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Lapachol Synthesis Essay

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2.1. Hybrid Compounds

A series of hybrid compounds was synthesized using as building blocks diterpenes and quinones. The quinones included lapachol (1) and its hydrogenation products 2 and 3. The quinone part of the molecule included an isoprenyl side chain, hydrogenated side chains with aromatic rings in the naphthoquinone moiety or hydrogenated side chains and aromatic rings. The diterpene junicedric acid (4) with an exocyclic double bond, its isomer 5 with an Δ8(9) double bond and the hydrogenation product 6 were used as the terpene part of the hybrid molecules. Eighteen hybrids, including six new compounds, were synthesized (Scheme 1), differing in some structural features while maintaining common characteristics to evaluate structure-activity trends. All compounds were characterized by spectroscopic and spectrometric means and are in agreement with the proposed structures. The structures of compounds 124 are shown in Figure 1, Figure 2 and Figure 3.

Scheme 1. General synthetic procedures for preparation of the hybrid compounds. Quinones: compounds 1, 2 and 3. Diterpenes: compounds 4, 5 and 6.

Scheme 1. General synthetic procedures for preparation of the hybrid compounds. Quinones: compounds 1, 2 and 3. Diterpenes: compounds 4, 5 and 6.

Reagents and conditions: (a): DCC, DMAP, CH2Cl2, rt; or (b): oxalyl chloride, CH2Cl2, quinone, TEA.

Figure 1. Structure of the starting quinones 13 and diterpenes 46.

Figure 1. Structure of the starting quinones 13 and diterpenes 46.

Figure 2. Diterpenylquinone derivatives of junicedric acid 712 and 17β-dihydrojunicedric acid 1318.

Figure 2. Diterpenylquinone derivatives of junicedric acid 712 and 17β-dihydrojunicedric acid 1318.

Figure 3. Diterpenylquinone derivatives of Δ8(9) junicedric acid 1924.

Figure 3. Diterpenylquinone derivatives of Δ8(9) junicedric acid 1924.

2.2. Cytotoxicity

The cytotoxicity of all compounds was evaluated on confluent cultures of human lung fibroblasts (MRC-5) and human gastric epithelial cells (AGS) to determine the working concentrations in the subsequent mechanism of action experiments. Cell viability was determined by means of the neutral red uptake assay and results are expressed as IC50 values (µM) in Table 1. Hydrogenation increases cytotoxicity for the lapachol derivatives 2 and 3. From the starting diterpenes, Δ8(9) junicedric acid (5) presented lower toxicity towards MRC-5 and AGS cells, with IC50 values of 834 and 666 µM, respectively. The diterpene with higher cytotoxicity was compound 6 (IC50 values of 163 and 283 µM for MRC-5 and AGS cells, respectively).

Among the six hybrid compounds derived from diterpene 4 (compounds 712), the carboxylic acid derivative 7 resulted more cytotoxic than the parent quinone and diterpene 1 and 4, respectively. A similar trend was observed for hybrids 9 and 11, prepared from quinones 2 and 3, and diterpene 4, respectively. Methylation of 7, 9 and 11 led to the less cytotoxic derivatives of the series 8, 10 and 12, indicating that the free C-19 carboxylic acid function is relevant for the cytotoxicity of this series. The highest cytotoxicity value was found for compound 11, in which both, the prenylated side chain and the benzene ring of the lapachol were reduced by hydrogenation.

The diterpene 5 differs from 4 in the position of the double bond, leading to different configurations for both compounds and their hybrid derivatives. Hybrid compounds containing diterpene 5 (compounds 1924), showed the same trend than derivatives from diterpene 4 when presenting a free C-19 COOH function. Therefore, compounds 19, 21 and 23 were more toxic than their corresponding C-19 COOMe derivatives 20, 22 and 24. As for the series from diterpene 4, the most cytotoxic compound for the derivative series of 5 resulted compound 23, with the prenylated side chain and the benzene ring of the lapachol completely reduced.

Among the hybrid compounds obtained from diterpene 6 (compounds 1318), it was observed that hybrids 13 and 17 bearing a carboxylic acid function at C-19, were more toxic than the corresponding methylester derivatives 14 and 18. However, when only hydrogenation of the prenylated side chain of the lapachol was present in the hybrid compounds (15 and 16), the ester derivative 16 showed higher cytotoxicity that the corresponding carboxylic acid derivative 15.

Table 1. .Cytotoxicity expressed as IC50 values (µM) towards confluent cultures of human lung fibroblasts (MRC-5) and gastric epithelial cells (AGS) treated with the compounds 124 and lansoprazole.

CompoundIC50 ± SD (µM) a
MRC-5AGS
Lapachol (1)>1000571 ± 28
Dihydroprenyl lapachol (2)611 ± 42292 ± 12
Dihydroprenyl-5,6,7,8-tetrahydrolapachol (3)307 ± 2287 ± 4
Junicedric acid (4)439 ± 26383 ± 23
Δ8(9) Junicedric acid (5)834 ± 51666 ± 35
17β-Dihydrojunicedric acid (6)163 ± 5283 ± 17
Lapachoyl junicedrate (7)221 ± 13143 ± 6
Lapachoyl junicedrate methyl ester (8)>1000>1000
Dihydroprenyl lapachol junicedrate (9)449 ± 2749 ± 10
Dihydroprenyl lapachoyl junicedrate methyl ester (10)>1000659 ± 42
Dihydroprenyl-5,6,7,8-tetrahydrolapachoyl junicedrate(11)42 ± 263 ± 4
Dihydroprenyl-5,6,7,8-tetrahydrolapachoyl junicedrate methyl ester (12)667 ± 40>1000
Lapachoyl 17β-dihydrojunicedrate (13)355 ± 21351 ± 29
Lapachoyl 17β-dihydrojunicedrate methyl ester (14)727 ± 48916 ± 54
Dihydroprenyl lapachol 17β-dihydrojunicedrate (15)>1000>1000
Dihydroprenyl lapachol 17β-dihydrojunicedrate methyl ester (16)596 ± 44643 ± 39
Dihydroprenyl-5,6,7,8-tetrahydrolapachoyl 17β-dihydrojunicedrate (17)44 ± 460 ± 5
Dihydroprenyl-5,6,7,8-tetrahydrolapachoyl 17β-dihydrojunicedrate methyl ester (18)369 ± 29884 ± 62
Lapachoyl Δ8(9) junicedrate (19)129 ± 9148 ± 9
Lapachoyl Δ8(9) junicedrate methyl ester (20)483 ± 31321 ± 24
Dihydroprenyl lapachoyl Δ8(9) junicedrate (21)601 ± 26278 ± 14
Dihydroprenyl lapachoyl Δ8(9) junicedrate methyl ester (22)848 ± 51340 ± 20
Dihydroprenyl-5,6,7,8-tetrahydrolapachoyl Δ8(9) junicedrate (23)20 ± 229 ± 2
Dihydroprenyl-5,6,7,8-tetrahydrolapachoyl junicedrate methyl ester (24)>1000>1000
Lansoprazole b316 ± 11168 ± 8

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