Synthetic Study toward Triterpenes from the Schisandraceae Family of Natural Products

Triterpenoid natural products from the Schisandraceae family have long presented a significant synthetic challenge. Lancifodilactone I, a member of the family not previously synthesized, was identified as a key natural product target, from which many other members could be synthesized. We envisaged that the core ring system of lancifodilactone I could be accessed by a strategy involving palladium-catalysed cascade cyclisation of a bromoenynamide, via carbopalladation, Suzuki coupling and 8π-electrocyclisation, to synthesize the core 7,8-fused ring system. Exploration of this strategy on model systems resulted in efficient syntheses of 5,6- and 5,8-fused systems in high yields, which represent the first such cyclisation where the ynamide nitrogen atom is ‘external’ to the forming ring system. The enamide functionality resident in the cascade cyclisation product was found to be less nucleophilic than the accompanying tri-/tetrasubstituted alkene(s), enabling regioselective oxidations. Application of this strategy to 7,6-, and 7,8-fused systems, and ultimately the ‘real’ substrate, was ultimately thwarted by the difficulty of 7-membered ring closure, leading to side product formation. Nevertheless, a tandem bromoenynamide carbopalladation, Suzuki coupling and 6/8π-electrocyclisation was shown to be a highly efficient tactic for the formation of bicyclic enamides, which may find applications in other synthetic contexts.


Introduction
Triterpenes from Schisandraceae species have attracted attention from the synthetic community due to their intriguing molecular frameworks and biological activity [1,2]. Efforts by the groups of Yang [3][4][5][6][7], Li [8,9], Tang [10,11] and Anderson [12,13] resulted in 11 total syntheses so far. However, previously reported strategies often suffer from lengthy synthetic routes and challenges in applications to other members of the family. We aimed to develop an efficient approach to the Schisandra triterpenes in which lancifodilactone I (1, Figure 1a), a previously unconquered target, was identified as a potential common precursor to other natural products in this family. We envisioned that the key 7,8-fused ring core of lancifodilactone I could be synthesized via a palladium catalysed cascade reaction: starting from bromoene-ynamide 2, oxidative addition followed by carbopalladation (3), Suzuki coupling (with a suitable dienyl organometallic) to give 4, and 8π electrocyclisation would deliver the 7,8-fused system 5 in one step. An electron-donating ynamide functionality in 2 was proposed to enhance the nucleophilicity of the resulting enamide in 5 for further functionalisation, while the bulkiness of the ynamide nitrogen substituent should prevent cis-trans isomerisation of the intermediate vinyl palladium species 3 [14].

Synthesis of 5,6-Fused Systems by Bromoenynamide Carbopalladation/Suzuki Coupling/6π-Electrocyclisation Cascade
To investigate the first of these questions, bromoenynamide 11 was synthesized in four steps from dimethyl malonate 15 ( Figure 2). Malonate deprotonation by sodium hydride, followed by allylation with 2,3-dibromopropene, gave the monoallylated product. Further deprotonation by sodium hydride and treatment with excess propargyl bromide Previous work from our group [15,16] demonstrated that this type of cascade can indeed be used to synthesize 7,8 systems (8, Figure 1b), and indeed carbopalladation/coupling/ electrocyclisation sequences are generally well-established [17][18][19][20]. However, we aimed to avoid the use of a toxic organotin coupling partner as had been employed in our previous work [15,16] by switching to an untested Suzuki/8π strategy. We have also shown that ynamides are viable partners for intramolecular carbopalladation cascades in a carbopalladation/Suzuki/6π sequence in which the ynamide nitrogen atom is located inside the tethering ring (9→10, Figure 1b) [21]. In our planned work, we now required the nitrogen atom to be positioned at the alkyne terminus, rather than internally. At the outset of this study, several questions therefore had to be answered ( Figure 1c):
Can the bromoenynamide-Suzuki cascade be extended to the synthesis of 8-membered rings (13)? 3.
Can the resulting 8-membered ring enamides undergo further selective functionalisation towards the carbocyclic D-ring core of lancifodilactone I (14)?

