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On the assignment of the absolute configuration at the isolated methyl branch in miyakosyne A, cytotoxic linear acetylene, from the deep-sea marine sponge Petrosia sp.
Tetrahedron 69 (2013) 11070e11073
Contents lists available at ScienceDirect
Tetrahedron journal homepage: www.elsevier.com/locate/tet
On the assignment of the absolute configuration at the isolated methyl branch in miyakosyne A, cytotoxic linear acetylene, from the deep-sea marine sponge Petrosia sp. Yuki Hitora, Kentaro Takada, Shigeki Matsunaga * Laboratory of Aquatic Natural Products Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 October 2013 Received in revised form 1 November 2013 Accepted 6 November 2013 Available online 15 November 2013
Absolute configuration of miyakosyne A at its isolated branched methyl stereogenic center has been studied by chemical degradation in combination with esterification with the Ohrui’s acid. Comparison of the 1H NMR data of the relevant diesters for the degradation product and the synthetic standards indicated the 14R-configuration. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Marine sponge Acetylene absolute configuration Ohrui’s reagent
1. Introduction Assignment of configurations of natural products has been facilitated by the development of NMR techniques, such as the modified Mosher’s analysis,1 J-based configurational analysis,2 and universal NMR database.3 However, when a molecule has a symmetric or nearly symmetric unit, signals in two parts of the molecule are overlapped beyond the resolution of NMR spectroscopy, rendering the differentiation of 2 equiv or almost equivalent segments impossible. Similarly, a long methylene chain placed between two chiral units invalidate the NMR interactions between each other, hampering the determination of the relative stereochemistry concerned with the two units.4,5 Such assignment problems in NMR are frequently found in linear molecules with one or more chiral units insulated from other parts of the molecule by methylene chain(s).4,5 These flexible molecules tend to resist crystallization. Miyakosyne A (1), a cytotoxic constituent of a deep-sea marine sponge Petrosia sp. is one of these difficult molecules to assign the absolute configuration.6 It is a linear compound with two identical chiral units placed at both termini and has a branched methyl in the
* Corresponding author. Fax: þ81 3 5841 8166; e-mail address: assmats@mail. ecc.u-tokyo.ac.jp (S. Matsunaga). 0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2013.11.013
middle of a long aliphatic chain. Even though we were able to determine the absolute configurations of both termini by application of the modified Mosher’s method,1 it was difficult to determine the position of the branched methyl and even more so to assign the absolute configuration of the chiral methine carbon by chemical or spectroscopic methods.6 Although miyakosyne A absorbed in porous metal complex was analyzed by X-ray crystallography, only the tentative assignment of the absolute configuration at C-14 was accomplished.7,8 Against this background, we studied the absolute configuration of miyakosyne A (1) at C-14 by a combination of chemical degradation and esterification with the Ohrui’s acid (2, Fig. 1).9
OH 3
OH
14
26
1
O N COOH O 2 Fig. 1. Structures of miyakosyne A (1) and (1S,2S)-2-(anthracene-2,3-dicarboximido) cyclohexanecarboxylic acid (2).
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2. Results and discussion We first attempted to examine whether miyakosyne A (1) is a diastereomeric mixture differing in the configuration at C-14 or not. In order to obtain diastereomers of 1, we randomized the configuration of two secondary alcohols by a sequence of oxidation and reduction reactions. Even though the modified Mosher’s analysis of the product demonstrated that the two oxygenated methines were racemic, the mixture gave only a single sharp peak in a variety of HPLC conditions.10 Then we set out to convert 1 into a linear 1,u-diol. It was reported that a pair of isomers of linear primary alcohols with anteiso-terminus, which are enantiomeric to each other in the absolute configuration of the methyl-substituted methine carbons, could be discernible by 1H NMR spectroscopy by conjugation with the Ohrui’s acid (2) (Ohrui’s method).9 It was also reported that the differences in chemical shifts between the enantiomers were not observed when the branching points were more distant than the 10th carbon atom.9 With this information in mind, miyakosyne A (1) was subjected to ozonolysis followed by reduction with NaBH4 to furnish 10-methylicosane-1,20-diol (3). In compound 3 the branched methyl group was placed at 10th and 11th carbon from each terminus, which were too far to apply the Ohrui’s method. Therefore, compound 3 was dehomologated by means of the Grieco elimination11 followed by ozonolysis and reduction with NaBH4 to give 9-methyl-1,18-octadecanediol (5), in which branched methyl group was placed at ninth and 10th position (Scheme 1).
Fig. 2. Expanded plots of the 1H NMR spectrum of the terminal oxygenated methylene protons (Left) and the branched methyl (Right) signals of the diesters. (a) S-5, (b) R-5, and (c) 5 prepared from miyakosyne A (1).
3. Conclusion The absolute configuration of the methyl branch in miyakosyne A (1) has been determined to be 14R by comparing the 1H NMR data of a degradation product with those of synthetic standards after derivatization with the Ohrui’s acid. This study demonstrates the efficiency of derivatization with the Ohrui’s acid to assign the absolute configuration of remote stereogenic centers once reference compounds are available.5 The differentiation of a methyl branch nine and 10 carbons away from the terminal hydroxyl groups was fulfilled by 1H NMR, permitting us to conclude that the natural miyakosyne A has the 3R,14R,26R-configuration. 4. Experimental 4.1. General
Scheme 1. Preparation of 9-methyl-1,18-octadecanediol (5) from miyakosyne A.
