Chemical Synthesis of the GHIJK Ring System and Further Experimental Support for the Originally Assigned Structure of Maitotoxin

Accepting the proposal: The originally proposed structure of maitotoxin has recently come under scrutiny based on biosynthetic and computational considerations. A newly synthesized ring framework corresponding to the GHIJK ring domain (see structure) of the molecule provided, through 13C NMR spectroscopic comparisons, strong experimental support for the originally proposed structure. Maitotoxin is a notorious marine neurotoxin that possesses unsurpassed molecular size and toxicity compared to any other known secondary metabolite.1–4 In a recent essay5 we provided computational support for the originally proposed structure2–4 of maitotoxin (1, Scheme 1), whose stereochemistry at the JK ring junction was questioned on the basis of rational biosynthetic considerations.6 Herein we provide further experimental support of the originally assigned structure of maitotoxin through chemical synthesis and NMR spectroscopic analysis of GHIJK ring system 2 (Scheme 1), a fragment that corresponds to the GHIJK domain of the naturally occurring molecule. Originally proposed structure of maitotoxin (1) and the targeted GHIJK ring system 2. Representing the relevant region of maitotoxin, 2 was targeted for synthesis with the intention of comparing its 13C NMR spectral data to those of the naturally occurring substance, an exercise of well-recognized diagnostic value in stereochemical assignments of complex molecules.4, 7 Concurrent with this objective, we had in mind the development of synthetic strategies suitable for an eventual total synthesis of larger fragments of maitotoxin for biological investigations. The devised synthetic plan for the construction of the GHIJK ring system 2 required the coupling and elaboration of fragments 13 (Scheme 2) and 33 (Scheme 5) to the targeted structure. Scheme 2 depicts the enantioselective construction of the G-ring intermediate 13 from prochiral furan derivative 3 and features a Noyori reduction with catalyst 68 as a means to introduce chirality into the emerging molecule. Thus, lithiation (nBuLi) of furan 39 in THF at 0 °C followed by the addition of Weinreb amide 410 at −78 °C afforded ketone 5 (91 % yield), which was then reduced asymmetrically with the (S,S)-Noyori catalyst 6 (5 mol %) in a 5:2 mixture of formic acid/triethylamine at 25 °C (94 % yield, >95 % ee) to afford, after selective pivaloate protection of the primary hydroxy group (PivCl, 2,6-lut., 92 % yield), chiral furan 7. An Achmatowicz rearrangement11 was then initiated by the action of NBS in the presence of NaOAc and NaHCO3 in aqueous THF to afford the corresponding hemiacetal, which proved quite unstable and was, therefore, reduced immediately with Et3SiH in the presence of BF3⋅Et2O in MeCN to pyran 8, obtained in 60 % overall yield as a single stereoisomer. The enone moiety within pyran 8 was then reduced selectively (β isomer) under Luche conditions12 (NaBH4, CeCl3⋅7 H2O, 94 % yield) to the corresponding allylic alcohol, which was protected as the PMB ether (PMBOC(NH)CCl3, La(OTf)3, 