a novel small-molecule inhibitor of PI3Kδ that was evaluated in clinical assays to study the treatment and basic science of Primary Sjogren’s Syndrome
[29]. This molecule is an ATP-competitive and highly selective PI3Kδ inhibitor. Phosphoinositide 3-kinases (PI3K) are enzymes regulating cellular survival, development, and function. They play a key role in immune cell development and function.