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Minassi, A. Synthesis of Natural Disesquiterpenoids. Encyclopedia. Available online: https://encyclopedia.pub/entry/8918 (accessed on 01 July 2024).
Minassi A. Synthesis of Natural Disesquiterpenoids. Encyclopedia. Available at: https://encyclopedia.pub/entry/8918. Accessed July 01, 2024.
Minassi, Alberto. "Synthesis of Natural Disesquiterpenoids" Encyclopedia, https://encyclopedia.pub/entry/8918 (accessed July 01, 2024).
Minassi, A. (2021, April 22). Synthesis of Natural Disesquiterpenoids. In Encyclopedia. https://encyclopedia.pub/entry/8918
Minassi, Alberto. "Synthesis of Natural Disesquiterpenoids." Encyclopedia. Web. 22 April, 2021.
Synthesis of Natural Disesquiterpenoids
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This entry is adapted from the peer-reviewed paper 10.3390/plants10040677

Natural disesquiterpenoids represent a small group of secondary metabolites characterized by complex molecular scaffolds and interesting pharmacological profiles. The intriguing architectures and the interesting pharmacological profile of sesquiterpene dimers attracted the attention of synthetic chemists in the attempt to duplicate the efficiency and the selectivity of natural processes under laboratory conditions.

natural compounds secondary metabolites disesquiterpenoids bioactive compounds

1. Introduction

Disesquiterpenoids, or sesquiterpene dimers, are a group of active molecules containing at least 30 carbons, with a high structural variance deriving from homo- or heterodimeric coupling of two sesquiterpenoids units. They are classified in three major classes depending on their biogenetic pathways and their structural features, namely true disesquiterpenoids, pseudo-disesquiterpenoids and di-merosesquiterpenoids [1]. The compounds belonging to the true disesquiterpenoids originate from farnesyl diphosphate, with the two sesquiterpenoids moieties linked at list by one C-C bond derived from cycloaddition or free-radical coupling reactions. The pseudo-disesquiterpenoids share the same biogenetic pathway of the previous group, but their structures are characterized by an ester, ether or other groups acting as “connectors” between the two sesquiterpenes subunits. The last group of dimers derives from hybrid biogenetic pathways leading chimeric structures with the terpenoid moiety linked by a C-C bond with other subunits belonging to different classes of secondary metabolites such as polyphenols and alkaloids.

2. Synthesis of Natural Disesquiterpenoids

2.1. Bisabolane Dimers

Meiogynin A (1) is a bisabolane dimer possessing an original cis-decalin skeleton with five stereocenters and a carboxylic moiety at the ring junction. Isolated from the bark of Meiogyne cylindrocarpa in 2009 [2], it displays a strong inhibition activity against the anti-apoptotic protein Bcl-xL. The biomimetic total synthesis of meiogynin A (1) was accomplished in 2010 by using a convergent approach allowing the determination of its absolute configuration, having an endo selective Diels-Alder reaction as key step [3]. The synthetic process was based on the synthesis of two synthons, the diene 2 and the dienophile 3, obtained respectively from the commercially available trans-1,4-cyclohexanedimethanol (4) and (S)-citronellal (5).

The (–)-perezoperezone (13) was isolated from the gorgonian octocoral Pseudopterogorgia rigida, a Carribean soft coral [4]. It displays a dimeric structure derived from the dimerization of two units of (-)-perezone (14) through an enzymatic radical mediated coupling. The total synthesis of this marine dimer was published in 2019, and it was based on a copper-catalyzed intermolecular [5+2] homodimerization of (-)-perezone (14) [5]. The synthetic process started from 2,3,5-trimethoxy-toluene (16) obtained from the commercially available 3,5-dimethoxy-toluene (15) via a known three-steps sequence [6].

2.2. Xanthane Dimers

The total synthesis of mogolides A-C (24–26) represents “a showcase of biomimetic-synthesis-guided discovery of new natural products,” wherein a natural product has been synthesized before being discovered in nature [7][8][9][10][11]. The total synthesis of these compounds, initially considered artifacts and then isolated from Xanthium mogolium Kitag plant, was accomplished in 2014 starting from xanthatin (23) by using photo and termal-dimerization processes [12].