92 % yield) and epoxidized with mCPBA to afford epoxide 9 as a single isomer in 75 % yield. Treatment of the latter compound with Ti(OBn)4 in toluene at 100 °C led to regioselective opening of its epoxide moiety and cleavage of its pivaloate ester to afford, after selective acetylation (AcCl, 2,6-lut.), hydroxy acetate 10 in 73 % overall yield. The obligatory inversion of stereochemistry of the secondary hydroxy group was then carried out within compound 10 by an oxidation/reduction sequence [(COCl)2, DMSO, Et3N; NaBH4] that afforded, after deacetylation with K2CO3 in MeOH, the desired diol 11 in 95 % overall yield. With all four asymmetric centers set in the desired configuration for ring G, diol 11 was then selectively converted into the primary iodide in 89 % yield through the action of iodine in the presence of Ph3P and imidazole. The remaining secondary hydroxy group was then protected as a TES ether (TESOTf, 2,6-lut., 93 % yield). At this point we were forced to call upon selenide 12 to serve as a precursor to the targeted ring G fragment 13 because of the failure of the corresponding iodide to furnish the desired elimination product in more than 40 % yield under a variety of basic conditions. Thus, displacement of the iodide from the last intermediate of the sequence (PhSeSePh, NaBH4) led to selenide 12 which was converted into 13 in 86 % overall yield by exposure to O3 at −78 °C followed by heating (80 °C) in benzene containing excess iPr2NH. Construction of G ring system 13. Reagents and conditions: a) 3 (2.5 equiv), nBuLi (2.5 M in hexanes, 2.5 equiv), THF, 0 °C, 30 min; then −78 °C, 4 (1.0 equiv), 2 h, 91 %; b) 6 (0.05 equiv), HCO2H/Et3N (5:2), 25 °C, 72 h, 94 % (>95 % ee); c) PivCl (1.4 equiv), 2,6-lut. (3.0 equiv), CH2Cl2, −78 °C, 1 h, 92 %; d) NBS (1.0 equiv), NaOAc (1.0 equiv), NaHCO3 (2.0 equiv), THF/H2O (3:1), 0 °C, 1 h; e) Et3SiH (5.0 equiv), BF3⋅Et2O (2.0 equiv), MeCN, 0 °C, 30 min, 60 % over two steps; f) CeCl3⋅7 H2O (0.5 equiv), NaBH4 (1.0 equiv), MeOH/CH2Cl2 (1:1), −10 °C, 10 min, 94 %; g) PMBOC(NH)CCl3 (1.5 equiv), La(OTf)3 (0.05 equiv), PhMe, 25 °C, 30 min, 92 %; h) mCPBA (5.0 equiv), CH2Cl2, 25 °C, 48 h, 75 %; i) Ti(OBn)4 (3.0 equiv), PhMe, 100 °C, 28 h, 77 %; j) AcCl (1.1 equiv), 2,6-lut. (3.0 equiv), CH2Cl2, −78 °C, 1 h, 95 %; k) (COCl)2 (3.0 equiv), DMSO (5.0 equiv), CH2Cl2, −78 °C, 2 h; then Et3N (7.0 equiv), 0 °C, 1 h; l) NaBH4 (4.0 equiv), MeOH, 0 °C, 15 min; then K2CO3 (5.0 equiv), 25 °C, 16 h, 95 % over two steps; m) I2 (2.0 equiv), Ph3P (2.0 equiv), imid. (2.0 equiv), THF, 25 °C, 2 h, 89 %; n) TESOTf (2.0 equiv), 2,6-lut. (3.0 equiv), CH2Cl2, 0 °C, 30 min, 93 %; o) (PhSe)2 (1.1 equiv), NaBH4 (2.0 equiv), EtOH/THF (5:3), 0 to 25 °C, 3 h; p) O3, CH2Cl2/MeOH (5:1), −78 °C, 10 min; q) iPr2NH:C6H6 (1:10), 80 °C, 3 h, 86 % over three steps. Bn=benzyl, TBS=tert-butyldimethylsilyl, THF=tetrahydrofuran, Piv=trimethylacetyl, lut.=lutidine, NBS=N-bromosuccinimide, PMB=para-methoxybenzyl, mCPBA=meta-chloroperbenzoic acid, DMSO=dimethyl sulfoxide, imid.