Lomatiol. Part II. Its Occurrence, Constitution, Relation to and Conversion into Lapachol. Also a Synthesis of Lapachol.^1,2

Boulbazine M., Djellala I., Boudjahem A.

Gadea A., Khazem M., Gaslonde T.

The present review details in a first part the structures of secondary metabolites discovered in Proteaceae up to now. In a second part, biological activities of bis-5-alkylresorcinols, a typical group of metabolites from Proteaceae, and their cyclized analogues are summarized. The scientific data required to make the inventory were exclusively compiled from books and journals. Using the list of plant genus, the bibliographic references were collected from (i) The Dictionary of Natural Products (DNP) and (ii) SciFinder Chemical abstract services. More than 130 references were carefully analyzed to found about 327 secondary metabolites. The most important classes of metabolites in Proteaceae were found as phenol glycosides (31%), derivatives of 5-alkylresorcinols (30%) and tropane alkaloids (11%).

Paengsri W., Promsawan N., Baramee A.

A series of 3-substituted-2-hydroxy-1,4-naphthoquinone derivatives with a variety of side chains were successfully synthesized by Mannich reaction of 2-hydroxy-1,4-naphthoquinone (lawsone) with selected amines and aldehydes. All substances (1-16) were evaluated for in-vitro antimalarial activity against strains of Plasmodium falciparum by microculture radioisotope technique. Bioassay data revealed that ten derivatives (1-8, 11 and 13) displayed significantly good activity with values of IC50 ranging from 0.77 to 4.05 µg/mL. The best biological profile (IC50 = 0.77 µg/mL) was observed in compound 1, possessing a n-butyl substituted aminomethyl group. Experimental results support the potential use of our active Mannich components as promising antimalarial agents in the fight against malaria infections and multidrug resistance problems.

de B. Oliveira A.L., Navegantes-Lima K.C., Monteiro V.V., Quadros L.B., de Oliveira J.P., dos Santos S.M., de A. Pontes A.C., Dorneles G.P., Romão P.R., Júnior L.C., de Oliveira A.B., Monteiro M.C.

Sepsis is characterized by a dysregulated immune response to infection characterized by an early hyperinflammatory and oxidative response followed by a subsequent immunosuppression phase. Although there have been some advances in the treatment of sepsis, mortality rates remain high, urging for the search of new therapies. β-Lapachone (β-Lap) is a natural compound obtained from Tabebuia avellanedae Lorentz ex Griseb. with several pharmacological properties including bactericidal, anti-inflammatory, and antioxidant activity. Thus, the aim of this study was to evaluate the effects of β-Lap in a mouse sepsis model. To this, we tested two therapeutic protocols in mice submitted to cecal ligation and puncture- (CLP-) induced sepsis. First, we found that in pretreated animals, β-Lap reduced the systemic inflammatory response and improved bacterial clearance and mouse survival. Moreover, β-Lap also decreased lipid peroxidation and increased the total antioxidant capacity in the serum and peritoneal cavity of septic animals. In the model of severe sepsis, the posttreatment with β-Lap was able to increase the survival of animals and maintain the antioxidant defense function. In conclusion, the β-Lap was able to increase the survival of septic animals by a mechanism involving immunomodulatory and antioxidant protective effects.

do Nascimento M.F., Borgati T.F., de Souza L.C., Tagliati C.A., de Oliveira A.B.

The natural naphthoquinones lapachol, α- and β-lapachone are found in Bignoniaceous Brazilian plant species of the Tabebuia genus (synonym Handroanthus) and are recognized for diverse bioactivities, including as antimalarial. The aim of the present work was to perform in silico, in vitro and in vivo studies to evaluating the antimalarial potential of these three naphthoquinones in comparison with atovaquone, a synthetic antimalarial. The ADMET properties of these compounds were predicted in silico by the preADMET program. The in vitro toxicity assays were experimentally determined in immortalized and tumoral cells from different organs. In vivo acute oral toxicity was also evaluated for lapachol. Several favorable pharmacokinetics data were predicted although, as expected, high cytotoxicity was experimentally determined for β-lapachone. Lapachol was not cytotoxic or showed low cytotoxicity to all of the cells assayed (HepG2, A549, Neuro 2A, LLC-PK1, MRC-5), it was nontoxic in the acute oral test and disclosed the best parasite selectivity index in the in vitro assays against chloroquine resistant Plasmodium falciparum W2 strain. On the other hand, α- and β-lapachone were more potent than lapachol in the antiplasmodial assays but with low parasite selectivity due to their cytotoxicity. The diversity of data here reported disclosed lapachol as a promising candidate to antimalarial drug development.

