Total synthesis of apratoxin A and B using Matteson's homologation approach

Abstract

Apratoxin A and B, two members of an interesting class of marine cyclodepsipeptides are synthesized in a straightforward manner via Matteson homologation. Starting from a chiral boronic ester, the polyketide fragment of the apratoxins was obtained via five successive homologation steps in an overall yield of 27% and very good diastereoselectivity. This approach is highly flexible and should allow modification also of this part of the natural products, while previous modifications have been carried out mainly in the peptide fragment.

---

Introduction

The apratoxins are a class of cyclodepsipeptides produced by cyanobacteria.¹ The first described member of this group, apratoxin A, was isolated by Moore and Paul et al. from the marine cyanobacterium Lyngbya majuscula in 2001.² It showed potent cytotoxicity towards a range of tumor cell lines in the sub nanomolar range,³ acting as a broad-spectrum Sec61 inhibitor⁴ targeting HER/ErbB family proteins.⁵ Over the following years, a wide range of further apratoxins have been isolated.⁶ Some of these structures are shown in Fig. 1.

Fig. 1 Selected apratoxins.

The apratoxins form a 25-membered ring consisting of a pentapeptide and a rather unusual polyketide fragment, containing, in most cases, a terminal t-butyl group. The apratoxins differ mainly in their methylation pattern (apratoxins A–D) or in the composition of the peptide fragment. While most apratoxins embody an unusual unsaturated prolonged cysteine unit, in some of the members of the apratoxin S family, artificial derivatives obtained by a medicinal chemistry campaign, this unit is (structurally) reduced.⁷ The unusual polyketide fragment is common to most family members. In case of apratoxin D, its carbon chain is even one carbon longer. Detailed studies indicate that an iron-dependent methyltransferase directs the tert-butyl group formation by initiating the assembly of the apratoxin A polyketide starter unit.⁸

Not surprisingly, the excellent cytotoxicities in the low nanomolar range, and the interesting mode of action initiated synthetic investigations for the synthesis of the polyketide fragment of the apratoxins,⁹ the apratoxins themselves¹⁰ as well as derivatives for SAR studies.¹¹ Common to all syntheses of apratoxin A is the late-stage assembly of the thiazoline moiety, which is oxidatively sensitive and potentially prone to unwanted side reactions, e.g. epimerization of the adjacent stereogenic centre. To synthesise the polyketide fragment, asymmetric cuprate additions or allyl isomerisations are used to introduce the internal methyl group, while the two stereogenic centres adjacent to the thiazoline ring are generated via different versions of aldol reactions.

Results and discussion

Since a couple of years our group is involved in the synthesis of natural products,¹² especially cyclic peptides with interesting biological activities.¹³ Recently, we have become interested in the synthesis of peptide/polyketide hybrids¹⁴ such as the apratoxins, with a focus on aldol-free polyketide synthesis. The aldol reaction,¹⁵ or alternatively an allylation/ozonolysis sequence,¹⁶ is perfectly suited to generating the 3, 5, 7, …-polyhydroxylated carboxylic acids found in many natural products, with methyl groups between the hydroxy functionalities, but these reactions are more or less restricted to this substitution pattern. With the synthesis of lagunamide A,¹⁷ we could show that also other approaches, such as Matteson homologations,¹⁸ which are more flexible, are well suited to the synthesis of complex polyketides. This elegant stereoselective prolongation of chiral boronic esters was introduced by Donald Matteson 40 years ago (Scheme 1).¹⁹

Scheme 1 Matteson homologation.

A key step of this protocol is the highly stereoselective formation of an α-chloro boronic ester A, which can be subjected to nucleophilic substitution under S_N2-conditions with a wide range of nucleophiles such as Grignard reagents, alkoxides or certain enolates.²⁰ Continual application of this procedure allows the stepwise stereoselective incorporation of a wide range of substituents and functionalities into a growing carbon chain.

We attempted to prepare the polyketide fragment of apratoxin A and B using the above-mentioned Matteson homologation approach. Starting from diisopropyl tert-butylboronate 1²¹ transesterification with (S,S)-DICHED (dicyclohexylethane-1,2-diol)²² gave the chiral boronic ester 2 (Scheme 2). While homologation of sterically demanding 2 to the α-chloroboronic ester proceeded smoothly under the usual conditions, the subsequent substitution to the alkoxyboronic ester 3 was sluggish and incomplete when we used only a slight excess (1.3 eq.) of NaOPMB. In contrast, with a larger excess (3.0 eq.) of alkoxide, we observed full conversion and could isolate 3 in excellent yield.

