Divergent Strategy in Marine Tetracyclic Meroterpenoids Synthesis

marine drugs Review Divergent Strategy in Marine Tetracyclic Meroterpenoids Synthesis Antonio Rosales Martínez 1,* , Ignacio Rodríguez-García2and Josefa L. López-Martínez 2   Citation: Rosales Martínez, A.; Rodríguez-García, I.; López-Martínez, J.L. Divergent Strategy in Marine Tetracyclic Meroterpenoids Synthesis. Mar. Drugs 2021,19, 273. https:// doi.org/10.3390/md19050273 Academic Editor: Emiliano Manzo Received: 29 April 2021 Accepted: 12 May 2021 Published: 13 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Chemical Engineering, Escuela Politécnica Superior, University of Sevilla, 41011 Sevilla, Spain 2Organic Chemistry, ceiA3, University of Almería, 04120 Almeria, Spain; [email protected] (I.R.-G.); [email protected] (J.L.L.-M.) *Correspondence: ar[email protected] Abstract: The divergent total synthesis strategy can be successfully applied to the preparation of families of natural products using a common late-stage pluripotent intermediate. This approach is a powerful tool in organic synthesis as it offers opportunities for the efficient preparation of structurally related compounds. This article reviews the synthesis of the marine natural product aureol, as well as its use as a common intermediate in the divergent synthesis of other marine natural and non-natural tetracyclic meroterpenoids. Keywords: divergent total synthesis; marine natural products; tetracyclic meroterpenoids; aureol 1. Introduction The original definition of divergent total synthesis (Figure 1) was reported by Boger et al. [ 1 ] and was defined as the synthesis in which “at least two members of the class of compounds” can be separately prepared from a common, advanced synthetic intermediate. Therefore, the most important challenge in a divergent synthesis is the choice of a common intermediate which could be transformed into a target array of natural products and non-natural derivatives. This strategy is a powerful tool that has attracted the attention of numerous research groups as it improves the efficiency of chemical processes [ 2 , 3 ], and attains special relevance when structure-activity studies are the ultimate goals. Later, other terms such as “diverted total synthesis” [ 4 ] and “collective total synthesis” [ 5 ] were introduced, thus extending the amplitude of divergent synthesis. In this way, “diverted total synthesis” can be applied to the preparation of a natural product-like compound library by appropriate transformations of a common intermediate, avoiding the limitations inherent to partial syntheses from the natural product caused by the presence of multiple similar functional groups. In addition, the term “collective total synthesis” is used when the common intermediate is endowed with functional characteristics suitable for the preparation of structurally diverse natural products belonging to different families. Mar. Drugs 2021, 19, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/marinedrugs Review Divergent Strategy in Marine Tetracyclic Meroterpenoids Synthesis Antonio Rosales Martínez 1, *, Ignacio Rodríguez-García 2 and Josefa L. López-Martínez 2 1 Department of Chemical Engineering, Escuela Politécnica Superior, University of Sevilla, 41011 Sevilla, Spain 2 Organic Chemistry, ceiA3, University of Almería, 04120 Almeria, Spain; [email protected] (I.R.-G.); [email protected] (J.L.L.-M.) * Correspondence: [email protected] Abstract: The divergent total synthesis strategy can be successfully applied to the preparation of families of natural products using a common late-stage pluripotent intermediate. This approach is a powerful tool in organic synthesis as it offers opportunities for the efficient preparation of structurally related compounds. This article reviews the synthesis of the marine natural product aureol, as well as its use as a common intermediate in the divergent synthesis of other marine natural and non-natural tetracyclic meroterpenoids. Keywords: divergent total synthesis; marine natural products; tetracyclic meroterpenoids; aureol 1. Introduction The original definition of divergent total synthesis (Figure 1) was reported by Boger et al. [1] and was defined as the