1,3-Diazepine: A privileged scaffold in medicinal chemistry

Privileged structures have been widely used as effective templates for drug discovery. While benzo-1,4-diazepine constitutes the first historical example of such a structure, the 1,3 analogue is just as rich in terms of applications in medicinal chemistry. The 1,3-diazepine moiety is present in numerous biological active compounds including natural products, and is used to design compounds displaying a large range of biological activities. It is present in the clinically used anticancer compound pentostatin, in several recent FDA approved β-lactamase inhibitors (e.g., avibactam) and also in coformycin, a natural product known as a ring-expanded purine analogue displaying antiviral and anticancer activities. Several other 1,3-diazepine containing compounds have entered into clinical trials. This heterocyclic structure has been and is still widely used in medicinal chemistry to design enzyme inhibitors, GPCR ligands, and so forth. This review endeavours to highlight the main use of the 1,3-diazepine scaffold and its derivatives, and their applications in medicinal chemistry, drug design, and therapy. We will focus more particularly on the development of enzyme inhibitors incorporating this scaffold, with a strong emphasis on the molecular interactions involved in the inhibition mechanism.

The Synthesis of 5-Amino-dihydrobenzo[b]oxepines and 5-Amino-dihydrobenzo[b]azepines via Ichikawa Rearrangement and Ring-Closing Metathesis

The combination of Ichikawa's rearrangement and a ring-closing metathesis reaction of allyl carbamates is presented as a method for the preparation of 5-amino-substituted 2,5-dihydro-benzo[b]oxepines, 2,5-dihydro-benzo[b]azepines, and 2,5-dihydro-benzo[b]thiepins. It was demonstrated that the use of nonracemic allyl carbamates enables the synthesis of enantioenriched benzo-fused seven-membered heterocycles. Finally, it was shown that further functionalization of the obtained structures allows access to pharmacologically active 5-amino-substituted 2,3,4,5-tetrahydro-1-benzo[b]oxepine scaffolds.

Investigations of possible prodrug structures for 2-(2-mercaptophenyl)tetrahydropyrimidines: reductive conversion from anti-HIV agents with pyrimidobenzothiazine and isothiazolopyrimidine scaffolds

3,4-Dihydro-2H,6H-pyrimido[1,2-c][1,3]benzothiazin-6-imine (PD 404182) and 3,4-dihydro-2H-benzo[4,5]isothiazolo[2,3-a]pyrimidine are the heterocyclic antiretroviral agents against human immunodeficiency virus type 1 (HIV-1) infection. On the basis of similar structure-activity relationships of anti-HIV activities toward the early-stage of viral infection between these unique scaffolds, the transformations under the bioassay conditions were investigated. The distinctive S-N bond in the isothiazolopyrimidine scaffold was immediately cleaved under reductive conditions in the presence of GSH to generate a thiophenol derivative. A similar rapid conversion of PD 404182 into the same thiophenol derivative was observed, suggesting that pyrimidobenzothiazine and isothiazolopyrimidine scaffolds may work as prodrug forms of the common bioactive thiophenol derivatives.

Identification of anti-HIV agents with a novel benzo[4,5]isothiazolo[2,3-a]pyrimidine scaffold

3,4-Dihydro-2H-benzo[4,5]isothiazolo[2,3-a]pyrimidine is a newly identified antiviral agent against human immunodeficiency virus type 1 (HIV-1) infection, derived from 3,4-dihydro-2H,6H-pyrimido[1,2-c][1,3]benzothiazin-6-imine (PD 404182). The introduction of the hydrophobic 8-aryl substituent on the benzene substructure improved its anti-HIV activity, resulting in the identification of 6-fold more potent analogs. In addition, it was demonstrated that these isothiazolopyrimidine derivatives exert anti-HIV effects at an early stage of viral infection.

