Cascade Metathesis Reactions for the Synthesis of Taxane and Isotaxane Derivatives

Abstract

Tricyclic isotaxane and taxane derivatives have been synthesized by a very efficient cascade ring‐closing dienyne metathesis (RCDEYM) reaction, which formed the A and B rings in one operation. When the alkyne is present at C13 (with no neighboring gem‐dimethyl group), the RCEDYM reaction leads to 14,15‐isotaxanes 16 a,b and 18 b with the gem‐dimethyl group on the A ring. If the alkyne is at the C11 position (and thus flanked by a gem‐dimethyl group), RCEDYM reaction only proceeds in the presence of a trisubstituted olefin at C13, which disfavors the competing diene ring‐closing metathesis reaction, to give the tricyclic core of Taxol 44.

Introduction

Taxol^® (paclitaxel), together with its derivatives Taxotere^® (docetaxel) and Jevtana^® (cabazitaxel) are the largest selling anticancer drugs of all time, with sales of over three billion USD per year for Taxotere alone in 2010.1 Originally indicated for the treatment of ovarian and breast cancers, they are now widely prescribed to treat a broad range of malignancies.2 The structures of these three compounds only differ in terms of the functionalization of the amine on the side chain and the hydroxyl groups at C10 and C7 (Scheme 1). Taxol is currently being manufactured through plant‐cell fermentation by Phyton Biotech, LLC, a DFB Pharmaceuticals Company for Bristol–Myers Squibb, whilst Taxotere and Jevtana are produced by semisynthesis from 10‐deacetylbaccatin III by Sanofi, which still requires an expensive extraction process of natural resources. There have been six total syntheses of Taxol by the groups of Holton,3 Nicolaou,4 Danishefsky,5 Wender,6 Mukaiyama7 and Kuwajima,8 as well as three formal syntheses by the groups of Takahashi,9 Nakada10 as well as Sato and Chida,11 but they all comprise at least 37 steps.12 An efficient synthesis of active taxoid analogues has yet to be achieved, because of the sterically hindered, complex and highly functionalized structure of these compounds.1

Scheme 1.

Taxol and derivatives; retrosynthesis of the ABC tricycle of Taxol featuring a ring‐closing metathesis (RCM).

A rapid synthesis of the tricyclic core of Taxol where all of the functional groups required for activity are present or in a latent form would facilitate access to a diverse array of novel taxoids with potential anticancer activity. We report here a synthetic strategy featuring a cascade ring‐closing dienyne metathesis (RCDEYM) reaction that allows access to the ABC tricyclic ring system of taxanes as well as taxane analogues that possess a novel skeleton and cannot be prepared by semi‐synthesis.13

Results and Discussion

Our initial retrosynthesis is outlined in Scheme 1. We aimed for a formal synthesis of Taxol, so we chose the intermediate 4 described by Holton during his synthesis of this natural product as our primary target.3 The A ring would be closed by a pinacol coupling between the ketones at C11 and C12 in compound 5, as previously described by Mukaiyama on a similar substrate.7 The ketone at C12 would be installed by hydration of alkyne 6. The eight‐membered B ring would be formed by a ring‐closing metathesis (RCM) reaction14 between the alkenes at C10 and C11 in compound 7. This key step was successful in our synthesis of model BC bicyclic systems of Taxol (with no hydroxyl group at C7 and a butyl side chain at C1).15 Finally, the metathesis precursor 7 would be assembled by a Shapiro reaction between hydrazone 8 and aldehyde 9. This coupling reaction has proved to be very diastereoselective on similar substrates during our previous approaches to taxoids.16

Our synthesis commenced with the preparation of aldehyde 9 (Scheme 2). Commercially available 3‐pentyn‐1‐ol was oxidized with the Dess–Martin periodinane17 (DMP) and the resulting aldehyde was subjected to a Barbier reaction with prenyl bromide under the Luche conditions18 to furnish alcohol 10 in excellent yield. Oxidation of alcohol 10 gave the corresponding ketone 11, which was submitted to trimethylsilyl cyanide in the presence of a the tertiary amine 1,4‐diazabicyclo[2.2.2]octane (DABCO) as a catalyst. The resulting cyanohydrin was reduced to the intermediate imine, which was hydrolyzed to give the racemic aldehyde (±)‐9 by exposure to silica gel. Optically active aldehyde 9 was also prepared in 99 % ee in a similar fashion19 using a chiral amine base for the cyanation reaction,20 but we chose to pursue the synthesis of the metathesis precursors with the racemic aldehyde to widen the array of taxoids generated, and to study the influence of the stereochemistry of the precursor on the RCM reaction outcome.2

Scheme 2.

