Keyword

22.1 Introduction

Marine red algae of the genus Laurencia have produced the thyrsiferol family natural products, triterpene polyethers biogenetically derived from squalene, and the family possess a variety of biological activities [1]. In 1978, Munro group isolated the first member thyrsiferol (1) from Laurencia thyrsifera [2], and afterward, dehydrothyrsiferol (2) [3], thyrsiferyl 23-acetate (3) [4], venustatriol (4) [5], (Fig. 22.1) and total about 30 congeners have been reported so far [6]. These compounds show eminent growth inhibitory activities on P388 murine leukemia cell lines (IC50 = 0.47–17 nM) [1]. The structures 1–4 have been determined based on NMR spectroscopy, X-ray crystallography, and chemical conversion, and have a bromine-bearing tetrahydropyran (THP) ring attached to C7 of a dioxabicyclo[4.4.0]decane framework (ABC ring system), common to the thyrsiferol family, and various alkyl substituents at C14. In the THP C ring, a twist-boat form was observed due to undesirable steric repulsion by C10- and C14-substituents in the chair conformation. The absolute configuration of 4 could successfully be elucidated by X-ray analysis; however, the relative and absolute configurations of the thyrsiferol family cannot easily be determined even by modern spectroscopic analysis, because there are acyclic tetrasubstituted chiral centers (red arrows) and the remote relationship between the stereoclusters ABC and D ring moieties. These examples show the limitations in modern NMR technology for the structure determination of complex natural products. In these cases, asymmetric total synthesis demonstrates its power [7, 8].

Fig. 22.1
A set of 5 chemical structures explains the total synthesis, stereochemical relationships, chemical conversions, configurations, and more for thyrsiferol, dehydrothyrsiferol, thyrsiferol 23-acetate, venustatriol, and a possible stereo structure of isodehydrothyrsiferol labeled 5 a, and a twist-boat confrontation of C ring.

Structures of the representative thyrsiferol family 1–4 and isodehydrothyrsiferol

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These unique structures and prominent biological properties, combined with the entire stereostructure elucidation, have promoted studies on total synthesis of the thyrsiferol family by the synthetic community. The first total syntheses of thyrsiferol and venustatriol were achieved by Shirahama and co-workers in 1988 [9,10,11], and subsequently Corey and Ha reported the total synthesis of venustatriol [12]. The absolute configuration of thyrsiferol (1) was determined by the asymmetric total synthesis of 1 by Shirahama et al., resulting in determination of the absolute configuration of dehydrothyrsiferol (2) which had chemically been converted into 1 [3]. Shirahama et al. have also reported the chemical synthesis and the absolute stereochemistry of thyrsiferyl 23-acetate (3) (IC50 = 0.47 nM on P388 cells), an inhibitor of serine/threonine protein phosphatase 2A [13], in 1988 [11, 14]. In 2000, the total syntheses of 1 and 3 have been reported by González and Forsyth [15, 16].

In 1996, Norte and co-workers reported a minor metabolite isodehydrothyrsiferol isolated from acetone extracts of Laurencia viridis, together with a major constituent dehydrothyrsiferol (2) [17, 18]. The compound exhibits cytotoxic activity with IC50 = 17 nM on P388 cells. The NMR analysis revealed the stereostructure of the ABC skeleton common to the thyrsiferol family and the relative configuration around the D ring moiety, but the relative configuration between the remote stereoclusters ABC and D moieties and the absolute stereochemistry of the compound remained undetermined. The structure of 5a represents one of possible stereostructures for isodehydrothyrsiferol. Although the absolute stereostructure of the ABC skeleton of isodehydrothyrsiferol was deduced to be of course the same as that of 2, there was no experimental evidence. Thus, we planned to determine the entire stereostructure of isodehydrothyrsiferol through the asymmetric total synthesis.

