Mollisin: A Promising Antifungal Natural Product - American Chemical

Chapter 14

Downloaded by CENTRAL MICHIGAN UNIV on December 8, 2015 | http://pubs.acs.org Publication Date (Web): November 4, 2015 | doi: 10.1021/bk-2015-1204.ch014

Mollisin: A Promising Antifungal Natural Product Christian Winter* and Christine Wiebe BASF SE, Global Research Crop Protection, GBA/FI – B009, Carl-Bosch-Strasse 38, 67056 Ludwigshafen, Germany *E-mail: [email protected]

Mollisin is a fungal secondary metabolite produced by several Mollisia species, which has been known for decades to exhibit antifungal activities. Recently, more than 60 years after its first isolation, the first total synthesis of this naphthoquinone pigment was reported enabling access to various analogues for further investigation of its biological activity. It was revealed that mollisin and its analogues show not only a good inhibition of phytopathogenic fungi, but also possess interesting pharmacological activities.

Introduction In 1952 Gremmen cultivated several different discomycete families on maltagar in order to investigate their biological properties (1). The focus of these studies soon pointed towards the two species Mollisia caesia and Mollisia fallens, which attracted attention due to their fast growth and the production of a yellow substance that crystallized on the surface of the agar. Isolation and structural investigations of the pigment by van der Kerk and Overeem (2) revealed a chlorinated naphthoquinone motif, that was soon after named mollisin. Initial structural assignment was of mollisin was incorrect, however, the correct structure was finally elucidated by the same authors in 1964 (Figure 1) (3) and was later confirmed through crystallography (4). More than 50 years after the isolation of mollisin (1) two further naphthoquinone metabolites produced by the same fungus were isolated and named mollisin A (2) and mollisin B (3) (5). Mollisin is not just interesting due to its unique chlorination pattern but also due to its biological activity. Gremmen already observed a strong growth © 2015 American Chemical Society In Discovery and Synthesis of Crop Protection Products; Maienfisch, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


inhibition of several fungi, such as Heterobasidion annosum (formerly known as Fomes annosum) which causes serious damage to coniferous forests (6). Furthermore, mollisin exhibits a strong antibiotic activity towards Gram-positive bacteria (7).

Figure 1. Structures of naphthoquinone metabolites isolated from Mollisia caesia.

Biosynthesis of Mollisin Although the biosynthesis of mollisin by Mollisia caesia has been intensively investigated, the exact pathway is still not entirely elucidated (8–11). Feeding experiments with different sodium acetate isotopes clearly showed that mollisin is synthesized following a polyketide pathway. However, there are still two possibilities: the construction of the naphthoquinone skeleton starting from one polyketide chain (pathway 1) or out of two polyketide fragments (pathway 2) (Scheme 1) (5). Pathway 1 constructs the naphthoquinone by intramolecular condensation of a C16-polyketide chain, whereas, in pathway 2 the bicycle is formed from C6- and a C10- chains. In both cases the loss of two C1-units is proposed. Feeding with chlorinated acetate did not lead to an incorporation of this building block, therefore, it is assumed that the chlorine atoms are introduced after the condensation to the naphthoquinone (12). This assumption is affirmed by the detection of a chloro peroxidase that is present in the fungus. Although pathway 2 cannot be excluded, several findings suggest that the biosynthesis of mollisin 1 follows the first route. First, feeding deuterated acetyl-CoA leads to high deuterium concentration at C9 only (5, 12). Pathway 2 would, on the other hand, lead to incorporation of deuterium at C2′ in addition to C9‘. Furthermore, chlorination through a chloro peroxidase usually proceeds at nucleophilic substrate positions. The position C2′ in the C16-polyketide chain of route 1 is activated by two carbonyls , thus chlorination of this chain should be more likely than chlorination at monactivated C2′ of the C6-chain (route 2). The fact that only mono- or dichlorination occurs in all three naphthoquinone metabolites (also at C10 of mollisin A and B) indicates chlorination of a double-activated methylene group and not of a methyl group (as in pathway 2). Thus, it is very likely that mollisin 1 and its derivatives are produced by pathway 1 (5).

196 In Discovery and Synthesis of Crop Protection Products; Maienfisch, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Scheme 1. Two possible biosynthetic pathways to mollisin 1.

