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J. Am. Chem. Soc., 122 (36), 8654 -8664, 2000. 10.1021/ja0015287 S0002-7863(00)01528-6 Web Release Date: August 26, 2000 Copyright © 2000 American Chemical Society Evolution of a Gram-Scale Synthesis of (+)-Discodermolide Amos B. Smith III,* Thomas J. Beauchamp, Matthew J. LaMarche, Michael D. Kaufman, Yuping Qiu, Hirokazu Arimoto, David R. Jones, and Kaoru Kobayashi Contribution from the Department of Chemistry, Monell Chemical Senses Center, and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received May 2, 2000 Abstract: An efficient, highly convergent, stereocontrolled total synthesis of the potent antimitotic agent (+)-discodermolide (1) has been achieved on gram scale. Key elements of the successful strategy include (1) elaboration of three advanced fragments from a common precursor (CP) which embodies the repeating stereochemical triad of the discodermolide backbone, (2) -bond installation of the Z trisubstituted olefin, exploiting a modified Negishi cross-coupling reaction, (3) synthesis of a late-stage phosphonium salt utilizing high pressure, and (4) Wittig installation of the Z disubstituted olefin and the terminal (Z)-diene. -------------------------------------------------------------------------------- In 1990, Gunasekera and co-workers1 at the Harbor Branch Oceanographic Institute reported the isolation of (+)-discodermolide (1), an architecturally novel antitumor agent derived from the deep-water marine sponge Discodermia dissoluta. The structure of (+)-discodermolide comprises a linear polypropionate backbone punctuated by (Z)-olefinic linkages at C(8,9), C(13,14), and C(21,22). Structure elucidation entailed extensive spectroscopic studies, including a combination of 1-D and 2-D NMR techniques; the relative stereochemistry was subsequently revealed by X-ray crystallography.1 The absolute configuration however remained undefined until Schreiber and co-workers2 synthesized both antipodes. Interestingly, the unnatural (-)-antipode also displays significant cytotoxicity.3 Initial biological studies revealed that (+)-discodermolide suppresses both the two-way mixed-lymphocyte reaction and the concanavalin A-induced mitogenesis of murine splenocytes in vitro (IC50 0.24 and 0.19 mM, respectively), with no associated cytotoxicity.4 (+)-Discodermolide also inhibits the graft vs host-splenomegaly response induced by injection of parental splenocytes into F1 recipient mice, with potency intermediate between those of cyclosporin A and FK506.5 These findings stimulated considerable interest in discodermolide as a possible immunosuppressant. Subsequently, ter Haar et al. reported that (+)-discodermolide is a potent cell growth inhibitory agent, which arrests cell development at the M phase by binding and stabilizing mitotic spindle microtubules.6 Thus, the ability of (+)-1 to inhibit cell growth was suggested to resemble the clinically proven anticancer agent Taxol (2, Figure 1). (Taxol is a registered trademark of Bristol-Myers Squibb.) Significantly, the binding affinity to microtubules is higher.6 -------------------------------------------------------------------------------- Figure 1 t -------------------------------------------------------------------------------- That the cell growth inhibitory effects of (+)-discodermolide and Taxol are indeed similar was confirmed by Day and co-workers.7 (+)-Discodermolide (1) also displays potent activity against multi-drug-resistant (MDR) carcinoma cell lines, including Taxol-resistant lines.8 In this regard, Horwitz et al.9 reported that 1 was unable to replace Taxol in a newly discovered Taxol-dependent human lung carcinoma cell line (A549-T12), whereas the natural products epothilone A and B (3, 4) and eleutherobin (5) were able to serve as substitutes for Taxol and thus maintain viability of the cell line.9 Importantly, the Taxol-dependent cell line proved to be 20-fold more sensitive to 1 in the presence of low concentrations of Taxol than in its absence.9 The significance of these results is 2-fold: (1) (+)-discodermolide may bind to microtubules at a site distinct from that of Taxol, the epothilones (3, 4), and eleutherobin (5) and (2) the combination of (+)-discodermolide and Taxol may constitute a promising synergistic cancer chemotherapeutic treatment regime. Unfortunately, the current scarcity of natural material (0.002% w/w from frozen sponge) has precluded further evaluation as an anticancer agent. Not surprisingly the remarkable biological activity and novel structure, in conjunction with the scarcity of the natural material, has led to considerable interest in discodermolide as a synthetic target.10 To date, six total syntheses of discodermolide, including our first-generation synthesis of the unnatural levorotatory congener, have been reported.2,11 In this, a full paper, we describe the evolution of a synthetic strategy which recently culminated in the preparation of 1 g of the natural congener.11b Synthetic Plan. Analysis of the discodermolide structure revealed a repeating triad of contiguous stereocenters (Scheme 1), separated by (Z)-olefinic linkages at C(8,9) and C(13,14). From the synthetic perspective we sought an efficient, highly convergent approach taking full advantage of this structural element, while also exploiting the