Pungiolides are a group of dimeric xanthanolides isolated from different plants of genus Xanthium (Compositae) [13][14]. From a biogenetic point of view, they derive from a dimerization process of two moities of 8-epi-xanthatin (31) through a head-to-tail [4+2] cycloaddition. Their pharmacological profile has been explored, showing a weak cytotoxic activity [15][16] and an interesting antiprotozoal action against pathogens responsible for HAT (Trypanosoma brucei rhodesiense) [17]. The collective synthesis of pungiolides A-E (32–36), L-N (37–39) was reported in 2017 using pungiolide D (35) and pre-pungiolide (40) as key precursors to access to all the other dimeric congeners [18].

2.3. Guaiane Dimers

Ainsliadimers A and B (43, 44) with gochnatiolides A–C (45, 46, 47) are members of the guaiane disesquiterpenoids dimers, showing complex and unique structures mainly characterized by an intriguing spiro [19][20] decane moiety. Ainsliadimer A and B (43, 44) [21][22] have been isolated from different plants of genus Ainsliaea, while the genera Gochnatia represents the major source of gochnatiolides A–C (45, 46, 47) [14][23][24]. Whereas (+)-ainsliadimer A (43) displayed an interesting anti-inflammatory activity, through the inhibition of the production of NO in RAW264.7 stimulated by LPS, [25] and gochnatiolide B (46) demonstrated a potent anti-bladder cancer activity inducing G1 arrest both in vitro and in vivo studies, [26] the biological profile of their congeners still remains an uncharted area. Four different biomimetic total syntheses have been published for both ainsliadimers and gochnatiolides sharing dehydrozaluzanin C (49) [27] as key monomer, and a hetero Diels-Alder reaction as dimerization step.

2.4. Lindenane Dimers

The (+)-chloranthalactone F (61) was first isolated from the leaves of Chlorantus glaber, and it is characterized by an intriguing structure with twelve stereogenic centers and two clyclopropane rings bearing two adjacent angular methyl groups next to a highly congested cyclobutane [28][29]. While the plants of the genera Chloranthus have been widely used in traditional Chinese medicine, the biological profile of this compound still remains an untouched area. Biogenetically, it is postulated that the complex architecture of chloranthalactone F (61) derives from an intermolecular [2+2] cycloaddition between two units of chloranthalactone A (63). This biogenetical hypothesis inspired two different biomimetic total synthesis.

Lindenane sesquiterpene dimers deriving from a formal [4+2] cycloaddition, represent a large group of natural compounds characterized by a basic framework of eight fused rings with more than eleven stereogenic centers. Together with an intriguing structural motif, these metabolites have an interesting pharmacological profile displaying a broad range of biological activities. Among all, shizukaol E (71) showed a strong inhibitory effect against the most popular NNRTI-resistant HIV-1 [30], while its congener chlorajaponilide C (72) was revealed to be a highly selective inhibitor of chloroquine resistant strains of Plasmodium falciparum, showing IC50 values in the low nanomolar range comparable to the potency of artemisinin [31].

2.5. Cadinane Dimers

The (−)-(R)-gossypol (108) is a polysubstituted salicylaldehyde dimer that was firstly discovered in 1886 in cotton seeds oil by Longmore [32] and purified in 1889 by cristallization from an acetic acid solution by MarchLewski [33]. Gossypol (108) displays different pharmacological properties and, among all, the spermicidal and antitumor activity are noteworthy. The contraceptive activity does not affect the hormone levels and derives from the inhibition of the enzyme systems that affect energy metabolism in sperm and spermatogenic cells [34]. On the other side, this polyphenolic dimer is actually on phase II clinical trials as antitumor agent against progressive or recurrent glioblastoma multiforme [35][36].

