Synthesis of 6-unsubstituted 2-oxo, 2-thioxo, and 2-amino-3,4-dihydropyrimidines and their antiproliferative effect on HL-60 cells

Palladium-catalyzed/copper-mediated carbon-carbon cross-coupling reaction for synthesis of 6-unsubstituted 2-aryldihydropyrimidines

Dihydropyrimidines (DPs) show a wide range of biological activities for medicinal applications. Among the DP derivatives, 2-aryl-DPs have been reported to display remarkable pharmacological properties. In this work, we describe a method for the synthesis of hitherto unavailable 6-unsubstituted 2-aryl-DPs by Pd-catalyzed/Cu-mediated carbon–carbon cross-coupling reaction of 1-Boc 2-methylthio-DPs with organostannane reagents. The Boc group of the substrate significantly increases the substrate reactivity. Aryl tributylstannanes having various substituents such as MeO, Ph, CF₃, CO₂Me, and NO₂ groups smoothly afforded the corresponding products in high yields. Various heteroaryl tributylstannanes having 2-, or 3-thienyl, 2-, or 3-pyridinyl groups were also applicable to the reaction. Regarding the substituents at the 4-position, the reactions of DPs bearing various aryl and alkyl substituents proceeded smoothly to give the desired products. The Boc group of the products was removed under a standard acidic condition to produce N-unsubstituted DP as a mixture of the tautomers in quantitative yields. The synthetic procedure was also applied to 4,4,6-trisubstituted 2-methylthio-DP to give novel 2,4,4,5,6-pentasubstituted DP. Therefore, the Pd-catalyzed/Cu-mediated reaction should help expand the DP-based molecular diversity, which would impact biological and pharmacological studies.

This protocol enables the synthesis of 6-unsubstituted 2-aryldihydropyrimidines using various substituents at the 2- and 4-positions, which would impact dihydropyrimidine-based biological and pharmacological studies.

Synthesis of 4,4-Disubstituted 3,4-Dihydropyrimidin-2(1H)-ones and -thiones, the Corresponding Products of Biginelli Reaction Using Ketone, and Their Antiproliferative Effect on HL-60 Cells

An efficient synthetic method for novel 4,4-disubstituted 3,4-dihydropyrimidin-2(1H)-ones 5 and -thiones 6 was developed. The cyclocondensation reaction of O-methylisourea hemisulfate salt 11 with 8 gives a tautomeric mixture of dihydropyrimidines 12 and 13 following acidic hydrolysis of the cyclized products to produce 5 in high yields. Thionation reaction of 5 at the 2-position smoothly proceeds to give 2-thioxo derivatives 6. These compounds 5 and 6, corresponding to the products of a Biginelli-type reaction using urea or thiourea, a ketone and a 1,3-dicarbonyl compound, have long been inaccessible and hitherto unavailable for medicinal chemistry. These methods are invaluable for the synthesis of 5 and 6, which have been inaccessible by conventional methods. Therefore, the synthetic methods established in this study will expand the molecular diversity of their related derivatives. These compounds were also assessed for their antiproliferative effect on a human promyelocytic leukemia cell line, HL-60. Treatment of 10 µM 6b and 6d showed high inhibitory activity similarly to 1 µM all-trans retinoic acid (ATRA), indicating that the 2-thioxo group and length of two alkyl substituents at the 4-position are strongly related to activity.

[Development of Efficient Synthetic Method for Tautomeric Dihydropyrimidines and Analysis of Their Functionality]

I here present the results of our studies on the synthesis and functional analysis of tautomeric dihydropyrimidines (DPs) and related compounds in two sections. In the first section, we describe our experimental and theoretical studies on the thermodynamics and properties of 2-substituted 1,4- and 1,6-dihydropyrimidine-5-carboxylates by ¹H NMR measurements and density functional theory (DFT) calculations, respectively. The concentration ratios of tautomers a/b of DPs 1, 2, and 3 were determined under various conditions to determine the effects of temperature, solvent, and concentration on thermodynamics data. The obtained free energy differences (ΔG), enthalpy differences (ΔH), and entropy differences (ΔS) are discussed in terms of the molecular structures, dipole moments (DM), and electrostatic potential maps obtained by DFT calculations to clarify the nature of DPs 4-8. In the second section, an efficient synthetic method developed for 6-unsubstituted 3,4-dihydropyrimidin-2(1H)-thiones 9 and 2-ones 10 is described. The novelties of the synthesis protocol are as follows: 1) the use of Lewis acid-mediated reaction, 2) good to high yields, and 3) its broad scope of applicability to aldehydes and ureas. Hitherto unavailable 6-unsubstituted 2-amino DP 11 and 2-aryl DP 12 were obtained from 9 by a substitution reaction with the amine and the Liebeskind-Srogl reaction, respectively. The compounds 9, 10, and related 6-methyl derivatives 19-21 were assessed for their antiproliferative effect on the human promyelocytic leukemia cell line HL-60. 4,4-Dipropyl derivative 20 showed relatively strong activity with an IC₅₀ value of 341 nM.