Synthesis of aldehyde (±)‐9. a) DMP, CH₂Cl₂; b) Zn, NH₄Cl, prenyl bromide, 99 % (over 2 steps); c) a) DMP, CH₂Cl₂, 95 %; d) TMSCN, DABCO, CH₂Cl₂; e) DIBAL‐H, CH₂Cl₂; SiO₂ (58 % over 2 steps). DMP=Dess–Martin periodinane, TMS=trimethylsilyl, DABCO=1,4‐diazabicyclo[2.2.2]octane, DIBAL‐H=diisobutylaluminium hydride.

In order to test the key metathesis reaction, we decided to use a 7‐deoxy C ring as a coupling partner in the Shapiro reaction. It is worth noting that removal of the functional group at C7 in Taxol did not result in a significant loss of bioactivity.21 When hydrazone 12 (Scheme 3), prepared in 76 % yield from the corresponding known ketone,22 was submitted to tBuLi for the Shapiro coupling, only degradation was observed.23 We surmised that this was due to the deprotonation at the allylic position, and thus the alkene was masked as a protected primary alcohol. Enantiopure hydrazone 13 15b was treated with aldehyde (±)‐9 using conditions we had developed previously.15b To our surprise, the reaction only proceeded in 20 % yield. Several additives were screened. Addition of MgBr₂ and ZnCl₂ did not lead to any of the desired product, but we observed a dramatic increase in yield when dry CeCl₃ was stirred for 30 min with the vinyllithium reagent derived from hydrazone 13 before addition of aldehyde (±)‐9, and diols 14 a,b were obtained in 85 % combined yield after hydrolysis of the TMS ether. The reason for this difference in reactivity between the model aldehyde (butyl side chain at C1) and (±)‐9(2‐butynyl side chain at C1) is unclear.15b As had been observed previously for the model aldehydes, this reaction was highly diastereoselective, giving the trans diol compounds24 14 a and 14 b after hydrolysis of the trimethylsilyl ether. The stereochemistry of 14 a and 14 b was assigned by comparing their proton NMR spectra with those of the corresponding model Shapiro adducts possessing a butyl side chain at C1.25 Diols 14 a and 14 b were then submitted separately to trityl ether hydrolysis, elimination of the resulting primary alcohol using the Grieco protocol26 and protection of the C1‐C2 diol as the cyclic carbonate ester to furnish the metathesis precursors 15 a and 15 b in 75 % and 65 % overall yields for the four steps, respectively. No intermediate purification was required for these transformations.3

Scheme 3.

Synthesis of metathesis precursors 15 a,b. a) tBuLi, CeCl₃, THF, −78 °C; b) 1 N aq. HCl, 14 a 45 % (over 2 steps), 14 b 40 % (over 2 steps); c) Amberlyst H‐15, MeOH; d) o‐NO₂C₆H₄SeCN, PBu₃, THF; e) Im₂CO, toluene, 110 °C; f) (NH₄)₆Mo₇O₂₄⋅4 H₂O, H₂O₂, H₂O, 15 a, 75 % (over 4 steps), 15 b 65 % (over 4 steps). THF=tetrahydrofuran, Im=imidazolyl.

We first tried out the key RCM reaction on carbonate 15 a, which possesses the opposite configuration at C1 and C2 compared to Taxol. Treatment of this compound with 10 mol % of the second‐generation Grubbs precatalyst in toluene at reflux for 24 h did not lead to the desired cyclooctene, but gave tricyclic derivative 16 a instead (Scheme 4). This product resulted from an enyne metathesis reaction between the alkene at C10 and the alkyne at C13, furnishing the intermediate bicycle 16 a′, which further cyclized by a diene metathesis to give 16 a in good yield. This intermediate 16 a′ could be isolated as a 1:1 mixture with 16 a if only 5 mol % of the precatalyst was used for the reaction. The first enyne metathesis reaction was not unexpected;27 what was more surprising to us was the ease of formation of the strained tricyclic system in compound 16 a. This 14,15‐isotaxane has an unprecedented skeleton, which is very similar to that of taxane derivatives, except that the C14 and C15 carbons have swapped positions, which places the gem‐dimethyl group in the A‐ring alone. In addition, the C4 stereogenic center possesses the undesired configuration for Taxol.