We then prepared both enantiomers of 9-methyl-1,18octadecanediol. (S)-Citronellal (S-6) was subjected to ozonolysis to afford S-7. S-7 was treated with the ylide prepared from phosphonium salt 8 to give S-9, which was hydrogenated to give S-5 (Scheme 2). R-5 was synthesized in the same way from R-citronellal (R-6).
Scheme 2. Synthesis of (S)-9-methyl-1,18-octadecanediol (S-5).
With 5 from 1 and both enantiomers of 5 in hand, we esterified each of them with 2. The 1H NMR spectra of the resulting three diesters appeared identical except for the chemical shift of the branched methyl signal (Fig. 2). The chemical shift of the methyl signal in the diester of natural 5 was identical with that of the diester of R-5, whereas it shifted down-field compared to that of the diester of S-5. From this result we considered that miyakosyne A has 14R-configuration.
1 H and 13C NMR spectra were recorded on a JEOL a 600 NMR spectrometer. Chemical shifts were referenced to solvent peaks: dH 7.24 and dC 77.0 for CDCl3. High resolution mass spectra were obtained on a JEOL JMS-700T mass spectrometer. Optical rotations were measured on a JASCO DIP-1000 digital polarimeter.
4.2. 10-Methyl-1,20-icosanediol (3) O3 was bubbled through a solution of miyakosyne A (1, 3.5 mg) in MeOH (2.5 mL) at 78 C until the color of the solution changed to blue. After excess O3 was purged with N2, NaBH4 was added and stirred for 1 h. To the reaction mixture water was added and the solution was extracted with EtOAc to afford 10-methyl-1,20icosanediol (3, 2.3 mg) as a colorless oil: 1H NMR (600 MHz, CDCl3) d 0.81 (d, J¼6.9 Hz, 3H), 1.06 (m, 2H), 1.19e1.33 (m, 28H), 1.52e1.57 (m, 5H), 3.62 (t, J¼6.9 Hz, 4H); HRFABMS calcd for C21H45O2 [MþHþ] 329.3414, found: 329.3412. 4.3. 10-Methyl-1,19-icosadiene (4) To a solution of 3 (1.5 mg) in THF (0.3 mL) o-NO2PhSeCN (5 mg) and Bu3P (6 mL) were added. After stirring for 4 h, the reaction mixture was cooled to 0 C, and H2O2 (0.2 mL) in THF (3 mL) was added. The reaction mixture was stirred at 0 C for 30 min, and allowed to warm to room temperature and stirred overnight. The reaction mixture was quenched by addition of saturated aqueous NaHCO3, and the mixture was extracted with EtOAc. The organic layer was concentrated, and the residue was subjected to silica gel column chromatography (n-hexane) to afford 10-methyl-1,19icosadiene (4, 1.5 mg) as a pale yellow solid: 1H NMR (600 MHz,
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CDCl3) d 0.81 (d, J¼6.9 Hz, 3H), 1.06 (m, 2H), 1.19e1.35 (m, 24H), 1.56 (m, 1H), 2.01 (m, 4H), 4.91 (br d, J¼9.6 Hz, 2H), 4.97 (br d, J¼17.1 Hz, 2H), 5.80 (m, 2H). 4.4. 9-Methyl-1,18-octadecanediol (5) Compound 4 (1.5 mg) was dissolved in CH2Cl2 and O3 was bubbled through the solution until the color turned blue. After the excess O3 was purged with N2, MeOH and NaBH4 were added and stirred for 1 h at room temperature. The reaction mixture was concentrated and partitioned between water and EtOAc. Concentration of the organic layer afforded 9-methyl-1,18-octadecanediol (5, 1.0 mg) as a colorless oil: 1H NMR (600 MHz, CDCl3) d 0.81 (d, J¼6.9 Hz, 3H), 1.05 (m, 2H), 1.23e1.33 (m, 24H), 1.52e1.66 (m, 5H), 3.62 (t, J¼6.9 Hz, 4H); HRFABMS calcd for C19H41O2 [MþHþ] 301.3101, found: 301.3100.
saturated water was added and extracted with EtOAc. The organic layer was concentrated and purified by silica gel column chromatography (n-hexane/EtOAc 1:3) to give (S)-9-methyl-6,12octadecadiene-1,18-diol (S-9, 11 mg) as a pale yellow oil: [a]23 D þ3.5 (c 0.48, CHCl3); 1H NMR (600 MHz, CDCl3) d 0.85 (d, J¼6.9 Hz, 3H), 1.12e1.54 (m, 23H), 3.60 (t, J¼6.9 Hz, 4H), 5.31e5.38 (m, 4H); 13 C NMR (200 MHz, CDCl3) d 19.5, 25.37, 25.39, 27.2, 27.3, 29.3, 29.4, 29.5, 32.56, 32.60, 33.0, 34.3, 36.6, 62.8, 128.4, 129.5, 130.1, 130.4; HRFABMS calcd for C19H37O2 [MþHþ] 297.2788, found: 297.2788. (R)-9-Methyl-6,12-octadecadiene-1,18-diol (R-9) was prepared 1 in the same way from R-7: [a]22 D 4.3 (c 0.34, CHCl3); H NMR (600 MHz, CDCl3) d 0.85 (d, J¼6.9 Hz, 3H), 1.12e1.58 (m, 23H), 3.62 (t, J¼6.9 Hz, 4H), 5.32e5.38 (m, 4H); 13C NMR (200 MHz, CDCl3) d 19.5, 25.3, 25.4, 27.0, 27.1, 29.2, 29.4, 29.5, 32.5, 32.9, 34.3, 36.6, 62.7, 128.4, 129.5, 130.1, 130.4; HRFABMS calcd for C19H37O2 [MþHþ] 297.2788, found: 297.2794.
