Confirmation of the Configuration of 10-Isothiocyanato-4-cadinene

Nov 19, 2012 - Journal of Natural Products .... John W. Blunt , Brent R. Copp , Robert A. Keyzers , Murray H. G. Munro ... Yasman, Edrada, Wray and Pr...
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Confirmation of the Configuration of 10-Isothiocyanato-4-cadinene Diastereomers through Synthesis Keisuke Nishikawa,† Taiki Umezawa,† Mary J. Garson,‡ and Fuyuhiko Matsuda*,† †

Division of Environmental Materials Science, Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan ‡ School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, QLD 4072, Australia S Supporting Information *

ABSTRACT: The marine sponge metabolite 10-isothiocyanato4-cadinene (1) was first isolated by Garson et al. from Acanthella cavernosa in 2000. The same structure 1 was later reported by Wright et al. from the nudibranch Phyllidiella pustulosa and its sponge diet, but with different NMR data. The syntheses of both enantiomers of 1 were accomplished through the isothiocyanation of 10-isocyano-4-cadinene (2) previously synthesized by our group. The correct spectroscopic data and specific rotation value of the structure 1 were determined on the basis of the syntheses. The NMR data of synthetic 1 matched those of the isothiocyanate isolated by Garson and differed from those reported by Wright. The spectroscopic data and specific rotation values of 10-epi-10-isothiocyanato-4cadinene (6) and di-1,6-epi-10-isothiocyanato-4-cadinene (8) were also established through the syntheses of these diastereomers. Structure 6 has been reported as a natural product by Mitome et al., but the NMR data for the synthetic sample of 6 differ from those of the natural isolate. he terpene 10-isothiocyanato-4-cadinene (1) was first isolated by Garson et al. from the marine sponge Acanthella cavernosa in 2000 along with the major metabolites axisonitrile-3 (3) and axisothiocyanate-3 (4).1,2 As a structural feature, 1 has four contiguous stereocenters including a quaternary carbon with an isothiocyanate group. The relative configuration of these stereocenters of 1 is the same as those of 10-isocyano-4-cadinene (2) isolated by Okino et al. from nudibranchs of the family Phyllidiidae in 1996.3 Although the relative configuration of 1 was determined using 1D and 2D NMR experiments, the absolute configuration of 1 has not yet been established. In 2003, Wright described the isolation of an isothiocyanate showing antiplasmodial activity from the nudibranch Phyllidiella pustulosa and its dietary sponge Phakellia carduus and which had the same structure and relative configuration as 1.4 However, the 1H and 13C NMR spectroscopic data of his isothiocyanate were different from those of 1 reported by Garson. Notably, the 13C NMR value (CDCl3) for C-10 was assigned as 64.7 ppm by Garson, whereas Wright reported 61.1 ppm. Comparing the 1H NMR data (CDCl3), there were differences in the chemical shift values reported for H-1, H-6, H-7, H-8ax, H2-9, and the methyl group attached to C-10 (Me-14). For example, the Garson isolate has a 1H NMR resonance for the Me-14 at 1.27 ppm, whereas Wright reported 1.37 ppm. The current paper describes the preparation of 1 and related diastereomers from isocyanides previously synthesized by our group and the use of these synthetic isothiocyanates in the determination of the configurations of the natural isolates.

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© 2012 American Chemical Society and American Society of Pharmacognosy