synthesis in which “at least two members of the class of compounds” can be separately prepared from a common, advanced synthetic intermediate. Therefore, the most important challenge in a divergent synthesis is the choice of a common intermediate which could be transformed into a target array of natural products and non-natural derivatives. This strategy is a powerful tool that has attracted the attention of numerous research groups as it improves the efficiency of chemical processes [2,3], and attains special relevance when structure-activity studies are the ultimate goals. Later, other terms such as “diverted total synthesis” [4] and “collective total synthesis” [5] were introduced, thus extending the amplitude of divergent synthesis. In this way, “diverted total synthesis” can be applied to the preparation of a natural product-like compound library by appropriate transformations of a common intermediate, avoiding the limitations inherent to partial syntheses from the natural product caused by the presence of multiple similar functional groups. In addition, the term “collective total synthesis” is used when the common intermediate is endowed with functional characteristics suitable for the preparation of structurally diverse natural products belonging to different families. Figure 1. Divergent total synthesis. Citation: Rosales Martinez, A.; Rodríguez-García, I.; López- Martínez, J.L. Divergent Strategy in Marine Tetracyclic Meroterpenoids Synthesis. Mar. Drugs 2021, 19, x. doi:10.3390/xxxxx Academic Editor Emiliano Manzo Received: 29 April 2021 Accepted: 12 May 2021 Published: 13 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Figure 1. Divergent total synthesis. On the other hand, tetracyclic meroterpenoids [ 6 , 7 ] are a unique class of marine natural compounds with significant biological activities. Representative examples of marine Mar. Drugs 2021,19, 273. https://doi.org/10.3390/md19050273 https://www.mdpi.com/journal/marinedrugs Mar. Drugs 2021,19, 273 2 of 15 natural and non-natural tetracyclic meroterpenoids, including (+)-aureol ( 1 ) [ 8 – 10 ], (+)- strongylin A (2) [8,11,12], (+)-smenoqualone (3) [8,13], (−)-cyclosmenospongine (4) [8,14], (+)-5-epi-aureol ( 5 ) [ 8 , 15 – 17 ], and (+)-5-epi-smenoqualone ( 6 ) [ 8 ] have been considered of interest by the chemical community due to their interesting biological properties and unique molecular structures (Scheme 1). In fact, structure–activity relationship (SAR) studies show that variations on the nature and substituents on the aromatic ring have a strong impact on the observed activity [ 8 , 18 ]. These natural products (Scheme 1) contain a compact tetracyclic system with a substituted benzopyran moiety, four consecutive asymmetric carbon atoms, and a well-defined trans- or cis-relationship between the two cyclohexane rings of the decalin system. Although several synthetic methods have been described, a divergent approach to this class of compounds has not been previously reported as such. Mar. Drugs 2021, 19, x FOR PEER REVIEW 2 of 16 On the other hand, tetracyclic meroterpenoids [6,7] are a unique class of marine natural compounds with significant biological activities. Representative examples of marine natural and non-natural tetracyclic meroterpenoids, including (+)-aureol (1) [8–10], (+)-strongylin A (2) [8,11,12], (+)-smenoqualone (3) [8,13], (−)-cyclosmenospongine (4) [8,14], (+)-5-epi-aureol (5) [8,15–17], and (+)-5-epi-smenoqualone (6) [8] have been considered of interest by the chemical community due to their interesting biological properties and unique molecular structures (Scheme 1). In fact, structure–activity relationship (SAR) studies show that variations on the nature and substituents on the aromatic ring have a strong impact on the observed activity [8,18]. These natural products (Scheme 1) contain a compact tetracyclic system with a substituted benzopyran moiety, four consecutive asymmetric carbon atoms, and a well-defined trans- or cis-relationship between the two cyclohexane rings of the decalin system. Although several synthetic methods have been described, a divergent approach to this class of compounds has not been previously reported as such. Scheme 1. Representative