dithiane coupling and - and -bond construction tactics that had proven so effective in our syntheses of FK506 and rapamycin.12 Thus, disconnections at C(8,9), C(14,15), and C(21,22) generated fragments A, B, and C, each envisioned to arise from a common precursor (CP) possessing the recurring triad of contiguous stereogenic centers. Union of A and B via either cuprate chemistry13 or palladium-catalyzed cross-coupling14 would then lead to an AB fragment possessing the C(13,14) trisubstituted (Z)-olefin; in turn the ylide derived from the AB fragment would be coupled via Wittig olefination with aldehyde C to generate the C(8,9) cis-alkene. Introduction of the terminal Z-diene would also exploit a Wittig olefination or like process. Completion of the discodermolide venture would then only require elaboration of the C(1) lactone carbonyl, selective incorporation of a carbamate moiety at C(19), and global deprotection. -------------------------------------------------------------------------------- Scheme 1 -------------------------------------------------------------------------------- The Common Precursor. The critical role of the common precursor CP demanded development of a route that would be amenable to large-scale production. To orchestrate the triad of contiguous stereogenic centers, we selected the Evans syn-aldol protocol.15 Given that the absolute stereochemistry of (+)-discodermolide was unknown at the outset (early 1993), we arbitrarily chose the commercially available (R)-(-) antipode of methyl 3-hydroxy-2-methylpropionate (6) as starting material. Ultimately this choice led to the unnatural (-)-antipode of 1.11a As detailed here, this strategy is equally amenable to the synthesis of (+)-discodermolide, utilizing the commercially available (S)-(+) antipode of 6. Although some of the transformations described in this paper were developed during our initial synthesis of (-)-discodermolide, for clarity of presentation, all transformations are depicted in the optical series of the natural product. The absolute stereochemistries with associated chiropic properties of all intermediates are available in the Supporting Information. Our point of departure for the construction of the common precursor CP required for natural (+)-discodermolide entailed protection of hydroxy ester (+)-6 as the p-methoxybenzyl (PMB) ether (Scheme 2), exploiting the Bundle trichloroimidate protocol.16 Reduction with LiAlH4 and distillation provided alcohol 7; Swern oxidation17 then furnished aldehyde 8, which was used without purification in an Evans aldol condensation with oxazolidinone 9.18 Weinreb amide formation19 completed construction of the common precursor CP. -------------------------------------------------------------------------------- Scheme 2 -------------------------------------------------------------------------------- While preparing for the large-scale synthesis of natural discodermolide (vide infra), we became aware of a report by Walkup and co-workers20 describing aldol adduct 12 as a crystalline solid. Since the phenylalaninol-derived oxazolidine 10 is noncrystalline and thus purified by silica gel chromatography, the prospect of utilizing 11 in place of 9 appeared attractive for the large-scale production of CP. Gratifyingly, the dibutylboron enolate of 11, readily prepared from (1S,2R)-norephedrine,15 did indeed produce a crystalline adduct with aldehyde 8. The yield of 12 was 64-70% on a 60-70 g scale. Significantly, 12 could be isolated directly from the reaction mixture by crystallization. In practice, the first three intermediates were carried forward without purification, and 12 was isolated by crystallization (55%, four steps, 60 g scale). The structure of 12 was confirmed by single-crystal X-ray analysis.21 Subsequent conversion of 12 to the common precursor CP proceeded as before without complication (98% yield). Purification of CP was facilitated by crystallization of the oxazolidinone auxiliary from the reaction mixture (80-90% recovery). The readily recovered auxiliary further augmented the efficiency of this sequence. Fragment A. The polypropionate structural motif of fragment A suggested a second asymmetric aldol reaction (Scheme 3). Initial generation of the p-methoxybenzylidene (PMP) acetal22 was intended to permit eventual late-stage deprotection of the C(21) and C(19) hydroxyls, facilitating selective introduction in turn of the terminal diene and carbamate moieties. Accordingly, treatment of CP with DDQ furnished crystalline acetal 13. Controlled reduction of the Weinreb amide23 provided crystalline aldehyde 14 which was directly coupled with oxazolidinone 9 via an Evans aldol reaction to provide crystalline alcohol 15 (83% yield, two steps), possessing the requisite five contiguous stereogenic centers of subunit A. The structures of 13 and 15 were confirmed by single-crystal X-ray analyses.21 Protection of the secondary hydroxyl as the TBS ether and removal of the chiral auxiliary (LiBH4, EtOH, THF)24 then furnished crystalline 17 (82% yield, two steps). Alcohol 17 was then converted to iodide A (95% yield), completing the synthesis of the C(17-21) subunit A in six steps and 56% overall yield from CP.25 The remarkable number of crystalline intermediates en route made this sequence well suited for the routine production of A on a large