In our previous study, the first enantioselective total synthesis of 2 and ent-2 was achieved by employing an intermolecular Diels−Alder reaction and a Barbier-type cyclization using samarium diiodide (SmI2) as the key steps.5 On the basis of the total synthesis, the absolute configuration of natural 2 was unambiguously determined to be (1S,6S,7R,10S). Compound 1 was synthesized via the isothiocyanation of 2 according to our synthetic pathway. The treatment of 2 with S, Et3N, and Se using the synthetic method mentioned by Kambe et al.6 afforded (1S,6S,7R,10S)-10-isothiocyanato-4-cadinene (1) in 42% yield (Scheme 1). Table 1 shows a comparison of the 13C NMR spectroscopic data of synthetic 1 with those reported by Garson1 and by Wright.4 The 13C NMR spectrum of synthetic 1 closely corresponded to that reported by Garson and differed from that reported by Wright. Other data (1H NMR and MS) of synthetic 1 were also comparable with those of the natural sample isolated by Garson. The 1H NMR spectroscopic data of the isothiocyanate isolated by Wright were different from those of synthetic 1. The specific rotation of synthetic 1, [α]D23 +21.2 Received: June 22, 2012 Published: November 19, 2012 2232

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Scheme 1. Syntheses of 1 and ent-1

Scheme 2. Syntheses of 6 and 8

Table 1. 13C NMR Data for Natural 1 and Synthetic 1 in CDCl3

definitively establish the configuration at C-1, nor is a value reported for the H-1/H-6 coupling constant as a result of signal overlap.4 Considering next the amorphene series of sesquiterpenes with a cis configuration at the ring junction, (1R,6S,7S,10S)-10isothiocyanato-4-amorphene (9),10−12 (1S,6R,7R,10R)-10-isothiocyanato-4-amorphene (ent-9),3,10,13 and (1R,*6S,*7R,*10R*)10-isothiocyanato-4-amorphene (axinisothiocyanate K) (10)12,14 have all been reported in the sponge literature (Figure 1).

natural 1 by Garson

synthetic 1

natural 1 by Wright

position

(125 MHz)

(100 MHz)

(100 MHz)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

48.7 24.1 30.6 135.2 121.3 38.4 46.2 20.6 40.7 64.7 25.9 15.0 21.4 20.0 23.7 not observed

48.8 24.3 30.7 135.1 121.2 38.5 46.3 20.8 40.9 64.8 26.0 15.2 21.5 20.2 23.8 not observed

47.8 25.0 30.9 135.3 121.6 39.7 46.3 21.7 41.8 61.1 26.0 15.1 21.4 19.2 23.6 not observed

Figure 1. Amorphene sesquiterpenes.

(c 0.25, CH2Cl2), was identical in sign to that of natural 1 isolated by Garson, [α]D +3 (c 0.15, CH2Cl2).7 We also synthesized the enantiomer (1R,6R,7S,10R)-10-isothiocyanato4-cadinene (ent-1) from (1R,6R,7S,10R)-ent-2 by the same synthetic procedure. The specific rotation of ent-1, [α]23D −24.8 (c 0.17, CH2Cl2), is opposite in sign of that of the natural product. The enantiospecific syntheses defined the absolute configurations of the two enantiomers, 1 and ent-1. However, the specific rotation value of natural 1 is small and close to zero. Evaluation of the optical purity of natural 1 is not possible, as the natural sample is no longer available. The same isothiocyanation of (1S,6S,7R,10R)-10-epi-10isocyano-4-cadinene (5) and (1R,6R,7R,10S)-di-1,6-epi-10isocyano-4-cadinene (7), previously synthesized by our group,5 provided (1S,6S,7R,10R)-10-epi-10-isothiocyanato-4cadinene (6)8 and (1R,6R,7R,10S)-di-1,6-epi-10-isothiocyanato-4-cadinene (8) in 36% and 39% yields (Scheme 2). However, the 1H and 13C NMR spectroscopic data of the isothiocyanates 6 and 8 were not identical to those reported by Wright. Therefore, the isothiocyanate isolated by Wright is neither of these diastereomers of 10-isothiocyanato-4-cadinene. Furthermore, both 89 and the remaining cadinene diastereomer, (1S,*6S,*7S,*10S*)-7-epi-10-isothiocyanato-4-cadinene, have the isopropyl group in an axial orientation. The 1H NMR data published by Wright support an axial position for each of H-6, H-7, and the C-10 methyl group (Me-14) and, hence, an equatorial orientation for the C-7 isopropyl group. On the other hand, the NOE difference data provided do not