=imidazole, TES=triethylsilyl, Tf=trifluoromethanesulfonyl. The G-ring coupling partner 13 was also synthesized through an alternative route starting from known tetraol 14 (available in seven steps from methyl-D-glucopyranoside)13 as shown in Scheme 3. Protection of the 1,3-diol system involving the primary alcohol within 14 (C-1, C-3) was accomplished with PhCH(OMe)2 and CSA (cat.) in 74 % yield. The less hindered secondary hydroxy group of the molecule (C-4) was then protected as the TBS ether (TBSCl, imid.), and the remaining free hydroxy group was converted into its PMB ether (PMBCl, TBAI, NaH) to give to the fully protected pyran system 15 (70 % yield for the two steps). Treatment of the latter intermediate with TBAF then effected removal of both its silyl groups, thereby furnishing the corresponding diol in 83 % yield. The secondary hydroxy moiety was then inverted through an oxidation/reduction sequence [(COCl)2, DMSO, Et3N; NaBH4, 86 % yield over two steps] to provide the desired diol 16. Benzylation of 16 (BnBr, TBAI, NaH, 88 % yield) followed by removal of the benzylidene group (TsOH, MeOH) then provided 1,3-diol 11 (85 % yield), which was converted into coupling partner 13 as described in Scheme 2. Alternate construction of G ring system 13. Reagents and conditions: a) PhCH(OMe)2 (1.5 equiv), CSA (0.02 equiv), 4-Å MS, CH2Cl2, 25 °C, 2 h, 74 %; b) TBSCl (2.0 equiv), imid. (3.0 equiv), DMF, 25 °C, 18 h, 88 %; c) PMBCl (10 equiv), TBAI (0.5 equiv), NaH (6.0 equiv), DMF, 25 °C, 18 h, 80 %; d) TBAF (5.0 equiv), THF, 25 °C, 18 h, 83 %; e) (COCl)2 (5.0 equiv), DMSO (10 equiv), CH2Cl2, −78 °C, 1 h; Et3N (20 equiv), 0 °C, 30 min; f) NaBH4 (2.2 equiv), MeOH, 0 °C, 86 % over two steps; g) BnBr (7.0 equiv), TBAI (0.2 equiv), NaH (5.0 equiv), DMF, 25 °C, 18 h, 88 %; h) TsOH (0.2 equiv), MeOH, 25 °C, 18 h, 85 %. TBDPS=tert-butyldiphenylsilyl, CSA=(±)-camphor-10-sulfonic acid, MS=molecular sieves, DMF=N,N-dimethylformamide, TBAI=tetra-n-butylammonium iodide, TBAF=tetra-n-butylammonium fluoride. The synthesis of the J-ring fragment 26 also started from a prochiral furan and proceeded through a route that introduced chirality in high enantioselectivity through a Noyori reduction as shown in Scheme 4. Lithiation of furan (17) with nBuLi in THF at 0 °C, followed by addition of γ-butyrolactone (18) at −78 °C afforded the corresponding keto alcohol (58 % yield), which was then converted into its pivaloate ester 19 (PivCl, py, 86 % yield). Asymmetric reduction of the carbonyl group in the latter compound with 2.5 mol % (S,S)-Noyori catalyst 6 in formic acid and triethylamine (5:2) provided chiral alcohol 20 in 89 % yield and greater than 95 % ee. An Achmatowicz rearrangement was then induced on 20 by treatment with NBS, NaOAc, and NaHCO3 in aqueous THF (96 % yield), and the resulting hemiacetal was protected as the pivaloate 21 (PivCl, DMAP, Et3N) in 64 % yield. The undesired anomer of 21 was also produced in this reaction (20 % yield). Enone 21 was then reduced selectively under Luche conditions (NaBH4, CeCl3⋅7 H2O, −78 °C, 100 % yield) to give the corresponding allylic alcohol, which was converted into its pivaloate ester 22 (PivCl, DMAP, Et3N, 89 % yield). The ensuing dihydroxylation of the olefinic bond within 22 (NMO, cat. OsO4) proceeded