scale.26 -------------------------------------------------------------------------------- Scheme 3 -------------------------------------------------------------------------------- Fragment B. As outlined in Scheme 1 our synthetic plan called for construction of a vinyl halide possessing the (Z)-configuration to couple with fragment A. We selected bromide 23 as our initial target (Scheme 4). Beginning again with CP, protection of the hydroxyl as the TBS ether, followed by DIBAL reduction, afforded aldehyde 19 in 89% overall yield. Unfortunately the one-step protocol of Smithers27 to introduce the vinyl halide via an -bromoalkylylide was not synthetically useful (e.g., 40% yield, 4:1 Z/E). We therefore adopted a stepwise approach, whereby 19 was first converted to the Z -bromo unsaturated ester 20 (Ph3P=CBrCO2Et,28 PhH, reflux; 75% yield). Reduction, followed by mesylation and displacement with hydride (LiBHEt3) then furnished the (Z)-vinyl bromide 23 in 77% yield from 20. -------------------------------------------------------------------------------- Scheme 4 -------------------------------------------------------------------------------- In seeking to verify the olefin geometry in 23 by sequential metalation, protonation, and 1H NMR analysis, we discovered that the initially formed vinyl anion was prone to 1,5-silyl migration, affording 24 when the lithium-halogen exchange (n-BuLi) was performed in THF.29 Although the unexpected retro-[1,5]-Brook rearrangement30 provided evidence that bromide 23 indeed possessed the desired (Z)-configuration (vide infra), our synthetic plan was now in jeopardy, unless the migration could be suppressed. Toward this end, halogen exchange with t-BuLi at -78 C in THF resulted in only 20% rearrangement, while treatment with t-BuLi in Et2O (-78 C) completely circumvented the problem. We attribute the observed difference in reactivity to stronger solvation of the organolithium aggregate in THF, an effect that renders the carbanion more nucleophilic.31 Protonation of the vinyl anion then led to 25 in 94% yield; the trans-olefinic geometry was confirmed by the vinylic coupling constant (e.g., J = 15.3 Hz).32 While the protecting groups in subunits A and B were carefully chosen to exploit most efficiently CP, our long-term strategy demanded removal of the primary PMB ether (derived from 23) in the presence of the PMP acetal in the AB coupled product (Scheme 1). It thus appeared prudent to explore the feasibility of this unprecedented tactic before committing to this protecting group scheme. Accordingly, exposure of an equimolar mixture of PMB ether 23 and PMP acetal 17 to DDQ (1.1 equiv) in CH2Cl2/H2O22 (Scheme 5) furnished acetal 17 largely intact and alcohol 26 in 83% yield. Presumably, the observed selectivity is due to the higher oxidation potential of the acetal which contains two electron-withdrawing oxygens. With these results in hand, we proceeded with confidence. -------------------------------------------------------------------------------- Scheme 5 -------------------------------------------------------------------------------- Fragment C. Our point of departure was TBS ether 18 (Scheme 6), previously prepared during the synthesis of fragment B (vide supra). Oxidative removal of the PMB group (DDQ, CH2Cl2, H2O) provided an unstable alcohol, 27, in yields ranging from 60% to 86%, accompanied by significant amounts of the corresponding -lactone, which could be recycled.33 Better results were obtained by hydrogenolysis of 18 with Pearlman's catalyst, which provided 27 in 95% yield. Oxidation with SO3·pyridine34 then furnished aldehyde 28 (98% yield); the two steps were routinely performed without purification of 27. Dithiane generation via modification of the Evans protocol35 [e.g., (TMSSCH2)2CH2, ZnCl2, Et2O] minimized elimination of the TBS ether, permitting efficient production of dithiane 29 (79%). DIBAL reduction (91% yield), followed by dimethyl acetal formation, then gave 31 (99%). Coupling of the dithiane with benzyl (S)-(+)-glycidyl ether 32 exploited the conditions developed earlier in our laboratory in connection with our FK506 and rapamycin syntheses.12 This sequence provided alcohol 33 in 79% yield. Unmasking of the ketone [(CF3CO2)2IPh, 80%]36 followed by Evans directed reduction37 furnished the anti-diol 35 (97%), which embodied all of the stereogenic centers required for fragment C. Acid-catalyzed cyclization then provided methoxy pyran 36 in 87% yield, as a separable mixture of and anomers (2:1). The large coupling constant in 36 (J4,5 = 9.8 Hz), diagnostic of the diaxial arrangement of the C(4) and C(5) hydrogens, confirmed the selectivity of the directed reduction.38 The configuration of the methoxy group was deduced by a series of NOE experiments.39 The epimeric mixture 36 was then converted to the corresponding TBS ethers 37 (: = 2:1, 97% yield). -------------------------------------------------------------------------------- Scheme 6 -------------------------------------------------------------------------------- In our initial synthetic strategy, we had planned to mask the C(1) carbonyl in fragment C as the mixed methyl acetal. However, under a variety of conditions, we were unable to hydrolyze the acetal moiety in the presence of the TBS ethers, a required endgame transformation. To circumvent this