Total Synthesis of (±)-7-epi-Nemorosone

Abstract

A concise total synthesis of (±)-7-epi-nemorosone is reported. Our synthetic approach establishes a viable route to polycyclic polyprenylated acylphloroglucinol natural products (PPAP’s) bearing a C-7 endo prenyl sidechain. Key steps include retro-aldol-vinyl cerium addition to a hydroxy adamantane core scaffold and palladium-mediated deoxygenation.

Polycyclic polyprenylated acylphloroglucinol natural products (PPAP’s) are a class of compounds that have attracted significant attention in the synthetic chemistry community, largely due to their challenging structures and biological activities.¹ PPAP’s generally possess a bicyclo [3.3.1] nonane-1, 3, 5-trione core structure which is comprised of a highly oxygenated framework and vicinal quaternary carbon centers (Figure 1). Recent biological studies have showed promising results for PPAP’s. For example, nemorosone (1) and its C-7 epimer 7-epi-nemorosone (2) both show antibacterial activity² and potent activity against the malaria parasite P. falciparum. ³ In addition, (1) and (2) exhibit cytotoxicity against a number of human cancer cell lines⁴ including breast, colon, brain, ovary, liver, and lung carcinomas.⁵ Recently, nemorosone (1) was found to be a potent protonophoric mitochondrial uncoupler which may form the basis of its cytotoxicity to cancer cells.⁶

Figure 1.

Representative Type A Polycyclic Polyprenylated Acylphloroglucinols (PPAP’s)

The type A⁷ PPAP nemorosone (1) has a C-7 exo prenyl moiety whereas 7-epi-nemorosone (2) bears a C-7 endo-prenyl substituent. The O-methyl ether of 2 was first isolated by Marsaioli and coworkers in 1999⁸ but was assigned as an isomeric structure. In 1999, Jacobs and coworkers reported the isolation of the enol ester isomers plukenetiones D and E (3 and 4). These compounds were found to have the same framework as nemorosone but the C-7 stereocenter was unassigned.⁹ In 2000, Jacobs and Grossman¹⁰ analyzed NMR data for 3 and 4 and proposed that these compounds were enol acetates of 7-epi-nemorosone (2). Subsequently, Marsaioli and coworkers reisolated the compound and corrected its structure to 2. ¹¹ Despite numerous synthetic efforts concerning type A PPAPs,¹² few have targeted construction of the C-7 endo prenyl moiety as found in 2 with the exception of a recent successful example reported by Plietker and coworkers.^{12 l} Herein, we report the first total synthesis of (±)-7-epi-nemorosone (2) via elaboration of a hydroxy adamantane core scaffold. ¹³

Retrosynthetically, we envisioned that 7-epi-nemorosone (2) may be derived from the bis-O-acylated bicyclo [3.3.1] nonane 7 after palladium(0)-mediated reduction (Figure 2). Bicyclic intermediate 7 may be derived from organocerium-mediated retro-aldol/vinyl metal addition to adamantane alcohol 8.¹³ Finally, adamantane 8 may be obtained from the readily available α-acetoxy enal 9 and acylphloroglucinol 10 using an alkylative dearomatization-annulation sequence.¹³

Figure 2.

Retrosynthetic analysis for (±)-7-epi-nemorosone 2

The synthesis commenced with treatment of acylphloroglucinol 10 with aldehyde 9 under basic conditions to afford the dearomatized adduct in high yield.¹³ (Scheme 1) The crude product was directly subjected to acidic conditions (conc. HCl, THF, rt) to yield adamantane alcohol 8 (50% yield, two steps).¹³ Exposure of adamantane 8 to a pre-activated CeCl₃/vinylmagnesium bromide mixture led to tandem retro-aldol condensation / Grignard addition.¹⁴ The crude product mixture was subjected to standard esterification conditions (Ac₂O/pyridine/DMAP) which afforded bis-acylated compound 11 as the major product (45% yield, two steps). A small amount of adamantane acetate 12 was also isolated in 3% yield via acylation of unreacted adamantane alcohol 8.

Scheme 1.

