Human immunodeficiency virus is considered as a group of retroviruses causing sicknesses characterized by a delay in the onset of symptoms after infection called lentivirus. This virus has two main types, HIV-1 which is mainly responsible for this epidemic, and HIV-2, which is related to the first type but it is widespread in western Africa . HIV-1 protease is classified as an aspartic acid protease which contains 99 amino acid residues that include the active site at the interface between the protein units with a homodimer function, and similar to all dimers, it is symmetrical, with two fold rotational (C2) symmetry . Lopinavir (ABT-378) (Figure 1) is a peptidomimetic HIV-1 protease inhibitor which is considered to interact with the active catalytic site of the protease. Therefore, it has the ability to prevent the transformation process of polyprotein precursors into the mature form, and as a result, it blocks the division and replication operation of the protease inside the virus. It was discovered in 2000 as an improved analog of Ritonavir, and was firstly prescribed to control the interactions between Val 82 active site residues of the HIV-1 protease with the inhibitor, however the residuals of the virus were considerably transformed into drug resistant strains .
Phosphinic peptides act as potent and selective inhibitors of metalloproteases (Matrix metalloproteinase, Angiotensin-converting enzyme, Neprilysin, etc.) that are involved in many pathophysiological conditions, such as cancer and hypertension, exhibiting high selectivity and low toxicity properties   . Phosphinic peptide inhibitors of aspartic acid proteases have also been reported for serine proteases  and serine kinases .
2. Results and Discussion
2.1. Retrosynthetic Strategy for PL1
According to the retrosynthetic analysis study, four main fragments are present in PL1, a-d, which come from the disconnections 1 - 3. As shown in Figure 2, fragment a corresponds to the Cbz-(R)-Phe-PO2H2 , fragment b come from an acid chloride, fragment c corresponds to (2-nitroallyl) benzene , and fragment d to a valine analog .
2.2. Synthesis of Fragment a
The synthesis of ((R)-1-(((benzyloxy) carbonyl) amino)-2-phenylethyl) phosphinic acid 7a (precursor of fragment a, Scheme 1) had taken place after several
Figure 1. Lopinavir and Ritonavir structures.
Figure 2. Main disconnections for PL1.
Scheme 1. Synthesis of fragment a.
steps, started from the addition of 3 to the in situ formed imine of 1 and 2 (Scheme 1). Removal of the diphenyl group by refluxing in strong acid followed by the protection of the amino group of the obtained product with benzyl carbonochloridate 6 to produce the product 7 (precursor of fragment a) in the form of racemic mixture (Scheme 1). Then, the resolution through the formation of diastereomeric salts using (R)-phenylethyl amine led to the desired (R)-isomer 7a . As shown in Scheme 1, the mixture of enantiomers of 7 was converted to salts with the chiral amine (R)-phenylethyl amine. The salt containing a mixture of two diastereomers, which was separated by crystallization. The procedure was repeated with (S)-phenylethyl amine to produce the (S)-isomer 7b. The enantiomeric purity of the 7a and 7b was determined by optical rotation measurements.
2.3. Synthesis of the Precursor for Fragment c
The most high yielding attempt to obtain 9 was the reduction of (E)-(2-nitrovinyl) benzene 8 as shown in Scheme 2, followed by olefination to get product 10.
2.4. Synthesis of Fragment d
As shown in Scheme 3, the synthesis of 13 (fragment d) was accomplished in high yield through two steps beginning from the conversion of L-valine 11 to N-phenoxycarbonyl-L-valine and then transformed to the derivative 13 .
2.5. Assembly of PL1
PL1 was synthesized according to the series of reactions shown in Scheme 4, started from the assembly of the precursor for fragment c (2-nitroallyl) benzene (10) with fragment a (7a) through Michael addition reaction to afford 14 in a good yield. Then, the reduction reaction of 14, which was achieved under very mild conditions leading to conversion of the nitro (−NO2) group to amine (−NH2) in a high yield without affecting the Cbz-protecting group to produce compound 15 as shown in Scheme 4.The next step was to assembly the resulted 15 with valine urea (fragment d) (13), which affording16 in a good yield. Removal of carboxybenzyl (Cbz-) group of16 was produced product 17 as shown in Scheme 4.The final step to obtain PL1 was the coupling of 17 with fragment b, which was accomplished in good yield (Scheme 4).
The synthesized PL1was tested against HIV-1 to test its potency as phosphinic inhibitor using HIV-1 Protease Inhibitor Assay Kit (Fluorometric) (ab211106) by Abcam. The result displayed that PL1 efficiently inhibits HIV-1 with an IC50 in the low nanomolar range (IC50 = 50 nm).
Scheme 2. Synthesis of the precursor for fragment c.
Scheme 3. Synthesis of fragment d.
Scheme 4. Assembly of PL1.
HIV/AIDS has become a devastating disease and resulted in an enormous human suffering. Lopinavir played a major role in the era of boosted protease inhibitor therapy, and in offering a good option to patients affected with this virus. Considering those findings, PL1, a new potent Lopinavir phosphinic analog as HIV-1 inhibitor was designed and synthesized. Its potency was assessed toward HIV-1 inhibitory and pharmacokinetic activities.
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