examples of marine natural and non-natural tetracyclic meroterpenoids and their reported biological activity. This article focuses on the synthetic efforts towards aureol (1), a marine natural meroterpenoid present in the Caribbean sponges Smenospongia aurea [9] and Verongula gigantea [10] which has shown an important biological profile [8,19,20]. Aureol (1) can be an excellent advanced and common synthetic intermediate for the divergent synthesis of other natural and non-natural tetracyclic terpenoids. In this article, we present a unified and versatile approach for the diversification of this class of compounds with the aim to contribute to the development of new desirable drugs for the pharmaceutical industry and the medicinal chemistry. The divergent synthesis of either natural or fully synthetic derivatives could be achieved through aureol (1) as a common intermediate, by adequate sequential functionalization of the aromatic ring, or by epimerization of the decalin core of aureol (1) to 5-epi-aureol (5) followed by functionalization of the aromatic ring (Scheme 2). H O OH 1: (+)-Aureol H O 4: (-)-Cyclosmenospongine H O 2: (+)-Strongylin A O O NH2 O 3: (+)-Smenoqualone O O OMe H OMe HO H O OH 5: (+)-5-epi-aureol O 6: (+)-5-epi-smenoqualone O O OMe H [antiviral, cytotoxic, antibiotic] [antiviral, cytotoxic, antibiotic] [antibiotic] [cytotoxic] [antibiotic] [antibiotic] Smenospongia aurea [9] Verongula gigantea [10] Strongylophora hartmani [11] Xestosgongia wiedenmuyeri [12] Smenospongia sp. [13] Spongia sp. [14] synthetic synthetic Scheme 1. Representative examples of marine natural and non-natural tetracyclic meroterpenoids and their reported biological activity. This article focuses on the synthetic efforts towards aureol ( 1 ), a marine natural meroterpenoid present in the Caribbean sponges Smenospongia aurea [ 9 ] and Verongula gigantea [ 10 ] which has shown an important biological profile [ 8 , 19 , 20 ]. Aureol ( 1 ) can be an excellent advanced and common synthetic intermediate for the divergent synthesis of other natural and non-natural tetracyclic terpenoids. In this article, we present a unified and versatile approach for the diversification of this class of compounds with the aim to contribute to the development of new desirable drugs for the pharmaceutical industry and the medicinal chemistry. The divergent synthesis of either natural or fully synthetic derivatives could be achieved through aureol ( 1 ) as a common intermediate, by adequate sequential functionalization of the aromatic ring, or by epimerization of the decalin core of aureol ( 1 ) to 5-epi-aureol ( 5 ) followed by functionalization of the aromatic ring (Scheme 2). Mar. Drugs 2021,19, 273 3 of 15 Mar. Drugs 2021, 19, x FOR PEER REVIEW 3 of 16 Scheme 2. Conceptual model of the divergent synthesis of tetracyclic meroterpenoids using aureol (1) as a pluripotential late-stage intermediate. Scheme 3 summarizes the last step of previous syntheses towards aureol (1). All of them have as key step a cationic cyclization of an olefinic intermediate (7–11). Scheme 3. Key olefinic intermediates of previous syntheses of aureol (1). The different synthetic sequences for aureol (1) are listed below, classified according to the olefinic key intermediate shown in Scheme 3. 2. Synthesis of Aureol 2.1. Synthesis of Aureol from Key Intermediates 7–9 2.1.1. Capon’s Synthesis of (+)-Aureol The first work on the synthesis of the marine product (+)-aureol (1) was published by the group of R. J. Capon [15] using natural sesquiterpene hydroquinones ((+)-avarol (7) and (+)-arenarol (8)) as starting materials (Scheme 4). In these processes (+)-aureol (1) could be formed via a concerted 1,2-migration of Me-12 and H-10. However, formation of (+)-epi-aureol (5) is better understood considering that, after methyl migration, there is a loss of the C-10 proton to give a Δ5,10 olefin intermediate, which would later suffer trans addition of the OH group. This lack of stereocontrol of the process was later confirmed by Lakshmi et al. [21], as they could determine the structure of 5 by X-ray analysis (Scheme 4b). H O OH 1: aureol OH HO HOH HO OH HO BF3.Et2O BF3.Et2O T. Magauer & col (2017) 11 10 I. S. Marcos & col (2010) J. H. George & col (2012) Y.