4.5. (S)-3-Methyl-1,6-hexanedial (S-7) 4.8. (S)-9-Methyl-1,18-octadecanediol (S-5) A solution of (S)-()-citronellal (S-6) (170 mg, 1.1 mmol) in CH2Cl2 (7 mL) was kept at 78 C and bubbled with ozone until the color of the solution turned blue. Me2S (180 mL) was added to the reaction mixture and gradually warmed to room temperature. The reaction mixture was stirred for overnight, and the solvent was removed by evaporation. The residue was purified by silica gel column chromatography (n-hexane/ethyl acetate 1:1) to afford a fraction enriched with (S)-3-methyl-1,6hexanedial (S-7 110 mg) as a colorless oil, which was used in 1 the next step: [a]23 D 12 (c 0.75, CHCl3 ); H NMR (600 MHz, CDCl3) d 0.95 (m, 3H), 1.47e1.56 (m, 1H), 1.69 (m, 1H), 1.95 (m, 1H), 2.16 (m, 1H), 2.30e2.44 (m, 3H), 9.71 (br s, 1H), 9.73 (br s, 1H); 13C NMR (200 MHz, CDCl3) d 19.0, 26.8, 28.0, 40.7, 50.0, 202.0, 202.1; HRFABMS calcd for C7H11O2 [MH], 127.0765, found: 127.0771. A fraction enriched with (R)-3-methyl-1,6-hexanedial (R-7) was prepared in the same way from (R)-(þ)-citronellal (R-6). The product was used in the next step without further purification: 1 [a]23 D þ8.4 (c 1.8, CHCl3); H NMR (600 MHz, CDCl3) d 0.95 (m, 3H), 1.47e1.56 (m, 1H), 1.69 (m, 1H), 1.95 (m, 1H), 2.16 (m, 1H), 2.30e2.44 (m, 3H), 9.71 (br s, 1H), 9.73 (br s, 1H); 13C NMR (200 MHz, CDCl3) d 19.0, 27.1, 28.0, 41.0, 50.2, 202.1, 202.2; HRFABMS calcd for C7H11O2 [MH], 127.0765, found: 127.0771. 4.6. (6-Hydroxyhexyl)-triphenyl phosphonium bromide (8) 6-Bromo-1-hexanol (500 mg) and triphenylphosphine (800 mg) in MeCN (4 mL) were refluxed at 90 C overnight. The reaction mixture was cooled to room temperature and concentrated. The resulting residue was dissolved in acetone and diluted with dry diethyl ether to precipitate the phosphonium salt. After stirring for 1 h, the supernatant was obtained by decantation and dried in vacuo to afford the desired phosphonium salt (8, 1.1 g) as 1 white solids: [a]24 D 0.65 (c 0.97, CHCl3); H NMR (600 MHz, CDCl3) d 1.43e1.77 (m, 8H), 2.03 (br s, 1H), 3.54e3.70 (m, 4H), 7.65e7.79 (m, 15H); 13C NMR (200 MHz, CDCl3) d 22.0 (d, J¼4.3 Hz), 22.2 (d, J¼49.9 Hz), 24.5, 29.3 (d, J¼15.9 Hz), 31.7, 61.0, 118.0 (d, J¼86.0 Hz), 130.3 (d, J¼12.3 Hz), 133.3 (d, J¼10.1 Hz), 134.8 (d, J¼2.2 Hz); HRFABMS calcd for C24H29OP [MþBr] 363.1872, found: 363.1869. 4.7. (S)-9-Methyl-6,12-octadecadiene-1,18-diol (S-9) To a suspension of 8 (250 mg) in dry THF (4 mL) was added LiHMDS (1.0 M in THF, 1.4 mL). The solution was stirred for 1 h under nitrogen atmosphere, and then a solution of S-7 (15 mg) was added. The reaction mixture was stirred for 4 h, and NH4Cl
S-9 (10 mg) was dissolved in methanol, and Pd/C (3 mg) was added to the solution. The suspension was stirred vigorously overnight under H2. The resultant mixture was filtered, and concentration of the filtrate afforded the diol (S-5, 7.0 mg) as colorless 1 oil: [a]22 D 0.52 (c 0.21, CHCl3); H NMR (600 MHz, CDCl3) d 0.81 (d, J¼6.9 Hz, 3H), 1.24e1.33 (m, 20H), 1.54e1.65 (m, 11H), 3.62 (t, J¼6.9 Hz, 4H); 13C NMR (200 MHz, CDCl3) d 19.7, 25.70, 25.71, 25.72, 27.0, 29.38, 29.42, 29.50, 29.54, 29.59, 29.61, 29.90, 29.94, 32.7, 32.8, 37.0, 63.0; HRFABMS calcd for 301.3101, found: 301.3122. R-5 was prepared in the same way using R-9: (R)-9-methyl-1,18octadecanediol (R-5); [a]23 þ0.64 (c 0.14, CHCl3); 1H NMR D (600 MHz, CDCl3) d 0.81 (d, J¼6.9 Hz, 3H), 1.24e1.33 (m, 20H), 1.54e1.65 (m, 11H), 3.62 (t, J¼6.9 Hz, 4H); 13C NMR (200 MHz, CDCl3) d 19.7, 25.70, 25.71, 27.0, 29.38, 29.40, 29.51, 29.55, 29.56, 29.60, 29.88, 29.93, 32.8, 37.0, 63.0; HRFABMS calcd for C19H41O2 [MþHþ] 301.3101, found: 301.3106. 