Total synthesis of (-)-basiliskamide A and NMR studies on the conversion of basiliskamide A to basiliskamide B

Abstracts

We describe herein our approach to the total synthesis of the antifungal polyketide (-)-basiliskamide A, as well as ¹H NMR studies on the migration of the cinnamoyl side chain of basiliskamide A to form basiliskamide B in CDCl3 solution.

antifungal polyketides; Stille cross-coupling; aldol reaction; NMR studies

Descrevemos neste trabalho nossa rota sintética para obtenção do policetídeo antifúngico (-)-basiliskamida A, assim como estudos de RMN de ¹H relacionados a migração da cadeia lateral cinamoíla da basiliskamida A para formar a basiliskamida B em CDCl3.

SHORT REPORT

Total synthesis of (-)-basiliskamide A and NMR studies on the conversion of basiliskamide A to basiliskamide B

Luiz C. Dias^*; Caroline C. S. Gonçalves

Instituto de Química, Universidade Estadual de Campinas, CP 6154, 13083-970 Campinas-SP, Brazil

ABSTRACT

We describe herein our approach to the total synthesis of the antifungal polyketide (-)-basiliskamide A, as well as ¹H NMR studies on the migration of the cinnamoyl side chain of basiliskamide A to form basiliskamide B in CDCl₃ solution.

Keywords: antifungal polyketides, Stille cross-coupling, aldol reaction, NMR studies

RESUMO

Descrevemos neste trabalho nossa rota sintética para obtenção do policetídeo antifúngico (-)-basiliskamida A, assim como estudos de RMN de ¹H relacionados a migração da cadeia lateral cinamoíla da basiliskamida A para formar a basiliskamida B em CDCl₃.

Introduction

The polyketides basiliskamides A (1) and B (2) were isolated from the marine bacterium PNG-276 from the coast of Papua New Guinea (Figure 1).^1,2 These antifungal polyketides show potent in vitro activity against Candida albicans and Aspergillus fumigatus,¹ as well as at least 4-fold less cytotoxicity for normal human fibroblast cells when compared to amphotericin B.^1,3 The relative configurations of basiliskamides A (1) and B (2) were proposed by Andersen and co-workers¹ and the absolute configurations have been confirmed by total synthesis.⁴

We have recently concluded the total synthesis of basiliskamide B.⁵ As the natural supply of basiliskamide A is restricted, and attracted by its potent cytotoxicity, we initiated a project directed towards an efficient and flexible total synthesis, in order to provide further material for more extensive biological studies, along with access to novel analogs.⁶

Results and Discussion

Our approach for the preparation of (-)-basiliskamide A (1) began with the synthesis of vinyl iodide 4, prepared in 7 steps and 19.1% overall yield from ethylketone 3 (Scheme 1).^5,7

The next step involved a Stille cross-coupling reaction between Z-vinylstannane 5⁸ and E-vinyl iodide 4 (Scheme 2).This was accomplished by treatment of 4 and 5 in DMF with catalytic amounts of Pd(MeCN)₂Cl₂ at 50 ºC, providing 6 in 60% yield after purification by silica-gel column chromatography. At this point only four synthetic operations remained to conclude the total synthesis of (-)-basiliskamide A. Removal of the isopropylidene acetal protecting group in 6 with 80% AcOH provided diol 7. Selective protection of the less hindered oxygen with TES chloride in the presence of pyridine gave alcohol 8 (67% yield, 2 steps).⁹ Treatment of secondary alcohol 8 with E-cinnamoyl chloride in the presence of DMAP and Et₃N gave ester 9, which, after treatment with HF in acetonitrile, provided (-)-basiliskamide A in 42% for the four-step sequence.

The spectroscopic and physical data (¹H and ¹³C NMR, IR, [α]^D, R_f) for synthetic (-)-basiliskamide A were identical in all respects with the published data for the natural product.^1,4

NMR studies

At this point, we decided to investigate the possibility of basiliskamide B being produced by cinnamoyl side chain migration from basiliskamide A. To this end, we did an NMR study to verify whether the conversion of basiliskamide A to basiliskamide B in mild acidic CDCl₃ solution was possible or not (Scheme 3). This conversion would occur through migration of the cinnamoyl side chain from the more hindered oxygen at C9 in basiliskamide A to the less hindered oxygen at C7.

In the ¹H NMR spectrum of basiliskamide A (1), the signal at δ 3.63 ppm (dt, J 3.5 and 10.0 Hz) is attributed to the hydrogen H7. In the ¹H NMR spectrum of basiliskamide B the signal at δ 3.35 ppm (dd, J 2.5 and 9.5 Hz) refers to the hydrogen H9 (Figure 2).