obstacle we converted 37 to the ethylthio derivative 39, a tactic first reported by Schreiber and co-workers in their discodermolide syntheses.2 Toward this end, hydrogenolysis of 37 afforded alcohol 38 quantitatively; exposure to ethanethiol and MgBr2 in Et2O40 then furnished a separable mixture of hemithioacetal 39 and the anomer (6:1) in 83% yield.39 Although in principle both epimers could be carried forward, it was more expedient to work with homogeneous material. Swern oxidation of 39 led to 40 (86% yield), our first-generation fragment C. The overall sequence proceeded in 14 steps from CP and in 24% overall yield (average yield/step 90%). Union of Fragments A and B. At the outset, we had planned to employ a vinyl cuprate to effect -bond construction of the C(13-14) Z trisubstituted olefin.13 Unfortunately, attempts to couple iodide A (Scheme 7) with the cuprate prepared from vinyl bromide 23, via lithium halogen exchange followed by the addition of a variety of Cu(I) salts [i.e., CuCN, CuI, (Th)Cu(CN)Li], by and large proved unsuccessful. Only the higher order cuprate exploiting the Corey reagent n-Bu4NCu(CN)241 furnished the desired AB segment, albeit in only 8% yield. -------------------------------------------------------------------------------- Scheme 7 -------------------------------------------------------------------------------- Attention quickly shifted to palladium-catalyzed cross-coupling reactions.14 Initial studies targeted the coupling of bromide 23 with organomagnesium, organozinc, or organoboron reagents prepared from A.14 To this end, lithiation of iodide A (2 equiv of t-BuLi, -78 C) followed by the addition of MgBr2, ZnCl2, or B-methoxy-9-BBN furnished the requisite organometallic species.42 Not surprisingly, the initial lithio species also proved susceptible to silyl migration to form 41, a significant side reaction at -45 C in Et2O or at -78 C in THF (cf. Scheme 4). Best results were obtained by metalation with t-BuLi at -78 C for ca. 10 min followed by addition of MgBr2. Although AB was only obtained in 14% yield, we were encouraged by the observation that most of the vinyl bromide could be recovered. The high recovery of 23 suggested slow oxidative insertion of the palladium into the carbon-bromine bond. We therefore examined the more reactive vinyl iodide 46 (Scheme 8). Initially, iodide 46 was prepared in one step (86% yield) from bromide 23 via Nickel-catalyzed bromine-iodine exchange.43 The overall length of this sequence from CP however was not acceptable (i.e., seven steps). We therefore explored the Zhao one-step protocol.44 Treatment of aldehyde 19 with the ylide derived from 4344 afforded 46 in 40-46% overall yield from CP.45 The Z/E selectivity ranged from 8:1 to 17:1. In practice, the sequence required only a single chromatographic purification, and could be performed routinely on a 30 g scale. -------------------------------------------------------------------------------- Scheme 8 -------------------------------------------------------------------------------- Upon scale-up, we noted the formation of significant amounts of an olefin lacking the iodide. A similar result was recently reported by Marshall and Johns.11 This side product 25 proved to be the direct result of incomplete incorporation of iodine in phosphonium salt 42, thereby leading to a mixture of ylides upon deprotonation with NaHMDS. The poor iodine incorporation was in turn the result of the low solubility of iodine in THF; increasing the amount of THF (0.1 M I2) during the preparation of 43, in conjunction with vigorous stirring at ambient temperature prior to cooling, circumvented this problem. As anticipated vinyl iodide 46 proved superior to bromide 23 in the palladium-catalyzed cross-coupling reactions, although not without significant initial experimentation. Best results involved modification of a Negishi protocol (Scheme 9).14b Specifically, treatment of iodide A and 1 equiv of ZnCl2 in ether with 3 equiv of t-BuLi at -78 C, followed by warming to ambient temperature and transfer to an intimate mixture of vinyl iodide 46 and Pd(PPh3)4 (5 mol %), furnished AB after workup in 66% yield (recrystallized). The efficiency of this cross-coupling reaction is particularly noteworthy. In contrast to most cross-coupling reactions which require a minimum of 1.5 equiv of alkyl iodide for comparable efficiency, only 1.1 equiv of A was required.46 Importantly, 3 equiv of t-BuLi is required for complete consumption of A. Treatment with 2 equiv of t-BuLi, followed by quenching with water, leads to a 1:1 mixture of starting iodide and the expected alkane. Accordingly, we speculate that a mixed tert-butyl-alkyl zinc intermediate (e.g., 47) is the reactive alkyl donor in the coupling process. -------------------------------------------------------------------------------- Scheme 9 -------------------------------------------------------------------------------- The AB Wittig Reagent. Conversion of AB to the requisite phosphonium iodide 48 (Scheme 10) began with selective removal of the PMB group. As anticipated from our earlier model study (Scheme 5), exposure of AB to 1 equiv of DDQ (CH2Cl2, H2O) effected selective removal of the PMB ether in the presence of the PMP acetal, furnishing 49 in 91% yield. The relative stereochemistry of 49 was confirmed by single-crystal