Chen T., Chen S., Fu S., Qin S., Liu B.

A homocoupling reaction of 2-naphthols with formation of a C–O bond through electrochemical oxidative dearomatization in the presence of catalytic amounts of ferrocene and a ruthenium complex was developed. Mechanistic studies revealed that the reaction might proceed through coupling between two identical radical species. Moreover, a gram-scale experiment was performed to illustrate the potential practicability of this methodology in organic synthesis.

Tsanakopoulou M., Tsovaltzi E., Tzani M.A., Selevos P., Malamidou-Xenikaki E., Bakalbassis E.G., Domingo L.R.

Selective monoacetalization of lawsone inverts the site-selectivity of the Diels–Alder reactions of its alkylidene derivatives favoring the protected β-lapachone derivatives.

Deans B.J., Tedone L., Bissember A.C., Smith J.A.

An investigation of the previously unexamined ancient Tasmanian clone Lomatia tasmanica W. M. Curtis (Proteaceae) and two other endemic species Lomatia tinctoria R. Br. and Lomatia polymorpha (Labill.) R. Br. was undertaken. This represents the first extensive natural products study in which individual phytochemical components have been isolated and identified from these three Lomatia species. Extraction of L. tasmanica leaves provided the naphthoquinone juglone (0.34% w/w), and n-alkanes nonacosane and heptacosane (0.30% w/w combined). L. polymorpha afforded the flavonoid glycosides dihydroquercetin 3-O-β-D-xyloside (0.22% w/w) and quercetin 3-O-β-d-glucose (0.14% w/w), as well as the naphthalene glucoside 1,4,8-trihydroxynaphthalene-1-O-β-d-glucose (0.04% w/w) and 4-O-p-coumaroyl-d-glucose (0.03% w/w). In addition, both L. polymorpha and L. tinctoria contained juglone (0.32% w/w and 0.58% w/w, respectively). L. polymorpha provided tetracosan-1-ol, hexacosan-1-ol and octacosan-1-ol (0.07% w/w combined), while L. tinctoria gave nonacosane (0.13% w/w). Analysis of three individual specimens from each of the three species demonstrated consistency in the respective phytochemical profiles of these populations and tentatively suggests limited intraspecific variation.

Borah A., Sharma A., Hazarika H., Gogoi P.

A synthetic strategy has been developed for the synthesis of 2,3-disubstituted-1,4-naphthoquinones. This synthetic protocol consists of metal-free tert-Butyl hydroperoxide (TBHP)-mediated oxidative benzoylation followed by Pd-catalyzed Suzuki-cross coupling reactions. Benzaldehyde, benzylalcohol and toluene were efficiently used as benzoyl radical source for the benzoylation of 1,4-naphthoquinones. By this protocol, a wide range of 1,4-naphthoquinones with different substitution patterns were efficiently prepared in a sequential way with good to excellent yields.

Eyong K.O., Chinthapally K., Senthilkumar S., Lamshöft M., Folefoc G.N., Baskaran S.

The biomimetic synthesis of lomatiol from lapachol and the subsequent transformation of lomatiol to biologically important furano- and pyrano-napthoquinone derivatives have been reported.

MATA-SANTOS T., PINTO N.F., MATA-SANTOS H.A., DE MOURA K.G., CARNEIRO P.F., CARVALHO T.D., DEL RIO K.P., PINTO M.D., MARTINS L.R., FENALTI J.M., DA SILVA P.E., SCAINI C.J.