Scheme 2 Synthesis of the apratoxin polyketide fragment.

In the following step, we introduced a CH₂ group by treating the intermediate α-chloroboronic ester with LiBHEt₃. Next, we used a methyl Grignard reagent as a nucleophile, which cleanly yielded 5. Introduction of a second CH₂ unit gave the prolonged boronic ester 6 in good yield. In the following homologation step, we applied the lithium enolate of tert-butyl propionate as a nucleophile. Similar reactions of simple α-haloboronic esters with enolates have already been described by Matteson et al.^20a,b but, surprisingly, this method has not been applied to the synthesis of more complex natural products so far. A reason for this may be that this reaction requires the use of α-bromoboronic esters which are more reactive, but also more prone to epimerization than their chloro-analogues.²³ Fortunately, we could completely suppress epimerization by quenching the formation of the α-bromoboronic ester at low temperature (−55 °C) and immediately treated the crude product with the enolate. While the homologations from 2 to 6 provided the products as single diastereomers, we obtained 8 as a mixture of syn and anti isomers, but with good anti selectivity (d.r. = 9 : 1 after oxidation of the boronic ester to alcohol 8). O-Troc protection of 8 followed by oxidative cleavage of the PMB ether provided alcohol 10, which we reacted with Fmoc-(S)-Pro-Cl²⁴ to ester 11 in almost quantitative yield and without epimerization of the α-stereogenic centre of proline. Acidic cleavage of the tert-butyl ester gave access to crude carboxylic acid 12. N-Boc deprotection of the modified cysteine building block 13, followed by coupling with acid 12, yielded amide 14, which is a known intermediate of the apratoxin A synthesis reported by Doi et al.^10c,11c To convert 14 into a thiazoline, we applied Kelly's method using Tf₂O and Ph₃PO.²⁵ For the subsequent cleavage of the Troc carbamate, we examined several conditions, e.g. with a Zn/Cu couple or Me₃SnOH²⁶ as a deprotection reagent, but Doi's protocol using zinc dust and aqueous NH₄OAc^10c,11c gave 15 in the best yield. Pd-catalysed cleavage of the allyl ester provided carboxylic acid 16.

We were also interested in preparing apratoxin B²⁷ for which, to our knowledge, there has not yet been a total synthesis reported. Starting from Boc-(S)-Ile-OAllyl (17b),²⁸ we synthesised the tripeptide building block 19b in two HATU-mediated peptide couplings, achieving excellent overall yield (Scheme 3). The corresponding apratoxin A tripeptide 19a, in which (S)-Ile is replaced by N-Me-(S)-Ile, was prepared according to known procedures.^10c,11c

Scheme 3 Synthesis of the apratoxin B tripeptide.

Coupling of the polyketide fragment 16 with Fmoc-deprotected 19a or 19b yielded the linear precursors 20a and 20b, which we cyclised to the natural products after cleavage of the allyl and Fmoc protecting groups (Scheme 4). As reported in previous syntheses,^10c,d,11c,d we observed partial epimerization of the stereogenic center adjacent to the thiazoline moiety during the steps from 15 to 21, and we obtained the cyclisation products as mixtures of two diastereomers 21a/b and epi-21a/b. Interestingly, no water elimination to the α,β-unsaturated thiazoline occurred under these conditions. In both cases, we were able to separate the isomers via preparative HPLC to isolate the diastereomerically pure natural products.

Scheme 4 Synthesis of apratoxin A and B.

Conclusions

In conclusion: we have shown that the Matteson homologation is perfectly suited to generating the unusual polyketide fragment of the apratoxins. Starting from a chiral boronic ester (2), we were able to prepare the desired building block 8 of apratoxin A and B via five successive Matteson homologations and subsequent oxidation, achieving an overall yield of 27% and very good diastereoselectivity. Obviously, even a sterically demanding tert-butyl group is well accepted in the boronic ester and, therefore, this protocol should also be perfectly suited for the synthesis of libraries of derivatives for SAR studies, simply by changing the nucleophilic coupling partners in the homologation steps. Further applications are currently under investigation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from Saarland University and the DFG (Grant Ka 880/13-1) is gratefully acknowledged.

