Studies Toward Elucidating the Stereochemical Structure of Iriomoteolide 1a

Abstract

A diastereomer of iriomoteolide 1a has been synthesized as part of our effort to identify the so far unknown stereochemical structure of the natural product. The synthetic route features a lithium acetylide-chloroformate coupling, the ring-closing metathesis to form the macrocyclic diolide, and a SmI₂-mediated intramolecular reductive allylation for formation of the cyclic hemiketal.

Marine microorganism is a rich source of structurally diverse secondary metabolites, many of which show interesting biological activities. One recent example is iriomoteolide 1a (1, Figure  [¹] ), which was the first of a group of three macrolides isolated from strain HYA024 of dinoflagelates of the Amphidinium sp., collected off the Iriomoto island of Japan. [¹] It was reported to be potently cytotoxic against human B lymphocyte DG-75 cells and Epstein-Barr virus-infected human B lymphocytes Raji cells with IC₅₀ values of 2 and 3 ng/mL, respectively. The complex molecular structure and potent cytotoxicity of iriomoteolide 1a attracted considerable attention from synthetic chemists. Indeed, a number of labs have been engaged in the synthesis of this natural product, [²] which was isolated in only minute quantities from its natural source. In 2010, Horne, Ghosh, and we independently arrived at the conclusion that the original structural assignment of iriomoteolide 1a was incorrect. While both Horne and Ghosh synthesized the originally assigned iriomoteolide 1a and found its spectra to be different from those reported for the natural product, [³] our conclusion that the trisubstituted Z-C2-C3 double bond of the original structure must had been misassigned prompted us to target the E-C2-C3 diastereomer (i.e., 2) of the original assignment. [4] It was found to be different from the natural product too. We also synthesized two additional 2E-diastereomers (i.e., 3 and 4). But they do not match the natural product either. More recently, Dai and co-workers reported their synthesis of 2 and another 2E-diastereomer 5, which were also found to be different from the natural product. [5] These results collectively show that, if the connectivity of the natural product has been correctly assigned, the original structure likely has been misassigned at more than one stereocenter.

The potent cytotoxicity that was reported for iriomoteolide 1a warrants further studies to elucidate the structure of the natural product, which will enable access and mode-of-action studies of this potent cytotoxin. As part of our effort toward this goal, we decided to synthesize macrolide 6. An important consideration is our observation of a significant ROESY (CDCl₃) correlation of H19 and 13-OH from the spectra that accompany the original report of iriomoteolide 1a by Tsuda. [6] Molecular mechanics simulation at the MM2 level suggests that H19 and 13-OH have to be oriented to the same side of the macrocycle in order for such a correlation to occur. This and other considerations prompted us to choose 6 as the target. [7] The goal for synthesizing 6 is to identify if it represents the real structure of iriomoteolide 1a or (if it does not), in combination with the diastereomers that have been previously synthesized, to provide a ‘training set’ for computational elucidation of the stereochemical structure of the natural product.

We embarked on the synthesis of 6 using a synthetic design shown in Scheme  [¹] , which involved formation of the macrocyclic diolide 7 from 8, 9, and 10, followed by the intramolecular reductive allylation of 7 to introduce the C9/C13-cyclic hemiketal.

The synthesis started from preparation of each of the fragments, some of which can be synthesized by adapting our early synthetic route or as previously described for compounds 10 or 9. [4] [8] However, stereoselective preparation of 8 by direct coupling of α,β-unsaturated aldehydes and allenic metal reagents was problematic. [9] For example, the reaction of 11 [²j] with 12 led to a mixture of the desired ­homopropargylic alcohol 13 and an unassigned diaste­reomer with little stereochemical control (1.4:1, Scheme  [²] ). What’s more, this diastereomeric mixture could not be separated even after the hydroxyl group has been silylated.