However, the 1H and 13C NMR spectroscopic data of the isothiocyanate isolated by Wright were different from those of 9 and 10. Moreover, on the basis of our evaluation of the 1H and 13 C NMR spectroscopic data of the isothiocyanates 1, 6, 8, 9, and 10 with those of the Wright isolate,10 it became clear that the 1H/13C NMR data described by Wright were closest to those of 1, despite the differences in chemical shift identified above. In their work on marine sponges, Crews et al. noted that (1S,*4R,*6S,*7R*)-4-isothiocyanato-9-amorphene (11) and (1R,*4R,*6R,*7S*)-4-thiocyanato-9-amorphene (12)15 found in marine sponges have nearly identical 13C NMR shifts, apart from those at C-4 and its substituents.11,16 In 12, the 13C shift value (CDCl3) for C-4 (having the tertiary thiocyanate group) is 58.5 ppm, quite similar to the 61.1 ppm value reported by Wright for the carbon bearing the tertiary isothiocyanate group. While the Wright NMR data clearly establish a C-4/C-5 (rather than C-9/C-10) double bond, there seems to be a possibility that the Wright isolate could be (1S,*6S,*7R,*10S*)-10thiocyanato-4-cadinene, i.e., with a thiocyanate in place of the isothiocyanate. Wright reported an IR band at 2149 cm−1 that is not inconsistent with the presence of a thiocyanate group; in isothiocyanates, the equivalent band is at 2150−2050 cm−1 and is usually described as broad and strong.15,17 Terpene thiocyanates can also be distinguished from their isothiocyanate counterparts by the 13C chemical shift of the −SCN (112−114 ppm) vs −NCS (126−132 ppm) group,15,17,18 although these 2233

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EIMS m/z 263 [M+]; HREIMS m/z 263.1709 [M+] (calcd for C16H25NS, 263.1708). Synthesis of (1R,6R,7S,10R)-10-Isothiocyanato-4-cadinene (ent-1). Using the same procedure starting from (1R,6R,7S,10R)-ent-2 (5.10 mg, 22.5 μmol), 2.42 mg (9.20 μmol, 41%) of (1R,6R,7S,10R)-ent-1 was obtained as a colorless oil: [α]23D −41.1 (c 0.19, CHCl3), [α]23D −24.8 (c 0.17, CH2Cl2). Synthesis of (1S,6S,7R,10R)-10-epi-10-Isothiocyanato-4-cadinene (6). Using the same procedure starting from (1R,6R,7S,10S)-5 (7.80 mg, 33.8 μmol), 3.20 mg (12.2 μmol, 36%) of (1R,6R,7S,10S)-6 was obtained as a colorless oil: [α]23D −57.6 (c 0.17, CHCl3); IR (neat) 2952, 2918, 2848, 2092, 1733, 1448, 1376, 1297, 1258, 1209, 1190, 1148, 1098, 1031, 875, 802 cm−1; 1H NMR (CDCl3, 400 MHz) δ 0.83 (3H, d, J = 6.8 Hz), 0.92 (3H, d, J = 6.8 Hz), 1.05 (1H, t, J = 11.4 Hz), 1.18 (1H, t, J = 11.7 Hz), 1.31−1.68 (5H, m), 1.43 (3H, s), 1.67 (3H, s), 1.91−2.11 (4H, m), 2.19 (1H, m), 5.52 (1H, brs); 13C NMR (CDCl3, 100 MHz) δ 15.2, 20.6, 21.5, 23.6, 23.8, 26.3, 27.5, 30.5, 38.5, 40.4, 46.2, 49.0, 63.9, 121.6, 134.3; EIMS m/z 263 [M+]; HREIMS m/z 263.1699 [M+] (calcd for C16H25NS 263.1708). Synthesis of (1R,6R,7S,10S)-10-epi-10-Isothiocyanato-4-cadinene (ent-6). Using the same procedure starting from (1S,6S,7R,10R)-ent-5 (8.50 mg, 34.4 μmol), 3.45 mg (13.1 μmol, 38%) of (1S,6S,7R,10R)ent-6 was obtained as a colorless oil: [α]23D +55.9 (c 0.15, CHCl3). Synthesis of (1R,6R,7R,10S)-Di-1,6-epi-10-isothiocyanato-4-cadinene (8). Using the same procedure starting from (1R,6R,7R,10S)-7 (29.3 mg, 0.127 mmol), 13.0 mg (49.4 μmol, 39%) of (1R,6R,7R,10S)8 was obtained as white crystals: mp 90−91 °C; [α]23D +135 (c 0.31, CHCl3); IR (KBr) 2918, 2952, 2850, 2142, 1728, 1444, 1374, 1288, 1259, 1189, 1149, 1104, 1025, 969, 927, 891, 848, 799 cm−1; 1H NMR (CDCl3, 400 MHz) δ 0.88 (3H, d, J = 6.7 Hz), 0.92 (3H, d, J = 6.7 Hz), 1.32−1.85 (8H, m), 1.42 (3H, s), 1.62 (3H, s), 1.88−2.06 (3H, m), 2.46 (1H, dd, J = 2.3, 10.6 Hz), 5.24 (1H, brs); 13C NMR (CDCl3, 100 MHz) δ 22.8, 23.4, 23.5, 23.6, 25.9, 26.2, 27.5, 30.0, 36.3, 41.3, 42.6, 44.3, 64.8, 127.1, 127.4, 130.5; EIMS m/z 263 [M+]; HREIMS m/z 263.1708 [M+] (calcd for C16H25NS 263.1708). Synthesis of (1S,6S,7S,10R)-Di-1,6-epi-10-isothiocyanato-4-cadinene (ent-8). Using the same procedure starting from (1S,6S,7S,10R)ent-7 (12.1 mg, 49.0 μmol), 5.28 mg (20.1 μmol, 41%) of (1S,6S,7S,10R)-ent-8 was obtained as white crystals: mp 88−89 °C; [α]23D −132 (c 0.28, CHCl3).