4.9. (1S,2S)-2-Diester of 5 derived from natural miyakosyne A To a solution of a 9-methyl-1,18-diol (5, 1 mg) in CH2Cl2, (1S,2S)2 (1.2 mg), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, hydrochloride (1 mg), and N,N-dimethyl-4-aminopyridine (0.2 mg) were added and stirred for 4 h. The solution was concentrated and the residue was purified by HPLC (COSMOSIL 5SL-II) to afford (1S,2S)-2-diester (0.2 mg) of 5, and a mixture of monoesters (0.2 mg). 1 H NMR (600 MHz, CDCl3) of the (1S,2S)-diester of 5 derived from miyakosyne A: d 0.722 (d, J¼6.9 Hz, 3H), 0.85e1.60 (m, 37H), 1.82e1.89 (m, 6H), 2.12e2.15 (m, 2H), 2.25 (dd, J¼3.6, 12.6 Hz, 2H), 3.50 (dt, J¼3.5, 11.8 Hz, 2H), 3.87 (dt, J¼1.4, 6.7 Hz, 4H), 4.43 (dt, J¼4.0, 11.8 Hz, 2H), 7.58 (dd, J¼3.2, 6.4 Hz, 4H), 8.04 (dd, J¼3.3, 6.5 Hz, 4H), 8.45 (s, 4H), 8.59 (s, 4H); HRFABMS calcd for C65H75N2O8 [MþHþ] 1011.5523, found: 1011.5532. 4.10. (1S,2S)-2-diester of S-5 (1S,2S)-2-Diester of S-5 was prepared in the same way using S-5: H NMR spectrum was almost superimposable on that of (1S,2S)-2diester of 5 except for the branched methyl (d 0.720); HRFABMS calcd for C65H75N2O8 [MþHþ] 1011.5523, found: 1011.5512. 1
4.11. (1S,2S)-2-Diester of R-5 (1S,2S)-2-Diester of R-5 was prepared in the same way using R5: (1S, 2S)-2-diester of R-5: 1H NMR was almost superimposable on
Y. Hitora et al. / Tetrahedron 69 (2013) 11070e11073
that of (1S,2S)-2-diester of 5 (chemical shift of the branched methyl d 0.722); HRFABMS calcd for C65H75N2O8 [MþHþ] 1011.5523, found: 1011.5573. Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas ‘Chemical Biology of Natural Products’ (23102007) from MEXT and by Grant-in-Aid for Scientific Research (A) (25252037) from JSPS to S.M. We thank Professor M. Fujita, Department of Applied Chemistry, The University of Tokyo, and Professor K. Mori, Professor Emeritus, The University of Tokyo, for valuable discussion. Supplementary data Full plots of the 1H NMR spectra of the diesters of S-5, R-5, and 5 from 1. Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.tet.2013.11.013.
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References and notes 1. Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc. 1991, 113, 4092e4096. 2. Matsumori, N.; Kaneno, D.; Murata, M.; Nakamura, H.; Tachibana, K. J. Org. Chem. 1999, 64, 866e876. 3. (a) Lee, J.; Kobayashi, Y.; Tezuka, K.; Kishi, Y. Org. Lett. 1999, 1, 2181e2184; (b) Kobayashi, Y.; Hayashi, N.; Kishi, Y. Org. Lett. 2001, 3, 2253e2255; (c) Higashibayashi, S.; Czechtizky, W.; Kobayashi, Y.; Kishi, Y. J. Am. Chem. Soc. 2003, 125, 14379e14393. 4. Curran, D. P.; Zhang, Q.; Lu, H.; Gudipati, V. J. Am. Chem. Soc. 2006, 128, 9943e9956. 5. Mori, K. In Determination of Structure Including Absolute Configuration of Bioactive Natural Products; Mander, L., Liu, H.-W., Eds.; Comprehensive Natural Products II Chemistry and Biology; Elsevier: Oxford, 2010; Vol. 9, pp 147e167. 6. Hitora, Y.; Takada, K.; Okada, S.; Matsunaga, S. Tetrahedron 2011, 67, 4530e4534. 7. Inokuma, Y.; Yoshioka, S.; Ariyoshi, J.; Arai, T.; Hitora, Y.; Takada, K.; Matsunaga, S.; Rissanen, K.; Fujita, M. Nature 2013, 495, 461e466. 8. Inokuma, Y.; Yoshioka, S.; Ariyoshi, J.; Arai, T.; Hitora, Y.; Takada, K.; Matsunaga, S.; Rissanen, K.; Fujita, M. Nature 2013, 501, 262. 9. Imaizumi, K.; Terasima, H.; Akasaka, K.; Ohrui, H. Anal. Sci. 2003, 19, 1243e1249. 10. The inability to HPLC separation of diastereomeric linear acetylenes with remote stereogenic centers are discussed in Sui, B.; Yeh, E. A.-H.; Curran, D. P. J. Org. Chem. 2010, 75, 2942e2954. 11. Grieco, P. A.; Gilman, S.; Nishizawa, M. J. Org. Chem. 1976, 41, 1485e1486.