To address this difficulty, we decided to apply the Evans chiral auxiliary approach for the synthesis of 8. While being stepwise, this approach was expected to be reliable in delivering the desired homopropargylic alcohol in a dia­stereoselective manner. To this end, the α,β-unsaturated aldehyde 11 was reacted with 14 under the standard syn-aldol reaction conditions (n-Bu₂BOTf, i-Pr₂NEt, CH₂Cl₂) to give 15. [¹0] This was followed by the Weinreb amidation with MeNH(OMe)˙HCl-Me₃Al and silylation of the hydroxyl group to give 16 in high yield. [¹¹] DIBAL-H reduction of the Weinreb amide gave an aldehyde intermediate, which was converted to 8 as a single diastereomer by the Colvin rearrangement under the conditions developed by Aoyama and Shioiri (Scheme  [³] ). [¹²] Whereas multiple steps had been involved, this procedure provided a reliable approach for preparation of adequate quantities of diastereo­merically pure 8 to complete the synthesis of the macrolide 6.

The coupling of 8 and 10 requires deprotonation of the terminal alkyne of 8 to form an acetylide prior to addition of chloroformate 10. Surprisingly, this reaction could not be consistently reproduced. After extensive experimentation, this was eventually resolved through addition of HMPA as a co-solvent. The coupling product 17 was subjected to the conjugate addition with the Gilman reagent in the presence of TMSCl, followed by acidic workup to stereoseletively give the E-trisubstituted C2-C3 alkenoic ester 18 in 99% yield.

To prepare for assembling the macrocyclic diolide, the C9-OH PMB protection of 18 was oxidatively removed. This was followed by the Mitsunobu reaction with 9 to give 19. [¹³] The macrocyclic diolide 20 was obtained by the ring-closing metathesis of 19 catalyzed by the Grubbs second-generation catalyst under high dilution (Scheme  [4] ). [¹4] Despite the presence of five double bonds in 19, the ring-closing metathesis reaction occurred at the two terminal double bonds selectively and only the desired E-isomer was formed.

Functional-group manipulations of 20 were carried out to prepare for synthesizing the C9/C13-cyclic hemiketal. This involved selective desilylation of the C26-hydroxyl group under the acidic conditions (PPTS, EtOH, reflux), halogenation (MsCl, Et₃N, CH₂Cl₂; LiCl, HMPA), and switching the protection of the hydroxyl groups to the more labile trimethylsilyl group to give 7. The allyl chloride of 7 was converted to the iodide by the Finkelstein ­reaction. [¹5] Despite the presence of a myriad of other functional groups, the C9/C13-cyclic hemiketal was selectively formed by treatment of the iodide with excess of SmI₂. [¹6] While this reaction itself was facile, the crude product had to be immediately desilylated under the buf­fered conditions (TBAF-HF˙pyridine-pyridine, THF). [¹7] The final product was obtained as a mixture of 6 and a minor isomer (ca. 7:1 ratio) which we tentatively assign as the ketol form of the C9/C13-cyclic hemiketal (Scheme  [5] ). These two isomers could be resolved by rapid preparative thin-layer chromatography. However, mixtures of the same composition were recovered when these resolved components were eluted, suggesting relatively rapid equilibration of the isomers. A similar equilibration was observed for 2, [4] but not for the natural product or the other diastereomers (i.e., 3-5). The NMR spectra of 6 were found to be different from those reported for the natural product. [¹8]

In summary, macrolide 6 was synthesized using a synthetic route that featured a lithium acetylide-chloroformate coupling, the ring-closing metathesis to form the macrocyclcic diolide, and a SmI₂-mediated intramolecular reductive allylation for formation of the cyclic hemiketal. Whereas 6 is still different from the natural product, it provides a useful reference for our future studies to elucidate the stereochemical structure of iriomoteolide 1a by computational methods.

Acknowledgment

We thank Mr. Thomas Kaiser and Ms. Fei Yang of Texas A&M University for helpful discussions and preparation of some of the starting materials, respectively. Financial support was provided by the Welch Foundation (A-1700) and Texas A&M University.

