Synlett 2021; 32(05): 532-538
DOI: 10.1055/s-0039-1690901
cluster
The Power of Transition Metals: An Unending Well-Spring of New Reactivity
© Georg Thieme Verlag Stuttgart · New York

Chiral P,Olefin Ligands with Rotamers for Palladium-Catalyzed Asymmetric Allylic Substitution Reactions

Takashi Mino
a  Graduate School of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
b  Molecular Chirality Research Center, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
c  Soft Molecular Activation Research Center, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan   Email: [email protected]
,
Daiki Yamaguchi
a  Graduate School of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
,
Manami Kumada
a  Graduate School of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
,
Junpei Youda
a  Graduate School of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
,
Hironori Saito
a  Graduate School of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
,
Junya Tanaka
a  Graduate School of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
,
Yasushi Yoshida
a  Graduate School of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
b  Molecular Chirality Research Center, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
c  Soft Molecular Activation Research Center, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan   Email: [email protected]
,
Masami Sakamoto
a  Graduate School of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
b  Molecular Chirality Research Center, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
› Author Affiliations
This work was supported by Grants-in-Aid for Scientific Research (C) (Nos. 26410109 and 18K05117) from the Japan Society for the Promotion of Science (JSPS).
Further Information

Publication History

Received: 16 March 2020

Accepted after revision: 29 March 2020

Publication Date:
16 April 2020 (online)


Abstract

We synthesized a series of phosphine–olefin-type chiral aminophosphines, and we confirmed that these each exists as two rotamers at the C(aryl)–N(amine) bond. We also investigated the ability of these aminophosphines to act as chiral ligands for Pd-catalyzed asymmetric allylic substitution reactions, such as the alkylation of allylic acetates with malonates or indoles, and we found they gave high enantio­selectivities (up to 98% ee).