X-ray analysis.21 Further elaboration of 49, however, presented unforeseen difficulties. Quite surprisingly, attempts to convert 49 to iodide 51 via tosylate 50 proved unsuccessful. While some tosylate appeared to be present in the reaction mixture (1H NMR), attempts to chromatograph the tosylate failed. Attempts to convert the tosylate directly to iodide 51 also proved unrewarding. Detailed 1H NMR analysis of the product mixture revealed the loss of the trisubstituted olefin, presumably the result of olefin-assisted displacement at C(9). Analogous olefin-assisted solvolyses of 5-hexenylsulfonates are well-known.47 However, such reactions usually require more vigorous conditions (e.g., formic acid, 75 C).47 -------------------------------------------------------------------------------- Scheme 10 -------------------------------------------------------------------------------- To understand better the nature of this reaction, we turned to model alcohol 52 (Scheme 11), prepared from aldehyde 19 via reaction with the ylide derived from isopropyltriphenylphosphonium iodide, followed by oxidative removal (DDQ) of the PMB ether. Again attempts to generate the iodide 54 from 52 via the tosylate proved unsuccessful. However, a modification of the Garegg conditions (PPh3, I2, imidazole, 3:1 Et2O/CH3CN)48 did permit conversion of 52 to iodide 54 (74%); minor amounts of 56 and 57 were also isolated and characterized (8% each). The observation of cyclopentanes 56 and 57 provides clear evidence for the competitive solvolysis process.49 -------------------------------------------------------------------------------- Scheme 11 -------------------------------------------------------------------------------- Analysis of the competing transition states (e.g., intramolecular vs bimolecular displacement) suggested that solvent polarity should influence the product distribution.50 Indeed, the use of CH3CN or CH3CN/Et2O (3:1) as solvent significantly increased cyclization vs displacement (e.g., 54:56:57 = 9:1:1; 90% yield) whereas the less polar medium, Et2O/PhH (2:1), provided a greater proportion of iodide 54 (18:1:1; 97% yield). To our delight similar treatment of alcohol 49 [PPh3, I2, Et2O/PhH (2:1)] furnished the requisite iodide 51 in near quantitative yield (Scheme 12). The sensitive iodide could be either purified by rapid silica gel chromatography (pretreated, Et3N) or used without purification. -------------------------------------------------------------------------------- Scheme 12 -------------------------------------------------------------------------------- Turning next to the preparation of the requisite Wittig reagent (Scheme 12), initial efforts involved treatment of 51 with PPh3 in solvent (not shown); again extensive cyclization and some decomposition occurred. To circumvent intramolecular cyclization, we turned to molten PPh3. In the event, treatment of 51 with excess triphenylphosphine (15 equiv) and i-Pr2NEt (3 equiv) at 80 C in the absence of additional solvent generated 48 in 37% yield from 49. The major byproduct was again cyclopentane 60 (35% yield). Critical to the success of this reaction was the use of an amine base to prevent the HI, produced during cyclization, from promoting decomposition. Similar treatment of model iodide 54 furnished phosphonium salt 61 in 38% overall yield from 52. In marked contrast, the fully saturated congener obtained by hydrogenation of olefin 52 afforded the corresponding saturated phosphonium salt (via the iodide) in almost quantitative yield (94%). Phosphonium Salts via Ultrahigh Pressure. Although the use of molten triphenylphosphine as solvent led to displacement of the iodide, the 37% yield was not acceptable for large-scale material advancement. We therefore turned to high pressure to facilitate displacement. In 1984 Dauben and co-workers reported that alkylation of phosphines can be dramatically accelerated by ultrahigh pressure.51 In the event, reaction of a mixture of iodide 54, triphenylphosphine (3 equiv), and i-Pr2NEt (0.5 equiv) in benzene/toluene (7:3) at 12.8 kbar (185 000 psi) for 72 h furnished an 86% yield of phosphonium salt 61 (Scheme 13). Gratifyingly, iodide 51 was similarly transformed into 48 via these conditions (6 d; 70% overall yield from alcohol 49). -------------------------------------------------------------------------------- Scheme 13 -------------------------------------------------------------------------------- A First-Generation Endgame. Deprotonation of phosphonium salt 48 with sodium bis(trimethylsilyl)amide in THF gave rise to an ylide which underwent high Z-selective coupling (>49:1 Z/E) with aldehyde 40, to furnish 62 in 76% yield (Scheme 14). DIBAL reduction52 then led to a primary alcohol (88% yield), which in turn was oxidized to aldehyde 64 (96% yield). The terminal (Z)-diene 65 was next installed via the Yamamoto protocol53 in 70% yield, completing assembly of the discodermolide backbone; the selectivity was 16:1 Z/E.54 Hydrolysis of the hemithio acetal followed by Kuzuhara-Fletcher oxidation55 provided lactone 66 in 82% yield for the two steps. Removal of the PMB group (DDQ, CH2Cl2, H2O, 95%) and introduction of the carbamate via the Kocovsky protocol (Cl3CCONCO, CH2Cl2; K2CO3, MeOH, 83%)56 then led to 68. Final deprotection with HF/CH3CN (48%, 1:9) furnished (-)-discodermolide (60% yield), identical in all respects, with an authentic sample provided by Schreiber (e.g., 500 MHz 1H NMR, 125 MHz 13C NMR, IR, HRMS, optical rotation, and TLC in four solvent systems).57 -------------------------------------------------------------------------------- Scheme 14 -------------------------------------------------------------------------------- A Second-Generation Endgame. Although the endgame shown in Scheme 14 permitted completion of (-)-discodermolide in 1995 (40 total steps; 28 in the longest linear sequence),11 the synthesis was far from optimal. Having subsequently learned of the potential of (+)-discodermolide as an anticancer agent, we felt compelled both to shorten the existing synthetic route and to improve the overall efficiency of the synthesis to permit preparation of significant quantities of this natural product for further clinical development. We set as our goal the synthesis of 1 g. Fragment C: A Second-Generation Strategy. In our initial synthesis of (-)-discodermolide, elaboration of subunit C (40) from the common precursor CP was clearly the least efficient aspect of the synthetic venture, requiring 14 steps (CP 40; Scheme 6). The length of this sequence was due, at least in part, to our desire to exploit the coupling of dithiane 31 with an epoxide (31 33; Scheme 6), a tactic utilized to great advantage in earlier syntheses in our laboratory. While the above protocol proved effective, a direct three-carbon extension of aldehyde 28 appeared more expedient (Scheme 15). The challenge, however, would entail stereocontrol in the addition of an appropriate nucleophile to 28, to generate the required anti relationship between C(4) and C(5) in 70.58 This strategy also held the promise of allowing the unprotected C(1) lactone to be carried throughout the endgame sequence. We thus set lactone-aldehyde 71 as our second-generation fragment C.59 -------------------------------------------------------------------------------- Scheme 15 -------------------------------------------------------------------------------- We selected as our initial nucleophile the silyl enol ether derived from mesityl oxide (72) (Scheme 16).60 There was of course no guarantee that we would be able to control the facial selectivity of aldehyde 28. Two options were however available. One would be to replace the TBS ether of 28 with a more Lewis basic protecting group to direct nucleophilic addition to the aldehyde via metal ion chelation.10k,l The alternative would be to use an external asymmetric promoter. In the interest of synthetic economy and the demonstrated effectiveness of the C(14) TBS ether in the first-generation endgame (vide supra), we elected to explore initially the addition of enol ether 72 to 28 (Scheme 16). To our delight treatment of a solution of 28 and TiCl4 (1 equiv) with enol ether 72 at -78 C proceeded in 77% yield with near complete anti-Felkin selectivity (ds > 20:1); lactone 74 was then available after acid-catalyzed cyclization of the intermediate hydroxy amide 73.61 -------------------------------------------------------------------------------- Scheme 16 -------------------------------------------------------------------------------- Anti-Felkin addition was also observed in the TiCl4-promoted reactions of allyltrimethylsilane and allyltributylstannane with 28 (ca. 49:1 and 19:1, respectively; Scheme 17). In contrast, BF3·OEt2-promoted the addition of allyltributylstannane with no selectivity.62 Taken together, these results suggest that bidentate complexation of the TiCl4 with the carbonyl groups of the Weinreb amide and the aldehyde, via an eight-membered chelate, is responsible for the observed high anti-Felkin selectivity.63,64 -------------------------------------------------------------------------------- Scheme 17 -------------------------------------------------------------------------------- With a method available to access 73, 74, and 77 with excellent stereocontrol at C(5), attention turned to introduction of the requisite -hydroxy aldehyde functionality. Initial studies entailed asymmetric dihydroxylation of olefin 77.65 Attempts to effect this transformation led, at best, only to a 3:1 mixture of diols. Attention thus shifted to the acyclic Mukaiyama aldol adduct 73. We reasoned that the stereogenicity of the C(5) hydroxyl could be employed to direct reduction of the enone carbonyl to the anti-diol similiar to a reduction reported by Evans.37,66 Having failed to observe high selectivity in the directed reduction of the C(7) enone in 73, we explored the reduction of the enone in lactone 74 (Scheme 18), readily available (77% yield) from 73 by acidic treatment (Scheme 16). A variety of reducing conditions were examined. Only the Selectride reagents furnished the desired allylic alcohol 89 as the predominant product.67 Higher selectivities were obtained in toluene compared to THF or ether. The selectivies also proved sensitive to the counterion; K-Selectride (89:90 = 9:1) was notably more selective than both N-Selectride (89:90 = 3:1) and L-Selectride (89:90 = 1.2:1). The stereochemistry of alcohol 89 was subsequently secured via single-crystal X-ray analysis (Scheme 19). -------------------------------------------------------------------------------- Scheme 18 Scheme 19 -------------------------------------------------------------------------------- With alcohol 89 in hand, conversion to our second-generation fragment C (71) proved straightforward. Protection as the TBS ether furnished 93 (87% yield), which was then subjected to ozonolysis to give 71 (95% yield). Thus, our second-generation subunit C was available in seven steps (47% overall yield, 89% average) from CP. While this synthesis was considerably shorter and more efficient than that of 40 (e.g.,14 steps, 24% overall yield), the vitality of 71 as a Wittig coupling partner was yet to be established. Chemoselectivity in the Wittig Coupling. Treatment of 71 (Scheme 20) with the ylide derived from 48 (KHMDS, toluene, 0 C) furnished olefin 94 in 57% yield with excellent Z/E selectivity (>20:1). The choice of both base and solvent proved critical for this coupling reaction. Subsequent exposure of 94 to excess DIBAL (10 equiv, -78 C 0 C) effected reductive cleavage of the PMP acetal, with concomitant reduction of the lactone carbonyl, to furnish 95. Reduction of the lactone proved of little consequence, since pyridinium dichromate (PDC) readily converted the primary hydroxyl and the lactol to lactone aldehyde 96 (62% yield, two steps). -------------------------------------------------------------------------------- Scheme 20 -------------------------------------------------------------------------------- The stage was thus set for incorporation of the terminal diene. Although our first synthesis of discodermolide (cf. 64, Scheme 14) made efficient use of the Yamamoto protocol for this transformation (70% yield, 16:1 Z/E), in this case only a modest yield of 66 with significantly lower Z selectivity (40-58%, 4:1 Z/E) was observed. Apparently the lactone moiety is sensitive to the reaction conditions, although it is difficult to account for the low Z selectivity (e.g., 65).68 The low efficiency of the diene installation stimulated us to explore an alternative route in which the diene would be incorporated at an earlier stage. Early Installation of the Terminal Diene: Completion of a Gram-Scale Synthesis of (+)-Discodermolide. We began with the previously prepared C(9-22) AB fragment (Scheme 21). To maintain eventual discrimination of the C(19) PMB ether, a protecting group interchange was required. Initially we selected the TBS moiety to protect the primary hydroxyl. However, during reductive-opening of the acetal, we experienced competitive removal of the silyl group.69 Accordingly, we exploited the trityl group. Reduction of 97 with DIBAL then gave the primary alcohol (Scheme 21) which in turn was subjected to oxidation with buffered Dess-Martin periodinane,70 installation of the diene via the Yamamoto diene protocol,53 and removal of the trityl group71 to furnish 98 in yields ranging from 50% to 74% (four steps) with good diastereocontrol (8-12:1 Z/E, 10 g scale). As will be described, the undesired (E)-congener eventually proved inconsequential since it could be readily removed at a later stage in the synthesis (vide infra). -------------------------------------------------------------------------------- Scheme 21 -------------------------------------------------------------------------------- Conversion of alcohol 98 to the corresponding phosphonium salt via our previously optimized high-pressure conditions proceeded without difficulty, providing phosphonium salt 99 in 75-82% yield on a multigram scale (Scheme 22). Chemoselective addition of the derived ylide also proceeded in good yield and with excellent selectivity (Z/E 15-24:1). Again the solvent of choice (vide supra) proved to be THF, with aldehyde 71 added to the ylide at -20 C. Addition at lower temperature resulted in considerable decomposition. The phosphonium salt 99 proved to be highly hygroscopic; thus, great care was necessary to remove residual water, which if present led to the formation of the corresponding diphenylphosphine oxide upon addition of base.72 Best results were obtained by azeotropic removal of water from the salt with toluene (three times), followed by drying in vacuo at 50 C for 12 h (ca. 0.1 Torr). Attention to these details led to 66 reproducibly in 59-69% yield up to the 2.0 g scale; importantly the phosphonium salt 99 could be recovered. The yield of 66 based on recovered 99 was 85-95%. -------------------------------------------------------------------------------- Scheme 22 -------------------------------------------------------------------------------- Having achieved assembly of the discodermolide carbon skeleton, we were poised to complete the large-scale synthesis. However, the Z/E selectivity (8-12:1) obtained in the diene synthesis demanded attention. Previously the minor (E)-diene was removed via AgNO3-impregnated silica gel chromatography; however, for large-scale work this was less than optimal. Fortunately we discovered that when 66, an 8:1 Z/E mixture at C(22,23), was subjected to DDQ oxidation to remove the C(19) PMB group, 67 was obtained as a single diastereomer along with an easily separable byproduct (not shown). The byproduct proved to be the result of a fortuitous Diels-Alder reaction between the (E)-diene and DDQ. The ability of the (E)-diene to adopt the s-cis-conformation required for cycloaddition to DDQ leads to the observed chemoselectivity, whereas the desired (Z)-isomer is prevented from