Scheme 4.

Attempts at metathesis and synthesis of isotaxanes. a) 5 mol % Grubbs 2, toluene, 110 °C, 48 h, 1:1 16 a/16 a’ 30 %; b) 10 mol % Grubbs 2, toluene, 110 °C, 24 h, 16 a 62 %; c) PhLi, THF, −78 °C, 17 a 54 %, 17 b 70 %; d) 5 mol % Grubbs 2, toluene, 80 °C, 2 h, 91 %; e) 5 mol % Grubbs 2, toluene, 110 °C, 1 h, 57 %. For the structure of Grubbs 2, see Table 1.

In an effort to assess the influence of the nature of the diol protecting group on the outcome of the metathesis reaction, which was shown to be crucial for model compounds,15b the benzoate 17 a was prepared by addition of phenyllithium to the carbonate 15 a (Scheme 4). Unfortunately, benzoate 17 a did not undergo metathesis when treated with the Grubbs 2 precatalyst, but slowly decomposed.

A similar cascade dienyne metathesis reaction was observed with stereoisomer 15 b, but the reaction proceeded under milder conditions and gave tricycle 16 b in 91 % yield (Scheme 4). This time, RCM of benzoate 17 b, obtained by phenyllithium addition to 15 b, furnished isotaxane 18 b in 57 % yield, underscoring the influence of the configuration at C1 and C2 on the outcome of the metathesis reactions.15b These isotaxanes possess the undesired configuration at C1. Isotaxane 16 b was crystalline, and its X‐ray crystallographic analysis28 (Figure 1) established its tricyclic structure and confirmed the configuration of the carbonate‐bearing stereocenters at C1 and C1

Figure 1.

ORTEP (50 % probability) representation of compound 16 b.

The isotaxanes 16 a, 16 b and 18 b represent a novel class of Taxol analogues, and could be transformed into potentially active compounds. Indeed, taxanes such as tasumatrols E, F and G (Figure 2), isolated from Taxus sumatrana, do not possess the classical ABC 6,8,6‐tricyclic system of Taxol; however, they exhibit more potent activity than Taxol in vitro against four human cancer cell lines.29, 2

Figure 2.

Structures of tasumatrols E, F and G.

An easy way to circumvent the unwanted dienyne metathesis cascade is to perform the alkyne hydration before the RCM step, and this has been achieved in excellent yield (Scheme 5). Diol 19 was prepared in three steps from the Shapiro adduct 14 b in 68 % overall yield. Treatment of alkyne 19 with the Gagosz catalyst30 in the presence of water did not give the corresponding ketone but hemiketal 20.31 Fortunately, compound 20 underwent ring‐closing metathesis in 98 % yield to form the BC ring system of Taxol 21. Work is in progress for the completion of the synthesis of the tricyclic core of Taxol according to the retrosynthesis shown in Scheme 1.5

Scheme 5.

Alkyne hydration followed by RCM. a) Amberlyst H‐15, MeOH, 98 %; b) PPh₃, imidazole, I₂, 82 %; c) NaH, DMF, 84 %; d) Ph₃PAuNTf₂, THF, H₂O, 80 %; e) 5 mol % Grubbs 2, CH₂Cl₂, reflux, 1.5 h, 98 %. DMF=dimethylformamide, Tf=trifluroromethanesulfonyl.