22.2 First Generation Retrosynthetic Analysis

The retrosynthetic analysis of isodehydrothyrsiferol is shown in Scheme 22.1. The disconnection at the C15–C16 bond predicted a convergent and effective strategy, wherein the ABC skeleton 6 and a borane unit 7 or its enantiomer ent-7 are linked using a cross-coupling reaction developed by Suzuki and Miyaura [19], due to the undetermined relative configuration between both segments. The A ring of 6 could be formed by challenging 6-endo bromoetherification of bishomoallylic alcohol 8. The fused BC ring skeleton of 8 would be constructed from triene 9 via two 6-exo oxacyclizations of trishomoallylic epoxy alcohols. The triene 9 was disconnected to geranyl phenyl sulfide (11) and terminal epoxide 10, which would be prepared from commercially available methyl tiglate (13) via epoxy alcohol 12. The coupling partner D ring 7 could be derived from commercially available geranyl acetate (15) through 6-endo oxacyclization of epoxy alcohol 14.

Scheme 22.1
A reaction schematic for Suzuki-Miyaura cross-coupling and convergent synthesis processes leads to 6-endo bromoetherification with multiple compounds and labeled reaction pathways. It includes compounds numbered 11 to 15, along with their respective transformations involving functional groups like O T B S, C O 2 M e, and more.

First generation retrosynthetic analysis of isodehydrothyrsiferol. TES = triethylsilyl, TBS = t-butyldimethylsilyl, Piv = pivaloyl, MOM = methoxymethyl

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22.3 Toward Construction of the ABC Skeleton

First, the synthesis of the ABC skeleton 6 was begun according to the aforementioned retrosynthetic analysis. The known diester 17 [20] was prepared by homocoupling of silyl ketene acetal 16, which was obtained from commercially available methyl tiglate (13) by dienolate formation and the trapping with TMSCl, with TiCl4 (Scheme 22.2). TBS protection of the known diol 18 [21] transformed by reduction of diester 17 afforded the desired monosilyl ether 19a [22, 23] along with recovered 18 and disilyl ether 19b, which was returned to diol 18. A catalytic Sharpless asymmetric epoxidation [24] of 19a provided the known epoxy alcohol 12 [22, 23] in 90% yield (98% ee). A Ti(Oi-Pr)4-mediated epoxide-opening reaction [25] of 2,3-epoxy alcohol 12 regio- and stereoselectively gave pivalate 20, which was converted into epoxide 10. The lithiation of geranyl phenyl sulfide (11) [22], which was prepared from commercially available geraniol [26], and addition of epoxide 10 to the anion yielded sulfide 21 as a diastereomeric mixture at C8. The resulting sulfide 21 was reduced to 22 with metallic sodium [11], and the diol 22 was treated with triethylsilyl chloride to selectively afford TES-protected 9.

Scheme 22.2
A chemical synthesis diagram for an A B C ring system 28 outlines a series of reactions with various reagents and conditions, including L D A in T H F at negative 78 degrees Celsius, and T B H P with D E T and molecular sieves in dichloromethane. It lists several intermediates and the yields of specific steps.

Attempt to synthesize ABC ring system 28. LDA = lithium diisopropylamide, THF = tetrahydrofuran, TMS = trimethylsilyl, rsm = recovered starting material, TBHP = t-butyl hydroperoxide, DET = diethyl tartrate, MS = molecular sieves, ee = enantiomeric excess, Ms = methanesulfonyl, TMEDA = N,N,N′,N'-tetramethylethylenediamine, acac = acetylacetonate, TBAF = tetrabutylammonium fluoride, DMSO = dimethyl sulfoxide, NBS = N-bromosuccinimide, HFIP = 1,1,1,3,3,3-hexafluoro-2-propanol

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The Sharpless oxidation of hydroxy alkene 9 found by Shirahama et al. provided epoxy alcohol 23 in a diastereoselective manner [27] via more stable transition state B rather than more unstable transition state A suffering from an allylic 1,3-strain [28]. MOM protection of 23 and subsequent deprotection of all silyl ethers gave hydroxy epoxide 24, which was subjected to basic conditions to furnish the desirable tetrahydropyranyl B ring 25 n 83% yield and a 6-exo selective manner. After deprotection of the MOM group, the allylic alcohol 26 was treated with a stoichiometric Katsuki-Sharpless conditions [29] at a low temperature and then the temperature was stepwise raised to room temperature and reflux, successfully constructing the THP C ring, that adopts a twist-boat conformation, through titanium-assisted 6-exo oxacyclization such as C [11, 25]. When this reaction was catalytically performed, the starting material 26 was not completely consumed to furnish a low yield of 27 together with recovered 26 and an epoxy alcohol intermediate corresponding to C. Oxidative cleavage of the vicinal diol 27 afforded ketone 8, and after constructing the A ring, we attempted to confirm the stereostructure of the expected product 28, an authentic sample reported by Shirahama et al. [11].

