A strategy toward the synthesis of C₁₃-oxidized cembrenolides

Abstract

An efficient strategy for the construction of C₁₃-oxidized cembrenolides is reported. Central to this strategy is the installation of the C₁₃ hydroxyl group prior to cembrane macrocyclization (via formation of the C₁–C₂ bond), allowing access to both C₁₃ alcohol epimers. The orientation of the C₁₃ alcohol was found to influence the cyclization mode of the cembranolide scaffold upon furan oxidation, leading to motifs reminiscent to bipinnatolide F, bielschowskysin, and verrillin.

Isolated from soft corals and octocorals, cembrenolides constitute a large family of natural products that are typified by a 14-membered cembrane skeleton. Oxidations at the periphery of this framework set the stage for various skeletal rearrangements and transannular cyclizations ultimately producing a wide array of polycyclic metabolites.¹ In addition to their ecological impact,² these compounds have been pursued both for their unusual structural motifs and for their biological and pharmacological potential. ^1a,3 For instance, oxidation at the C₁₃ center of the cembrane motif is found in several structurally intricate and biologically unexplored cembrenolides ( Fig. 1).^2a Among them, bielschowskysin (1), a complex hexacyclic cembrane, was shown to exhibit very potent cytotoxicity against nonsmall cell lung cancer (EKVX, GI₅₀ ca. 10 nM) and renal cancer (CAKI-1, GI₅₀ ca. 0.5 μM).^4,5 Moreover, 1 has demonstrated potent anti-malarial activity against Plasmodium falciparum (IC₅₀ ca. 10 μg/ml).⁴ Lophotoxin (4)⁶ was found to be a potent nicotinic acetylcholine receptor inhibitor with an LD₅₀ of 8 mg/kg^2a in mice and, since its isolation, it has become a synthetic conundrum.⁷ On the other hand, the bioactivities of verrillin (2)⁸ and bipinnatolide F (3)⁹ have not yet been fully explored.

Figure 1.

Proposed biosynthetic pathway to bielshowskysin and verrillin.

Evaluation of the polycyclic motifs of bielschowskysin (1) and verrillin (2) suggests that both compounds can derive biosynthetically from the same precursor 5a (R₁ = Me) whose cembrane macrocycle contains a furan in close proximity to a butenolide ring ( Fig. 2). Furan oxidation of 5a could produce intermediate 6 in which the electron rich enol could react with the pendant butenolide to form intermediate 7. If the C₁₃ β-alcohol is modified as an acetate (R₂ = OAc), the resulting C₁₂ carbanion could cyclize at the C₆ center creating the strained cyclobutane motif reminiscent to that found in 1.^1b,c Both photochemical^5a–c and Lewis-acid induced conditions¹⁰ could account for the formation of this ring. However, if the C₁₃ β-alcohol is available (R₂ = H), it could cyclize at the C₆ center of 7, forming a more structurally favorable six membered hemiketal ring encountered in the motif of verrillin (2).

Figure 2.

Proposed biosynthetic pathway to bielshowskysin and verrillin.

The combination of structural intricacy and biological potential of these metabolites prompted our studies toward the synthesis of the C₁₃ oxidized cembranolide 5b (R₁ = H). From a synthetic standpoint, 5b could derive from the union of three components: (a) a C₇–C₁₂ fragment containing a masked butenolide; (b) a C₁₃–C₁ aldehyde motif; and (c) a C₂–C₆ framework containing a furan ring.

The synthesis of 5b began with construction of the C₇–C₁₂ fragment 11, in which the butenolide ring is introduced as an α-selenolactone (Scheme 1).^7d To this end, propargyl ester 8, available from but-3-yn-1-ol in 3 steps,¹¹ was reduced to saturated ester 9 using excess NaBH₄ and CuI¹² (Scheme 1). The crude material was then treated with cat. TsOH in benzene to form γ-lactone 10 (79% yield over two steps). It is worth noting that this approach represents a significant improvement over the previously reported synthesis of compound 10.¹³ Treatment of 10 with LiHMDS/PhSeBr then afforded 11 in 76% yield.^7f,13

Scheme 1.