On the other hand, we also wanted to take advantage of this very efficient metathesis cascade to synthesize the ABC tricycle of Taxol, and our revised retrosynthesis is shown in Scheme 6. The 2‐ene‐1,4‐diol unit of compound 4 would be installed by a Ti^III radical‐mediated opening of the corresponding 1,3‐diepoxide, which can be generated from the 1,3‐diene moiety at C10‐C13 of compound 22.12b A hydroboration/oxidation sequence of the C3‐C4 olefin would lead to the ketone at C4.5 Tricycle 22 would be formed by a metathesis cascade reaction from dienyne 23, where the alkyne at C11 and the alkene at C13 have swapped positions compared to those in compound 15 b. In order to direct the metathesis cascade reaction so it starts with the olefin at C10 and not with the one at C13, we elected to have a disubstituted olefin at C13, which would react more slowly with the metathesis precatalysts. It is important to note that this extra methyl group will not be present in the metathesis product 22, but will be part of the propene released after the diene metathesis reaction. Disconnection of dienyne 23 through the C2−C3 bond reveals the two precursors aldehyde 24 and hydrazone 6.

Scheme 6.

Novel retrosynthesis of taxol featuring a cascade ring‐closing dienyne metathesis (RCDEYM).

The synthesis of aldehyde 24 in its racemic form was not as straightforward as the synthesis of the corresponding aldehyde 9. It started with ester 26,32 obtained by propargylation of ethyl isobutyrate (Scheme 7). Attempts to isomerize the terminal alkyne of 26 into the internal one with potassium tert‐butoxide only resulted in degradation products. Fortunately, this isomerization reaction was successful on the corresponding acid 27, and acid 28 was obtained in 94 % yield. Addition of crotylmagnesium chloride to the corresponding Weinreb amide (compound 41, see Scheme 9 for structure) furnished a complex mixture of products, so we next turned to the crotylation of aldehyde 29. Treatment of this aldehyde with crotyl magnesium chloride in the presence of aluminum trichloride33 led to a 1:1.5 mixture of α and γ crotylation products. Fortunately, allyl transfer from 2,3‐dimethyl‐4‐penten‐2‐ol catalyzed by tin(II) triflate34 gave the desired alcohol 30 (as an inconsequential 3:1 mixture of E/Z isomers) in 76 % yield after 2 days. Oxidation of 30 with 2‐iodoxybenzoic acid (IBX) followed by homologation of the resulting ketone 31 furnished aldehyde (±)‐24 35 in good overall yield.7

Scheme 7.

Synthesis of aldehyde (±)‐24. a) KOH, H₂O, MeOH, 93 %; b) tBuOK, DMSO, 75 °C, 94 %; c) LiAlH₄, THF; d) (COCl)₂, DMSO, Et₃N, CH₂Cl₂, 76 % (over 2 steps); e) 2,3‐dimethyl‐4‐penten‐2‐ol, Sn(OTf)₂, CH₂Cl₂, 76 %; f) IBX, DMSO, THF, 93 %; g) TMSCN, ZnI₂, CH₂Cl₂, reflux; g) DIBAL‐H, hexane; SiO₂ (62 % over 2 steps). DMSO=dimethyl sulfoxide, IBX=2‐iodoxybenzoic acid.

Scheme 9.

Synthesis of aldehyde (±)‐36. a) 2,3,3‐Trimethyl‐4‐penten‐2‐ol, Sn(OTf)₂, CH₂Cl₂, 15–61 %; b) IBX, DMSO, THF, 85 %; c) TMSCN, ZnI₂, CH₂Cl₂; d) DIBAL‐H, hexane; SiO₂ (83 % over 2 steps); e) 1,2‐ethanedithiol; BF₃⋅OEt₂, CH₂Cl₂, 75 %; f) BuLi, prenyl bromide, THF, −78 °C, 99 %; g) MeI, CaCO₃, MeCN, H₂O, 83 %; h) Mg, THF, prenyl chloride, 95 %.

The dienynes 32 a,b and 34 a,b were prepared using a similar reaction sequence to the one used for compounds 15 a,b and 17 a,b, as described in the preliminary account of our work.13 Metathesis reactions of carbonates 32 a,b and benzoates 34 a,b with Grubbs 2 precatalyst did not produce tricyclic compounds, but led to the bicycles 33 a,b and 35 a,b, respectively, resulting from a simple diene RCM between the olefins at C10 and C13 (Scheme 8).13, 8

Scheme 8.