Upon subjection of bishomoallylic alcohol 8 to the optimal conditions (NBS in HFIP) [30], the desired 6-endo bromoetherification proceeded, but epimerization at C14 also occurred under the conditions, giving 14-epi-28 as the product. Our synthetic compound 14-epi-28 with C14-H at 4.07 ppm (dd, J = 12.1, 3.0 Hz) was not identical to the reported data of 28 with C14-H at 3.99 ppm (dd, J = 6.6, 2.2 Hz). The unfavorable 1,3-diaxial interaction in 28 would lead to the epimerization at C14 under our conditions, employing a polar and protic solvent HFIP (pKa = 9.3) [31]. This process occurs through an enol intermediate 29, resulting in the formation of 14-epi-28, where such interaction is absent (Fig. 22.2). Thus, it was found that the ketone α-C14-H of the THP C ring tends to easily epimerize in the 6-endo bromoetherification and the bromination yield is low; therefore, we decided to carry out the A ring formation at a final stage of the total synthesis.

Fig. 22.2
A chemical reaction sequence transforms structure 28 with C r, O, C, H, C F 3, and double-bond O atoms, including a 1, 3-diaxial interaction with structure 14-epi-28 with a B r, H, M e, C, H, O, and double-bond O atoms, through intermediate structure 29, with B r, O, H, and C F 3 atoms.

Possible mechanism for epimerization at C14

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22.4 Second Generation Retrosynthetic Analysis and Synthesis of BC Ring System

The second generation retrosynthetic analysis of isodehydrothyrsiferol is depicted in Scheme 22.3. The bromoetherification of hydroxy alkene 30 would finally form the bromine-containing tetrahydropyranyl ring. The bond formation between C15 and C16 would convergently produce the penultimate 30 via a cross-coupling reaction by Suzuki and Miyaura using enol phosphate 31a or enol triflate 31b and borane 7 or ent-7. Practically, enol phosphate 31a and enol triflate 31b were derived from ketone 8 via kinetic enolate formation followed by phosphorylation and trifluoromethanesulfonylation [32], respectively, after MOM protection of 8.

Scheme 22.3
A reaction schematic of Suzuki-Miyaura cross-coupling and 6-endo bromoetherification processes leads to convergent synthesis with multiple compounds and labeled reaction pathways. It includes compounds numbered 5 a, 30, 31 a, and 31 b along with their respective transformations involving L D A, Comins reagent, M O M C I reagent, and more.

Second generation retrosynthetic analysis of isodehydrothyrsiferol and synthesis of BC ring system 31. LHMDS = lithium hexamethyldisilazide, Tf = trifluoromethanesulfonyl

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22.5 Synthesis of D Ring

The synthesis of alkene 39 required as a precursor of borane 7, a cross-coupling partner for 31, began with Sharpless asymmetric dihydroxylation [33, 34] of commercially available geranyl acetate (15) with AD-mix-β to afford the known diol 33 [35] in 93% yield and 98% ee (Scheme 22.4). Selective TES protection of the secondary hydroxy group and subsequent MOM protection provided acetate 34, and after deacetylation of 34 the resulting allylic alcohol was treated with catalytic Sharpless oxidation conditions to yield epoxy alcohol 35. Parikh-Doering oxidation [36] of alcohol 35, Wittig olefination of aldehyde 36, and removal of the TES ether gave bishomoallylic epoxide 14, which was utilized in the next reaction without purification because of the instability. According to the reaction conditions of the 6-endo selective cyclization in a vinylic epoxide substrate similar to 14 reported by Hioki and co-workers [37], the crude vinylic epoxide 14 was treated with CSA in CH2Cl2 at – 78 °C to furnish the desired 6-endo THP 38a in 61% yield over 2 steps in addition to byproduct 38b. After chromatographic separation of products 38a and 38b, MOM ether 38a was deprotected and the diol was reprotected as a TES ether to afford the desirable alkene 39.

Scheme 22.4
A reaction schematic. It begins with compound 15 followed by compounds 34 to 37, 38 a and b and 39.

