A Concise Route for the Synthesis of Tetracyclic Meroterpenoids: (±)-Aureol Preparation and Mechanistic Interpretation

A new concise general methodology for the synthesis of different tetracyclic meroterpenoids is reported: (±)-aureol (1), the key intermediate of this general route. The synthesis of (±)-aureol (1) was achieved in seven steps (28% overall yield) from (±)-albicanol. The key steps of this route include a C–C bond-forming reaction between (±)-albicanal and a lithiated arene unit and a rearrangement involving 1,2-hydride and 1,2-methyl shifts promoted by BF3•Et2O as activator and water as initiator.


Introduction
Marine sponges appear to have become an almost inexhaustible source of new natural compounds, showing a broad spectrum of biological activities and different structural patterns. Among these compounds there is a structurally unique class of natural products, the meroterpenoids, which are constituted by a sesquiterpene unit linked to a phenolic or quinone moiety [1]. Important examples of tetracyclic meroterpenoids ( Figure 1) include (+)-aureol (1) [2,3], (+)-strongylin A (2) [4], (-)-cyclosmenospongine (3) [5] and (+)-smenoqualone (4) [6]. (+)-Aureol (1) was initially isolated and characterized by Faulker et al. [2] from the Caribbean sponge Smeonspongia aurea. It was later also found in some other species of Caribbean sponges, (+)-Aureol (1) was initially isolated and characterized by Faulker et al. [2] from the Caribbean sponge Smeonspongia aurea. It was later also found in some other species of Caribbean sponges, Mar. Drugs 2020, 18 Verongula gigantea and Smenospongia sp. [7]. (+)-Aureol (1) is a tetracyclic meroterpenoid with a unique structure that combines a cis-decalin system with a substituted benzopyran moiety. It shows anti-influenza-A virus activity [8] and selective cytotoxicity against human tumor cells, including colon adenocarcinoma HT-29 cells [9] and nonsmall cell lung cancer A549 [9]. Although the tetracyclic meroterpenoids have exclusive structural features and a wide assortment of biological activities, only one highly modular and robust platform for the synthesis of this class of natural products has been reported to date [10]. The rest of the reported routes are synthetic operations (10-27 linear steps) that have not enabled straightforward access to the whole family of these interesting natural products [11][12][13][14][15][16][17][18][19][20].

Results and Discussion
As a continuation of our research on the synthesis of marine natural bioactive compounds [18,[21][22][23], we have developed a new concise route for the synthesis of tetracyclic meroterpenoids. In this new synthetic route, aureol (1) is the key intermediate from which other tetracyclic meroterpenoids, such as 2, 3 and 4, can be easily synthesized by simple functional modification of its aromatic ring.
We thought the synthesis of 1 could be achieved through a coupling of albicanal (6) with 2-lithiohydroquinone dimethyl ether and a biogenetic-type rearrangement (previously explored by us) as pivotal steps (Scheme 1). Verongula gigantea and Smenospongia sp. [7]. (+)-Aureol (1) is a tetracyclic meroterpenoid with a unique structure that combines a cis-decalin system with a substituted benzopyran moiety. It shows anti-influenza-A virus activity [8] and selective cytotoxicity against human tumor cells, including colon adenocarcinoma HT-29 cells [9] and nonsmall cell lung cancer A549 [9]. Although the tetracyclic meroterpenoids have exclusive structural features and a wide assortment of biological activities, only one highly modular and robust platform for the synthesis of this class of natural products has been reported to date [10]. The rest of the reported routes are synthetic operations (10-27 linear steps) that have not enabled straightforward access to the whole family of these interesting natural products [11][12][13][14][15][16][17][18][19][20].

Results and Discussion
As a continuation of our research on the synthesis of marine natural bioactive compounds [18,[21][22][23], we have developed a new concise route for the synthesis of tetracyclic meroterpenoids. In this new synthetic route, aureol (1) is the key intermediate from which other tetracyclic meroterpenoids, such as 2, 3 and 4, can be easily synthesized by simple functional modification of its aromatic ring.
We thought the synthesis of 1 could be achieved through a coupling of albicanal (6) with 2lithiohydroquinone dimethyl ether and a biogenetic-type rearrangement (previously explored by us) as pivotal steps (Scheme 1). Scheme 1. Retrosynthesis of tetracyclic meroterpenoids.
Mar. Drugs 2020, 18, x FOR PEER REVIEW 3 of 8 already been reported [10]. From these two compounds, aureol (1) and 5-epi-aureol (11), adequate functionalization sequences can lead to (-)-cyclomenospongine (3), (+)-strongylin A (2) and (+)-smenoquealone (4), sequences that can be considered alternative formal syntheses of these tetracyclic compounds [9,28]. In this way, the methodology here described can be considered a general method for the synthesis of tetracyclic meroterponoids. The transformation of the exocyclic alkene 7 into the rearranged products 8 and 9 can be rationalized as depicted in Scheme 3. It is known that pure Lewis acids, such as boron trifluoride, are not effective initiators in alkene cationic polymerization [29], which makes more likely a pathway involving a proton transfer. On the other hand, it is well known that BF3•Et2O is very moisturesensitive, and inevitably over time the HF that forms from the hydrolysis of BF3 will react with excess BF3 to form HBF4, which is a strong acid and possibly triggers the cationic rearrangement. Thus, when the exocyclic alkene group in the bicyclic compound 7 is activated by a proton, the tertiary carbocation intermediate I is formed. Since the cleavage of a C-H bond is usually easier than a C-C bond, the hydrogen on C9 has a higher migratory aptitude than the alkyl group. In addition, migration of any of the hydrogens on C7 would lead to a secondary carbocation, less stable. The transformation of the exocyclic alkene 7 into the rearranged products 8 and 9 can be rationalized as depicted in Scheme 3. It is known that pure Lewis acids, such as boron trifluoride, are not effective initiators in alkene cationic polymerization [29], which makes more likely a pathway involving a proton transfer. On the other hand, it is well known that BF 3 •Et 2 O is very moisture-sensitive, and inevitably over time the HF that forms from the hydrolysis of BF 3 will react with excess BF 3 to form HBF 4 , which is a strong acid and possibly triggers the cationic rearrangement. Thus, when the exocyclic alkene group in the bicyclic compound 7 is activated by a proton, the tertiary carbocation intermediate I is formed. Since the cleavage of a C-H bond is usually easier than a C-C bond, the hydrogen on C9 has a higher migratory aptitude than the alkyl group. In addition, migration of any of the hydrogens on C7 would lead to a secondary carbocation, less stable. In this way, the carbocationic intermediate  Scheme 3), which can react with the aromatic ring by electrophilic substitution to generate the minor tetracyclic by-product 9. In both pathways, a H + is liberated, which can react with more alkene 7 to continue the catalytic cycle. On the other hand, the simultaneous formation of 8 and 9 suggests that the all of the abovementioned rearrangements leading from 7 to 8 are not part of a concerted process, but proceed through a series of rapidly interconverting carbocations.
Mar. Drugs 2020, 18, x FOR PEER REVIEW 4 of 8 H + is liberated, which can react with more alkene 7 to continue the catalytic cycle. On the other hand, the simultaneous formation of 8 and 9 suggests that the all of the abovementioned rearrangements leading from 7 to 8 are not part of a concerted process, but proceed through a series of rapidly interconverting carbocations.

