We report herein the synthesis of 4-amino-2,8-dichloropyrido[3,2-d]pyrimidine derivatives 2 and their regioselective diversification through S(N)Ar and metal-catalyzed cross-coupling reactions. While amination of 2 took place selectively at C-2, the regioselectivity of thiol or thiolate addition depended on the reaction conditions. Selective C-8 addition was obtained in DMF with Hünig's base and C-2 addition in (i)PrOH. These C-2 or C-8 regioselective thiolations provided an opportunistic way to selectively activate either of the two positions toward the metal-catalyzed cross-coupling reaction. The chloride could be efficiently substituted by Suzuki-Miyaura reaction and the sulfanyl group by Liebeskind-Srogl cross-coupling reaction, demonstrating the orthogonality of both reactive centers. The development of regioselective conditions for these different transformations yielded the synthesis of 4-amino-2,6,8-trisubstituted pyrido[3,2-d]pyrimidine derivatives, with various substituents.

Antagonism of the CRTH2 receptor represents a very attractive target for a variety of allergic diseases. Most CRTH2 antagonists known to date possess a carboxylic acid moiety, which is essential for binding. However, potential acid metabolites O-acyl glucuronides might be linked to idiosynchratic toxicity in humans. In this communication, we describe a new series of compounds that lack the carboxylic acid moiety. Compounds with high affinity (K i < 10 nM) for the receptor have been identified. Subsequent optimization succeeded in reducing the high metabolic clearance of the first compounds in human and rat liver microsomes. At the same time, inhibition of the CYP isoforms was optimized, giving rise to stable compounds with an acceptable CYP inhibition profile (IC 50 CYP2C9 and 2C19 > 1 μM). Taken together, these data show that compounds devoid of carboxylic acid groups could represent an interesting alternative to current CRTH2 antagonists in development.

We report herein an efficient route for the synthesis of 2,4,8-trichloropyrido[3,2-d]pyrimidines 1 with R(1) substituents at C-6. The potential of such scaffolds was demonstrated by the possibility to displace regioselectively each aromatic chloride to introduce diversity. Sequential sulfur nucleophilic addition followed by Liebeskind-Srogl cross-coupling reaction yielded unprecedented aryl introduction at C-4 on a trichloropyrido[3,2-d]pyrimidine derivative. The reactivity difference of the remaining two chlorides toward S(N)Ar reactions was investigated. Amination yielded high C-2 regioselectivity, while thiolation was influenced by C-6 substituents, resulting in medium to high C-2 versus C-8 regioselectivity. The last chloride was efficiently displaced by S(N)Ar, Suzuki-Miyaura cross-coupling reaction, or reduction. C-2 arylation as a final step was also possible by Liebeskind-Srogl cross-coupling reaction on the previously introduced C-2 thioether. A concise and highly divergent synthetic use of 1 was developed, thereby providing an efficient approach to explore the structure-activity relationship of pyrido[3,2-d]pyrimidine derivatives such as 9, 10, 15, and 16.

Access and Regioselective Transformations of 6-Substituted 4-Aryl-2,8-dichloropyrido[3,2-d]pyrimidine Compounds. -Treatment of the uracil (I) with the acrylates (II) followed by cyclization and chlorination provides an efficient approach to 2,4,8-trichloropyridopyrimidines. Their reaction with NaSMe and subsequent Liebeskind-Srogl coupling allows convenient and regioselective access to 4-aryl-2,8-dichloropyridopyrimidines. Under appropriate conditions the remaining chlorides can also undergo regioselective substitution yielding diverse products such as (XI), (XV), and (XIX). -(BOUSCARY-DESFORGES, G.; BOMBRUN, A.; AUGUSTINE, J. K.; BERNARDINELLI, G.; QUATTROPANI*, A.; J. Org. Chem. 77 (2012) 10, 4586-4595, http://dx.doi.org/10.1021/jo300189q ; Merck Serono S.A., CH-1202 Geneva, Switz.; Eng.) -Jannicke 37-164

The use of appropriate conditions and reagents allows regioselective formation of a number of differently substituted pyridopyrimidines.