Synthesis of D ring 39. CSA = camphorsulfonic acid, DMAP = N,N-dimethyl-4-aminopyridine

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22.6 Examination of the Suzuki–Miyaura Cross-Coupling

First, we tried to investigate the conditions for Suzuki–Miyaura cross-coupling using manageable and stable enol phosphate 31a [38]. The results are given in Table 22.1. The coupling reaction between phosphate 31a and borane 7, which was in situ generated form alkene 39 with a large excess of 9-BBN [39], was carried out using Pd(PPh3)4 catalyst to only afford a complex mixture including the starting material 31a (entry 1). The addition of Ph3As gave the same result (entry 2). Since it seemed that the starting material 31a remained in the complex mixture, we felt the oxidative addition to enol phosphate 31a did not occur. Although we used Pt-Bu3 and Bu3P as more electron donating ligands and increased an amount of Pd catalysts, the same results were obtained (entries 3–6). Further the reaction temperature was elevated, but there was no effect (entries 7–9). Therefore, we decided to perform the coupling reaction using intractable but more reactive enol triflate 31b.

Table 22.1 Examination of Suzuki–Miyaura cross-coupling using enol phosphate 31aa
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The results using enol triflate 31b are indicated in Table 22.2. Although three kinds of Pd catalysts were tested in the presence of Cs2CO3 at room temperature, a complex mixture was only obtained (entries 1–3). Considering the instability of enol triflate 31b, crude 31b without purification was used but the results were not improved (entries 4 and 5). Referring to Jamison’s conditions [40] in the similar cross-coupling, we conducted the reaction with 30 mol% of Pd(dppf)Cl2 at 50 °C once again and were very much delighted to be able to obtain the desired coupling product 40 despite a low yield (entry 6). To increase the stability of the palladium catalyst in the reaction system [19], lithium bromide was added to the reaction and consequently the increase of the yield was observed (entry 7). The addition of lithium chloride further increased the yield (entry 8). For the purpose of inhibiting reductive detriflation of 31b, in addition to lithium chloride triphenylarsine [41] was also added to provide the coupling product 40 in 66% yield (entry 9). Thus, we could find the optimized conditions for the Suzuki–Miyaura cross-coupling of triflate 31b and borane 7.

22.7 Examination of 6-endo Bromoetherification and Synthesis of the Target Structure 5a

For final construction of the bromine-containing tetrahydropyranyl ring, MOM and two TES groups of the coupling product 40 were deprotected with a Brønsted acid to yield triol 30 (Scheme 22.5). In previous total syntheses of the thyrsiferol family, all the construction of the A ring has been carried out by bromoetherification (TBCO in MeNO2) of bishomoallylic alcohols such as 30 [11, 12, 16]. In those reactions, major products were undesirable 5-exo-cyclized THFs and the desired 6-endo THPs were minor products. Therefore, we investigated the bromoetherification reaction using synthetic intermediate 27 before the final step.

Scheme 22.5
A chemical reaction sequence illustrates the transformation of molecule 40 to molecule 30, leading to bromoetherification. It includes organic compounds, with structural formulas, arrows indicating reaction progression, and text annotations describing reaction conditions and yields. The minor output is 6-endo 3-epi-T H P and the major output includes 5-exo T H F.

Synthesis of bishomoallylic alcohol 30 and previous selectivities of bromoetherification for substrates such as 30

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The results are indicated in Table 22.3. The same conditions as those used for 8 afforded 13% yield of the desired 6-endo THP 41a along with a mixture of 5-exo THFs 41b and 41c in 18% yield (entry 1). The conditions (reagent A) reported by Gulder et al. [42] slightly improved the yield of 41a (26%), but many THFs were produced (entry 2). Next, the conditions using NBS and a catalytic amount of thiourea 42 (reagent B) by Sakakura et al. [43] were tested. The reactions in CH2Cl2 resulted in at most 19% yield of 41a together with 5-exo 41b and 41c as major products (entries 3–5). The reactions in different solvents gave similar results (entries 6 and 7). Next, we examined the reaction utilizing bromodiethylsulfonium bromopentachloroantimonate (BDSB) as a brominating reagent developed by Snyder and co-workers [44, 45]. The original conditions (reagent C) in MeNO2 provided 41a in 30% yield in addition to 41b and 41c in 30% and 9% yields, respectively (entry 8), with the best 6-endo:5-exo ratio ever achieved. Other solvents were also examined, but the circumstances were not improved (entries 9–11). Although unsatisfied, we tried the bromoetherification by BDSB for bishomoallylic alcohol 30.