Notes and references

1. (a) M. Gutiérrez, T. L. Suyama, N. Engene, J. S. Wingerd, T. Matainaho and W. H. Gerwick, J. Nat. Prod., 2008, 71, 1099–1103; (b) S. Matthew, P. J. Schupp and H. Luesch, J. Nat. Prod., 2008, 71, 1113–1116 (correction: J. Nat. Prod. 2018, 81, 217–217); (c) C. C. Thornburg, E. S. Cowley, J. Sikorska, L. A. Shaala, J. E. Ishmael, D. T. A. Youssef and K. L. McPhail, J. Nat. Prod., 2013, 76, 1781–1788; (d) E. M. Tarsis, E. J. Rastelli, S. E. Wengryniuk and D. M. Coltart, Tetrahedron, 2015, 71, 5029–5044.
2. H. Luesch, W. Y. Yoshida, R. E. Moore, V. J. Paul and T. H. Corbett, J. Am. Chem. Soc., 2001, 123, 5418–5423.
3. (a) Y. Liu, B. K. Law and H. Luesch, Mol. Pharmacol., 2009, 76, 91–104; (b) H. Luesch, S. K. Chanda, R. M. Raya, P. D. DeJesus, A. P. Orth, J. R. Walker, J. C. Izpisúa Belmonte and P. G. Schultz, Nat. Chem. Biol., 2006, 2, 158–167; (c) X. Wan, J. D. Serrill, I. R. Humphreys, M. Tan, K. L. McPhail, I. G. Ganley and J. E. Ishmael, Mar. Drugs, 2018, 16, 77.
4. (a) K.-C. Huang, Z. Chen, Y. Jiang, S. Akare, D. Kolber-Simonds, K. Condon, S. Agoulnik, K. Tendyke, Y. Shen, K.-M. Wu, S. Mathieu, H.-W. Choi, X. Zhu, H. Shimizu, Y. Kotake, W. H. Gerwick, T. Uenaka, M. Woodall-Jappe and K. Nomoto, Mol. Cancer Ther., 2016, 15, 1208–1216; (b) A. O. Paatero, J. Kellosalo, B. M. Dunyak, J. Almaliti, J. E. Gestwicki, W. H. Gerwick, J. Taunton and V. O. Paavilainen, Cell Chem. Biol., 2016, 23, 561–566.
5. S. Kazemi, S. Kawaguchi, C. E. Badr, D. R. Mattos, A. Ruiz-Saenz, J. D. Serrill, M. M. Moasser, B. P. Dolan, V. O. Paavilainen, S. Oishi, K. L. McPhail and J. E. Ishmael, Biochem. Pharmacol., 2021, 183, 114317.
6. (a) Q. Chen, Y. Liu and H. Luesch, ACS Med. Chem. Lett., 2011, 2, 861–865; (b) C. C. Thornburg, E. S. Cowley, J. Sikorska, L. A. Shaala, J. E. Ishmael, D. T. A. Youssef and K. L. McPhail, J. Nat. Prod., 2013, 76, 1781–1788.
7. (a) B. Qiu, A. Tan, A. B. Veluchamy, Y. Li, H. Murray, W. Cheng, C. Liu, J. M. Busoy, Q.-Y. Chen, S. Sistla, W. Hunziker, C. M. G. Cheung, T. Y. Wong, W. Hong, H. Luesch and X. Wang, Invest. Ophthalmol. Visual Sci., 2019, 60, 3254–3263; (b) W. Cai, Q.-Y. Chen, L. H. Dang and H. Luesch, ACS Med. Chem. Lett., 2017, 8, 1007–1012 (correction: ACS Med. Chem. Lett. 2017, 8, 1342).
8. (a) M. A. Skiba, A. P. Sikkema, N. A. Moss, C. L. Tran, R. M. Sturgis, L. Gerwick, W. H. Gerwick, D. H. Sherman and J. L. Smith, ACS Chem. Biol., 2017, 12, 3039–3048; (b) M. A. Skiba, A. P. Sikkema, N. A. Moss, A. N. Lowell, M. Su, R. M. Sturgis, L. Gerwick, W. H. Gerwick, D. H. Sherman and J. L. Smith, ACS Chem. Biol., 2018, 13, 1640–1650.
9. (a) J. Chen and C. J. Forsyth, Org. Lett., 2003, 5, 1281–1283; (b) Z. Xu, Z. Chen and T. Ye, Tetrahedron: Asymmetry, 2004, 15, 355–363; (c) A. Gilles, J. Martinez and F. Cavelier, C. R. Chim., 2011, 14, 437–440; (d) S. Dey, S. E. Wengryniuk, E. M. Tarsis, B. D. Robertson, G. Zhou and D. M. Coltart, Tetrahedron Lett., 2015, 56, 2927–2929.