Contents lists available at ScienceDirect
Tetrahedron journal homepage: www.elsevier.com/locate/tet
On the assignment of the absolute configuration at the isolated methyl branch in miyakosyne A, cytotoxic linear acetylene, from the deep-sea marine sponge Petrosia sp. Yuki Hitora, Kentaro Takada, Shigeki Matsunaga * Laboratory of Aquatic Natural Products Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 October 2013 Received in revised form 1 November 2013 Accepted 6 November 2013 Available online 15 November 2013
Absolute configuration of miyakosyne A at its isolated branched methyl stereogenic center has been studied by chemical degradation in combination with esterification with the Ohrui’s acid. Comparison of the 1H NMR data of the relevant diesters for the degradation product and the synthetic standards indicated the 14R-configuration. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Marine sponge Acetylene absolute configuration Ohrui’s reagent
1. Introduction Assignment of configurations of natural products has been facilitated by the development of NMR techniques, such as the modified Mosher’s analysis,1 J-based configurational analysis,2 and universal NMR database.3 However, when a molecule has a symmetric or nearly symmetric unit, signals in two parts of the molecule are overlapped beyond the resolution of NMR spectroscopy, rendering the differentiation of 2 equiv or almost equivalent segments impossible. Similarly, a long methylene chain placed between two chiral units invalidate the NMR interactions between each other, hampering the determination of the relative stereochemistry concerned with the two units.4,5 Such assignment problems in NMR are frequently found in linear molecules with one or more chiral units insulated from other parts of the molecule by methylene chain(s).4,5 These flexible molecules tend to resist crystallization. Miyakosyne A (1), a cytotoxic constituent of a deep-sea marine sponge Petrosia sp. is one of these difficult molecules to assign the absolute configuration.6 It is a linear compound with two identical chiral units placed at both termini and has a branched methyl in the
* Corresponding author. Fax: þ81 3 5841 8166; e-mail address: assmats@mail. ecc.u-tokyo.ac.jp (S. Matsunaga). 0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2013.11.013
middle of a long aliphatic chain. Even though we were able to determine the absolute configurations of both termini by application of the modified Mosher’s method,1 it was difficult to determine the position of the branched methyl and even more so to assign the absolute configuration of the chiral methine carbon by chemical or spectroscopic methods.6 Although miyakosyne A absorbed in porous metal complex was analyzed by X-ray crystallography, only the tentative assignment of the absolute configuration at C-14 was accomplished.7,8 Against this background, we studied the absolute configuration of miyakosyne A (1) at C-14 by a combination of chemical degradation and esterification with the Ohrui’s acid (2, Fig. 1).9
OH 3
OH
14
26
1
O N COOH O 2 Fig. 1. Structures of miyakosyne A (1) and (1S,2S)-2-(anthracene-2,3-dicarboximido) cyclohexanecarboxylic acid (2).
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2. Results and discussion We first attempted to examine whether miyakosyne A (1) is a diastereomeric mixture differing in the configuration at C-14 or not. In order to obtain diastereomers of 1, we randomized the configuration of two secondary alcohols by a sequence of oxidation and reduction reactions. Even though the modified Mosher’s analysis of the product demonstrated that the two oxygenated methines were racemic, the mixture gave only a single sharp peak in a variety of HPLC conditions.10 Then we set out to convert 1 into a linear 1,u-diol. It was reported that a pair of isomers of linear primary alcohols with anteiso-terminus, which are enantiomeric to each other in the absolute configuration of the methyl-substituted methine carbons, could be discernible by 1H NMR spectroscopy by conjugation with the Ohrui’s acid (2) (Ohrui’s method).9 It was also reported that the differences in chemical shifts between the enantiomers were not observed when the branching points were more distant than the 10th carbon atom.9 With this information in mind, miyakosyne A (1) was subjected to ozonolysis followed by reduction with NaBH4 to furnish 10-methylicosane-1,20-diol (3). In compound 3 the branched methyl group was placed at 10th and 11th carbon from each terminus, which were too far to apply the Ohrui’s method. Therefore, compound 3 was dehomologated by means of the Grieco elimination11 followed by ozonolysis and reduction with NaBH4 to give 9-methyl-1,18-octadecanediol (5), in which branched methyl group was placed at ninth and 10th position (Scheme 1).
Fig. 2. Expanded plots of the 1H NMR spectrum of the terminal oxygenated methylene protons (Left) and the branched methyl (Right) signals of the diesters. (a) S-5, (b) R-5, and (c) 5 prepared from miyakosyne A (1).
3. Conclusion The absolute configuration of the methyl branch in miyakosyne A (1) has been determined to be 14R by comparing the 1H NMR data of a degradation product with those of synthetic standards after derivatization with the Ohrui’s acid. This study demonstrates the efficiency of derivatization with the Ohrui’s acid to assign the absolute configuration of remote stereogenic centers once reference compounds are available.5 The differentiation of a methyl branch nine and 10 carbons away from the terminal hydroxyl groups was fulfilled by 1H NMR, permitting us to conclude that the natural miyakosyne A has the 3R,14R,26R-configuration. 4. Experimental 4.1. General
Scheme 1. Preparation of 9-methyl-1,18-octadecanediol (5) from miyakosyne A.