References

• 1a Tsuda M. Oguchi K. Iwamoto R. Okamoto Y. Kobayashi J. Fukushi E. Kawabata J. Ozawa T. Masuda A. Kitaya Y. Omasa K. J. Org. Chem.  2007,  72:  4469 
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• 2d Xie J. Ma Y. Horne DA. Org. Lett.  2009,  11:  5082 
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• 2e Wang S.-Y. Chen Y.-J. Loh T.-P. Synthesis  2009,  3557 
• 2f Ye Z. Deng L. Qian S. Zhao G. Synlett  2009,  2469 
• 2g Chin Y.-J. Wang S.-Y. Loh T.-P. Org. Lett.  2009,  11:  3674 
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• 2h Xie J. Horne DA. Tetrahedron Lett.  2009,  50:  4485 
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• 2i Ghosh AK. Yuan H. Tetrahedron Lett.  2009,  50:  1416 
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• 2j Fang L. Xue H. Yang J. Org. Lett.  2008,  10:  4645 
  Search in Google Scholar
• 3a Xie J. Ma Y. Horne DA. Tetrahedron  2011,  67:  7485 
  Search in Google Scholar
• 3b Ghosh AL. Yuan H. Org. Lett.  2010,  12:  3120 
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  Search in Google Scholar
• For a recent synthesis of the proposed iriomoteolide 1b:
• 3d Ye Z. Gao T. Zhao G. Tetrahedron  2011,  67:  5979 
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• 4 Fang L. Yang J. Yang F. Org. Lett.  2010,  12:  3124 
  Search in Google Scholar
• 5a Liu Y. Feng G. Wang J. Wu J. Dai W.-M. Synlett  2011,  1774 
• 5b

  The spectra reported by Dai and co-workers for macrolide 2 are different from ours. In particular, a chemical shift of δ = 96.8 ppm was observed for 2 at C13 (and δ = 97.0 ppm for 6) in our hands, but δ = 99.5 ppm by Dai and co-workers and δ = 99.7 ppm for the natural product. It is not clear what is the source of this discrepancy.
• 8 Zhang D. Bleasdale C. Golding BT. Watson WP. Chem. Commun.  2000,  1141 
• 9 Marshall JA. J. Org. Chem.  2007,  72:  8153 
  Search in Google Scholar
• 10 Evans DA. Bartroli J. Shih TL. J. Am. Chem. Soc.  1981,  103:  2127 
  Search in Google Scholar
• 11a Levin JI. Turos E. Weinreb SM. Synth. Commun.  1982,  12:  989 
  Search in Google Scholar
• 11b Basha A. Lipton M. Weinreb SM. Tetrahedron Lett.  1977,  18:  4171 
  Search in Google Scholar
• 12a Miwa K. Aoyama T. Shioiri T. Synlett  1994,  107 
• For a recent review, see:
• 12b Habrant D. Rauhala V. Koskinen AMP. Chem. Soc. Rev.  2010,  39:  2007 
  Search in Google Scholar
• For two reviews of the Mitsunobu reaction, see:
• 13a Mitsunobu O. Synthesis  1981,  1 
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  Search in Google Scholar
• 14 Scholl M. Ding S. Lee CW. Grubbs RH. Org. Lett.  1999,  1:  953 
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• 15 Finkelstein H. Ber. Dtsch. Chem. Ges.  1910,  43:  1528 
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• 16 Heumann LV. Keck GE. Org. Lett.  2007,  9:  1951 
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• 17a Gaffney BL. Jones RA. Tetrahedron Lett.  1982,  23:  2257 
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• 17b Coleman RS. Li J. Navarro A. Angew. Chem. Int. Ed.  2001,  40:  1736 
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Supporting Information of ref. 1.

Fang, L.; Yang, J. unpublished results.