-C. Wu & col (2018) A. Rosales Martínez & col (2015, 2020) H 4 11 12 1 7:3(4), Me-12 8:4(11), Me-12 9:4(11), Me-12 3 R. J. Capon & col (1994) T. Katoh & col (2002, 2002, 2008) BF3.Et2O Scheme 2. Conceptual model of the divergent synthesis of tetracyclic meroterpenoids using aureol (1) as a pluripotential late-stage intermediate. Scheme 3summarizes the last step of previous syntheses towards aureol ( 1 ). All of them have as key step a cationic cyclization of an olefinic intermediate (7–11). Mar. Drugs 2021, 19, x FOR PEER REVIEW 3 of 16 Scheme 2. Conceptual model of the divergent synthesis of tetracyclic meroterpenoids using aureol (1) as a pluripotential late-stage intermediate. Scheme 3 summarizes the last step of previous syntheses towards aureol (1). All of them have as key step a cationic cyclization of an olefinic intermediate (7–11). Scheme 3. Key olefinic intermediates of previous syntheses of aureol (1). The different synthetic sequences for aureol (1) are listed below, classified according to the olefinic key intermediate shown in Scheme 3. 2. Synthesis of Aureol 2.1. Synthesis of Aureol from Key Intermediates 7–9 2.1.1. Capon’s Synthesis of (+)-Aureol The first work on the synthesis of the marine product (+)-aureol (1) was published by the group of R. J. Capon [15] using natural sesquiterpene hydroquinones ((+)-avarol (7) and (+)-arenarol (8)) as starting materials (Scheme 4). In these processes (+)-aureol (1) could be formed via a concerted 1,2-migration of Me-12 and H-10. However, formation of (+)-epi-aureol (5) is better understood considering that, after methyl migration, there is a loss of the C-10 proton to give a Δ5,10 olefin intermediate, which would later suffer trans addition of the OH group. This lack of stereocontrol of the process was later confirmed by Lakshmi et al. [21], as they could determine the structure of 5 by X-ray analysis (Scheme 4b). H O OH 1: aureol OH HO HOH HO OH HO BF3.Et2O BF3.Et2O T. Magauer & col (2017) 11 10 I. S. Marcos & col (2010) J. H. George & col (2012) Y.-C. Wu & col (2018) A. Rosales Martínez & col (2015, 2020) H 4 11 12 1 7:3(4), Me-12 8:4(11), Me-12 9:4(11), Me-12 3 R. J. Capon & col (1994) T. Katoh & col (2002, 2002, 2008) BF3.Et2O Scheme 3. Key olefinic intermediates of previous syntheses of aureol (1). The different synthetic sequences for aureol ( 1 ) are listed below, classified according to the olefinic key intermediate shown in Scheme 3. 2. Synthesis of Aureol 2.1. Synthesis of Aureol from Key Intermediates 7–9 2.1.1. Capon’s Synthesis of (+)-Aureol The first work on the synthesis of the marine product (+)-aureol ( 1 ) was published by the group of R. J. Capon [ 15 ] using natural sesquiterpene hydroquinones ((+)-avarol ( 7 ) and (+)-arenarol ( 8 )) as starting materials (Scheme 4). In these processes (+)-aureol ( 1 ) could be formed via a concerted 1,2-migration of Me-12 and H-10. However, formation of (+)-epi- aureol ( 5 ) is better understood considering that, after methyl migration, there is a loss of the C-10 proton to give a ∆5,10 olefin intermediate, which would later suffer trans addition of the OH group. This lack of stereocontrol of the process was later confirmed by Lakshmi et al. [21], as they could determine the structure of 5by X-ray analysis (Scheme 4b). Mar. Drugs 2021,19, 273 4 of 15 Mar. Drugs 2021, 19, x FOR PEER REVIEW 4 of 16 Scheme 4. (a): Acid-induced rearrangement of (+)-avarol (7). (b): Acid-induced rearrangement of (+)-arenarol (8). 2.1.2. Katoh´s Synthesis of (+)-Aureol Katoh and colleagues [22–24] reported the first enantioselective total synthesis of (+)-aureol (1) in 2002 [22], a process they later improved in 2003 [23]. The retrosynthetic plan of the improved synthesis is shown in Scheme 5a. This approach obtains aureol (1) in one step by acid-induced rearrangement/cyclization of (−)-neoavarol (9). In turn, 9 can be prepared by reduction of the quinone moiety present in (−)-neovarone (13), a compound which can be readily obtained by strategic salcomine oxidation of 14. This can be assembled by stereocontrolled reductive alkylation of (+)-5-methyl-Wieland-Miescher ketone (15) with 2-methoxybenzyl bromide (16) (Scheme 5a). As shown in