We then prepared both enantiomers of 9-methyl-1,18octadecanediol. (S)-Citronellal (S-6) was subjected to ozonolysis to afford S-7. S-7 was treated with the ylide prepared from phosphonium salt 8 to give S-9, which was hydrogenated to give S-5 (Scheme 2). R-5 was synthesized in the same way from R-citronellal (R-6).
Scheme 2. Synthesis of (S)-9-methyl-1,18-octadecanediol (S-5).
With 5 from 1 and both enantiomers of 5 in hand, we esterified each of them with 2. The 1H NMR spectra of the resulting three diesters appeared identical except for the chemical shift of the branched methyl signal (Fig. 2). The chemical shift of the methyl signal in the diester of natural 5 was identical with that of the diester of R-5, whereas it shifted down-field compared to that of the diester of S-5. From this result we considered that miyakosyne A has 14R-configuration.
1 H and 13C NMR spectra were recorded on a JEOL a 600 NMR spectrometer. Chemical shifts were referenced to solvent peaks: dH 7.24 and dC 77.0 for CDCl3. High resolution mass spectra were obtained on a JEOL JMS-700T mass spectrometer. Optical rotations were measured on a JASCO DIP-1000 digital polarimeter.
4.2. 10-Methyl-1,20-icosanediol (3) O3 was bubbled through a solution of miyakosyne A (1, 3.5 mg) in MeOH (2.5 mL) at 78 C until the color of the solution changed to blue. After excess O3 was purged with N2, NaBH4 was added and stirred for 1 h. To the reaction mixture water was added and the solution was extracted with EtOAc to afford 10-methyl-1,20icosanediol (3, 2.3 mg) as a colorless oil: 1H NMR (600 MHz, CDCl3) d 0.81 (d, J¼6.9 Hz, 3H), 1.06 (m, 2H), 1.19e1.33 (m, 28H), 1.52e1.57 (m, 5H), 3.62 (t, J¼6.9 Hz, 4H); HRFABMS calcd for C21H45O2 [MþHþ] 329.3414, found: 329.3412. 4.3. 10-Methyl-1,19-icosadiene (4) To a solution of 3 (1.5 mg) in THF (0.3 mL) o-NO2PhSeCN (5 mg) and Bu3P (6 mL) were added. After stirring for 4 h, the reaction mixture was cooled to 0 C, and H2O2 (0.2 mL) in THF (3 mL) was added. The reaction mixture was stirred at 0 C for 30 min, and allowed to warm to room temperature and stirred overnight. The reaction mixture was quenched by addition of saturated aqueous NaHCO3, and the mixture was extracted with EtOAc. The organic layer was concentrated, and the residue was subjected to silica gel column chromatography (n-hexane) to afford 10-methyl-1,19icosadiene (4, 1.5 mg) as a pale yellow solid: 1H NMR (600 MHz,
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CDCl3) d 0.81 (d, J¼6.9 Hz, 3H), 1.06 (m, 2H), 1.19e1.35 (m, 24H), 1.56 (m, 1H), 2.01 (m, 4H), 4.91 (br d, J¼9.6 Hz, 2H), 4.97 (br d, J¼17.1 Hz, 2H), 5.80 (m, 2H). 4.4. 9-Methyl-1,18-octadecanediol (5) Compound 4 (1.5 mg) was dissolved in CH2Cl2 and O3 was bubbled through the solution until the color turned blue. After the excess O3 was purged with N2, MeOH and NaBH4 were added and stirred for 1 h at room temperature. The reaction mixture was concentrated and partitioned between water and EtOAc. Concentration of the organic layer afforded 9-methyl-1,18-octadecanediol (5, 1.0 mg) as a colorless oil: 1H NMR (600 MHz, CDCl3) d 0.81 (d, J¼6.9 Hz, 3H), 1.05 (m, 2H), 1.23e1.33 (m, 24H), 1.52e1.66 (m, 5H), 3.62 (t, J¼6.9 Hz, 4H); HRFABMS calcd for C19H41O2 [MþHþ] 301.3101, found: 301.3100.
saturated water was added and extracted with EtOAc. The organic layer was concentrated and purified by silica gel column chromatography (n-hexane/EtOAc 1:3) to give (S)-9-methyl-6,12octadecadiene-1,18-diol (S-9, 11 mg) as a pale yellow oil: [a]23 D þ3.5 (c 0.48, CHCl3); 1H NMR (600 MHz, CDCl3) d 0.85 (d, J¼6.9 Hz, 3H), 1.12e1.54 (m, 23H), 3.60 (t, J¼6.9 Hz, 4H), 5.31e5.38 (m, 4H); 13 C NMR (200 MHz, CDCl3) d 19.5, 25.37, 25.39, 27.2, 27.3, 29.3, 29.4, 29.5, 32.56, 32.60, 33.0, 34.3, 36.6, 62.8, 128.4, 129.5, 130.1, 130.4; HRFABMS calcd for C19H37O2 [MþHþ] 297.2788, found: 297.2788. (R)-9-Methyl-6,12-octadecadiene-1,18-diol (R-9) was prepared 1 in the same way from R-7: [a]22 D 4.3 (c 0.34, CHCl3); H NMR (600 MHz, CDCl3) d 0.85 (d, J¼6.9 Hz, 3H), 1.12e1.58 (m, 23H), 3.62 (t, J¼6.9 Hz, 4H), 5.32e5.38 (m, 4H); 13C NMR (200 MHz, CDCl3) d 19.5, 25.3, 25.4, 27.0, 27.1, 29.2, 29.4, 29.5, 32.5, 32.9, 34.3, 36.6, 62.7, 128.4, 129.5, 130.1, 130.4; HRFABMS calcd for C19H37O2 [MþHþ] 297.2788, found: 297.2794.