2.
NaH, THF, rt 63% To study the desired carbopalladation/Suzuki/6π cascade, three vinylboronic acid derivatives (18)(19)(20), and a vinylzinc species (21), were investigated as coupling partners. Only the potassium vinyltrifluoroborate salt 20 was found to be effective (Table 1), with other partners resulting only in degradation or no observed reaction. Further optimisation (Table 1) led to identification of tetrakis(triphenylphosphine)palladium(0) and potassium carbonate in THF/water 10:1 at 80 °C as optimal conditions to deliver the desired product 12 in 89% isolated yield. These results demonstrate that the position of the nitrogen atom on the alkyne (internal vs. external), and therefore the alkyne polarisation, does not influence the carbopalladation and subsequent Suzuki coupling reactivity, with both types of ynamide undergoing the desired cascade in excellent yields [21].  To study the desired carbopalladation/Suzuki/6π cascade, three vinylboronic acid derivatives (18)(19)(20), and a vinylzinc species (21), were investigated as coupling partners. Only the potassium vinyltrifluoroborate salt 20 was found to be effective (Table 1), with other partners resulting only in degradation or no observed reaction. Further optimisation (Table 1) led to identification of tetrakis(triphenylphosphine)palladium(0) and potassium carbonate in THF/water 10:1 at 80 • C as optimal conditions to deliver the desired product 12 in 89% isolated yield.

2.
NaH, THF, rt 63% To study the desired carbopalladation/Suzuki/6π cascade, three vinylboronic acid derivatives (18)(19)(20), and a vinylzinc species (21), were investigated as coupling partners. Only the potassium vinyltrifluoroborate salt 20 was found to be effective (Table 1), with other partners resulting only in degradation or no observed reaction. Further optimisation ( Table 1) led to identification of tetrakis(triphenylphosphine)palladium(0) and potassium carbonate in THF/water 10:1 at 80 °C as optimal conditions to deliver the desired product 12 in 89% isolated yield. These results demonstrate that the position of the nitrogen atom on the alkyne (internal vs. external), and therefore the alkyne polarisation, does not influence the carbopalladation and subsequent Suzuki coupling reactivity, with both types of ynamide undergoing the desired cascade in excellent yields [21].

Synthesis of 5,8-Fused Systems by Bromoenynamide Carbopalladation/Suzuki Coupling/8π-Electrocyclisation Cascade
To address the second question, of extending the cascade to formation of 5,8-fused systems from bromoenynamides, a dienylboronic acid equivalent 23 was required ( Figure  3). After examination of several different routes, this simple but synthetically challenging fragment was synthesized from 2-methyl-3-butyn-2-ol 22 in two steps. Dehydration [24], followed by rhodium catalysed trans-hydroboration developed by Miyaura [25], gave the  These results demonstrate that the position of the nitrogen atom on the alkyne (internal vs. external), and therefore the alkyne polarisation, does not influence the carbopalladation and subsequent Suzuki coupling reactivity, with both types of ynamide undergoing the desired cascade in excellent yields [21].

Synthesis of 5,8-Fused Systems by Bromoenynamide Carbopalladation/Suzuki Coupling/8π-Electrocyclisation Cascade
To address the second question, of extending the cascade to formation of 5,8-fused systems from bromoenynamides, a dienylboronic acid equivalent 23 was required ( Figure 3).
After examination of several different routes, this simple but synthetically challenging fragment was synthesized from 2-methyl-3-butyn-2-ol 22 in two steps. Dehydration [24], followed by rhodium catalysed trans-hydroboration developed by Miyaura [25], gave the desired pinacolboronic ester as a mixture of geometric isomers. It was possible to obtain the pure Z isomer by preparative HPLC. It is important to note that the stereochemical purity of the diene coupling partner 23 is crucial to the success of the planned cascade: any E isomer present in the mixture would preclude an 8π-electrocyclisation, therefore leading to side product formation and yield deterioration. In the event, subjection of this dienyl pinacolboronic ester to the previously optimized conditions cleanly afforded the 5,8-fused product 13 in an excellent yield of 96% ( Figure 3).

