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.

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.

Dihydropyrimidines (DPs) show a wide range of biological activities suitable for medicinal applications. Among DP derivatives, 2-aryl-DPs have been reported to display remarkable pharmacological properties. In this work, we describe a method of synthesizing hitherto unavailable fully substituted pentasubstituted 2-aryl-DPs as tautomeric mixture using a Pd(PPh₃)₄-catalyzed/CuBr-mediated 2-arylation reaction. The reaction using aryl tributylstannanes with various substituents such as MeO, Me, Ph, CF₃, CO₂Me, and NO₂ groups efficiently afforded the corresponding 2-aryl-DPs in high yields. Heteroaryl tributylstannanes having 2-thienyl, 3-thienyl, or 2-pyridinyl groups were also suitable for the reaction. Regarding the substituents at the 4-, 5-, and 6-positions of DPs, the reactions of DPs bearing substituents such as Me, n-C₃H₇, n-C₅H₁₁, -(CH₂)₅- , phenyl and fluorenylidene groups proceeded smoothly to give the desired products. The synthetic method was also applied to a 2-thioxo-DP to give the 2-aryl-DP. Therefore, the reaction will help expand DP-based molecular diversity, which may impact biological and pharmacological studies. Received: 16 December 2022 Accepted after revision: 08 February 2023 Accepted Manuscript online: 08 February 2023 © . Thieme. All rights reserved. Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

Programmed death 1 ligand (PD-L1), an important immune checkpoint molecule, is mainly expressed on cancer cells and has been shown to exert an immunosuppressive effect on T-cell function by binding to programmed cell death 1 (PD-1) expressed on T-cells. Recently, immune checkpoint inhibitors using antibody drugs such as nivolumab and atezolizumab have attracted attention. However, clinical challenges, including limitations to the scope of their application, are yet to be addressed. In this study, we developed a novel immune checkpoint inhibitor that targets PD-L1 using lipid-siRNA conjugates (lipid-siPDL1s). The inhibitory effect of lipid-siPDL1s on PD-L1 expression was evaluated and found to strongly suppress mRNA expression. Notably, lipid-siPDL1s exerted a significantly stronger effect than unmodified siPDL1. Interestingly, lipid-siPDL1s strongly inhibited PD-L1 expression despite cancer cell stimulation by interferon-gamma, which induced the overexpression of PD-L1 genes. These results strongly suggest that lipid-siPDL1s could be used as novel immune checkpoint inhibitors.