signals are often of low intensity in the 13C NMR spectra unless an extended delay time (>5 s) is used. LiAlH4 reduction of thiocyanates to a thiol product provides a convenient chemical approach to confirm the presence of the thiocyanate functionality.17,19 In 2004, Mitome et al. reported isothiocyanate 6 as a natural product from the Okinawan marine sponge Stylissa sp., but with only the relative configuration and a positive specific rotation, [α]23D +75.7 (c 2.4, CHCl3).8 Our synthetic sample of (1S,6S,7R,10R)-10-epi-10-isothiocyanato-4-cadinene (6) had an [α]23D −57.6 (c 0.17, CHCl3), suggesting that natural 6 was (1R,6R,7S,10S)-10-epi-10-isothiocyanato-4-cadinene (ent-6). For a direct comparison, we prepared (1R,6R,7S,10S)-ent-6 by the same synthetic procedure,9 with an [α]23D +55.9 (c 0.15, CHCl3). However, when the 1H and 13C NMR spectroscopic data of synthetic 6 and ent-6 were compared with those published by Mitome et al.,8 it was apparent that the structure of the natural isolate did not correspond to 6. We also noted that the 13C NMR data of the Mitome isolate were very similar to those of axinisothiocyanate K (10), the 10-isothiocyanato-4amorphene isolated by Zubiá et al. from a sponge of the genus Axinyssa,12 but that the published 1H NMR spectrum (500 MHz) of the Mitome isothiocyanate did not match the 1 H NMR spectrum (400 MHz) of 10. In conclusion, the syntheses of both enantiomers of 10-isothiocyanato-4-cadinene, 1 and ent-1, were achieved by conversion of the isonitrile group of 10-isocyano-4-cadinene into the isothiocyanato group. On the basis of the syntheses, the correct spectroscopic data and specific rotation value of 10-isothiocyanato-4-cadinene were determined. Moreover, the correct spectroscopic data and specific rotation values of 10-epi-10isothiocyanato-4-cadinene (6) and di-1,6-epi-10-isothiocyanato4-cadinene (8) and their enantiomers were also established through the syntheses of these stereoisomers. The 1H and 13C NMR spectroscopic data of the three synthetic cadinene derivatives 1, 6, and 8 have highlighted complications in the structure determination of terpene isothiocyanates by both Wright4 and Mitome et al.8 Our work has clearly revealed major challenges in the correct assignment of configurations for terpenes in the amorphene/cadinene structural classes.