Table 22.2 Examination of Suzuki–Miyaura cross-coupling using enol triflate 31b
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Table 22.3 Examination of bromoetherification using bishomoallylic alcohol 27
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The bromoetherification of 30 by BDSB in MeNO2 predominantly afforded the desired 6-endo-cyclized compound 5a in 36% yield, in addition to 5-exo-cyclized byproduct 43 (20%) (Scheme 22.6). This is the first example in that the 6-endo cyclization predominated over the 5-exo one on the occasion of the A ring formation in the total syntheses of the thyrsiferol family. The stereochemistries of synthetic compounds 5a and 43 including synthetic intermediates 27 and 38a have unambiguously been determined by their NOESY spectra. The regio- and diastereoselectivity in the bulky BDSB-mediated bromoetherification of bishomoallylic alcohol 30 could be explained as follows. The attack of BDSB to the double bond between C2 and C3 of 30 from the Re-face could reversibly generate bromonium ion intermediates. In that time products, 5a and 43 would be formed through 6-endo chair-like TS D and 5-exo TS E with similar stability, respectively. On the other hand, the Si-face attack could reversibly generate bromonium ion intermediates as well. In that time, 6-endo chair-like TS F with repulsive 1,3-diaxial interaction or the strained boat-like TS G leading to 3-epi-5a and 5-exo TS H with steric repulsion between the blue hydrogen and methyl leading to 3-epi-43 would be less stable than D and E without such strain and blue steric repulsion. Therefore, bromonium ion intermediates generated by the Si-face attack would return to the starting material 30. The polar nitromethane solvent would be useful to stabilize the ionic reagent and bromonium ion intermediates.

Scheme 22.6
A set of chemical reaction pathways illustrates the transformation of reactant molecule 30 to the final product molecule 5 a through intermediate compounds, reactions, and conditions. The labels include steric repulsion, boat-like transition state, chair-like transition state, B D S B, Si-face, Re-face attack, and more.

Synthesis of target structure 5a and regio- and diastereoselectivity in the BDSB-mediated bromoetherification. TS = transition state

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Unfortunately, the NMR spectra (1H and 13C) of compound 5a did not coincide with those of authentic isodehydrothyrsiferol [17], but this was predictable enough because 5a only represented one of possible stereostructures for isodehydrothyrsiferol. Thus, these circumstances prompted us to synthesize another diastereomeric compound 5b, wherein the absolute configuration of D ring is opposite to that of 5a.

22.8 Total Synthesis and Complete Assignment of the Stereostructure of Isodehydrothyrsiferol

The D ring borane ent-7 required for the synthesis of another diastereomer 5b was brought from the starting material 15 through the same sequence of reactions as those of 7, except for AD-mix-α for the known diol ent-33 [35] and a chiral ligand d-(–)-DET for epoxy alcohol ent-35 (Scheme 22.7). The Suzuki–Miyaura cross-coupling reaction of borane ent-7 and triflate 31b afforded a coupling product 45 in 65% yield. Removal of protective groups in 45 under acidic conditions and subsequent bromoetherification provided the desired 6-endo diastereomer 5b and 5-exo byproduct 47 in each 36% yield. Expectedly, the NMR spectra (1H and 13C) of compound 5b were identical to those of authentic isodehydrothyrsiferol [17]; however, surprisingly, the signs in optical rotations of compound 5b, [α]27D – 7.6 (c 0.25, CHCl3), and the authentic isodehydrothyrsiferol, [α]25D + 6.5 (c 0.23, CHCl3) [17], were the reverse to each other.

Scheme 22.7
A complex chemical reaction sequence illustrates the transformation of molecule 15 to the total synthesis of isodehydrothyrsiferol e n t-5 b through various intermediate processes. They include relative and absolute configurations, different conditions, and reaction progressions.