selectively from the face opposite the pivaloate groups, thus furnishing the expected 1,2-diol, whose selective monobenzylation (nBu2SnO, BnBr, TBAI, benzene, Δ, 98 % yield) proceeded smoothly at the equatorial hydroxy group (C-3). This was followed by acetylation at the remaining axial hydroxy moiety (C-2) to afford the fully protected intermediate 23 (97 % yield). Addition of allylTMS to 23 in the presence of BF3⋅Et2O (MeCN, 60 °C) then afforded stereoselectively the allyl derivative 24 (87 % yield). The acetate in 24 was then exchanged for a TBS group (K2CO3, MeOH; TBSCl, imid., 72 % overall yield) and the terminal double bond was migrated inside the carbon chain by using RhCl3⋅H2O14 (EtOH, 80 °C) to afford E-olefin 25. Ozonolysis of the latter compound (O3, CH2Cl2/MeOH, −78 °C; Ph3P) then led to the targeted aldehyde 26 in 96 % yield for the last two steps. Construction of J ring aldehyde 26. Reagents and conditions: a) nBuLi (1.05 equiv), THF, 0 °C, 1 h; then 18 (2.0 equiv), −78 °C, 1 h, 58 %; b) PivCl (1.25 equiv), py (3.0 equiv), CH2Cl2, 0 °C, 4 h, 86 %, c) 6 (2.5 mol %), HCO2H/Et3N (5:2), 30 °C, 48 h, 89 % (>95 % ee); d) NBS (1.0 equiv), NaOAc (1.0 equiv), NaHCO3 (2.0 equiv), THF/H2O (3:1), 0 °C, 1 h, 96 %; e) PivCl (1.5 equiv), Et3N (2.5 equiv), DMAP (0.05 equiv), CH2Cl2, −78 °C, 2 h, 64 % (+20 % other anomer); f) CeCl3⋅7 H2O (0.5 equiv), NaBH4 (1.0 equiv), MeOH/CH2Cl2 (1:1), −78 °C, 30 min, 100 %; g) PivCl (1.5 equiv), Et3N (3.0 equiv), DMAP (0.05 equiv), CH2Cl2, 0 °C, 6 h, 89 %; h) OsO4 (0.02 equiv), NMO (2.0 equiv), acetone/H2O (10:1), 25 °C, 48 h, 93 %; i) nBu2SnO (1.0 equiv), C6H6, reflux, 18 h; then BnBr (1.4 equiv), TBAI (1.0 equiv), 3 h, 98 %; j) Ac2O (4.0 equiv), DMAP (0.05 equiv), py (8.0 equiv), CH2Cl2, 25 °C, 12 h, 97 %; k) allylTMS (5.0 equiv), BF3⋅Et2O (2.5 equiv), MeCN, 60 °C, 4 h, 87 %; l) K2CO3 (0.1 equiv), MeOH, 25 °C, 6 h, 84 %; m) TBSCl (2.0 equiv), imid. (4.0 equiv), DMF, 40 °C, 24 h, 85 %; n) RhCl3⋅H2O (0.05 equiv), EtOH, 80 °C, 3 h; o) O3, CH2Cl2/MeOH (5:1), −78 °C, 5 min; then Ph3P (1.5 equiv), 96 % over two steps. py=pyridine, DMAP=4-dimethylaminopyridine, NMO=4-methylmorpholine N-oxide, TMS=trimethylsilyl. Scheme 5 summarizes the construction of the IJK building block 33 from the J-ring aldehyde 26. Thus, aldehyde 26 was added to the lithium anion derived from cyclohexylacetylene (27) and nBuLi in THF at −78 °C to afford, after oxidation of the resulting propargylic alcohol with DMP,15 ynone 28 in 89 % overall yield. Exposure of the latter compound to aqueous HF in MeCN resulted in the cleavage of the TBS group to afford the corresponding hydroxy ynone intermediate, whose ring closure required considerable experimentation. It was finally discovered that treatment of this substrate with AgOTf in CH2Cl2 at 40 °C cleanly promoted the desired cyclization to afford pyranone 29 in 84 % overall yield from 28. Subsequent reduction of 29 under Luche conditions (NaBH4, CeCl3⋅7 H2O) led stereoselectively to the corresponding hydroxy compound (α isomer) whose hydroboration/oxidation (BH3⋅THF; NaOH, H2O2) proceeded regio- and stereoselectively to afford, after protection of the