Selective Functionalisation of 5,6-and 5,8-Fused Ring Systems
To investigate selective oxidations of the 5,6-and 5,8-fused ring scaffolds, hydrolysis of enamide 12 to the corresponding ketone was attempted. Subjection of 12 to a range o strongly or weakly acidic conditions resulted in either no reaction, or decomposition o the starting material ( Figure 4a). Attempts to cleave the sulfonamide under common re ductive conditions (Mg, MeOH; Na, naphthalene, DME; SmI2, pyrrolidine, THF/water also led to either no reaction, or decomposition of the starting material. To our surprise m-CPBA-mediated epoxidation of 12 selectively epoxidized the tetrasubstituted (non-en amide) alkene (26, 59%). Upjohn dihydroxylation of 12 gave a mixture of products, with the major product being 25 (30%).  These results stand in contrast to our anticipation that the electron donating capabil

Selective Functionalisation of 5,6-and 5,8-Fused Ring Systems
To investigate selective oxidations of the 5,6-and 5,8-fused ring scaffolds, hydrolysis of enamide 12 to the corresponding ketone was attempted. Subjection of 12 to a range of strongly or weakly acidic conditions resulted in either no reaction, or decomposition of the starting material ( Figure 4a). Attempts to cleave the sulfonamide under common reductive conditions (Mg, MeOH; Na, naphthalene, DME; SmI 2 , pyrrolidine, THF/water) also led to either no reaction, or decomposition of the starting material. To our surprise, m-CPBAmediated epoxidation of 12 selectively epoxidized the tetrasubstituted (non-enamide) alkene (26, 59%). Upjohn dihydroxylation of 12 gave a mixture of products, with the major product being 25 (30%).

Selective Functionalisation of 5,6-and 5,8-Fused Ring Systems
To investigate selective oxidations of the 5,6-and 5,8-fused ring scaffolds, hydrolysi of enamide 12 to the corresponding ketone was attempted. Subjection of 12 to a range o strongly or weakly acidic conditions resulted in either no reaction, or decomposition o the starting material ( Figure 4a). Attempts to cleave the sulfonamide under common re ductive conditions (Mg, MeOH; Na, naphthalene, DME; SmI2, pyrrolidine, THF/water also led to either no reaction, or decomposition of the starting material. To our surprise m-CPBA-mediated epoxidation of 12 selectively epoxidized the tetrasubstituted (non-en amide) alkene (26, 59%). Upjohn dihydroxylation of 12 gave a mixture of products, with the major product being 25 (30%).  These results stand in contrast to our anticipation that the electron donating capabil ities of a sulfonamide group, through conjugation of the enamide nitrogen lone pair with its alkene, would render the enamide more nucleophilic and therefore reactive toward These results stand in contrast to our anticipation that the electron donating capabilities of a sulfonamide group, through conjugation of the enamide nitrogen lone pair with its alkene, would render the enamide more nucleophilic and therefore reactive toward oxidation. It was reasoned that conjugation of the nitrogen lone pair with the double bond is reduced or even prevented due to steric congestion in systems 12 and 14, such that the lone pair may be (near) orthogonal to the alkene π system. With the mesomeric electrondonating effect removed, the inductive electron-withdrawing effects of the sulfonamide group mean the 'enamide' is in fact an electron poor olefin, and therefore less reactive toward oxidation.