10. (a) J. Chen and C. J. Forsyth, J. Am. Chem. Soc., 2003, 125, 8734–8735; (b) J. Chen and C. J. Forsyth, Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 12067–12072; (c) T. Doi, Y. Numajiri, A. Munakata and T. Takahashi, Org. Lett., 2006, 8, 531–534; (d) T. Doi, Yo. Numajiri, T. Takahashi, M. Takagi and K. Shin-ya, Chem. – Asian J., 2011, 6, 180–188; (e) B. D. Robertson, S. E. Wengryniuk and D. M. Coltart, Org. Lett., 2012, 14, 5192–5195; (f) Y. Masuda, J. Suzuki, Y. Onda, Y. Fujino, M. Yoshida and T. Doi, J. Org. Chem., 2014, 79, 8000–8009; (g) P. Wu, W. Cai, Q.-Y. Chen, S. Xu, R. Yin, Y. Li, W. Zhang and H. Luesch, Org. Lett., 2016, 18, 5400–5403; (h) T. Doi, Y. Masuda and M. Yoshida, J. Synth. Org. Chem., Jpn., 2018, 76, 1170–1175.
11. (a) B. Zou, J. Wei, G. Cai and D. Ma, Org. Lett., 2003, 5, 3503–3506; (b) D. Ma, B. Zou, G. Cai, X. Hu and J. O. Liu, Chem. – Eur. J., 2006, 12, 7615–7626; (c) Y. Numajiri, T. Takahashi and T. Doi, Chem. – Asian J., 2009, 4, 111–125; (d) T. Doi, Chem. Pharm. Bull., 2014, 62, 735–743; (e) M. Yoshida, Y. Onda, Y. Masuda and T. Doi, Biopolymers, 2016, 104, 404–414; (f) R. Yin, W. Zhang, G. Liu, P. Wu, C. Lau and Y. Li, Tetrahedron, 2016, 72, 3823–3831; (g) P. Wu, H. Xu, Z. Li, Y. Zhou, Y. Li and W. Zhang, Tetrahedron Lett., 2017, 58, 3333–3336; (h) Y. Onda, Y. Masuda, M. Yoshida and T. Doi, J. Med. Chem., 2017, 60, 6751–6765; (i) Z.-Y. Mao, C.-M. Si, Y.-W. Liu, H.-Q. Dong, B.-G. Wie and G.-Q. Lin, J. Org. Chem., 2017, 82, 10830–10845; (j) E. J. Rastelli and D. M. Coltart, Tetrahedron, 2018, 74, 2269–2290; (k) W. Cai, R. Ratnayake, M. H. Gerber, Q.-Y. Chen, Y. Yu, H. Derendorf, J. G. Trevino and H. Luesch, Invest. New Drugs, 2019, 37, 364–374.
12. (a) A. Ullrich, Y. Chai, D. Pistorius, Y. A. Elnakady, J. E. Herrmann, K. J. Weissman, U. Kazmaier and R. Müller, Angew. Chem., Int. Ed., 2009, 48, 4422–4425 ( Angew. Chem. , 2009 , 121 , 4486–4489 ); (b) A. Ullrich, J. Herrmann, R. Müller and U. Kazmaier, Eur. J. Org. Chem., 2009, 6367–6378; (c) P. Servatius, T. Stach and U. Kazmaier, Eur. J. Org. Chem., 2019, 3163–3168; (d) P. Servatius, L. Junk and U. Kazmaier, Synlett, 2019, 1289–1302.
13. (a) P. Barbie and U. Kazmaier, Org. Lett., 2016, 18, 204–207; (b) L. Junk and U. Kazmaier, Angew. Chem., Int. Ed., 2018, 57, 11432–11435 ( Angew. Chem. , 2018 , 130 , 11602–11606 ); (c) L. Junk and U. Kazmaier, J. Org. Chem., 2019, 84, 2489–2500; (d) A. Kiefer, C. D. Bader, J. Held, A. Esser, J. Rybniker, M. Empting, R. Müller and U. Kazmaier, Chem. – Eur. J., 2019, 25, 8894–8902.
14. (a) L. Karmann, K. Schultz, J. Herrmann, R. Müller and U. Kazmaier, Angew. Chem., Int. Ed., 2015, 54, 4502–4507 ( Angew. Chem. , 2015 , 127 , 4585–4590 ); (b) D. Becker and U. Kazmaier, Eur. J. Org. Chem., 2015, 2591–2602; (c) S. Kappler, L. Karmann, C. Prudel, J. Herrmann, G. Caddeu, R. Müller, A. Vollmar, S. Zahler and U. Kazmaier, Eur. J. Org. Chem., 2018, 6952–6965.