After 24 h, the ¹H NMR spectrum of basiliskamide A (1) shows a new signal at δ 3.35 ppm, which corresponds to the hydrogen H9 of basiliskamide B (Figure 3). After 20 days, we can observe that the signal at δ 3.35 ppm increases leading to a 1.00:0.45 ratio of basiliskamide A (1) and basiliskamide B (2), respectively. This clearly shows that the migration of the cinnamoyl side chain is occurring and that it is possible that basiliskamide B is an artifact of basiliskamide A.¹⁰

Conclusions

In summary, a highly convergent and efficient total synthesis of (-)-basiliskamide A has been accomplished. The synthesis required 15 steps from ethylketone 3 and produced the desired product in 3.2% overall yield. This approach compares very well with the published routes to (-)-basiliskamide A⁴ and the synthetic strategy presented here is, in principle, readily applicable for the preparation of additional analogues.

Experimental

(-)-Basiliskamide A (1)

The diene 6 (68.5 mg, 0.23 mmol) was dissolved in 80% acetic acid (5 mL). The reaction was protected from light and stirred at 60 ºC for 3 h. The reaction was then allowed to reach ambient temperature and adjusted to pH 7.0 with NH₄OH. The aqueous phase was extracted with EtOAc (2 × 10 mL), the combined organic extracts were dried over MgSO₄, filtered and concentrated under reduced pressure to give the corresponding diol 7. R_f= 0.15 (hexane:EtOAc, 50:50).= +15 (c = 0.35, CHCl₃). ¹H NMR (DMSO-d₆, 250 MHz):δ 0.69 (d, 3H, J 7.0 Hz); 0.73 (d, 3H, J 6.3 Hz); 0.84 (t, 3H, J 7.3 Hz); 1.35 (m, 3H), 1.61 (m, 1H); 2.05 (m, 1H); 2.28 (m, 1H); 3.24 (m, 1H); 3.81 (m, 1H); 4.49 (d, 1H, J 5.3 Hz); 4.7 (d, 1H, J 4.7 Hz); 5.58 (d, 1H, J 5.6 Hz); 6.03 (dt, 1H, J 6.0 Hz); 6.37 (t, 1H, J 11.5 Hz); 6.86 (brs, 1H); 7.37 (brs, 1H);7.45 (dd, 1H, J 11.5 e 15.5 Hz). ¹³C NMR (DMSO-d₆, 62.5 MHz):δ 11.3; 12.4; 12.5; 27.22; 35.9; 36.8; 41.9; 71.9; 74.8; 119.6; 128.5; 141.2; 141.3; 168.1.IR (film) ν_max/cm^-1:3350, 2964, 2930, 2871, 1666, 1454, 1327. HRMS: [C₁₄H₂₆NO₃⁺]: expected: 256.1907; observed: 256.1913.

A solution of the corresponding crude diol 7 obtained before (44.8 mg; 0.18 mmol) in pyridine (3 mL) was treated with TESCl (0.033 mL; 0.19 mmol) at 0 ºC, and stirred at 25 ºC for 2 h. The reaction was quenched with aqueous saturated NaHCO₃ solution and extracted with EtOAc providing alcohol 8. R_f=0.4 (hexane:EtOAc, 50:50). ¹H NMR (C₆D₆, 250 MHz):δ 0.73 (q, 6H, J 8 Hz); 0.84 (d, 3H, J 6.8 Hz); 1.01 (m, 6H); 1.12 (t, 9H, J 8 Hz); 1.49 (m, 3H); 2.02 (m, 1H); 2.43 (t, 2H, J 6.3 Hz); 2.67 (brd, 1H, J 3.3 Hz); 3.44 (brd, 1H, J 8.5 Hz); 4.23 (q, 1H, J 5.3 Hz); 4.72 (brs, 1H); 5.19 (d, 1H, J 11.3 Hz); 5.59 (brs, 1H); 6.25 (dt, 1H, J 7.3 e 15.5 Hz); 6.39 (t, 1H, J 11.3 Hz); 8.17 (dd, 1H, J 11.5 e 15.3 Hz).¹³C NMR (C₆D₆, 62.5 MHz):δ 5.5; 7.2; 11.3; 11.8; 12.4; 27.6; 36.8; 37.2; 42.1; 75.2; 75.6; 118.2; 129.6; 141.1; 142.6; 168.1.