Synthesis The first aproach to synthesize the naphtho-1,4-quinone metabolite mollisin was made in 1964 by van der Kerk and Overeem (13). However, they were not able to synthesize mollisin itself, but some close derivatives like dechloromollisin methyl ether (11) (Scheme 2). In the first step of their synthesis two molecules of diacetylacetone (5) reacted in a triple Knoevenagel condensation catalyzed by piperidine to give the 2-acetyl-3,6-dimethylnaphthalene-1,8-diol (6). Deacetylation led to naphthol-1,8-diol 7, which was sensitive to oxidation and, thus, immediately converted into the highly-stable dimethyl derivative 8 by methylation with dimethyl sulfate. For the synthesis of mollisin (1) it would have been necessary to install the dichloroacetyl group at this stage by a Friedel-Crafts reaction. However, all attempts to produce the dichloro-acetylated naphthol 9 failed. Due to this, the authors decided to prepare dechloromollisin methyl ether by treating naphthol 8 with excess acetyl chloride and subsequent oxidation to the naphthoquinone derivative 11. In 2013 the first successful total synthesis of mollisin was achieved by Opatz and coworkers (14). Based on the synthesis of dechloromollisin methyl ether by van der Kerk and Overeem naphtholether 8 was chosen as key intermediate in their synthesis (Scheme 3).



Scheme 2. Synthesis of dechloromollisin methyl ether (11).

As described by van der Kerk and Overeem naphthol ether 8 could not be acylated directly with the dichloroacetyl chloride. However, Opatz showed that the 4-position could be activated towards an electrophilic attack by bromination and subsequent conversion of the bromide 12 into stannane 13. Subsequent ipsosubstitution at the 4-position is favored due to the higher electrophilicity of this position and due to the weak C–Sn bond, which makes the trimethylstannyl cation a good leaving group. Hence, the dichloroacetyl group could be introduced by a Friedel–Crafts-like acylation. In the next step formation of the naphthoquinone 15 was achieved by oxidation of ether 14 and subsequent demethylation with AlCl3 to provide mollisin (1) as yellow crystals in 27% yield over five steps.



Scheme 3. Total synthesis of mollisin (1).

Although Friedel-Crafts acylation of 8 did not work with dichloro acetyl chloride as shown in Scheme 2, reaction with acetyl chloride or even with trifluoroacetyl chloride led to the formation of monoacetylated naphthols 17 (Scheme 4). Further oxidation to the quinones 18 and demethylation led to dechloromollisin 19a and its trifluoro analogue 19b (15). Recently a variety of synthetic analogues of mollisin have been reported by BASF (16). Most derivatives contain the quinone moiety typical for mollisin and differ only in the attached acyl group (Scheme 5). However, the synthesis of naphthatriol 20 was also achieved. With the synthesis route described above (see Schemes 3 and 4) not only halogenated acyl groups could be installed, but also various aliphatic and aromatic ketones, which were all tested for their antifungal properties against phytopathogenic fungi.



Scheme 4. Synthesis of dechloromollisin (19a) and its trifluoro analogue 19b.

Scheme 5. Synthetic derivatives of mollisin.


Biological Properties Mollisin, the fungi that produce itas well as other derivatives possess various antifungal and interesting pharmacological activities. The natural product mollisin (1) itself, its derivatives and the deuteromycetes Mollisia sp. and Mollisia fallens have been investigated for their biological properties.


Fungicidal Activity of Mollisia sp. The fungus Mollisia sp. was investigated for its properties against several phytopathogenic fungi occurring on trees. It showed strong antibiotic activities against the ascomycete Pezicula cinnamomea (DC), which causes Pezicula canker of Red Oak (Quercus rubra L.) (17), and the basidiomycete Heterobasidion annosum, one of the most destructive basidiomycetes in coniferous forests. In addition activity against the less-investigated ascomycetes Amphiporthe leiphaemia and Diaporthe carpini (Fr.) was found (5). Fungicidal Activity of Mollisia caesia The culture filtrate of Mollisia caesia showed a good antagonistic activity against the post-harvest pathogens Pezicula malicorticis and Nectria galligena, which can cause severe loss in stored apples (18). Fungicidal Activity of Mollisin The isolated natural product mollisin (1) shows various antifungal activities. For example mollisin is active against the basidiomycete fungus Heterobasidion annosum, an economically important forest pathogen and Sclerophoma pityophila, a pathogen that affects pines (5). It also inhibits growth of the wide-spread pests Dothichiza populea and Pollaccia radiosa (6). Furthermore mollisin is also active against some economically-relevant phytopathogenic fungi. Sclerotinia trifoliorum, which can affect spruces (6), Magnaporthe oryzae, also known as rice blast fungus, Phytophthora infestans, an oomycete that causes serious potatoe disease, and Botrytis cinerea, a necrotrophic fungus that affects many plant species are all inhibited by mollisin (2, 16). Activity against Penicillium notatum has been investigated for mollisin (1) and its metabolites mollisin A (2) and B (3). An activity increase from the dichloro (1) to the trichloro (2) to the tetrachloro compound (3) has been observed (5). During their studies BASF investigated the influence of several mollisin analogues on the spore germination of Magnaporthe oryzae, Botrytis cinerea and Phytophthora infestans (16). As also reported for mollisin itself, all of the derivatives show a good inhibition of Magnaporthe oryzae at all tested concentrations (Table 1). The spore germination inhibition of Phytophthora infestans, however, was not as consistent. Whereas smaller substituents (R) appeared to be favourable for the inhibition, aromatic acyl groups had a lower influence on germination. This effect is even more distinct with Botrytis cinerea. In this case only compounds with an acyl (19a) or trifluoroacyl group (19b and 201 In Discovery and Synthesis of Crop Protection Products; Maienfisch, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