Scheme 5b, Katoh´s synthesis of (+)-aureol (1) used enantiopure (+)-5-methyl-Wieland-Miescher ketone (15) as the starting material. A C-C bond-forming reaction between 15 and a lithiated arene unit, prepared from 2-methoxybenzyl bromide (16), gave the coupling product 17 as a single diastereomer in 74%. The Wittig methylenation of 17 produced the exo-double bond present in decaline 18 in 86% yield. Removal of the acetal protective group by acid treatment (97% yield), followed by hydrogenation of the exocyclic double bond present in the resulting ketone 19, led to the product 20 (80% yield) together with its C8 epimer (13% yield). Subsequent Wittig methylenation of 20 quantitatively gave 21, which was submitted to a deprotection of the O-methyl group in order to form the phenol 14 (92% yield). O2/salcomine oxidation of 14 gave the quinone 13 (91% yield). Finally, NaBH4 reduction of quinone 13 gave (−)-neoavarol (9) (86% yield). Scheme 4. ( a ): Acid-induced rearrangement of (+)-avarol ( 7 ). ( b ): Acid-induced rearrangement of (+)-arenarol (8). 2.1.2. Katoh’s Synthesis of (+)-Aureol Katoh and colleagues [ 22 – 24 ] reported the first enantioselective total synthesis of (+)-aureol ( 1 ) in 2002 [ 22 ], a process they later improved in 2003 [ 23 ]. The retrosynthetic plan of the improved synthesis is shown in Scheme 5a. This approach obtains aureol ( 1 ) in one step by acid-induced rearrangement/cyclization of ( − )-neoavarol ( 9 ). In turn, 9 can be prepared by reduction of the quinone moiety present in ( − )-neovarone ( 13 ), a compound which can be readily obtained by strategic salcomine oxidation of 14 . This can be assembled by stereocontrolled reductive alkylation of (+)-5-methyl-Wieland-Miescher ketone (15) with 2-methoxybenzyl bromide (16) (Scheme 5a). As shown in Scheme 5b, Katoh’s synthesis of (+)-aureol ( 1 ) used enantiopure (+)-5- methyl-Wieland-Miescher ketone ( 15) as the starting material. A C-C bond-forming reaction between 15 and a lithiated arene unit, prepared from 2-methoxybenzyl bromide ( 16 ), gave the coupling product 17 as a single diastereomer in 74%. The Wittig methylenation of 17 produced the exo-double bond present in decaline 18 in 86% yield. Removal of the acetal protective group by acid treatment (97% yield), followed by hydrogenation of the exocyclic double bond present in the resulting ketone 19, led to the product 20 (80% yield) together with its C8 epimer (13% yield). Subsequent Wittig methylenation of 20 quantitatively gave 21 , which was submitted to a deprotection of the O-methyl group in order to form the phenol 14 (92% yield). O 2 /salcomine oxidation of 14 gave the quinone 13 (91% yield). Finally, NaBH 4 reduction of quinone 13 gave ( − )-neoavarol ( 9 ) (86% yield). Once 9 was synthesized, the crucial step was the BF 3. Et 2 O-induced rearrangement of 9, which led to Mar. Drugs 2021,19, 273 5 of 15 the desired (+)-aureol ( 1 ) (93% yield). This rearrangement occurred via stereospecific 1,2- hydride and methyl shifts, as shown in Scheme 5b. In this reaction the three intermediate carbocations I , II , and III are involved. This synthesis was completed in nine steps (33% overall yield) from the starting material 15. Mar. Drugs 2021, 19, x FOR PEER REVIEW 5 of 16 Once 9 was synthesized, the crucial step was the BF3.Et2O-induced rearrangement of 9, which led to the desired (+)-aureol (1) (93% yield). This rearrangement occurred via stereospecific 1,2-hydride and methyl shifts, as shown in Scheme 5b. In this reaction the three intermediate carbocations I, II, and III are involved. This synthesis was completed in nine steps (33% overall yield) from the starting material 15. Scheme 5. Strategy for the synthesis of (+)-aureol (1) according to Katoh and colleagues [22–24]. (a): Retrosynthetic plan. (b): Synthesis of (+)-aureol (1). HMPA = hexamethylphosphoramide; salcomine = N,N′-bis(salicylidene)ethylenediamino cobalt(II); DMF = N,N′-dimethylformamide. Scheme 5. Strategy for the synthesis of (+)-aureol ( 1 ) according to Katoh and colleagues [ 22 – 24 ]. ( a ): Retrosynthetic plan. ( b ): Synthesis of (+)-aureol ( 1 ). HMPA = hexamethylphosphoramide; salcomine =N,N0-bis(salicylidene)ethylenediamino cobalt(II); DMF = N,N0-dimethylformamide. Mar. Drugs 2021,19, 273 6 of 15 2.2. Synthesis of Aureol from Key Intermediate 10 Magauer’s Synthesis of (+)-Aureol The synthesis of (+)-aureol ( 1 ) reported by Magauer and colleagues [ 8 ] used a highly robust and modular synthetic platform, developed for the preparation of natural and fully synthetic analogues of tetracyclic meroterpenoids. The retrosynthetic plan for (+)- aureol ( 1 ) (Scheme 6a) is based on the stereospecific acid-promoted cyclization of the ∆5(6) olefin intermediate 10 . Its methoxymethyl ether derivative 22 could be prepared from 23 following the Barton–McCombie deoxygenation protocol. This diastereoisomeric mixture of benzyl alcohols results from the addition of 2-lithiohydroquinone dimethyl ether to aldehyde 24, which can be obtained by Fukuyama’s reduction [25] of thioester 25. Mar. Drugs 2021, 19, x FOR PEER REVIEW 6 of 16 2.2. Synthesis of Aureol from Key Intermediate 10 Magauer´s Synthesis of (+)-Aureol The synthesis of (+)-aureol (1) reported by Magauer and colleagues [8] used a highly robust and modular synthetic platform, developed for the preparation of natural and fully synthetic analogues of tetracyclic meroterpenoids. The retrosynthetic plan for (+)-aureol (1) (Scheme 6a) is based on the stereospecific acid-promoted cyclization of the Δ5(6) olefin intermediate 10. Its methoxymethyl ether derivative 22 could be prepared from 23 following the Barton–McCombie deoxygenation protocol. This diastereoisomeric mixture of benzyl alcohols results from the addition of 2-lithiohydroquinone dimethyl ether to aldehyde 24, which can be obtained by Fukuyama’s reduction [25] of thioester 25. Scheme 6. Strategy for the synthesis of (+)-aureol (1) according to Magauer et al. [8]. (a): Retrosynthetic plan. (b): Synthesis of (+)-aureol (1). TMEDA = N,N,N′,N′-tetramethylethane-1,2-diamine; NaHDMS = sodium bis(trimethylsilyl)amide; AIBN = 2,2′-azobis(2-methylpropionitrile). Scheme 6. Strategy for the synthesis of (+)-aureol ( 1 ) according to Magauer et al. [ 8 ]. ( a ): Retrosynthetic plan. ( b ): Synthesis of (+)-aureol ( 1 ). TMEDA = N,N,N 0 ,N 0 -tetramethylethane-1,2-diamine; NaHDMS = sodium bis(trimethylsilyl)amide; AIBN = 2,20-azobis(2-methylpropionitrile). Mar. Drugs 2021,19, 273 7 of 15 The keystone in the asymmetric process is the formation of the three chiral centers present in 25 through the asymmetric Diels–Alder reaction [ 26 , 27 ] between 26 and enantiopure 27 followed by lithium ethanethiolate removal of the chiral auxiliar (Scheme 6a). Scheme 6b details the synthesis of (+)-aureol ( 1 ) from diene 26 . The first key step was the asymmetric construction of the 5,6-dehydrodecaline component 28 employing an exo-selective Diels–Alder cycloaddition between diene 26 and tiglic acid-derived dienophile 27 to afford 28 (61% yield). The oxazolidinone chiral auxiliar was replaced by nucleophilic 1,2-addition of lithium ethanethiolate to the carbonyl group of the Diels–Alder product 28 , a process which afforded thioester 25 in 98% yield. A smooth Fukuyama reduction [ 25 ] of 25 gave aldehyde 24 (85% yield), which was coupled with the lithiated arene unit to afford a mixture of diastereomeric benzylic alcohols 23 (86% yield). The free hydroxy group of 23 was removed using the two-step Barton–McCombie deoxygenation protocol, which afforded 22 in 84% yield (two steps). The subsequent deprotection of 22 with HCl/MeOH gave hydroquinone sesquiterpene 10 , which was directly subjected to cyclization conditions to give (+)-aureol ( 1 ) (83% yield). In this reaction, the proton formed by coordination of BF 3. OEt 2 to one of the OH-groups in 10 possibly triggers the cationic rearrangement. When the cyclization is carried out under kinetic conditions (at temperatures below − 10 ◦ C), a cis-decaline framework is formed exclusively. On the other hand, under thermodynamic control, only the trans-decaline is obtained. This total synthesis was achieved in eight steps (30% overall yield) from the starting material 26. 