The crude alcohol 8 (0.029 mmol) in CH₂Cl₂ (0.7 mL) was treated with Et₃N (0.034 mL; 0.24 mmol) and DMAP (1.7 mg, 0.014 mmol) and cooled to 0 ºC. The solution was treated with (E)-cinnamoyl chloride (0.02 g; 0.12 mmol). The reaction was protected from light and stirred at ambient temperature for 18 h, and the reaction was then quenched with water. The aqueous phase was extracted with CH₂Cl₂ and the solvent was removed under reduced pressure to provide ester 9. R_f= 0.55 (EtOAc:CH₂Cl₂, 40:10).= -77 (c 0.26, MeOH). ¹H NMR (CDCl₃, 500 MHz):δ 0.83 (d, 3H, J 7.0 Hz); 0.84 (d, 3H, J 7.0 Hz); 0.87 (t, 3H, J 7.5 Hz); 0.90 (d, 3H, J 7.0 Hz); 1.11 (m, 1H) 1.25 (m, 1H); 1.67 (m, 1H); 1.99 (m, 1H); 2.06 (m, 1H); 2.28 (m, 1H); 3.49 (m, 1H); 4.57 (d, 1H, J 5.0 Hz); 4.92 (dd, 1H; J 2.0 e 9.5 Hz); 5.55 (d, 1H, J 11.0 Hz); 5.91 (dt, 1H, J 7.0 e 15.0 Hz); 6.31 (brt, 1H, J 11.0 Hz); 6.61 (d, 1H, J 16.0 Hz); 6.83 (s, 1H); 7.31 (s, 1H); 7.40 (dd, 1H, J 11.0 e 15.0 Hz); 7.41 (m, 2H); 7.65 (d, 1H, J 16.0 Hz); 7.71 (m, 2H). ¹³C NMR (CDCl₃, 100 MHz):δ 10.1; 11.6; 12.8; 26.4; 34.7; 35.5; 40.8; 69.7; 76.3; 118.0; 119.3; 128.2; 128.4; 128.9; 130.4; 134.0; 140.56; 140.58; 144.6; 166.1; 167.5. IR (film) ν_max/cm^-1: 3367, 2966, 2936, 1701, 1670, 1588.

The crude compound 9 (0.029 mmol) was treated with 0.4 mL of a solution of HF (40% aqueous solution): CH₃CN (1:18). After 10 min at r.t. the reaction was quenched with NaHCO₃ (4 mg) and stirred for 10 min, filtered and concentrated in vacuo. Purification by flash column chromatography (EtOAc:CH₂Cl₂, 4:2) afforded (-)-basiliskamide A (1) (4.6 mg, 42% yield, 4 steps). R_f= 0.6 (EtOAc:CH₂Cl₂, 40:10). ¹H NMR (DMSO-d₆, 5000 MHz): δ 7.72 (2H, m); 7.66 (1H, d, 16.0 Hz); 7.41 (3H, m); 7.41 (3H, m); 7.39 (1H, m); 7.33 (1H, brs); 6.84 (1H, brs); 6.62 (1H, d, 16.0 Hz); 6.31 (1H, apt, 11.5 Hz); 5.91 (1H, dt, 7.4 and 14.9 Hz); 5.55 (1H, d, 11.5 Hz); 4.92 (1H, dd, 2.6 and 9.5 Hz); 4.58 (1H, d, 5.4 Hz); 3.48 (1H, m); 2.28 (1H, m); 2.02 (1H, m); 2.02 (1H, m), 1.67 (1H, m); 1.25 (1H, m); 1.11 (1H, m); 0.89 (3H, d, 6.6 Hz); 0.87 (3H, t, 7.7 Hz); 0.84 (3H, d, 7.1 Hz) ppm. ¹³C NMR (DMSO-d₆, 125 MHz): δ 167.5; 166.1; 144.6; 140.56; 140.51; 134.0; 130.4; 128.9; 128.4; 128.3; 119.4; 118.0; 76.3; 69.7; 40.8; 35.5; 34.7; 26.4; 12.8; 11.7; 10.1 ppm.

Supplementary Information

Supplementary data are available free of charge at http://jbcs.sbq.org.br, as a PDF file.

Acknowledgments

We are grateful to FAEP-Unicamp, FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and INCT-INOFAR Proc. CNPq 573.564/2008-6 for financial support. We also thank Prof. Carol H. Collins (Unicamp), for helpful suggestions about English grammar and style, Leonardo José Steil (UFABC) for helpful comments and Professor Carlos Roque Duarte Correia (Unicamp) for suggestions and comments about the NMR studies.

References

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10. When submitting (-)-basiliskamide B to the same experiment we can not observe the signal at d 3.63 ppm (dt, J 3.5 and 10.0 Hz) that corresponds to hydrogen H7 of basiliskamide A. However, the formation of another compound that shows three well defined signals (d 3.65, 4.94 and 5.04 ppm) was observed.

Submitted: May 4, 2010

Published online: August 3, 2010

Supplementary Information

Publication Dates

• Publication in this collection
  01 Dec 2010
• Date of issue
  2010

History

• Received
  04 May 2010
• Accepted
  03 Aug 2010