Synthesis of the 7-epi-nemorosone core

Efforts to convert the bis-acetate 11 to its reduced form 13 were problematic as a competing pathway leading to the formation of the undesired adamantane 14 was generally observed. This can presumably be attributed to a facile intramolecular cyclization which occurred prior to reduction (Scheme 2). After evaluation of reaction conditions including palladium (0) and hydride sources,¹⁵ an optimized yield (45%) for 13 was obtained using Pd(Ph₃P)₄ with added PBu₃15c using NH₄CO₂H at 68 °C and a reaction concentration of 0.2 M. A relatively high Pd(0) catalyst loading (50%) was found to be necessary to reduce the reaction time and avoid decomposition of product 13 under the reaction conditions. In addition, under these conditions adamantane product 14 was also produced (~10%). Finally, alkene cross metathesis of 13 with isobutylene catalyzed by the Grubbs II catalyst¹⁶ afforded plukenetione E acetate (4) albeit in inconsistent yield, likely due to cleavage of the labile enol acetate protecting group under thermal conditions.¹⁷ Analytical data for synthetic 4 were in agreement with the data reported by Jacobs and coworkers.^9,17

Scheme 2.

Cyclization vs. Reduction Processes

Interestingly, treatment of acylated adamantane 12 with K₂CO₃/MeOH led to unexpected formation of the fragmentation product 15 in nearly quantitative yield. The structure of 15 was confirmed by X-ray analysis (Figure 3).¹⁷ A proposed mechanism is shown in Scheme 3. Adamantane acetate 12 may be initially deacylated by methoxide to generate alkoxide 16. The proximity between the emerged alkoxide and C-4 ketone (~2.9 Ǻ) should facilitate formation of the oxetane intermediate 17. Fragmentation of 17 followed by protonation may then lead to formation of lactone 18 which may subsequently undergo ring-opening by methoxide to afford 19. Finally, dehydration of alcohol 19 via β-elimination may afford bicyclic product 15. A similar fragmentation process was reported by Nicolaou and coworkers enroute to the PPAP natural product hyperforin.¹⁸

Figure 3.

X-ray crystal structure of fragmentation product 15

Scheme 3.

An Unexpected Fragmentation Process

Due to inconsistent yields observed for reduction of 11 and in the cross metathesis of derived product 13, we considered altering the protecting group for the vinylogous acid moiety in order to increase its stability. After considerable experimentation, we found the pivalate group to be an excellent candidate for the sequence. Starting from adamantane 8, tandem retro-aldol/vinyl Grignard addition followed by sequential acylations led to the formation of 20 (45% yield, 3 steps) as a single enol pivalate^17,19 isomer with only a single purification required (Scheme 4). Palladium-mediated deoxygenation of 20 occurred smoothly to generate 21 in consistent overall yield (61%). Moreover, due to the increased stability of the pivalate protecting group, adamantane byproduct 14 (cf. Scheme 1) was not observed.

Scheme 4.

Synthesis of (±)-7-epi-nemorosone

In the final stages of the synthesis, global cross metathesis of 21 afforded the triprenylated 7-epi-nemorosone core structure 22 in high yield (Scheme 4). Tetrabutylammonium hydroxide-mediated deprotection of 22 effected removal of the pivalate protecting group affording the natural product 2. Attempted purification of the crude product mixture via conventional silica gel chromatography led to unexpected decomposition of 2.^12d,h Fortunately, purification of synthetic 2 using preparative HPLC was successful employing 99:1 CH₃CN: H₂O = with 0.01% TFA20 in the eluant buffer leading to the production of (±)-7-epi-nemorosone (2) (78%). Analytical data including ¹H and ¹³C NMR analyses for 2 were in agreement with those obtained from a sample provided by the Jacobs group.

In conclusion, we have achieved the first total synthesis of (±)-7-epi-nemorosone, a type A PPAP natural product bearing a C-7 endo prenyl sidechain. Key steps include retro-aldol-vinyl cerium addition to a hydroxy adamantane core structure and palladium-mediated deoxygenation. The use of an enol pivalate protecting group for a vinylogous acid moiety was found to be helpful in order to prevent undesirable cyclizations. Further studies on the synthesis of PPAP’s and their chemical reactivity are ongoing in our laboratory and will be reported in due course.