Total Synthesis of (−)-Laulimalide

Laulimalide (1), also known as figianolide B, a 20-membered macrolide isolated from the Indonesian sponge Hyattella sp., has shown remarkable antitumor activities.¹ Recently, Laulimalide was also isolated from an Okinawan sponge Fasciospongia rimosa.² It displayed potent cytotoxicity against the KB cell line with an IC₅₀ value of 15 ng/mL.¹ Furthermore, it has shown cytotoxicity against P388, A549, HT29, and MEL28 cell lines in the range of 10–50 ng/mL (IC₅₀ values).^2b The structure of 1 was initially established by NMR studies. Subsequently, its absolute configuration was established by X-ray analysis by Higa and coworkers.² The significant clinical potential of laulimalide has stimulated considerable interest in its synthesis and structure-function studies.³ Herein, we report the first synthesis of (−)-laulimalide 1.

As outlined in Figure 1, our synthetic strategy of laulimalide is convergent and involves the assembly of C₃−C₁₆ segment 2 and C₁₇−C₂₈ segment 3 by Julia olefination, followed by an intramolecular Horner-Emmons reaction between the C₁₉ phosphonoacetate and C₃ aldehyde. The sensitive epoxide at C₁₆−C₁₇ was selectively introduced at the final stage of the synthesis by Sharpless epoxidation.⁴ The construction of both dihydropyran rings of laulimalide was achieved by ring-closing olefin metathesis utilizing Grubbs’ catalyst as the key step.⁵

Figure 1.

The synthesis of C₃−C₁₆ segment is accomplished by modifications of the previously published sequence.^3a As outlined in Scheme 1, diastereomerically pure δ-lactone 5 was prepared efficiently by exposure of acryloyl ester 4 to Grubbs’ catalyst (10 mol %) in CH₂Cl₂.^3a DIBAL reduction of 5 at −78 °C in CH₂Cl₂ followed by reaction with ethanol and CSA afforded the ethyl glycoside.⁶ Reaction of this ethyl acetal with tert-butyldimethylsilyl vinyl ether and Montmorillonite K-10 as the Lewis acid in CH₂Cl₂ at 23 °C followed by NaBH₄ reduction of the resulting aldehyde afforded the corresponding dihydropyran as a single isomer (by ¹H and ¹³C NMR). Protection of the alcohol with TBSCl and imidazole furnished the TBS ether 6. Removal of the benzyl group by lithium in liquid ammonia provided the alcohol which was subsequently converted to iodide 7. To install the C₁₃ methylene unit and the C₁₅ hydroxyl group, alkylation of lactone 8 was carried out by treatment with NaH in DMF at 0 °C for 15 min followed by reaction with iodide 7 at 23 °C for 15 min and then 60 °C for 12 h, which furnished lactone 9 as a mixture (4.2:1 by ¹H NMR) of isomers. Reduction of the mixture of lactone 9 by Red-Al provided the diol 10. Benzoylation of 10 afforded the corresponding dibenzoate which was exposed to Na−Hg in MeOH at −20 °C to provide the olefin 11.⁷ Protection of the C₁₅ hydroxyl group as a MOM ether, removal of the PMB group and Swern oxidation of the resulting alcohol furnished the C₃−C₁₆ segment, aldehyde 2.

Scheme 1^a.

^a (a) Dibal-H, −78 °C then CSA, EtOH, 23 °C; (b) K-10, CH₂ = CHOTBS, 23 °C; (c) NaBH₄, MeOH, 0 °C (54%); (d) TBSCl, imidazole, DMF, 23 °C (75%); (e) Li, NH₃ (95%); (f) I₂, PPh₃, Imidazole (96%); (g) 8, NaH, DMF, 0°C then iodide 7, 60 °C (89%); (h) Red-Al, THF, 0 °C; (i) PhCOCl, Et₃N, DMAP (cat.); (j) Na(Hg), Na₂HPO₄, MeOH, −20° to 23 °C (72%); (k) MOMCl, i-Pr₂NEt, 23 °C; (l) DDQ, pH 7 buffer, 23 °C (81%); (m) DMSO, (COCl)₂, i-Pr₂NEt, −60 °C (85%).

The synthesis of the C₁₇−C₂₈ segment 3 has been achieved by modification of reaction sequences published previously.^3b Optically active dihydropyran derivative 12 was prepared in multigram quantity by utilizing ring-closing olefin methathesis⁵ and Corey-Fuchs’ homologation⁸ reactions as the key steps. The alkynyl anion of 12 was readily obtained by treatment with n-BuLi at −78 °C for 1 h followed by warming to 23 °C for 1 h (Scheme 2). Reaction of the resulting alkynyl anion with optically active aldehyde 13 at −78 °C furnished a diastereomeric mixture (1.8: 1) of alcohols 14 and 15, which upon oxidation with Dess-Martin periodinane⁹ gave the corresponding alkynyl ketone. L-Selectride reduction of this ketone at −78 °C furnished the desired synalkynyl alcohol 15 as a single diastereomer (by ¹H NMR and ¹³C NMR). Red-Al reduction of 15 set the C₂₁−C₂₂ trans-olefin geometry. Removal of the C₁₉ PMB group by exposure to TFA¹⁰ and subsequent reaction of the resulting diol with p-methoxybenzylidene acetal and CSA provided acetal 16. DIBAL reduction of 16 at −78 °C in CH₂Cl₂ afforded the C₁₇−C₂₈ segment 3 as a single regio isomer (by ¹H and ¹³C NMR).