20) showed a significant inhibition. Notably, the naphthatriol 20 showed a very good inhibition of spore germination of all tested fungi at all concentrations (16).

Table 1. Test results of spore germination inhibition assaya Spore Germination Inhibition @ 10, 25, 50 µg/mL M. oryzae


Cmp.

a

B. cinerea

P. infestans

10

25

50

10

25

50

10

25

50

19a

+++

+++

+++

++

+++

+++

+++

+++

+++

19b

+++

+++

+++

-

+++

+++

+

+++

+++

19c

+++

+++

+++

-

-

-

++

+++

+++

19f

+++

+++

+++

-

-

-

+

+++

+++

19g

-

+++

+++

-

-

-

-

-

-

19h

+++

+++

+++

-

-

-

-

++

+++

19i

+++

+++

+++

-

-

-

-

-

-

19k

+++

+++

+++

-

-

-

+

+

+

19l

+++

+++

+++

-

-

-

++

+++

+++

19m

+++

+++

+++

-

-

-

-

-

-

19n

+++

+++

+++

-

-

-

-

-

-

19o

+++

+++

+++

-

-

-

-

-

-

19p

+++

+++

+++

-

-

-

-

-

-

20

+++

+++

+++

++

+++

+++

+++

+++

+++

Rating: 0-25% (-), 25-50% (+), 50-75% (++), >75% (+++).

Antiproliferative Activity of Mollisin Mollisin (1) and its two isolated metabolites 2 and 3 have been tested in search for antiproliferative drugs. Hereby, mollisin (1) and mollisin A (2) showed very good antiproliferative activities against two cell lines: the L-929 (murine fibroblasts) and K-562 (human leukemia). Furthermore, both compounds exhibit a weaker cytotoxic effect against the HeLa (human cervix carcinoma) cell line. Mollisin B (3) showed a reduced level of activity against these three cell lines (5). Antiinflammatory Activity of Mollisin A similar activity order was found with respect to phagocytosis-modulating properties. Phagocytic cells are attracted and activated by inflammatory stimuli to attack microbes by effector mechanisms. The phagocytes generate after such stimuli large amounts of reactive oxygen species (ROS).The ROS produced may be of importance in a number of inflammatory diseases, including arthritis, 202 In Discovery and Synthesis of Crop Protection Products; Maienfisch, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


atherosclerotic lesions, and ischemic tissue injury. The degree of release of ROS can be modulated by compounds or drugs. Mollisin B (3) showed a strong inhibitory activity of ROS release of phagocytic cells (5). In the search for new anti-inflammatory drugs, the inhibition of 3α-hydroxysteroid dehydrogenase (3α-HSD) can be a suitable measurement (19). The 3a-HSD enzyme catalyzes the reduction of 5β-dihydrocortisone under consumption of NADPH. The NADPH consumption can be determined photometrically by measuring the decrease of UV/VIS extinction at 340 nm. Indomethacin and ibuprofen were used as reference compounds. Mollisin (1) shows no activity, but mollisin A (2) and B (3) are good inhibitors (5).

Summary More than 60 years after its first isolation from several Mollisia species, mollisin and several close analogues have been investigated for their biological activity against phytopathogenic fungi and pharmacological properties. These studies were enabled by the first total synthesis of this naphthoquinone metabolite and the easy access to its derivatives. It has been shown that several fungal pathogens that can cause significant crop failures are affected by these compounds. Furthermore, interesting pharmacological activities have been found. The biological properties of mollisin make it a promising starting point for combating phytopathogens as well as for pharmaceutical studies.

References 1.

2.

3.

4.

5.

6.

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7. 8. 9. 10.


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19.

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Mollisin: A Promising Antifungal Natural Product - American Chemical

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