2.3. Synthesis of Aureol from Key Intermediate 11 2.3.1. Marcos’s Synthesis of (−)-Aureol Marcos and colleagues [ 28 ] reported the total synthesis of the ( − ) enantiomer of aureol (ent- 1 ) from the methyl ester of natural ent-halimic acid (Scheme 7a). Their approach was based on: (a) the acid-induced cyclization of sesquiterpene hydroquinone ent- 11 , (b) the Barton decarboxylation reaction/p-benzoquinone addition sequence and the subsequent reduction with Raney ® nickel (ent- 11 from 29 ), and (c) the side-chain degradation of enthalimic acid methyl ester 30 and the subsequent reduction of C-18 methyl ester. As shown in Scheme 7b the synthesis of ( − )-aureol (ent- 1 ) used ent-halimic acid methyl ester 30 as the starting material. The degradation of the side chain of 30 was achieved [ 29 , 30 ] by oxidation with OsO 4 followed by Pb(OAc) 4 , which gave ketone 31 (94% yield, two steps). The synthesis of the endo-olefin 33 required the Wittig methylenation of 31 (87% yield) and subsequent acid isomerization of 32 (99% yield). In order to remove the C-18 methyl ester, a three steps sequence from 33 to 36 was used, a process which gave a very good global yield. The synthesis of product 39 was achieved in four steps: a) the chemoselective epoxidation of the side-chain double bond in 36 (98% yield), b) the oxidative cleavage with H 5 IO 6 in H 2 O/THF to afford 37 (94% yield); c) reduction with LiAlH 4 (99% yield) to give 38 (99% yield), and d) the acetylation of the hydroxy group in 38 to afford 39 (99% yield). The isomerization of the olefin double bond present in acetate 39 with HI (97% yield) followed by the saponification of the acetoxy group (98% yield) gave the rearranged product ent- 40 . Finally, the oxidation of ent- 40 to acid 29 via aldehyde 41 was achieved with pyridinium dichromate (PDC) in a moderate yield. Once intermediate 29 was available, the key precursor ent- 11 of ( − )-aureol (ent- 1 ) could be readily prepared by Barton decarboxylation reaction in the presence of p-benzoquinone, a methodology reported by Theodorakis and colleagues [ 31 , 32 ] for the synthesis of ilimaquinone. In this way, when 29 was treated with 2-mercaptopyridine N-oxide in the presence of N,N 0 - dicyclohexylcarbodiimide (DCC) a photo labile thio-hydroxamic ester ( 42 ) was obtained. Then, 43 was prepared by light-induced decarboxylation (halogen lamp 500W) of 42 in the presence of benzoquinone in a 65% yield from 29 . The subsequent reduction of 43 with Raney ® nickel gave ent- 11 in a 99% yield. With the key precursor ent- 11 in their hands, the treatment of this compound with BF 3. Et 2 O at low temperature exclusively afforded ( − )-aureol (ent- 1 ) with complete stereoselectivity (60% yield). This total synthesis was Mar. Drugs 2021,19, 273 8 of 15 achieved in 19 steps (10.3% overall yield) from ent-halimic acid methyl ester 30 as the chiral pool starting material. Mar. Drugs 2021, 19, x FOR PEER REVIEW 8 of 16 achieved in 19 steps (10.3% overall yield) from ent-halimic acid methyl ester 30 as the chiral pool starting material. Scheme 7. Strategy for the synthesis of (−)-aureol (ent-1) according to Marcos and colleagues [26]. (a): Retrosynthetic plan. (b): Synthesis of (−)-aureol (ent-1). NMO = N-methylmorpholine-N-oxide; TPAP = tetra-n-propylammonium perruthenate; mCPBA = m-chloroperoxybenzoic acid; PDC = pyridinium dichromate; DCC = N,N′-dicyclohexylcarbodiimide. 2.3.2. George´s Synthesis of (+)-Aureol George´s group [33] published in 2012 the second total synthesis of (+)-aureol (1). Their biosynthetically inspired retrosynthesis of (+)-aureol (1) (Scheme 8a) rests upon the biomimetic acid-mediated cyclization of the key tetrasubstituted olefin intermediate 11, which could be prepared through a process involving the addition of an aryllithium derivative to aldehyde 44. This aldehyde could be formed using a one-carbon dehomologation sequence from 45. Another key step in the process is the biomimetic sequence of 1,2- hydride and 1,2-methyl shifts, which converts alcohol 46 into 45. Finally, the reduction and selective protection of the commercially available enantiopure starting material (+)-sclareolide (47) would form the intermediate 46. Scheme 7. Strategy for the synthesis of ( − )-aureol (ent- 1 ) according to Marcos and colleagues [ 26 ]. ( a ): Retrosynthetic plan. ( b ): Synthesis of ( − )-aureol (ent- 1 ). NMO = N-methylmorpholine-N-oxide; TPAP = tetra-n-propylammonium perruthenate; mCPBA = m-chloroperoxybenzoic acid; PDC = pyridinium dichromate; DCC = N,N0-dicyclohexylcarbodiimide. 