resulting diol with TESOTf in the presence of 2,6-lutidine, the bis-TES silyl ether 30 in 65 % overall yield for the three steps. Lactone 31 was then derived from bis-pivaloate 30 by reduction with DIBAL-H, followed by oxidation of the resulting diol mediated by a PhI(OAc)2/TEMPO catalyst16 (82 % overall yield). Lactone 31 was finally converted into the requisite coupling partner, vinyl triflate 33, in 93 % yield by treatment with KHMDS and Comin's reagent (32) in THF at −78 °C, thus setting the stage for the casting of the final two rings. Construction of IJK ring system 33. Reagents and conditions: a) nBuLi (2.0 equiv), 27 (2.0 equiv), THF, −78 °C, 1 h; then 26 (1.0 equiv), 15 min, 93 %; b) DMP (1.5 equiv), CH2Cl2, 25 °C, 1 h, 96 %; c) 48 % aq HF/MeCN (1:3), 25 °C, 18 h, 94 %; d) AgOTf (0.1 equiv), CH2Cl2, 40 °C, 18 h, 89 %; e) CeCl3⋅7 H2O (0.2 equiv), NaBH4 (1.1 equiv), MeOH/CH2Cl2 (1:1), 0 °C, 15 min; f) BH3⋅THF (1.0 M in THF, 10 equiv), THF, 0 °C, 3 h; then NaOH (1.0 M aq), H2O2 (35 % aq), 1 h, 71 % over two steps; g) TESOTf (15 equiv), 2,6-lut. (20 equiv), CH2Cl2, 0 °C, 2 h, 92 %; h) DIBAL-H (1.0 M in CH2Cl2, 10 equiv), CH2Cl2, −78 °C, 10 min; i) TEMPO (0.1 equiv), PhI(OAc)2 (3.0 equiv), CH2Cl2, 25 °C, 18 h, 82 % over two steps; j) 32 (2.0 equiv), KHMDS (0.5 M in THF, 2.0 equiv), THF, −78 °C, 10 min, 93 %. DMP=Dess–Martin periodinane, DIBAL-H=diisobutylaluminum hydride, TEMPO=2,2,6,6-tetramethyl-1-piperidinyloxy, KHMDS=potassium bis(trimethylsilyl)amide. Scheme 6 depicts the completion of the synthesis of the GHIJK ring system 2. Hydroboration of the G-ring alkene 13 with 9-BBN in THF at 50 °C furnished the expected alkylborane which underwent smooth B-alkyl Suzuki coupling17 with vinyl triflate 33 in the presence of a Pd(OAc)2 catalyst, SPhos ligand (cat.),18 and KHCO3 (1.0 M aq, 20 equiv) to afford tetracycle 34 in 78 % yield. The hydroboration of 34 with BH3⋅THF in THF at 0 °C proceeded regio- and stereoselectively to afford the expected hydroxy compound (α isomer, 71 % yield) which was oxidized to the corresponding ketone in 95 % yield through the action of DMP. Heating this compound with TsOH in MeOH at 50 °C for 48 h resulted in the formation of pentacyclic trihydroxy methyl acetal 35, through removal of the PMB and TES groups and ring closure, in 85 % yield. The methoxy group was then reductively removed from 35 by the action of Et3SiH in the presence of TMSOTf in MeCN at 0 °C to afford, after hydrogenolysis of the three benzyl groups (H2, 20 % Pd(OH)2/C (cat.), EtOH, 69 % overall yield), the targeted GHIJK ring system 2 (see the Experimental Section).19 Completion of the synthesis of GHIJK ring system 2. Reagents and conditions: a) 13 (2.0 equiv), 9-BBN (4.0 equiv), THF, 50 °C, 3 h; then KHCO3 (1.0 M aq, 20 equiv), 33 (1.0 equiv), SPhos (0.2 equiv), Pd(OAc)2 (0.1 equiv), 25 °C, 48 h, 78 %; b) BH3⋅THF (1.0 M in THF, 10 equiv), THF, 0 °C, 18 h; then NaOH (1.0 M aq), H2O2 (35 % aq), 1 h, 71 %; c) DMP (1.5 equiv), CH2Cl2, 25 °C, 2 h, 95 %; d) TsOH (1.0 equiv), MeOH, 50 °C, 48 h, 85 %; e) Et3SiH (5.0 equiv), TMSOTf (2.0 equiv), MeCN, 0 °C, 15 min, 98 %; f) H2, 20 % Pd(OH)2/C (25 % w/w), EtOH, 25 °C, 18 h, 70 %. 