Attempts at metathesis cascade. a) 10 mol % Grubbs 2, toluene, 110 °C, 12 h, 33 a 63 %, 35 a 78 %, 33 b 68 %, 35 b 86 %.

Compound 33 a was crystalline, and its X‐ray crystallographic analysis36 (Figure 3) confirmed the configuration at C1 and C2 of the metathesis precursors 32 a and 34 a.3

Figure 3.

ORTEP (50 % probability) representation of compound 33 a.

We had assumed that in the case of compounds 15 and 17 (Scheme 4), the initial enyne methathesis (between C10 and C13) would be favored compared to the alternative diene metathesis (between C10 and C11) because it would lead to a more stable tricyclic product after subsequent diene metathesis, but it seems that in all cases the first RCM takes place with the less hindered unsaturated functional group having no neighboring gem‐dimethyl group. Since this gem‐dimethyl group is part of the Taxol skeleton, it is not possible to relieve the steric hindrance at the propargylic position in compounds 32 and 34, but another option is to increase the steric hindrance of the alkene at C13, so the undesired diene RCM is disfavored. We thus embarked on the synthesis of metathesis precursors bearing a trisubstituted olefin at C13. The synthesis of the aldehyde (±)‐36 required for their preparation is shown in Scheme 9. Prenyl transfer to aldehyde 29 from 2,3,3‐trimethyl‐4‐penten‐2‐ol was unreliable, with yields of 37 ranging from 15 to 61 %. Ketone 38 was then obtained by IBX oxidation. An umpolung synthesis of 38 was also achieved. Prenylation of dithiane 39, prepared from aldehyde 29, furnished 40 in excellent yield. Hydrolysis of the dithiane moiety gave ketone 38. However, this route was not very convenient on large scale, so as a last resort prenylation of the Weinreb amide 41 derived from acid 28 (Scheme 7) was also attempted. To our surprise, this reaction was very clean and afforded ketone 38 in 95 % yield. In this fashion, aldehyde (±)‐36 37 was obtained after homologation of 38 in 7 steps and 66 % overall yield from ethyl isobutyrate.9

Compounds 42 a,b and 43 a,b bearing a trisubstituted olefin at C13 were synthesized in the same way as compounds 32 a,b and 34 a,b as previously described, then subjected to the Grubbs 2 precatalyst in toluene at reflux (Scheme 10).13 We were disappointed to find out that carbonate 42 a and benzoate 43 a, possessing the undesired configurations at C1 and C2, furnished the bicyclic compounds 33 a and 35 a that we had already observed for the metathesis reactions of 32 a and 34 a (Scheme 8). Taxol‐like benzoate 43 b also underwent diene RCM to produce 35 b. However, Taxol‐like carbonate 42 b furnished compound 44 after RCDEYM, which corresponds to the tricyclic core of Taxol, along with the undesired bicyclic compound 33 b.13, 10

Scheme 10.

Attempts at metathesis cascade and synthesis of the ABC tricycle of taxol. a) 10 mol % Grubbs 2, toluene, 110 °C, 24 h, 33 a 79 %, 35 a 80 %, 44 45 % and 33 b 45 %, 35 b 90 %.

In order to confirm the structure of the highly strained tricyclic product 44, we converted it to the crystalline p‐nitrobenzoate derivative 45 by hydrolysis of the carbonate and acylation of the resulting secondary alcohol (Scheme 11). X‐ray crystallographic analysis of 45 38 not only confirmed the tricyclic structure, but also established without ambiguity the configurations at C1 and C2 for the Taxol‐like series of compounds.11

Scheme 11.

Synthesis and ORTEP (50 % probability) representation of p‐nitrobenzoate 45. a) 2 N aq. NaOH, 1,4‐dioxane, 80 %; b) p‐nitrobenzoyl chloride, DMAP, Et₃N, CH₂Cl₂, 80 %.