Attempted Application of the Carbopalladation/Suzuki Coupling/8π-Electrocyclisation Cascade towards the ABCD Rings of Schisandra Triterpenes
To test if the developed cascade could be used in the synthesis of the 7,8-and 7,6-systems found in Schisandra triterpenes, model bromoenynamide 34 was synthesized in four steps from dimethyl malonate (Figure 5a). Monoallylated malonate derivative 29 was first subjected to conjugate addition to acrolein, giving aldehyde 30 (47%). Meanwhile, benzyl tosyl amine 31 was transformed to dichloroenamide 32 (73%), which served as a precursor to lithiated ynamide 33 under conditions developed by our group [26]. Treatment of aldehyde 30 with 33 gave a secondary alcohol, which was protected as a tert-butyldimethylsilyl ether 34 (30% over two steps).
benzyl tosyl amine 31 was transformed to dichloroenamide 32 (73%), which served as a precursor to lithiated ynamide 33 under conditions developed by our group [26]. Treatment of aldehyde 30 with 33 gave a secondary alcohol, which was protected as a tertbutyldimethylsilyl ether 34 (30% over two steps).
To our disappointment, under the previously developed conditions using potassium vinyltrifluoroborate, bromoenynamide 34 was converted to a mixture of the 'direct' coupling product 38, and side product 37, which derives from a formal 7-endo-trig Heck-type cyclisation [14] (Figure 5a). Similar reactions of bromoenynamide 34 with diene 23 under a range of conditions resulted only in complex mixtures of unidentifiable products (not shown). These results suggest that in the setting of a 7-membered tether, the vinylpalladium species arising from oxidative addition (35) undergoes direct Suzuki coupling at a comparable rate to carbopalladation, and that once carbopalladation has occurred, the resulting dienylpalladium intermediate 36 undergoes intramolecular Heck-type cyclisation to side product 37 much faster than the desired Suzuki coupling.
It was hoped that the more conformationally constrained nature of the 'real' substrate (featuring the Schisandra AB rings) would reduce the entropic penalty of seven membered ring formation, and hence favour the desired cascade over direct coupling. This substrate 39 was synthesized from aldehyde 6 [27] by addition of the lithiated ynamide derived from 32. Unfortunately, with only milligram quantities of 39 to hand, attempts to apply the developed carbopalladation, Suzuki coupling, 8π-electrocyclisation cascade resulted only in the formation of complex mixtures of unidentifiable products.  To our disappointment, under the previously developed conditions using potassium vinyltrifluoroborate, bromoenynamide 34 was converted to a mixture of the 'direct' coupling product 38, and side product 37, which derives from a formal 7-endo-trig Heck-type cyclisation [14] (Figure 5a). Similar reactions of bromoenynamide 34 with diene 23 under a range of conditions resulted only in complex mixtures of unidentifiable products (not shown). These results suggest that in the setting of a 7-membered tether, the vinylpalladium species arising from oxidative addition (35) undergoes direct Suzuki coupling at a comparable rate to carbopalladation, and that once carbopalladation has occurred, the resulting dienylpalladium intermediate 36 undergoes intramolecular Heck-type cyclisation to side product 37 much faster than the desired Suzuki coupling.
It was hoped that the more conformationally constrained nature of the 'real' substrate (featuring the Schisandra AB rings) would reduce the entropic penalty of seven membered ring formation, and hence favour the desired cascade over direct coupling. This substrate 39 was synthesized from aldehyde 6 [27] by addition of the lithiated ynamide derived from 32. Unfortunately, with only milligram quantities of 39 to hand, attempts to apply the developed carbopalladation, Suzuki coupling, 8π-electrocyclisation cascade resulted only in the formation of complex mixtures of unidentifiable products.

General
All reactions were performed open to air and without precautions to exclude air/moisture unless specified otherwise. Reagents and solvents were purchased from commercial sources and used without further purification unless specified otherwise. NMR spectra were recorded on Bruker AVIII HD 400, NEO 400, AVIII HD 500 and AVII 500 spectrometers. Chemical shifts (δ) are quoted in parts per million (ppm). 1 H and 13 C NMR spectra are referenced to residual protons in chloroform-d (δ H = 7.26, δ C = 77.16) and acetone-d 6 (δ H = 2.05, δ C = 28.95). Peak multiplicities are defined as s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet) and br (broad). Coupling constants (J) are reported to the nearest 0.1 Hz. High-resolution mass spectra (HRMS) were recorded on a Thermo Scientific exactive mass spectrometer (Waters Equity autosampler and pump) for electrospray ionisation (ESI). Flash chromatography refers to normal phase column chromatography on silica gel (Merck Si 60, 0.040-0.063 mm) under a positive pressure of nitrogen.