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ASSOCIATED CONTENT

S Supporting Information *

1

H and 13C NMR spectra in CDCl3 for compounds 1, 6, and 8, and comparison of 13C NMR data (CDCl3) of 1, 6, and 8 and published data for 9 and 10 with those reported by Wright and Mitome et al. This material is available free of charge via the Internet at http://pubs.acs.org.

EXPERIMENTAL SECTION

General Experimental Procedures. The optical rotations were obtained on a Horiba SEPA-300 polarimeter. The IR spectra were recorded on a JASCO IR Report 100 spectrometer using a NaCl cell or KBr disk. The 1H and 13C NMR spectra were recorded using a JNM-EX 400 (400 and 100 MHz) spectrometer. Chemical shifts were reported in ppm downfield from the peak of Me4Si used as the internal standard. All commercially obtained reagents were used as received. Analytical and preparative TLC was carried out using precoated silica gel plates (Macherey-Nagel DC-Fertigplatten SIL G-25 UV254). Synthesis of (1S,6S,7R,10S)-10-Isothiocyanato-4-cadinene (1). To a solution of (1S,6S,7R,10S)-2 (4.40 mg, 19.0 μmol) in THF (0.19 mL) were added S (6.10 mg, 0.190 mmol), Se (0.60 mg, 7.60 μmol), and Et3N (26.6 μL, 0.190 mmol) at room temperature under an Ar atmosphere. The mixture was refluxed for 3 h. After the solvent was removed in vacuo, the residue was purified by HPLC (Mightysil Si-60, 4.6 × 250 mm, EtOAc/hexane, 0.3:99.7, flow rate 1.0 mL/min) to afford (1S,6S,7R,10S)-1 (2.10 mg, 7.98 μmol, 42%) as a colorless oil: [α]23D +43.9 (c 0.27, CHCl3), [α]23D +21.2 (c 0.25, CH2Cl2); IR (neat) 2954, 2922, 2850, 2072, 1734, 1463, 1452, 1411, 1380, 1259, 1209, 1098, 1034, 864, 802 cm−1; 1H NMR (CDCl3, 400 MHz) δ 0.74 (3H, d, J = 7.0 Hz), 0.90 (3H, d, J = 7.0 Hz), 0.98−1.19 (2H, m), 1.28 (3H, s), 1.33 (1H, m), 1.42−1.83 (4H, m), 1.66 (3H, s), 1.88−2.20 (5H, m), 5.45 (1H, brs); 13C NMR (CDCl3, 100 MHz), see Table 1;

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-11-706-4520. E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Global COE Research Assistantship for doctoral candidates (to K.N.) and the Australian Research Council and The University of Queensland for financial support (to M.J.G.).

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REFERENCES

(1) Clark, R. J.; Stapleton, B. L.; Garson, M. J. Tetrahedron 2000, 56, 3071−3076. (2) (a) Garson, M. J.; Simpson, J. S. Nat. Prod. Rep. 2004, 21, 164− 179. (b) Jumaryatno, P.; Stapleton, B. L.; Hooper, J. N. A.; Brecknell, D. J.; Blanchfield, J. T.; Garson, M. J. J. Nat. Prod. 2007, 70, 1725− 1730.