4.5. (S)-3-Methyl-1,6-hexanedial (S-7) 4.8. (S)-9-Methyl-1,18-octadecanediol (S-5) A solution of (S)-()-citronellal (S-6) (170 mg, 1.1 mmol) in CH2Cl2 (7 mL) was kept at 78 C and bubbled with ozone until the color of the solution turned blue. Me2S (180 mL) was added to the reaction mixture and gradually warmed to room temperature. The reaction mixture was stirred for overnight, and the solvent was removed by evaporation. The residue was purified by silica gel column chromatography (n-hexane/ethyl acetate 1:1) to afford a fraction enriched with (S)-3-methyl-1,6hexanedial (S-7 110 mg) as a colorless oil, which was used in 1 the next step: [a]23 D 12 (c 0.75, CHCl3 ); H NMR (600 MHz, CDCl3) d 0.95 (m, 3H), 1.47e1.56 (m, 1H), 1.69 (m, 1H), 1.95 (m, 1H), 2.16 (m, 1H), 2.30e2.44 (m, 3H), 9.71 (br s, 1H), 9.73 (br s, 1H); 13C NMR (200 MHz, CDCl3) d 19.0, 26.8, 28.0, 40.7, 50.0, 202.0, 202.1; HRFABMS calcd for C7H11O2 [MH], 127.0765, found: 127.0771. A fraction enriched with (R)-3-methyl-1,6-hexanedial (R-7) was prepared in the same way from (R)-(þ)-citronellal (R-6). The product was used in the next step without further purification: 1 [a]23 D þ8.4 (c 1.8, CHCl3); H NMR (600 MHz, CDCl3) d 0.95 (m, 3H), 1.47e1.56 (m, 1H), 1.69 (m, 1H), 1.95 (m, 1H), 2.16 (m, 1H), 2.30e2.44 (m, 3H), 9.71 (br s, 1H), 9.73 (br s, 1H); 13C NMR (200 MHz, CDCl3) d 19.0, 27.1, 28.0, 41.0, 50.2, 202.1, 202.2; HRFABMS calcd for C7H11O2 [MH], 127.0765, found: 127.0771. 4.6. (6-Hydroxyhexyl)-triphenyl phosphonium bromide (8) 6-Bromo-1-hexanol (500 mg) and triphenylphosphine (800 mg) in MeCN (4 mL) were refluxed at 90 C overnight. The reaction mixture was cooled to room temperature and concentrated. The resulting residue was dissolved in acetone and diluted with dry diethyl ether to precipitate the phosphonium salt. After stirring for 1 h, the supernatant was obtained by decantation and dried in vacuo to afford the desired phosphonium salt (8, 1.1 g) as 1 white solids: [a]24 D 0.65 (c 0.97, CHCl3); H NMR (600 MHz, CDCl3) d 1.43e1.77 (m, 8H), 2.03 (br s, 1H), 3.54e3.70 (m, 4H), 7.65e7.79 (m, 15H); 13C NMR (200 MHz, CDCl3) d 22.0 (d, J¼4.3 Hz), 22.2 (d, J¼49.9 Hz), 24.5, 29.3 (d, J¼15.9 Hz), 31.7, 61.0, 118.0 (d, J¼86.0 Hz), 130.3 (d, J¼12.3 Hz), 133.3 (d, J¼10.1 Hz), 134.8 (d, J¼2.2 Hz); HRFABMS calcd for C24H29OP [MþBr] 363.1872, found: 363.1869. 4.7. (S)-9-Methyl-6,12-octadecadiene-1,18-diol (S-9) To a suspension of 8 (250 mg) in dry THF (4 mL) was added LiHMDS (1.0 M in THF, 1.4 mL). The solution was stirred for 1 h under nitrogen atmosphere, and then a solution of S-7 (15 mg) was added. The reaction mixture was stirred for 4 h, and NH4Cl
S-9 (10 mg) was dissolved in methanol, and Pd/C (3 mg) was added to the solution. The suspension was stirred vigorously overnight under H2. The resultant mixture was filtered, and concentration of the filtrate afforded the diol (S-5, 7.0 mg) as colorless 1 oil: [a]22 D 0.52 (c 0.21, CHCl3); H NMR (600 MHz, CDCl3) d 0.81 (d, J¼6.9 Hz, 3H), 1.24e1.33 (m, 20H), 1.54e1.65 (m, 11H), 3.62 (t, J¼6.9 Hz, 4H); 13C NMR (200 MHz, CDCl3) d 19.7, 25.70, 25.71, 25.72, 27.0, 29.38, 29.42, 29.50, 29.54, 29.59, 29.61, 29.90, 29.94, 32.7, 32.8, 37.0, 63.0; HRFABMS calcd for 301.3101, found: 301.3122. R-5 was prepared in the same way using R-9: (R)-9-methyl-1,18octadecanediol (R-5); [a]23 þ0.64 (c 0.14, CHCl3); 1H NMR D (600 MHz, CDCl3) d 0.81 (d, J¼6.9 Hz, 3H), 1.24e1.33 (m, 20H), 1.54e1.65 (m, 11H), 3.62 (t, J¼6.9 Hz, 4H); 13C NMR (200 MHz, CDCl3) d 19.7, 25.70, 25.71, 27.0, 29.38, 29.40, 29.51, 29.55, 29.56, 29.60, 29.88, 29.93, 32.8, 37.0, 63.0; HRFABMS calcd for C19H41O2 [MþHþ] 301.3101, found: 301.3106. 