Scheme 3.
Proposed reaction mechanisms for the formation of tetrasubstituted alkene 8 and by-product 9.

General Methods
All reagents were used as received from commercial sources. All solvents were distilled before use. THF was refluxed over Na and CH2Cl2 over calcium hydride before being distilled under an Ar atmosphere. Reaction products were purified by conventional column chromatography on Merck silica gel 50. Analytical thin-layer chromatography (TLC) was performed on 0.2 mm DC-Fertigfolien Alugram ® Xtra Sil G/UV254 silica gel plates and visualized under a UV lamp or by immersion in an ethanol solution of phosphomolybdic acid (7%) followed by heating. 1 H and 13 C NMR spectra were recorded in Varian spectrometers operating at 300, 500 or 600 MHz. CDCl3 was always used as NMR solvent. (±)-Albicanol was prepared from commercial farnesol according to a known procedure [22,24]. Copies of 1 H and 13 C NMR spectra of relevant known compounds are provided in Supplementary Materials.

General Methods
All reagents were used as received from commercial sources. All solvents were distilled before use. THF was refluxed over Na and CH 2 Cl 2 over calcium hydride before being distilled under an Ar atmosphere. Reaction products were purified by conventional column chromatography on Merck silica gel 50. Analytical thin-layer chromatography (TLC) was performed on 0.2 mm DC-Fertigfolien Alugram ® Xtra Sil G/UV254 silica gel plates and visualized under a UV lamp or by immersion in an ethanol solution of phosphomolybdic acid (7%) followed by heating. 1 H and 13 C NMR spectra were recorded in Varian spectrometers operating at 300, 500 or 600 MHz. CDCl 3 was always used as NMR solvent. (±)-Albicanol was prepared from commercial farnesol according to a known procedure [22,24]. Copies of 1 H and 13 C NMR spectra of relevant known compounds are provided in Supplementary Materials.

Synthesis of Cis-Decaline 7
Hydroquinone dimethyl ether (0.83 g, 6.0 mmol) was dissolved in Et 2 O (13 mL) and sec-BuLi (3.1 mL, 1.3 M in cyclohexane) was added at 0 • C. After stirring the mixture for 3 h at room temperature, a solution of (±)-albicanal (6) (440 mg, 2.0 mmol) in Et 2 O (3 mL) was dropwise added. The reaction was stirred for 5 min before dropwise addition of NH 4 Cl (0.3 mL of saturated solution). To the mixture was then added 3 mL of saturated NaCl-solution, the organic phase dried over anhydrous Na 2 SO 4 and the solvent removed in vacuo.
A mixture of liquid NH 3 (24 mL), THF (13 mL) and Li (70 mg, 10 mmol, granulate, Merck) at −78 • C was prepared and stirred for 15 min. To this mixture was added a solution of the former reaction crude in THF (7 mL). The reaction was then stirred for 15 min at the same temperature. After that, NH 4 Cl (1.4 g) was added in portions (a change in color was observed from dark blue to colorless). Next, the mixture was allowed to reach room temperature to allow the evaporation of NH 3 (2 h) and finally the reaction mixture was extracted with EtOAc. The combined organic layers were washed with brine, dried (anhydrous Na 2 SO 4 ) and the solvent removed in vacuo. Column chromatography (Hexane/AcOEt 9:1) of the residue yielded the coupling product 7 (618 mg, 1. as activator and water as initiator. We are currently engaged in a computational study of the reaction mechanism, which will be published in due course. (±)-Aureol (1) and (±)-5-epi-aureol (5) obtained by this route are key intermediates for the synthesis of a large number of natural and synthetic derivative tetracyclic meroterpenoids, which will be used for further analysis as antitumor and antiviral agents.