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(3) Okino, T.; Yoshimura, E.; Hirota, H.; Fusetani, N. Tetrahedron 1996, 52, 9947−9952. (4) Wright, A. D. Comp. Biochem. Physiol. 2003, 134A, 307−313. (5) (a) Nishikawa, K.; Nakahara, H.; Shirokura, Y.; Nogata, Y.; Yoshimura, E.; Umezawa, T.; Okino, T.; Matsuda, F. Org. Lett. 2010, 12, 904−907. (b) Nishikawa, K.; Nakahara, H.; Shirokura, Y.; Nogata, Y.; Yoshimura, E.; Umezawa, T.; Okino, T.; Matsuda, F. J. Org. Chem. 2011, 76, 6558−6573. (6) Fujiwara, S.; Shin-Ike, T.; Sonoda, N.; Aoki, M.; Okada, K.; Miyoshi, N.; Kambe, N. Tetrahedron Lett. 1991, 32, 3503−3506. (7) The concentration value in ref 1 was cited as g/mL rather than g/ 100 mL. (8) Mitome, H.; Shirato, N.; Miyaoka, H.; Yamada, Y.; van Soest, R. W. M. J. Nat. Prod. 2004, 67, 833−837. (9) The synthesis of (1S,6S,7S,10R)-di-1,6-epi-10-isothiocyanato-4cadinene (ent-8) from (1S,6S,7S,10R)-di-1,6-epi-10-isocyano-4-cadinene (ent-7) was also carried out by using the same synthetic method; see Experimental Section. Note that the di-1,6-epimers (1R,6R,7R,10S)-8 and (1S,6S,7S,10R)-ent-8 are also the di-7,10epimers of (1R,6R,7S,10R)-ent-1 and (1S,6S,7R,10S)-1, respectively. (10) For the comparison of 13C NMR shift values for the synthetic cadinenes 1, 6, and 8 and the natural amorphenes 9 and 10 with those reported by Wright and Mitome et al., see Supporting Information. Although Scheuer et al. first isolated ent-9 from a deep-water Halichondria sp. and determined its absolute configuration in 1975 (ref 13), Crews et al. described an isolation of the enantiomer 9 from the Fiji sponge Axinyssa fenestratus and a complete assignment of its 13 C NMR spectrum (CDCl3) with the 13C shift value for −NCS in 1991 (ref 11). Therefore, we compared the 13C NMR values for these sesquiterpenes by using the 13C NMR data of 9 provided in ref 11. (11) Alvi, K. A.; Tenenbaum, L.; Crews, P. J. Nat. Prod. 1991, 54, 71−78. (12) Zubía, E.; Ortega, M. J.; Hernández-Guerrero, C. J.; Carballo, J. L. J. Nat. Prod. 2008, 71, 608−614. (13) Burreson, B. J.; Christophersen, C.; Scheuer, P. J. Tetrahedron 1975, 31, 2015−2018. (14) Structure 10 is not only the di-7,10-epimer of 9 but also the di1,6-epimer of ent-9; the absolute configuration of axinisothiocyanate K has not yet been determined. See ref 12. (15) He, H.-Y.; Faulkner, D. J.; Shumsky, J. S.; Hong, K.; Clardy, J. J. Org. Chem. 1989, 54, 2511−2514. (16) Crews et al. described opposite configurations at C-1, C-6, and C-7 for this isothiocyanate (11) and thiocyanate (12) pair, but only relative configurations have been assigned for 11 and 12; see ref 11. (17) Pham, A. T.; Ichiba, T.; Yoshida, W. Y.; Scheuer, P. J.; Uchida, T.; Tanaka, J.; Higa, T. Tetrahedron Lett. 1991, 32, 4843−4846. (18) Fusetani, N.; Wolstenholme, H. J.; Shinoda, K.; Asai, N.; Matsunaga, S.; Onuki, H.; Hirota, H. Tetrahedron Lett. 1992, 33, 6823−6826. (19) Simpson, J. S.; Garson, M. J. Tetrahedron Lett. 1998, 39, 5819− 5822.

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