Reagents and conditions: (a) 6 equiv NaBH₄, 2 equiv CuI, MeOH, −78 °C, 15 min; (b) 0.1 equiv TsOH, PhH, 25 °C, 30 min, 79% (over 2 steps); (c) 1.1 equiv LiHMDS, −78 °C, then 1 equiv PhSeBr, THF, −78 to 25 °C, 30 min, 76%.

Aldehyde 15, representing the C₁₃–C₁ component, was prepared beginning from propargyl alcohol 12 (Scheme 2). Methyl zirconation/iodination¹⁴ of 12 followed by protection of the pendant allylic alcohol afforded 13 in 50% combined yield. Lithiation of the resulting vinyl iodide and quenching of the reaction with oxirane yielded 14 in 55% yield.¹⁵ Oxidation of 14 with Dess–Martin periodinane¹⁶ cleanly afforded β,γ-unsaturated aldehyde 15 that was used without further purification.

Scheme 2.

Reagents and conditions: (a) (i) 2.5 equiv Me₃Al, 0.2 equiv Cp₂ZrCl₂, DCE, 25 °C, 24 h, then 2.0 equiv I₂, −40 °C, 30 min, (ii) 1.1 equiv TBSCl, 3.0 equiv imid, DCM, 0 °C, 1 h, 50% (over 2 steps); (b) 2.0 equiv t-BuLi, PhMe, −78 °C, 30 min, then oxirane (excess), −78 to 25 °C over 30 min, 55%; (c) 1.3 equiv Dess–Martin [O], 6.0 equiv NaHCO₃, 25 °C, 20 min, 95%.

The construction of the C₁₃-hydroxylated cembrenolide motif via sequential coupling of the three components is described in Scheme 3. Compound 11 was lithiated at the C₁₂ center (LiHMDS, −78 °C) and alkylated with aldehyde 15 to produce 16 as a mixture of 4 diastereomers. The crude mixture was oxidatively de-selenated^7d,17 to afford butenolide 17 as a 1:1 mixture of C₁₃ diastereomers (17a, 17b) that were easily separated by column chromatography (78% combined yield). The stereochemistry of the C₁₃ hydroxyl group was determined after macrocyclization (see Scheme 4). Protection of 17a and 17b with TBSCl produced the di-silylated compounds that, after selective deprotection of the primary TBS group using PPTS in ethanol,¹⁸ produced primary alcohols 18a and 18b (ca. 82% yield over 2 steps).

Scheme 3.

Reagents and conditions: (a) 1.1 equiv LiHMDS, THF, −78 °C, 1 h, then 15, −78 °C, 20 min; (b) 1.5 equiv H₂O₂, 10 equiv NaHCO₃, THF/EtOAc 1:1, 25 °C, 10 min, 78% (over 2 steps); (c) (i) 1.1 equiv TBSCl, 3.0 equiv imid, DCM, 25 °C, 12 h; (ii) 1.0 equiv PPTS, EtOH, 25 °C, 12 h, 82% (over 2 steps); (d) 4 mol % Pd(PPh₃)₄, 4 mol % CuI, then 19, DMF, 25 °C, 2 h, 75%; (e) 1.1 equiv NBS, 1.1 equiv PPh₃, DCM, −20 °C, 30 min, 88%; (f) 12.0 equiv CrCl₂, 3.0 equiv NiCl₂(DME), THF, 25 °C, 12 h, 83%.

Scheme 4.

Reagents and conditions: (a) 5.0 equiv Et₃N·3HF, MeCN, mw 80 °C, 20 min, 91%; (b) 1.2 equiv TFA, 1.5 equiv TESH, DCM, 0 °C, 15 min, 81%; (c) 1.3 equiv TBHP, 0.05 equiv Triton B, THF, 0 °C, 70%.