Scheme 2^a.

^a (a) n-BuLi, −78 °C, 1 h and 23 °C, 1 h then 13, −78 °C (64%); (b) Dess−Martin, CH₂Cl₂, 23 °C (81%); (c) L-Selectride, THF, −78 °C (87%); (d) Red-Al, THF, −20 °C (81%); (e) CF₃CO₂H, CH₂Cl₂, 23 °C; (f) p-MeO-Ph-CH(OMe)₂, CSA, CH₂Cl₂, 23 °C (71%); (g) Dibal-H, CH₂Cl₂, −78 °C (74%).

Our subsequent synthetic strategy calls for the assembly of fragments 2 and 3 by Julia olefination (Scheme 3). Thus, lithiation of sulfone derivative 3 with 2.1 equiv of n-BuLi in THF at −78 °C for 15 min followed by reaction of the resulting dianion with the aldehyde 2 at −78 °C to −40 °C for 2 h furnished the expected R-hydroxy sulfone derivatives. The hydroxy sulfone was transformed into the corresponding C₁₆−C₁₇ olefin in a two step sequence involving (1) acylation of the hydroxy sulfone derivatives with Ac₂O, Et₃N and DMAP (cat.) and (2) exposure of the resulting acetates to Na(Hg) in methanol at −20 °C for 2 h followed by warming the reaction to 23 °C for 30 min. The C₁₆−C₁₇ trans olefin 17 was obtained in 34% yield along with 10% cis-olefin, which was readily separated by silica gel chromatography.

Scheme 3^a.

^a (a) n-BuLi, −78°C, 15 min, Then 2, −78 to −40 °C, 2 h; (b) Ac₂O, Et₃N, DMAP (cat.); (c) Na(Hg), Na₂HPO₄, MeOH, −20 to 23 °C (34%); (d) (CF₃CH₂O)₂P(O)CH₂CO₂H, Cl₃C₆H₂COCl, i-Pr₂NEt, DMAP; (e) AcOH-THF-H₂O (3:1:1), 23 °C (99%); (f) Dess−Martin, CH₂Cl₂, 23 °C (79%); (g) K₂CO₃, 18-C-6, −20 to 0 °C (84%); (h) hv, Et₂O, 50 min (66%); (i) PPTS, t-BuOH, 84 °C (45%) (j) Ti(OⁱPr)₄, (+)-DET, t-BuOOH, −20 °C; (k) DDQ, pH 7, 23 °C (48%).

Subsequent elaboration to the macrolactone possessing C₂−C₃ cis-olefin geometry proved to be a formidable task. We finally relied upon an intramolecular Horner-Emmons reaction of C₁₉ phosphonoacetate and C₃ aldehyde using Still’s protocol.¹¹ Thus, acylation of C₁₉ hydroxyl group with bis-(2,2,2-trifluoroethyl)-phosphonoacetic acid followed by removal of the TBS group by exposure to aqueous acetic acid in THF at 23 °C furnished the acetate derivative 18 in near quantitative yield.¹² Oxidation of 18 with Dess−Martin periodinane provided the C₃ aldehyde which upon treatment with K₂CO₃ in the presence of 18-C-6 at −20 °C for 30 min and then at 0 °C for 2.5 h furnished a mixture (2:1) of macrolactones 19 and 20 in 84% yield (Scheme 3). Both cisand trans-lactones were separated by silica gel chromatography. Horner-Emmons reaction of the corresponding (diphenylphosphono)acetate derivative provided slight improvement of the cis-selectivity (cis:trans =1:1.7, 69% isolated yield).¹³ Further attempts to improve the ratio for 20 by changing reaction conditions or C₂₀ protecting group have been unsuccessful. The overall yield of the desired cis-macrolactone 20 was however improved to 47% after photoisomerization of the trans-macro-lactone 19.¹⁴ Thus, irradiation of 19 in ether under UV in a Rayonet photochemical reactor for 50 min afforded a mixture of trans-lactone 19 (33%) and the cis-lactone 20 (33%) which were separated by chromatography. The identity of the cis-olefin geometry of 20 was established by its observed coupling constant (J = 11.6 Hz). Macrolactone 20 was converted to synthetic (−)-laulimalide 1 as follows: removal of the MOM group by refluxing with PPTS in t-BuOH,¹⁵ exposure of the resulting alcohol to Sharpless epoxidation⁴ with (+)-DET and removal of the C₂₀ PMB ether by exposure to DDQ. Spectral data (¹H and ¹³C NMR) of synthetic 1 ([α]²³_D −196 c 0.23, CHCl₃) are identical to that from a sample of natural laulimalide (lit.^2a [α]²⁹_D −200 c 1.03, CHCl₃) kindly provided by Professor Higa.

Thus, a stereocontrolled synthesis of (−)-laulimalide has been achieved. Considering its clinical potential as an antitumor agent, the present synthesis will enable important structure−function studies as well as synthesis of structural variants of laulimalide. Further improvement in synthesis and biological studies are currently in progress.