Anthelmintics used for intestinal helminthiasis treatment are generally effective; however, their effectiveness in tissue parasitosis (i.e. visceral toxocariasis) is moderate. The aim of this study was to evaluate the in vitro activity of lapachol, β-lapachone and phenazines in relation to the viability of Toxocara canis larvae. A concentration of 2 mg/mL (in duplicate) of the compounds was tested using microculture plates containing Toxocara canis larvae in an RPMI-1640 environment, incubated at 37 °C in 5% CO2 tension for 48 hours. In the 2 mg/mL concentration, four phenazines, lapachol and three of its derivatives presented a larvicide/larvistatic activity of 100%. Then, the minimum larvicide/larvistatic concentration (MLC) test was conducted. The compounds that presented the best results were nor-lapachol (MLC, 1 mg/mL), lapachol (MLC 0.5 mg/mL), β-lapachone, and β-C-allyl-lawsone (MLC, 0.25 mg/mL). The larvae exposed to the compounds, at best MLC with 100% in vitro activity larvicide, were inoculated into healthy BALB/c mice and were not capable of causing infection, confirming the larvicide potential in vitro of these compounds.

Schuck D.C., Ferreira S.B., Cruz L.N., da Rocha D.R., Moraes M.S., Nakabashi M., Rosenthal P.J., Ferreira V.F., Garcia C.R.

The hydroxynaphthoquinones have been extensively investigated over the past 50 years for their anti-malarial activity. One member of this class, atovaquone, is combined with proguanil in Malarone®, an important drug for the treatment and prevention of malaria. Anti-malarial activity was assessed in vitro for a series of 3-alkyl-2-hydroxy-1,4-naphthoquinones (N1-N5) evaluating the parasitaemia after 48 hours of incubation. Potential cytotoxicity in HEK293T cells was assessed using the MTT assay. Changes in mitochondrial membrane potential of Plasmodium were measured using the fluorescent dye Mitrotracker Red CMXROS. Four compounds demonstrated IC50s in the mid-micromolar range, and the most active compound, N3, had an IC50 of 443 nM. N3 disrupted mitochondrial membrane potential, and after 1 hour presented an IC50ΔΨmit of 16 μM. In an in vitro cytotoxicity assay using HEK 293T cells N3 demonstrated no cytotoxicity at concentrations up to 16 μM. N3 was a potent inhibitor of mitochondrial electron transport, had nanomolar activity against cultured Plasmodium falciparum and showed minimal cytotoxicity. N3 may serve as a starting point for the design of new hydroxynaphthoquinone anti-malarials.

Sharma A., Santos I.O., Gaur P., Ferreira V.F., Garcia C.R., da Rocha D.R.

A series of 36 new phenylsulfanylmethyl[1,4]naphthoquinones (7-42) were synthesized by a three-component reaction that involves lawsone, the appropriate aldehyde and thiols with variable substitution patterns. These reactions involve the in situ generation of o-quinone methides (o-QM) via Knoevenagel condensation and 1,4-nucleophilic addition under conventional heating or microwave irradiation. The new naphthoquinones obtained by this methodology were shown to have moderate to good in vitro antimalarial activity against Plasmodium falciparum (3D7).

Pires S.M., Paula R.D., Simões M.M., Silva A.M., Domingues M.R., Santos I.C., Vargas M.D., Ferreira V.F., Neves M.G., Cavaleiro J.A.

The biomimetic oxidation of lapachol (1) using aqueous hydrogen peroxide as oxidant and chloro[5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrinatomanganese(III)] (Mn-Porph) as catalyst is described. A comparison between the obtained results and those described for the oxidation of lapachol using meta-chloroperoxybenzoic acid (m-CPBA) reveals, besides different reaction products, a completely different selectivity. Unlike the m-CPBA approach, where ortho-naphthoquinones are obtained, para-naphthoquinones are highly favoured when using Mn-Porph and H2O2. Moreover, a new lactone is isolated and characterized in the present work.

Kimachi T., Torii E., Kobayashi Y., Doe M., Ju-ichi M.

Synthesis of dehydoriso-β-lapachone (1) in both racemic and enantioenriched forms is achieved starting from reduced naphthoquinone equivalents. As for the synthesis of enantioenriched dehydroiso-β-lapachone, introduction of the asymmetric center was carried out by catalytic asymmetric epoxidation of the unfunctionalized trisubstituted olefin using Shi epoxidation diketal catalyst. The construction of isopropenylfurano-1,2-(β)-naphthoquinone was carried out by acidic ring-opening reaction of the epoxynaphthalene and the following diammonium cerium(IV) nitrate (CAN) oxidation. The absolute configuration of naturally occurring (-)-dehydroiso-β-lapachone was finally determined as (R) by comparing the measured optical rotation value of the synthetic (R)-dehydroiso-β-lapachone.