4.9. (1S,2S)-2-Diester of 5 derived from natural miyakosyne A To a solution of a 9-methyl-1,18-diol (5, 1 mg) in CH2Cl2, (1S,2S)2 (1.2 mg), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, hydrochloride (1 mg), and N,N-dimethyl-4-aminopyridine (0.2 mg) were added and stirred for 4 h. The solution was concentrated and the residue was purified by HPLC (COSMOSIL 5SL-II) to afford (1S,2S)-2-diester (0.2 mg) of 5, and a mixture of monoesters (0.2 mg). 1 H NMR (600 MHz, CDCl3) of the (1S,2S)-diester of 5 derived from miyakosyne A: d 0.722 (d, J¼6.9 Hz, 3H), 0.85e1.60 (m, 37H), 1.82e1.89 (m, 6H), 2.12e2.15 (m, 2H), 2.25 (dd, J¼3.6, 12.6 Hz, 2H), 3.50 (dt, J¼3.5, 11.8 Hz, 2H), 3.87 (dt, J¼1.4, 6.7 Hz, 4H), 4.43 (dt, J¼4.0, 11.8 Hz, 2H), 7.58 (dd, J¼3.2, 6.4 Hz, 4H), 8.04 (dd, J¼3.3, 6.5 Hz, 4H), 8.45 (s, 4H), 8.59 (s, 4H); HRFABMS calcd for C65H75N2O8 [MþHþ] 1011.5523, found: 1011.5532. 4.10. (1S,2S)-2-diester of S-5 (1S,2S)-2-Diester of S-5 was prepared in the same way using S-5: H NMR spectrum was almost superimposable on that of (1S,2S)-2diester of 5 except for the branched methyl (d 0.720); HRFABMS calcd for C65H75N2O8 [MþHþ] 1011.5523, found: 1011.5512. 1
4.11. (1S,2S)-2-Diester of R-5 (1S,2S)-2-Diester of R-5 was prepared in the same way using R5: (1S, 2S)-2-diester of R-5: 1H NMR was almost superimposable on
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that of (1S,2S)-2-diester of 5 (chemical shift of the branched methyl d 0.722); HRFABMS calcd for C65H75N2O8 [MþHþ] 1011.5523, found: 1011.5573. Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas ‘Chemical Biology of Natural Products’ (23102007) from MEXT and by Grant-in-Aid for Scientific Research (A) (25252037) from JSPS to S.M. We thank Professor M. Fujita, Department of Applied Chemistry, The University of Tokyo, and Professor K. Mori, Professor Emeritus, The University of Tokyo, for valuable discussion. Supplementary data Full plots of the 1H NMR spectra of the diesters of S-5, R-5, and 5 from 1. Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.tet.2013.11.013.
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References and notes 1. Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc. 1991, 113, 4092e4096. 2. Matsumori, N.; Kaneno, D.; Murata, M.; Nakamura, H.; Tachibana, K. J. Org. Chem. 1999, 64, 866e876. 3. (a) Lee, J.; Kobayashi, Y.; Tezuka, K.; Kishi, Y. Org. Lett. 1999, 1, 2181e2184; (b) Kobayashi, Y.; Hayashi, N.; Kishi, Y. Org. Lett. 2001, 3, 2253e2255; (c) Higashibayashi, S.; Czechtizky, W.; Kobayashi, Y.; Kishi, Y. J. Am. Chem. Soc. 2003, 125, 14379e14393. 4. Curran, D. P.; Zhang, Q.; Lu, H.; Gudipati, V. J. Am. Chem. Soc. 2006, 128, 9943e9956. 5. Mori, K. In Determination of Structure Including Absolute Configuration of Bioactive Natural Products; Mander, L., Liu, H.-W., Eds.; Comprehensive Natural Products II Chemistry and Biology; Elsevier: Oxford, 2010; Vol. 9, pp 147e167. 6. Hitora, Y.; Takada, K.; Okada, S.; Matsunaga, S. Tetrahedron 2011, 67, 4530e4534. 7. Inokuma, Y.; Yoshioka, S.; Ariyoshi, J.; Arai, T.; Hitora, Y.; Takada, K.; Matsunaga, S.; Rissanen, K.; Fujita, M. Nature 2013, 495, 461e466. 8. Inokuma, Y.; Yoshioka, S.; Ariyoshi, J.; Arai, T.; Hitora, Y.; Takada, K.; Matsunaga, S.; Rissanen, K.; Fujita, M. Nature 2013, 501, 262. 9. Imaizumi, K.; Terasima, H.; Akasaka, K.; Ohrui, H. Anal. Sci. 2003, 19, 1243e1249. 10. The inability to HPLC separation of diastereomeric linear acetylenes with remote stereogenic centers are discussed in Sui, B.; Yeh, E. A.-H.; Curran, D. P. J. Org. Chem. 2010, 75, 2942e2954. 11. Grieco, P. A.; Gilman, S.; Nishizawa, M. J. Org. Chem. 1976, 41, 1485e1486.