adopting a planar conformation due to significant steric interaction between the C(24) vinyl and C(20) methine moieties. With pure 67 in hand, completion of the 1 g synthesis of (+)-discodermolide now only required installation of the carbamate and global desilylation. Importantly, carbamate formation (Cl3CCONCO; Al2O3) proceeded in 90-95% yield. For global deprotection we had employed HF/CH3CN in our 1995 synthesis. However, on scale-up we found these conditions to be capricious (e.g., yields ranging from 20% to 60%). Screening a series of conditions revealed that slow addition of 3 N HCl to a methanol solution of 68 over a 12 h period produced the natural product cleanly and reproducibly up to the 200 mg scale. Utilizing this route, we were able to prepare 1.06 g of (+)-discodermolide; crystallization from acetonitrile73 furnished 1.043 g of the natural product (mp 117-120 C). Importantly, in contrast to the original report on the isolation, we found (+)-discodermolide to be stable as a crystalline solid. However, when (+)-discodermolide was concentrated from CDCl3, decomposition occurred, due to residual acid derived from the deuteriochloroform. Thus, the reported instability of natural discodermolide is most likely an artifact. Summary. A highly convergent total synthesis of discodermolide (1) has been achieved, exploiting a common precursor (CP) to permit rapid and efficient construction of three advanced subtargets. The subtargets, each of which embody the common triad of contiguous stereogenic centers, were then coupled via highly stereoselective -bond (palladium-catalyzed cross-coupling) and -bond (Wittig) constructions. Importantly, the synthesis is highly efficient, proceeding in 6% overall yield (longest linear sequence, 24 steps), stereocontrolled, and amenable to gram-scale production of this potentially important natural product. Acknowledgment Financial support was provided by the National Institutes of Health (Institute of General Medical Sciences) through Grant GM-29028, and NIH Postdoctoral Fellowship to T.J.B and M.D.K. We also thank Novartis for financial support and for supplying spectral data for (+)-discodermolide in CD3CN. Finally, we thank Drs. George T. Furst and Patrick J. Carroll and Mr. John Dykins of the University of Pennsylvania Spectroscopic Service Center for assistance in securing and interpreting high-field NMR spectra, X-ray crystal structures, and mass spectra, respectively. Supporting Information Available Experimental procedures and characterization data for all synthetic intermediates (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. * In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed. 1. Gunasekera, S. P.; Gunasekera, M.; Longley, R. E.; Schulte, G. K. J. Org. Chem. 1990, 55, 4912. Correction: Ibid. 1991, 56, 1346. 2. Nerenberg, J. B.; Hung, D. T.; Somers, P. K.; Schreiber, S. L. J. Am. Chem. Soc. 1993, 115, 12621. (b) Hung, D. T.; Nerenberg, J. B.; Schreiber, S. L. Chem. Biol. 1994, 1, 67. (c) Hung, D. T.; Nerenberg, J. B.; Schreiber, S. L. J. Am. Chem. Soc. 1996, 118, 11054.[Full text - ACS] 3. 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In addition, reductive removal of the auxiliary resulted in competitive opening of the oxazolidinone ring; thus, oxazolidinone 9 was the auxiliary of choice for the construction of fragment A. 27. Smithers, R. H. J. Org. Chem. 1978, 43, 2833. 28. Boeckman, R. K., Jr.; Ko, S. S. J. Am. Chem. Soc. 1980, 102, 7146. 29. For leading references on silicon migration from oxygen to carbon, see: (a) Jung, M. E.; Nichols, C. J. J. Org. Chem. 1996, 61, 9065.[Full text - ACS] (b) Lautens, M.; Delanghe, P. H. M.; Goh, J. B.; Zhang, C. H. J. Org. Chem. 1995, 60, 4213. (c) Marumoto, S.; Kuwajima, I. J. Am. Chem. Soc. 1993, 115, 9021. (d) For a related 1,4-O sp2C silyl rearrangement, see: Kim, K. D.; Magriotis, P. A. Tetrahedron Lett. 1990, 31, 6137. 30. Brook, A. G. Acc. Chem. Res. 1974, 77. 31. McGarrity, J. F.; Ogle, C. A. J. Am. Chem. Soc. 1985, 107, 1805. 32. Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds; John Wiley: New York, 1994; Chapter 9. 33. 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The relative stereochemistries of 74 and the C(5) epimer 75 were established by NMR: 74, J4,5 = 10.0 Hz; 75, J4,5 = 2.4 Hz, J2,3 = 9.8 Hz. 62. The BPS ether aldehyde 79, a substrate lacking the Weinreb amide moiety furnished the Felkin addition product 80 over the anti-Felkin adduct 81 (3:1): 63. For a discussion of chelation-controlled carbonyl additions see: Reetz, M. T. Acc. Chem. Res. 1993, 26, 462. 64. Interestingly the structurally similar aldehyde ester 82 has also been postulated to participate in bidentate coordination with both TiCl4 and SnCl4 to afford anti-Felkin products upon reaction with crotylsilanes and crotylstannanes. 13C NMR data strongly supports coordination of Sn(IV) with the carbonyls of 82. (a) Santelli-Rouvier, C. Tetrahedron Lett. 1984, 25, 4371. (b) Yamamoto, Y.; Yatagai, H.; Ishihara, Y.; Maeda, N.; Maruyama, K. Tetrahedron 1984, 40, 2239. (c) Yamamoto, Y.; Nemoto, H.; Kikuchi, R.; Komatsu, H.; Suzuki, I. J. Am. Chem. Soc. 1990, 112, 8598. 65. Kolb, H. 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We thank Novartis for informing us that discodermolide is stable in acetonitrile |
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