Advances in the Solid-Phase Synthesis of Pyrimidine Derivatives

This Review describes the existing synthetic approaches for the solid-phase synthesis (SPS) of differently substituted and fused pyrimidine derivatives. These synthetic strategies are classified on the basis of the different synthetic routes leading to the particular type of pyrimidine heterocycle formed. The Review discusses the application of a variety of polystyrene derived supports for the construction of pyrimidine rings. The effect of microwave heating on the solid-phase synthesis is also addressed in the review.

New pyrimido[5,4-b]indoles and [1]benzothieno[3,2-d]pyrimidines: High affinity ligands for the α1-adrenoceptor subtypes

A number of new pyrimido[5,4-b]indole and [1]benzothieno[3,2-d]pyrimidine derivatives were synthesized and evaluated for their binding and functional properties at alpha(1)-adrenergic receptor (alpha(1)-AR) subtypes. They behaved as potent alpha(1)-AR antagonists. In binding experiments, some of them (RC24 and RC23) showed very high affinity for the alpha(1D)-AR subtype.

Deazapurine Solid-Phase Synthesis: Combinatorial Synthesis of a Library of N3,N5,C6-Trisubstituted Pyrrolo[3,2-d]pyrimidine Derivatives on Cross-Linked Polystyrene Bearing a Cysteamine Linker

Solid-phase methodology for the preparation of pyrrolo[3,2-d]pyrimidine-6-carboxylates with diversity at the N3 pyrmidine nitrogen has now been elaborated to allow for the generation of pyrrolopyrimidine libraries with members possessing diversity at the N3, N5, and C6 positions. The diversification of the N5 position was achieved by treating the parent resin-bound pyrrolo[3,2-d]pyrimidines 3 with an alkyl halide in the presence of Cs2CO3 in DMF. Modification of the C6 carboxylate of resin-bound pyrrolopyrimidines 3-5 was first achieved by hydrolysis of the benzyl ester using LiOH in a mixture of THF/H2O/MeOH. Further alteration of the C6 position of resin-bound pyrrolo[3,2-d]pyrimidine-6-carboxylic acids 6-8 was then performed by activation with triphosgene and treatment with an amine to furnish resin-bound pyrrolo[3,2-d]pyrimidine-6-amides. Twenty-two pyrrolo[3,2-d]pyrimidines 1a-v with different substituents at the N3, N5, and C6 positions were obtained in yields of 21-83% and purities of 61-98% after cleavage from the solid support.

Deazapurine Solid-Phase Synthesis: Construction of 3-Substituted Pyrrolo[3,2-d]pyrimidine-6-carboxylates on Cross-Linked Polystyrene Bearing a Cysteamine Linker

The first solid-phase methodology for the preparation of pyrrolo[3,2-d]pyrimidines is presented. Merrifield resin bearing a cysteamine "traceless" linker was treated with 4-oxo-N-(PhF)proline benzyl ester (10; PhF = 9-(9-phenylfluorenyl)) to provide resin-bound aminopyrrole 20, which was treated with ethyl, phenyl, 4-phenoxyphenyl, and 2,4-dimethoxyphenyl isocyanates to furnish resin-bound ureidopyrroles 21a-d. Resin-bound pyrrolo[3,2-d]pyrimidines 22a-d were then obtained by acylation of 21 using trichloroacetyl chloride in dioxane followed by treatment with Cs2CO3 in DMF. Cleavage of pyrrolo[3,2-d]pyrimidines 22a-d from the resin was achieved in two steps, by oxidation of the sulfur to the sulfone followed by beta-elimination in the presence of t-BuONa. Four pyrrolo[3,2-d]pyrimidines, 24a-d, with different alkyl and aryl substituents at the N3 pyrimidine nitrogen, were thus obtained in overall yields of 42-50% and purities of 90-100%.

Diversity-oriented synthesis of functionalized pyrrolo[3,2-d]pyrimidines with variation of the pyrimidine ring nitrogen substituents

Nine 2,4-dioxo-2,3,4,5-tetrahydro-1H-pyrrolo[3,2-d]pyrimidine-6-carboxylic acid benzyl esters 12 were synthesized in four steps from 4-oxo-N-(PhF)proline benzyl ester 7 by a general method in which elements of molecular diversity were readily added onto the pyrimidine nitrogens. Conversion of 4-oxoproline 7 into the corresponding aminopyrrole 8 using benzyl-, allyl-, and isopropylamine followed by treatment with phenyl, allyl, and ethyl isocyanate gave nine different ureas 9. 4-Ureido-1H-pyrrole-2-carboxylic acid benzyl esters 9 were then converted into the respective pyrrolo[3,2-d]pyrimidines 12 using trichloroacetyl chloride in acetonitrile followed by treatment with Cs(2)CO(3). Crystallization from toluene gave the desired deazapurines in 37-55% overall yield from proline 7.

New pyrimido[5,4-b]indoles as ligands for alpha(1)-adrenoceptor subtypes

A new series of compounds were designed as structural analogues of the alpha(1)-AR ligand RN5 (4), characterized by a tricyclic 5H-pyrimido[5,4-b]indole-(1H,3H)2,4-dione system connected through an alkyl chain to a phenylpiperazine (PP) moiety. These compounds were synthesized and tested in binding assays on human alpha(1A)-AR, alpha(1B)-AR, and alpha(1D)-AR subtypes expressed in HEK293 cells. Several structural modifications were performed on the PP moiety, the tricyclic system, and the connecting alkyl chain. Many of the new molecules showed a preferential affinity for the alpha(1D)-AR subtype. Some compounds, including 39 and 40, displayed substantial alpha(1D)-AR selectivity with respect to alpha(1A)-AR, alpha(1B)-AR, serotonergic 5-HT(1A), 5-HT(1B), 5-HT(2A), and dopaminergic D(1) and D(2) receptors. Two conformationally rigid analogues of 4, useful for studying the architecture of the receptor/ligand complex, were also prepared and tested. A subset of the new compounds was then used to evolve a preliminary pharmacophore model for alpha(1D)-AR antagonists, based on a more generalized model we had developed for alpha(1)-AR antagonists. This new model rationalized the relationships between structural properties and biological data of the pyrimido[5,4-b]indole compounds, as well as other compounds.