9-BBN=9-borabicyclo[3.3.1]nonane, SPhos=2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl, Ts=p-toluenesulfonyl. The much-anticipated comparison of the 13C chemical shifts of the GHIJK ring system 2 with those of the corresponding region of maitotoxin (1)2d was then made (Figure 1). As shown, the 13C NMR chemical shifts (ppm) for the two compounds are in excellent agreement, with an average difference (Δδ/ppm) of less than 0.1 ppm and a maximum deviation of 0.6 ppm for carbon atoms C-42 to C-53. The larger differences between the values of the two compounds for carbon atoms C-39 to C-41 and C-54 to C-55 are apparently due to the special functional groups present on rings G (a sulfate moiety) and K (a dihydroxypyran) of maitotoxin (1) as compared to the simpler model system 2 which contains only free hydroxy groups on ring G and a cyclohexyl moiety on ring K (Figure 1). These observations lend strong support for our computationally derived conclusion5 that the originally proposed structure2–4 for maitotoxin (1) is, indeed, most likely correct, at least in this region of the molecule, despite the noted biosynthetic anomaly.6 Differences in the 13C chemical shift (Δδ/ppm) between GHIJK fragment 2 and the values reported for maitotoxin (1) in the same solvent system. 13C chemical shifts of 2 [150 MHz, 1:1 [D4]methanol/[D5]pyridine (reported values for maitotoxin in parentheses)]19: C-39: 74.5 (72.3), C-40: 73.2 (78.9), C-41: 70.3 (68.5), C-42: 81.2 (80.6), C-43: 69.8 (69.6), C-44: 36.3 (36.3), C-45: 77.8 (77.7), C-46: 77.4 (77.5), C-47: 37.7 (37.4), C-48: 67.7 (67.8), C-49: 85.8 (85.8), C-50: 70.1 (70.1), C-51: 74.9 (74.9), C-52: 72.1 (72.1), C-53: 79.3 (79.4), C-54: 71.3 (69.8), C-55: 85.0 (78.2). The described chemistry provides further experimental support for the originally proposed stereochemical assignment for the JK junction of maitotoxin, and should facilitate the construction of larger fragments of this notable marine neurotoxin. 2: Rf=0.27 (silica gel, EtOAc/MeOH 4:1); [α]=−5.7 deg cm3 g−1 dm−1 (c=0.11 g cm−3, MeOH); IR (film): max=3453, 3414, 2925, 2850, 1648, 1446, 1350, 1069 cm−1; 1H NMR (600 MHz, [D4]MeOD/[D5]pyridine): δ=4.30 (t, J=2.4 Hz, 1 H), 4.22 (t, J=4.8 Hz, 1 H), 3.92 (dt, J=9.0, 3.0 Hz, 1 H), 3.88 (dd, J=10.8, 8.4 Hz, 1 H), 3.88–3.82 (m, 2 H), 3.81–3.77 (m, 2 H), 3.74 (t, J=8.4 Hz, 1 H), 3.74–3.70 (m, 1 H), 3.61 (dd, J=10.8, 4.8 Hz, 1 H), 3.55 (t, J=9.6 Hz, 1 H), 3.43 (dd, J=9.6, 3.0 Hz, 1 H), 3.25 (dd, J=9.6, 1.8 Hz, 1 H), 3.16–3.08 (m, 3 H), 2.40 (dt, J=10.8, 3.6 Hz, 1 H), 2.35 (dt, J=10.8, 4.2 Hz, 1 H), 2.33–2.27 (m, 1 H), 1.93–1.87 (m, 1 H), 1.75 (ddt, J=14.4, 8.4, 6.0 Hz, 1 H), 1.68–1.63 (m, 1 H), 1.63–1.53 (m, 3 H), 1.52–1.44 (m, 3 H), 1.31 (dq, J=12.6, 3.0 Hz, 1 H), 1.21–1.11 (m, 2 H), 1.09–1.01 (m, 1 H), 0.98–0.90 ppm (m, 1 H); 13C NMR (150 MHz, CDCl3): δ=85.8, 85.0, 81.2, 79.3, 77.8, 77.5, 74.9, 74.5, 73.2, 72.1, 71.3, 70.3, 70.1, 69.8, 67.7, 59.6, 38.7, 37.7, 36.6, 36.3, 31.4, 27.7, 27.31, 27.26, 26.2 ppm; HRMS [electrospray ionization (ESI)]: calcd for C25H40O11Na+ [M+Na+]: 539.2463, found: 539.2451. 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