15. (a) G. Ehrlich, J. Hassfeld, U. Eggert and M. Kalesse, Chem. – Eur. J., 2008, 14, 2232–2247; (b) H. Steinmetz, K. Gerth, R. Jansen, N. Schläger, R. Dehn, S. Reinecke, A. Kirschning and R. Müller, Angew. Chem., Int. Ed., 2011, 50, 532–536 ( Angew. Chem. , 2011 , 123 , 553–557 ); (c) I. Paterson and N. Y. S. Lam, J. Antibiot., 2018, 71, 215–233; (d) S. Scheeff and D. Menche, Org. Lett., 2019, 21, 271–274; (e) N. Y. S. Lam and I. Paterson, Eur. J. Org. Chem., 2020, 2310–2320.
16. (a) W. R. Roush, A. D. Palkowitz and K. Ando, J. Am. Chem. Soc., 1990, 112, 6348–6359; (b) A. Arefolov and J. S. Panek, J. Am. Chem. Soc., 2005, 127, 5596–5603; (c) A.-M. R. Dechert-Schmitt, D. C. Schmitt, X. Gao, T. Itoh and M. J. Krische, Nat. Prod. Rep., 2014, 31, 504–513; (d) M. Pantin, J. G. Hubert, T. Söhnel, M. A. Brimble and D. P. Furkert, J. Org. Chem., 2017, 82, 11225–11229; (e) K. Spielmann, G. Niel, R. M. de Figueiredo and J.-M. Campagne, Chem. Soc. Rev., 2018, 47, 1159–1173.
17. (a) J. Gorges and U. Kazmaier, Org. Lett., 2018, 20, 2033–2036; (b) O. Andler and U. Kazmaier, Chem. – Eur. J., 2021, 27, 949–953.
18. Reviews: (a) D. S. Matteson, J. Org. Chem., 2013, 78, 10009–10023; (b) S. Kirupakaran, H.-G. Korth and C. Hirschhäuser, Synthesis, 2018, 50, 2307–2322.
19. (a) R. Ray and D. S. Matteson, Tetrahedron Lett., 1980, 21, 449–450; (b) D. S. Matteson and R. Ray, J. Am. Chem. Soc., 1980, 102, 7590–7591.
20. (a) D. S. Matteson, K. M. Sadhu and M. L. Peterson, J. Am. Chem. Soc., 1986, 108, 810–819; (b) D. S. Matteson and T. J. Michnick, Organometallics, 1990, 9, 3171–3177; (c) D. S. Matteson and J.-J. Yang, Tetrahedron: Asymmetry, 1997, 8, 3855–3861; (d) H. W. Man, W. C. Hiscox and D. S. Matteson, Org. Lett., 1999, 1, 379–382.
21. H. C. Brown, M. Srebnik and T. E. Cole, Organometallics, 1986, 5, 2300–2303.
22. W. C. Hiscox and D. S. Matteson, J. Org. Chem., 1996, 61, 8315–8316.
23. (a) D. S. Matteson and H.-W. Man, J. Org. Chem., 1994, 59, 5734–5741; (b) D. S. Matteson, J. Organomet. Chem., 1999, 581, 51–65.
24. L. A. Carpino, B. J. Cohen, K. E. Stephens, S. Y. Sadat-Aalaee, J. H. Tien and D. C. Langridge, J. Org. Chem., 1986, 51, 3732–3734.
25. S. You, H. Razavi and J. W. Kelly, Angew. Chem., Int. Ed., 2003, 42, 83–85 ( Angew. Chem. , 2003 , 115 , 87–89 ).
26. B. M. Trost, C. A. Kalnmals, J. S. Tracy and W. Bai, Org. Lett., 2018, 20, 8043–8046.
27. H. Luesch, W. Y. Yoshida, R. E. Moore and V. J. Paul, Bioorg. Med. Chem., 2002, 10, 1973–1978.
28. J. Hur, J. Jang, J. Sim, W. S. Son, H. Ahn, T. S. Kim, Y. Shin, C. Lim, S. Lee, H. An, S. Kim, D.-C. Oh, E. Jo, J. Jang, J. Lee and Y. Suh, Angew. Chem., Int. Ed., 2018, 57, 3069–3073 ( Angew. Chem. , 2018 , 130 , 3123–3127 ).

---

Footnote

† Electronic supplementary information (ESI) available: Experimental details, compound characterization, copies of ¹H and ¹³C NMR spectra and HPLC chromatograms. See DOI: 10.1039/d1ob00713k

---