Total synthesis of isodehydrothyrsiferol (ent-5b) and its enantiomer 5b and the absolute configuration of ent-5b

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This fact claims the correct absolute stereostructure of the natural product has to be ent-5b enantiomeric to 5b. To confirm these findings, the stereostructure ent-5b was synthesized in the same way as that of 5b from borane 7 and triflate ent-31b, which was prepared from allylic alcohol 19a via Sharpless asymmetric epoxidation using d-(–)-DET for the known epoxy alcohol ent-12 [46] and triol ent-27. The NMR spectra (1H and 13C) and the optical rotation, [α]24D + 5.9 (c 0.24, CHCl3), of compound ent-5b were consistent with those of authentic isodehydrothyrsiferol. Thus, we have accomplished the asymmetric chemical synthesis and total assignment of the relative and absolute configurations of isodehydrothyrsiferol, a new member of the thyrsiferol family [47].

It has been found that the absolute configuration of the ABC ring system of isodehydrothyrsiferol (ent-5b) is opposite to that common to the other congeners 1–4 through its asymmetric total synthesis. This phenomenon we call a phenomenon of enantiodivergence [48] in the structure common to congeners is very rare in natural products [49] and greatly surprised us, because the identical Laurencia viridis produced isodehydrothyrsiferol (ent-5b) and dehydrothyrsiferol (2) [18]. We thought about these facts as follows.

Okino group has proposed one of the key enzymes responsible for the production of brominated compounds from marine red algae of the genus Laurencia is vanadium-dependent bromoperoxidases (VBPOs) [50, 51], which bring about the generation of a bromocationic species from hydrogen peroxide and bromide [52]. Therefore, VBPO enzymes seem to be related to the biogenesis of the thyrsiferol congeners produced by the genus Laurencia. On the basis of their biogenetic considerations mentioned by Shirahama [11] and Fernández [1], the biogenetic pathway of ent-5b and 2 is proposed through the epoxide-opening cascade reaction from pentaepoxide 49 triggered by the VBPO-generated bromocation, although the timing of each cyclization is unclear (Scheme 22.8). The bromonium intermediate I would be generated in a major path via the Re-face attack of the bromocation to the 2,3-alkene in pentaepoxide 49, which is enantioselectively derived from squalene via squalene tetraepoxide 48 proposed as a plausible precursor for many triterpenoids [1, 11, 53, 54]. Dehydrothyrsiferol (2), a major metabolite from Laurencia viridis, would be biosynthesized through the mode of cyclization and addition of water at C15 and C23, as shown in the intermediate I, and subsequent dehydration at the C15 position of the resulting thyrsiferol (1). In a minor path, bromohydrin 50 would be generated via the Si-face attack of the bromocation in 49 [55] and subsequent addition of water at C2 in the bromonium intermediate J. Isodehydrothyrsiferol (ent-5b), a minor metabolite from Laurencia viridis, would be biosynthesized through the mode of cyclization and addition of water at C23, as shown in the bromohydrin 50, and subsequent dehydration at the C15 position of the resultant compound 51.

Scheme 22.8
A reaction schematic. It begins with the treatment of squalene with epoxidase to obtain compounds 48 to 50, followed by thyrsiferol and a minor metabolite isodehydrothyrsiferol.

Hypothetical epoxide-opening cascade biogenesis of isodehydrothyrsiferol (ent-5b) and dehydrothyrsiferol (2) initiated by VBPO

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22.9 Conclusion

In this contribution, the enantioselective chemical synthesis of a marine bromotriterpenoid isodehydrothyrsiferol, a member of the thyrsiferol family, has been achieved, featuring two 6-exo oxacyclizations of trishomoallylic epoxy alcohols (BC rings), 6-endo oxacyclization for the D ring formation, and 6-endo bromoetherification for the A ring construction. The total synthesis enabled complete assignment of the relative and absolute configurations depicted in ent-5b for the undetermined stereostructure of isodehydrothyrsiferol and revealed that the absolute configuration of the ABC ring system is opposite to that common to the other congeners 1–4 from the same red algae. In addition, such enantiodivergency also occurred between dehydrothyrsiferol (2) and isodehydrothyrsiferol (ent-5b) originating from the identical red alga Laurencia viridis. It is generally described in textbooks that enzymes precisely recognize substrates and enantio- or diastereoselectively catalyze each reaction; however, these facts prove an enantiodivergent phenomenon can occur in spite of natural products originating from a single species. There would be no these findings without asymmetric chemical synthesis.