2.6. Miscellaneous Dimers

Bisacutifolone A and B (127, 128) are two of the few examples of pinguisane-type sesquiterpenoids dimers so far identified [37][38]. They have been isolated from liverwort Porella acutifolia subsp. tosana, and despite this liverwort shown a broad range of interesting biological activities, the pharmacological profiles of the two natural dimers still remain an unexplored area. From a biogenetic point of view, it is postulated that the dimeric framework of 127 and 128 derives from an intermolecular [4+2] cycloaddition between two units of acutifolone A (129) [39][40]. All the reported compounds share an unusual [4.3.0]nonane framework with four contiguous chiral centers oriented in an all-cis fashion.

Vannusals A and B (142, 143) are two secondary metabolites with an unusual dimeric framework, characterized by six rings and thirteen stereocenters, isolated from the tropical strains of the interstitial marine ciliate Euplotes vannus [41]. The structural assignment was elucidated by Guella, and then revised by Nicolaou after comparison of the NMR spectral data of synthetic 144 (original assigned structure) and natural vannusal B (143) [41][42][43]. The campaign to elucidate the true structures of the vannusals required the synthesis of eight diastereoisomers of vannusal B (143), in the attempt to pinpoint the correct configuration(s) of the stereocenter(s). Herein reported is the final synthetic pathway applied to obtain the desired natural dimers 142 and 143 [44].

2.7. Sesquiterpenoid Alkaloid Dimers

Dimeric nuphar thioalkaloids are a unique group of sulfur-containing secondary metabolites deriving from the dimerization of monomers possessing a regular sesquiterpenic skeleton incorporated into 3-furyl substituted piperidines. They have been isolated for the first time in 1964 from Nuphar lutea (L.) Sm., a yellow water lily belonging to the aquatic family of Nymphaeaceae, and three different series are known whose structures differ in the relative configuration at the thiaspirane junction [45]. Recently they have displayed an interesting anticancer activity with IC50 values in the high nanomolar range in in vitro assays against murine (B16) melanoma cells proliferation, and crude extracts of N. lutea have shown a synergistic effect enhancing the cytotoxicity of cisplatin toward Hodgkin’s lymphoma-derived cells (L428) [46][47]. Additionally, these extracts have shown an interesting anti-inflammatory activity through the inhibition of the nuclear factor kB (NFkB) [47]. Although these natural compounds have been discovered in the second half of the last century, only in 2013 was reported the first total synthesis of a thiaspirane nuphar dimer in enantiomerically pure form [48].

2.8. Merosesquiterpenoid Dimers

Indole sesquiterpenoids are a small group of secondary metabolites characterized by a dichotomous structure with an indole moiety fused to a terpenic unit [49]. While different monomeric members have been isolated from Streptomyces species, displaying interesting biological activities such as anti-HIV and antibiotic [50][51][52], dimeric congeners still remain a mere curiosity given the small number of derivatives identified. Among them, while the two atropoisomeric dixiamycin A and B (182, 183) present a N-N linkage acting as connector, dixiamycin C (184) is characterized by C6-N1′ bond linking the two monomeric units [53][54].

3. Summary

Bioactive natural compounds are the products of continuous adaptation of living organisms to a constantly changing environment, magnifying the ability of nature in the construction of complex architectures. Dimeric sesquiterpenoids are an excellent example of this adaptation, being characterized by unique and complex frameworks combined with a wide range of biological activities. In the last decade, more than 400 disesquiterpenoids have been isolated, showing more potent activities than their corresponding monomeric congeneres. Despite their interesting pharmacological profile, further studies are blocked or slowed down by the difficulties in obtaining these secondary metabolites in good amount and purity. The attempt to duplicate the efficiency and the selectivity of natural processes under laboratory conditions still remains a challenging problem in organic chemistry, stimulating the development of new synthetic methodologies to build complex frameworks in a shorter time and more efficient manner. This review has summarized all the latest (2010–2020) efforts devoted to the synthesis of disequiterpenoids using new biomimetic approaches that allowed the access to interesting secondary metabolites otherwise produced by nature in very small amounts.

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Alberto Minassi
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