2.3.2. George’s Synthesis of (+)-Aureol George’s group [ 33 ] published in 2012 the second total synthesis of (+)-aureol ( 1 ). Their biosynthetically inspired retrosynthesis of (+)-aureol ( 1 ) (Scheme 8a) rests upon the biomimetic acid-mediated cyclization of the key tetrasubstituted olefin intermediate 11 , which could be prepared through a process involving the addition of an aryllithium derivative to aldehyde 44 . This aldehyde could be formed using a one-carbon dehomologation sequence from 45 . Another key step in the process is the biomimetic sequence of 1,2-hydride and 1,2-methyl shifts, which converts alcohol 46 into 45 . Finally, the reduc- Mar. Drugs 2021,19, 273 9 of 15 tion and selective protection of the commercially available enantiopure starting material (+)-sclareolide (47) would form the intermediate 46. Mar. Drugs 2021, 19, x FOR PEER REVIEW 9 of 16 Scheme 8. Strategy for the synthesis of (+)-aureol (1) according to George and colleagues [33]. (a) Retrosynthetic plan. (b) Synthesis of (+)-aureol (1). DMAP = 4-(dimethylamino)pyridine; NMO= N-methylmorpholine-N-oxide; TBAF = tetrabutylammonium fluoride. As shown in Scheme 8b, George´s synthesis of (+)-aureol (1) used 11 as the key intermediate, which was prepared from natural (+)-sclareolide (47). Its reduction with LiAlH4 gave a diol, which was selectively protected at the primary hydroxy group with Ac2O in pyridine to afford the acetate 46 in an 84% yield (two steps). Monoacetate 46 was stereoselectively converted to the single stereoisomer olefin 45 (70% yield) in a rearrangement induced by BF3.Et2O, which occurred via stereospecific sequential 1,2-hydride and 1,2-me- thyl shifts. Saponification of the acetate in 45 gave alcohol 40 (83% yield), which was readily converted into aldehyde 44 through a one-carbon dehomologation sequence using the Grieco–Sharples elimination protocol [34,35] (67% yield in two steps) followed by oxidative cleavage of the resulting terminal alkene 48 (45% yield in two steps). With aldehyde 44 in their hands, the coupling between 44 and an aryllithium species gave the mixture of diastereomeric benzylic alcohols 49. In order to remove the OH group, this mixture of alcohols (49) was treated with lithium in liquid ammonia followed by NH4Cl aqueous solution to afford deoxygenated compound 50 in a 78% yield (two steps). Removal of the TBS protecting groups in 50 with tetrabutylammonium fluoride provided the key intermediate 11 in an 86% yield. To complete the synthesis of (+)-aureol (1), the intermediate Scheme 8. Strategy for the synthesis of (+)-aureol ( 1 ) according to George and colleagues [ 33 ]. ( a ) Retrosynthetic plan. ( b ) Synthesis of (+)-aureol ( 1 ). DMAP = 4-(dimethylamino)pyridine; NMO= N-methylmorpholine-N-oxide; TBAF = tetrabutylammonium fluoride. As shown in Scheme 8b, George’s synthesis of (+)-aureol (1) used 11 as the key intermediate, which was prepared from natural (+)-sclareolide (47). Its reduction with LiAlH4 gave a diol, which was selectively protected at the primary hydroxy group with Ac 2 O in pyridine to afford the acetate 46 in an 84% yield (two steps). Monoacetate 46 was stereoselectively converted to the single stereoisomer olefin 45 (70% yield) in a rearrangement induced by BF 3. Et 2 O, which occurred via stereospecific sequential 1,2-hydride and 1,2-methyl shifts. Saponification of the acetate in 45 gave alcohol 40 (83% yield), which was readily converted into aldehyde 44 through a one-carbon dehomologation sequence using the Grieco–Sharples elimination protocol [ 34 , 35 ] (67% yield in two steps) followed by oxidative cleavage of the resulting terminal alkene 48 (45% yield in two steps). With aldehyde 44 in their hands, the coupling between 44 and an aryllithium species gave the mixture of diastereomeric benzylic alcohols 49 . In order to remove the OH group, this mixture of alcohols ( 49 ) was treated with lithium in liquid ammonia followed by NH 4 Cl aqueous solution to afford deoxygenated compound 50 in a 78% yield (two steps). Removal of the TBS protecting groups in 50 with tetrabutylammonium fluoride provided