We then set to optimize the yield of the desired tricyclic compound 44. The 44/33 b ratio was the same under different concentrations ranging from 3×10⁻³ to 15×10⁻³  m,13 so all metathesis reactions were performed at 5×10⁻³  m. Toluene at reflux proved to be a better choice than 1,2‐dichloroethane at reflux (80 °C) or xylene at reflux (140 °C).13 Various precatalysts were then screened (Table 1). No reaction was observed with the less reactive Grubbs 1 precatalyst, so we tested second‐generation ruthenium complexes. The Hoveyda–Grubbs precatalyst HG2 gave an improved yield of the desired compound 44 compared to the Grubbs 2 precatalyst (69 vs. 45 %), and so did the Grela complex, which possesses a nitro substituent on the benzylidene ligand. Pleasingly, the HG2 derivative Zhan‐1B, which possesses a N,N‐dimethylsulfonamido group gave the best yield (70 %) of compound 44. It seems that RCDYEM is favored with precatalysts possessing high initiation rates.1

Table 1.

Optimization of the formation of 44.^[a]

Precatalyst	Yield of 44 [%]	Yield of 33 b [%]
Grubbs 1	0[b]	0
Grubbs 2	45	45
Hoveyda–Grubbs 2	59	38
Grela	55	45
Zhan‐1 B	70	20

[a] Reaction conditions: a) 10 mol % precatalyst, toluene, 5×10⁻³  m, 110 °C. [b] No reaction was observed.

The different ratios observed with the Hoveyda–Grubbs‐type precatalysts cannot be easily rationalized. Indeed, metathesis of substrate 42 b with any precatalyst will result in the same carbene (Scheme 12). This intermediate should then lead to the same ratios of 44 and 33 b after cyclization, releasing the same isopropylidene catalyst. The only difference between the reactions is the ligand released after the first catalytic cycle, which could recombine with the isopropylidene catalyst to reform the precatalyst. To probe the influence of the ligand, a metathesis experiment was run with 10 mol % of the Hoveyda–Grubbs 2 precatalyst and 300 mol % of the corresponding ligand, but the observed ratio of 44 and 33 b was very similar to the one observed without any added ligand.12

Scheme 12.

Metathesis scheme. Conditions: toluene, reflux.

Attempts to convert bicyclic product 33 b to the desired tricycle 44 by heating it in toluene at reflux in the presence of the Grubbs 2 or Zhan‐1B precatalyst were unsuccessful, even under microwave conditions. When 10 mol % of precatalyst was used, only 33 b was recovered, and with 100 mol % of precatalyst decomposition occurred. Ring opening of 33 b in the presence of ethylene was not considered, because it would lead to a carbene unsubstituted at C13, carbene’ (Scheme 13), which would undergo diene metathesis preferentially. In an effort to regenerate the carbene with a trisubstituted olefin at C13, bicycle 33 b was submitted to the above conditions in the presence of 2‐methyl‐2‐butene, the reagent which Grubbs and co‐workers have employed for the synthesis of trisubstituted olefins from their terminal homologues by cross metathesis,39 but to no avail (Scheme 13). These results seem to indicate that the formation of compound 33 b is not reversible, and that the metathesis reactions leading to 33 b and 44 are under kinetic control.13

Scheme 13.

Equilibration attempts.

Conclusions

In summary, we have synthesized Taxol analogues with an unprecedented skeleton as well as the tricyclic core of Taxol in a very efficient fashion. The key step in these syntheses is a cascade ring‐closing dienyne metathesis (RCDEYM) reaction, leading to either 14,15‐isotaxane tricyclic ring systems or the tricylic ring system of Taxol in one operation from simple precursors, by judicious choice of the position of the alkyne (C13 for isotaxanes or C11 for taxanes). Furthermore, in the case of the taxane synthesis, we have shown that we can direct the course of the crucial metathesis reaction by adding a temporary methyl substituent to the olefin at C13, which does not appear in the structure of the tricycle. Calculations rationalizing the different outcomes of the metathesis reactions of compounds 42 a,b and 43 a,b, which strongly depend on the stereochemistry and the protecting group of the diol at the C1 and C2 positions in the metathesis precursors, will be reported in due course.

Experimental Section

All experimental details can be found in the Supporting Information. The material includes compound characterization, crystal structures of 16 b and 33 a, and copies of spectra for all new compounds.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

C. Ma, A. Letort, R. Aouzal, A. Wilkes, G. Maiti, L. J. Farrugia, L. Ricard, J. Prunet, Chem. Eur. J. 2016, 22, 6891.