Dimethyl 2-(3-((N-benzyl-4-methylphenyl)sulfonamido)prop-2-yn-1-yl)-2-(2-bromoallyl)-malonate (11).
To a stirred solution of alkyne 16 (50 mg, 0.17 mmol, 1.0 eq.) in acetone (0.34 mL) at room temperature was added AgNO 3 (2.9 mg, 10 mol%). After stirring for 5 min, N-bromosuccinimide (34 mg, 0.19 mmol, 1.1 eq.) was added and the resulting mixture was stirred for a further 4 h at rt. The reaction mixture was concentrated, then pentane was added, and the suspension was filtered through cotton wool to remove the white precipitate. The resulting solution was concentrated to obtain the corresponding bromoalkyne 17 (58 mg, 0.16 mmol), which was of sufficient purity to be used without further purification. Note 1: Because of similar R f values of reactant and product, the reaction was monitored by NMR aliquot (conversion of singlet at 5.84 to 5.80 ppm). Note 2: If acetone is not evaporated completely before trituration with pentane, some succinimide will dissolve and contaminate the product. Note 3: The product was used immediately in the next step due to its tendency to decompose on storage.
To a stirred solution of the crude alcohol in DMF (1 mL) at room temperature was added imidazole (14 mg, 0.19 mmol, 1.5 eq.) and TBSCl (20 mg, 0.13 mmol, 1.0 eq.), and the mixture was stirred for 1 h until complete (as monitored by TLC). Water and Et 2 O were added, then the organic layer was separated and washed with brine, dried (MgSO 4 ) and concentrated. Since the ynamide impurity (resulting from protonation of excess lithiated ynamide from previous step) has a similar R f value to the product, it was removed by washing the crude product with pentane several times (until the yellow colour faded). A solid, white impurity that is not soluble in pentane remains as a precipitate. The pentane washes were concentrated, and the residue was purified via flash chromatography (pentane/EtOAc 85:15) to afforded 34 (  To a vial loaded with bromoynamide 34 (11 mg, 0.016 mmol, 1.0 eq.) and potassium vinyltrifluoroborate 20 (3.1 mg, 0.023 mmol, 1.5 eq.) was added dry and degassed THF (0.40 mL), degassed K 2 CO 3 solution in water (6.4 mg in 40 µL water, 0.047 mmol, 3.0 eq.) and Pd(PPh 3 ) 4 (1.8 mg, 0.002 mmol, 10 mol%). The mixture was further degassed by bubbling N 2 , then sealed and heated overnight at 80 • C. After cooling to rt, water and EtOAc were added, the organic layer was separated and washed with brine, dried over MgSO 4  To an oven dried, argon flushed flask was added 1,2-dichloroenamide 32 (18 mg, 0.051 mmol, 3.0 eq.) and anhydrous THF (0.5 mL), and the mixture was cooled to −78 • C. A solution of phenyllithium (1.9 M solution in dibutyl ether, 25 µL, 2.8 eq.) was then added dropwise, and the mixture was left to stir at −78 • C for 15 min. After almost complete conversion to the lithiated ynamide (the 1,2-dichloroenamide is in excess, so a small amount remains), the aldehyde 6 (5.4 mg, 0.017 mmol) in anhydrous THF (0.5 mL) was added at −78 • C and stirred for 1 h. The reaction was quenched by the addition of NH 4 Cl (at −78 • C, sat., aq.), then warmed to room temperature and extracted with Et 2 O. The organic extract was washed with brine, dried (MgSO 4 ) and concentrated. The product was purified via flash chromatography to afford 39 (5.1 mg, 0.008 mmol, 50%) as a 2:1 mixture of diastereomeric alcohols.

Conclusions
In conclusion, a tandem carbopalladation, Suzuki coupling and 6π-or 8π-electrocyclisation reaction of bromoenynamides in which the ynamide nitrogen is exocyclic to the forming ring was explored in the context of synthesis of terpenes from the Schisandraceae family. It was shown that this tactic can be applied to the synthesis of 5,6-and 5,8-fused bicyclic ring systems in high yields. A route to a versatile dienylboronic ester building block was also developed. The reactivity of the resulting enamides was explored, and it was found that reactivity patterns differ to those observed with ynamides. Synthesis of 7,5-and 7,8 ring systems using the developed cascade was tested; however, the greater difficulty of 7-membered ring closure and propensity to form 7,4-fused ring by-products hindered the application of this cascade. Nevertheless, for smaller ring tethers, the tandem carbopalladation, Suzuki coupling and 6/8π electrocyclisation was shown to be a highly efficient tactic, allowing a rapid increase in molecular complexity, and with a novel positioning of the ynamide nitrogen group. This process may therefore enable future applications in the synthesis of other classes of polycyclic natural products.
Author Contributions: P.K. and E.A.A. conceived the work. P.K. carried out the experimental work and analyzed the data. P.K. and E.A.A. wrote and edited the manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: This research was part-funded by the EPSRC, grant number EP/S013172/1. PK thanks the Clarendon Fund, Oxford, for a scholarship.