A novel and potent inhibitor of dimethylarginine dimethylaminohydrolase: a modulator of cardiovascular nitric oxide

PD 404182 [6H-6-imino-(2,3,4,5-tetrahydropyrimido)[1,2-c]-[1,3]benzothiazine], a heterocyclic iminobenzothiazine derivative, is a member of the Library of Pharmacologically Active Compounds (LOPAC) that is reported to possess antimicrobial and anti-inflammatory properties. In this study, we used biochemical assays to screen LOPAC against human dimethylarginine dimethylaminohydrolase isoform 1 (DDAH1), an enzyme that physiologically metabolizes asymmetric dimethylarginine (ADMA), an endogenous and competitive inhibitor of nitric oxide (NO) synthase. We discovered that PD 404182 directly and dose-dependently inhibits DDAH. Moreover, PD 404182 significantly increased intracellular levels of ADMA in cultured primary human vascular endothelial cells (ECs) and reduced lipopolysaccharide-induced NO production in these cells, suggesting its therapeutic potential in septic shock-induced vascular collapse. In addition, PD 404182 abrogated the formation of tube-like structures by ECs in an in vitro angiogenesis assay, indicating its antiangiogenic potential in diseases characterized by pathologically excessive angiogenesis. Furthermore, we investigated the potential mechanism of inhibition of DDAH by this small molecule and found that PD 404182, which has striking structural similarity to ADMA, could be competed by a DDAH substrate, suggesting that it is a competitive inhibitor. Finally, our enzyme kinetics assay showed time-dependent inhibition, and our inhibitor dilution assay showed that the enzymatic activity of DDAH did not recover significantly after dilution, suggesting that PD 404182 might be a tightly bound, covalent, or an irreversible inhibitor of human DDAH1. This proposal is supported by mass spectrometry studies with PD 404182 and glutathione.

Structure-activity relationship study of phenylpyrazole derivatives as a novel class of anti-HIV agents

The structure-activity relationship of phenylpyrazole derivative 1 was investigated for the development of novel anti-HIV agents. Initial efforts revealed that the diazenyl group can be replaced by an aminomethylene group. In addition, we synthesized various derivatives by the reductive amination of benzaldehydes with 5-aminopyrazoles and carried out parallel structural optimization on the benzyl group and the pyrazole ring. This optimization led to a six-fold more potent derivative 32j than the lead compound 1, and this derivative has a 3',4'-dichloro-(1,1'-biphenyl)-3-yl group.

Design and synthesis of biotin- or alkyne-conjugated photoaffinity probes for studying the target molecules of PD 404182

To investigate the mechanism of action of the potent antiviral compound PD 404182, three novel photoaffinity probes equipped with a biotin or alkyne indicator were designed and synthesized based on previous structure-activity relationship studies. These probes retained the potent anti-HIV activity of the original pyrimidobenzothiazine derivatives. In photoaffinity labeling studies using HIV-1-infected H9 cells (H9IIIB), eight potential proteins were observed to bind PD 404182.

Structure-activity relationship study of pyrimido[1,2-c][1,3]benzothiazin-6-imine derivatives for potent anti-HIV agents

3,4-Dihydro-2H,6H-pyrimido[1,2-c][1,3]benzothiazin-6-imine (PD 404182) is an antiretroviral agent with submicromolar inhibitory activity against human immunodeficiency virus-1 (HIV-1) and HIV-2 infection. In the current study, the structure-activity relationships of accessory groups at the 3- and 9-positions of pyrimido[1,2-c][1,3]benzothiazin-6-imine were investigated for the development of more potent anti-HIV agents. Several different derivatives containing a 9-aryl group were designed and synthesized using Suzuki-Miyaura cross-coupling and Ullmann coupling reactions. Modification of the m-methoxyphenyl or benzo[d][1,3]dioxol-5-yl group resulted in improved anti-HIV activity. In addition, the 2,4-diazaspiro[5.5]undec-2-ene-fused benzo[e][1,3]thiazine derivatives were designed and tested for their anti-HIV activities. The most potent 9-(benzo[d][1,3]dioxol-5-yl) derivative was two-threefold more effective against several strains of HIV-1 and HIV-2 than the parent compound, PD 404182.