Characterization Data for Compound 6
[α]_D -11.07 (c 0.6, CHCl₃). IR (thin film): 2967, 2928, 2854, 1661 1638, 1427, 1013, 871 cm^-¹. ^¹H NMR (500 MHz, CDCl₃): δ = 5.72 (s, 1 H), 5.72-5.70 (m, 2 H), 5.62-5.57 (m, 1 H), 5.45 (dd, J = 16.2, 9.1 Hz, 1 H), 4.87 (s, 1 H), 4.85-4.80 (m, 1 H), 4.83 (s, 1 H), 3.77 (t, J = 9.1 Hz, 1 H), 3.64 (dd, J = 11.9, 2.4 Hz, 1 H), 3.61-3.53 (m, 1 H), 3.03 (s, 1 H), 2.50-2.46 (m, 1 H), 2.44-2.35 (m, 1 H), 2.33 (d, J = 8.1 Hz, 1 H), 2.25 (dd, J = 13.6, 10.3 Hz, 1 H), 2.17-2.11 (m, 3 H), 2.08 (s, 3 H), 2.06-1.98 (m, 1 H), 1.93-1.88 (m, 1 H), 1.84-1.79 (m, 1 H), 1.51-1.45 (m, 1 H), 1.28 (d, J = 6.8 Hz, 3 H), 1.15 (s, 3 H), 1.17-1.11 (m, 1 H), 1.07 (d, J = 6.3 Hz, 3 H), 1.02 (d, J = 7.1 Hz, 3 H), 0.85 (d, J = 6.8 Hz, 3 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 168.1, 162.6, 141.2, 139.0, 135.0, 128.0, 120.7, 115.7, 111.3, 97.0, 79.9, 75.8, 75.7, 73.7, 72.4, 48.4, 45.9, 43.8, 38.0, 37.0, 36.2, 35.4, 33.6, 23.2, 21.5, 20.4, 17.7, 16.9, 15.9. ESI-HRMS: m/z calcd for C₂₉H₄₆O₇Na⁺
[M + Na⁺]: 529.3141; found: 529.3152.

References

• 1a Tsuda M. Oguchi K. Iwamoto R. Okamoto Y. Kobayashi J. Fukushi E. Kawabata J. Ozawa T. Masuda A. Kitaya Y. Omasa K. J. Org. Chem.  2007,  72:  4469 
  Search in Google Scholar
• 1b Tsuda M. Oguchi K. Iwamoto R. Okamoto Y. Fukushi E. Kawabata J. Ozawa T. Masuda A. J. Nat. Prod.  2007,  70:  1661 
  Search in Google Scholar
• 2a Liu Y. Wang J. Li H. Wu J. Feng G. Dai W.-M. Synlett  2010,  2184 
• 2b Paterson I. Rubenbauer P. Synlett  2010,  571 
• 2c Li S. Chen Z. Xu Z. Ye T. Chem. Commun.  2010,  46:  4773 
  Search in Google Scholar
• 2d Xie J. Ma Y. Horne DA. Org. Lett.  2009,  11:  5082 
  Search in Google Scholar
• 2e Wang S.-Y. Chen Y.-J. Loh T.-P. Synthesis  2009,  3557 
• 2f Ye Z. Deng L. Qian S. Zhao G. Synlett  2009,  2469 
• 2g Chin Y.-J. Wang S.-Y. Loh T.-P. Org. Lett.  2009,  11:  3674 
  Search in Google Scholar
• 2h Xie J. Horne DA. Tetrahedron Lett.  2009,  50:  4485 
  Search in Google Scholar
• 2i Ghosh AK. Yuan H. Tetrahedron Lett.  2009,  50:  1416 
  Search in Google Scholar
• 2j Fang L. Xue H. Yang J. Org. Lett.  2008,  10:  4645 
  Search in Google Scholar
• 3a Xie J. Ma Y. Horne DA. Tetrahedron  2011,  67:  7485 
  Search in Google Scholar
• 3b Ghosh AL. Yuan H. Org. Lett.  2010,  12:  3120 
  Search in Google Scholar
• 3c Xie J. Ma Y. Horne DA. Chem. Commun.  2010,  46:  4770 
  Search in Google Scholar
• For a recent synthesis of the proposed iriomoteolide 1b:
• 3d Ye Z. Gao T. Zhao G. Tetrahedron  2011,  67:  5979 
  Search in Google Scholar
• 4 Fang L. Yang J. Yang F. Org. Lett.  2010,  12:  3124 
  Search in Google Scholar
• 5a Liu Y. Feng G. Wang J. Wu J. Dai W.-M. Synlett  2011,  1774 
• 5b