Coupling of compounds 18a and 18b with stannylated furfural 19 was performed via a modified Stille reaction¹⁹ (Scheme 3) to afford 20a and 20b in 75% average yield. Bromination of the allylic alcohol using Appel conditions²⁰ produced allylic bromides 21a and 21b in 88% yield. We were concerned that the presence of the new C₁₃ stereocenter, containing a bulky TBS ether on a linear uncyclized motif, might disrupt the high diastereoselectivity of the macrocylization that has been observed in previous systems.²¹ However, to our satisfaction, this macrocylization proceeded smoothly using CrCl₂/NiCl₂(DME) and formed compound 22a and 22b in 83% average yield. As was previously observed,²¹ the diastereoselectivity of this reaction is controlled by the chirality of the butenolide motif.

Removal of the extraneous silyl group was achieved using TEA-buffered HF reagent²² under microwave irradiation conditions. This treatment afforded cleanly diols 23a and 23b in 91% average yield. (Scheme 4). Reduction of the furylic C₂ alcohol of 23a and 23b with TFA and TESH gave rise to 24a and 24b respectively, the stereochemistry of which was determined via a single crystal X-ray analysis.²³ Epoxidation across the C₁₁–C₁₂ alkene proceeded selectively under TBHP/Triton B conditions,²⁴ to create, irrespectively of the orientation of the adjacent C₁₃ alcohol, α-epoxides 25a and 25b (70% average yield).

We then explored the effect of the C₁₃ hydroxyl functionality during oxidative cyclizations of furanocembrenolides (Scheme 5).²⁵ With an eye toward the scaffold of bipinnatolide F (3), we treated 23b under oxidative conditions mediated by CAN^5c,26 (benzene/water: 20/1 at 10 °C). Unfortunately, only a complex mixture of products was observed. On the other hand, when identical oxidative conditions were applied to 23a, containing the C₁₃-hydroxyl group at the α-face of the cembranolide scaffold, we observed the formation of 5,6-spiro ketal 27.²³ This spiroketal motif is presumably stabilized by the diaxial orientation of the ketal oxygens (anomeric effect), despite the axial orientation of the C₁₃ butenolide side chain. Motif 27 is similar to 6, a proposed intermediate in the biosynthesis of bielschowskysin (1) and verrillin (2),^1b,c where the C₆–C₇ enol ether is proposed to undergo cyclization with the C₁₁–C₁₂ butenolide to furnish the strained cyclobutane motif. Interestingly, CAN-mediated oxidation^5c,26 of 25b (benzene/water: 20/1 at 10 °C) led to isolation of compound 26²³ in which an intermediate enedione, produced upon furan oxidation, underwent hemiketalization at the C₃ center by the pendant C₁₃ β-hydroxyl group. Hemiketal 26 is stabilized by the diaxial orientation of the oxygen substituents (anomeric effect)²⁷ and by the equatorial orientation of the C₁ and C₁₃ side-chains. As projected, the structure of 26 is reminiscent to that of bipinnatolide F (3).⁶

Scheme 5.

Reagents and conditions: (a) 2.0 equiv CAN, PhH/H₂O 20:1, 0 °C, 15 min, 50%; (b) 2.0 equiv CAN, PhH/H₂O 20:1, 0 °C, 20 min, 77%.

In conclusion, we present here an efficient strategy toward C₁₃-oxidized cembrane scaffolds that paves the way for the synthesis of complex cembrenolides. The developed approach allows construction of such scaffolds in 13 steps from readily available starting materials. Our studies suggest that the stereochemistry of the C₁₃ hydroxyl group affects the mode of cyclization upon oxidation of the furan-containing starting materials. The resulting polycyclic motifs bear structural similarities to more intricate natural products. These studies could set the stage for a divergent, biomimetic synthesis of various bioactive natural products of the cembrenolide family.