  The spectra reported by Dai and co-workers for macrolide 2 are different from ours. In particular, a chemical shift of δ = 96.8 ppm was observed for 2 at C13 (and δ = 97.0 ppm for 6) in our hands, but δ = 99.5 ppm by Dai and co-workers and δ = 99.7 ppm for the natural product. It is not clear what is the source of this discrepancy.
• 8 Zhang D. Bleasdale C. Golding BT. Watson WP. Chem. Commun.  2000,  1141 
• 9 Marshall JA. J. Org. Chem.  2007,  72:  8153 
  Search in Google Scholar
• 10 Evans DA. Bartroli J. Shih TL. J. Am. Chem. Soc.  1981,  103:  2127 
  Search in Google Scholar
• 11a Levin JI. Turos E. Weinreb SM. Synth. Commun.  1982,  12:  989 
  Search in Google Scholar
• 11b Basha A. Lipton M. Weinreb SM. Tetrahedron Lett.  1977,  18:  4171 
  Search in Google Scholar
• 12a Miwa K. Aoyama T. Shioiri T. Synlett  1994,  107 
• For a recent review, see:
• 12b Habrant D. Rauhala V. Koskinen AMP. Chem. Soc. Rev.  2010,  39:  2007 
  Search in Google Scholar
• For two reviews of the Mitsunobu reaction, see:
• 13a Mitsunobu O. Synthesis  1981,  1 
• 13b Kumara Swamy KC. Bhuvan Kumar NN. Balaraman E. Pavan Kumar KVP. Chem. Rev.  2009,  109:  2551 
  Search in Google Scholar
• 14 Scholl M. Ding S. Lee CW. Grubbs RH. Org. Lett.  1999,  1:  953 
  Search in Google Scholar
• 15 Finkelstein H. Ber. Dtsch. Chem. Ges.  1910,  43:  1528 
  Search in Google Scholar
• 16 Heumann LV. Keck GE. Org. Lett.  2007,  9:  1951 
  Search in Google Scholar
• 17a Gaffney BL. Jones RA. Tetrahedron Lett.  1982,  23:  2257 
  Search in Google Scholar
• 17b Coleman RS. Li J. Navarro A. Angew. Chem. Int. Ed.  2001,  40:  1736 
  Search in Google Scholar

Supporting Information of ref. 1.

Fang, L.; Yang, J. unpublished results.

Characterization Data for Compound 6
[α]_D -11.07 (c 0.6, CHCl₃). IR (thin film): 2967, 2928, 2854, 1661 1638, 1427, 1013, 871 cm^-¹. ^¹H NMR (500 MHz, CDCl₃): δ = 5.72 (s, 1 H), 5.72-5.70 (m, 2 H), 5.62-5.57 (m, 1 H), 5.45 (dd, J = 16.2, 9.1 Hz, 1 H), 4.87 (s, 1 H), 4.85-4.80 (m, 1 H), 4.83 (s, 1 H), 3.77 (t, J = 9.1 Hz, 1 H), 3.64 (dd, J = 11.9, 2.4 Hz, 1 H), 3.61-3.53 (m, 1 H), 3.03 (s, 1 H), 2.50-2.46 (m, 1 H), 2.44-2.35 (m, 1 H), 2.33 (d, J = 8.1 Hz, 1 H), 2.25 (dd, J = 13.6, 10.3 Hz, 1 H), 2.17-2.11 (m, 3 H), 2.08 (s, 3 H), 2.06-1.98 (m, 1 H), 1.93-1.88 (m, 1 H), 1.84-1.79 (m, 1 H), 1.51-1.45 (m, 1 H), 1.28 (d, J = 6.8 Hz, 3 H), 1.15 (s, 3 H), 1.17-1.11 (m, 1 H), 1.07 (d, J = 6.3 Hz, 3 H), 1.02 (d, J = 7.1 Hz, 3 H), 0.85 (d, J = 6.8 Hz, 3 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 168.1, 162.6, 141.2, 139.0, 135.0, 128.0, 120.7, 115.7, 111.3, 97.0, 79.9, 75.8, 75.7, 73.7, 72.4, 48.4, 45.9, 43.8, 38.0, 37.0, 36.2, 35.4, 33.6, 23.2, 21.5, 20.4, 17.7, 16.9, 15.9. ESI-HRMS: m/z calcd for C₂₉H₄₆O₇Na⁺
[M + Na⁺]: 529.3141; found: 529.3152.

• Supplementary Material