Supporting Information

 
  • References and Notes


    • For selected reviews, see:
    • 1a Liu Y, Han S.-J, Liu W.-B, Stoltz BM. Acc. Chem. Res. 2015; 48: 740
    • 1b Trost BM. Tetrahedron 2015; 71: 5708
    • 1c Trost BM. Org. Process Res. Dev. 2012; 16: 185
    • 1d Guerrero Rios I, Rosas-Hernandez A, Martin E. Molecules 2011; 16: 970
    • 1e Diéguez M, Pàmies O. Acc. Chem. Res. 2010; 43: 312
    • 1f Mohr JT, Stoltz BM. Chem. Asian J. 2007; 2: 1476
    • 1g Guiry PJ, Saunders CP. Adv. Synth. Catal. 2004; 346: 497
    • 1h Trost BM. J. Org. Chem. 2004; 69: 5813
    • 1i Trost BM, Crawley ML. Chem. Rev. 2003; 103: 2921
    • 1j Graening T, Schmalz H.-G. Angew. Chem. Int. Ed. 2003; 42: 2580
    • 1k Trost BM, Van Vranken DL. Chem. Rev. 1996; 96: 395
  • 2 For a review, see: Feng X, Du H. Chin. J. Org. Chem. 2015; 35: 259
    • 3a Kamikawa K, Tseng Y.-Y, Jian J.-H, Takahashi T, Ogasawara M. J. Am. Chem. Soc. 2017; 139: 1545
    • 3b Gandi VR, Lu Y, Hayashi T. Tetrahedron: Asymmetry 2015; 26: 679
    • 3c Liu Y, Feng X, Du H. Org. Biomol. Chem. 2015; 13: 125
    • 3d Ogasawara M, Tseng Y.-Y, Arae S, Morita T, Nakaya T, Wu W.-Y, Takahashi T, Kamikawa K. J. Am. Chem. Soc. 2014; 136: 9377
    • 3e Wang H.-L, Hu R.-B, Zhang H, Zhou A.-X, Yang S.-D. Org. Lett. 2013; 15: 5302
    • 3f Liu Z, Du H. Org. Lett. 2013; 15: 740
    • 3g Liu Y, Cao Z, Du H. J. Org. Chem. 2012; 77: 4479
    • 3h Cao Z, Liu Z, Liu Y, Du H. J. Org. Chem. 2011; 76: 6401
    • 3i Shintani R, Narui R, Tsutsumi Y, Hayashi S, Hayashi T. Chem. Commun. 2011; 47: 6123
    • 3j Liu Z, Cao Z, Du H. Org. Biomol. Chem. 2011; 9: 5369
    • 3k Cao Z, Liu Y, Liu Z, Feng X, Zhuang M, Du H. Org. Lett. 2011; 13: 2164
    • 3l Liu Z, Du H. Org. Lett. 2010; 12: 3054
    • 3m Stemmler RT, Bolm C. Synlett 2007; 1365
    • 3n Duan W.-L, Iwamura H, Shintani R, Hayashi T. J. Am. Chem. Soc. 2007; 129: 2130
    • 3o Kasák P, Arion VB, Widhalm MA. Tetrahedron: Asymmetry 2006; 17: 3084
    • 3p Shintani R, Duan W.-L, Okamoto K, Hayashi T. Tetrahedron: Asymmetry 2005; 16: 3400
    • 3q Shintani R, Duan W.-L, Nagano T, Okada A, Hayashi T. Angew. Chem. Int. Ed. 2005; 44: 4611
    • 3r Marie P, Deblon S, Breher F, Geier J, Böhler C, Rüegger H, Schönberg H, Grützmacher H. Chem. Eur. J. 2004; 10: 4198
    • 4a Mino T, Youda J, Ebisawa T, Shima Y, Nishikawa K, Yoshida Y, Sakamoto M. J. Oleo Sci. 2018; 67: 1189
    • 4b Mino T, Nishikawa K, Asano M, Shima Y, Ebisawa T, Yoshida Y, Sakamoto M. Org. Biomol. Chem. 2016; 14: 7509
  • 5 Mino T, Yamaguchi D, Masuda C, Youda J, Ebisawa T, Yoshida Y, Sakamoto M. Org. Biomol. Chem. 2019; 17: 1455
  • 6 Hattori T, Komuro Y, Hayashizaka N, Takahashi H, Miyano S. Enantiomer 1997; 2: 203
  • 7 Mino T, Tanaka Y, Sakamoto M, Fujita T. Tetrahedron: Asymmetry 2001; 12: 2435
    • 8a Mino T, Tanaka Y, Sakamoto M, Fujita T. Heterocycles 2000; 53: 1485
    • 8b Hattori T, Sakamoto J, Hayashizaka N, Miyano S. Synthesis 1994; 199
  • 9 [2-(Diphenylphosphoryl)-6-methoxyphenyl][(1S)-1-phenylethyl]amine [(S)-8a] A 1.6 M solution of BuLi in hexane (9.4 mL, 15.0 mmol) was slowly added to a solution of [(1S)-1-phenylethyl]amine (1.82 g, 15.0 mmol) in THF (35 mL) at –80 °C. Phosphine oxide 7a (1.69 g, 5.0 mmol) was added at r.t., and the mixture was stirred for 21 h at r.t. The mixture was then diluted with Et2O, and the reaction was quenched with sat. aq NH4Cl. The organic layer was washed with brine, dried (MgSO4), and concentrated under reduced pressure. The residue was purified by chromatography [silica gel, hexane–EtOAc (2:1)] to give a beige solid; yield: 1.60 g (75%, 3.74 mmol); mp 124–126 °C; [α]D 20 +99.1 (c 0.50, CHCl3). 1H NMR (300 MHz, CDCl3): δ = 7.68–7.53 (m, 6 H), 7.50–7.41 (m, 4 H), 7.36 (br s, 1 H), 7.15–7.12 (m, 2 H), 7.06–7.03 (m, 3 H), 6.79 (d, J = 7.1 Hz, 1 H), 6.56 (dt, J = 3.6 and 7.8 Hz, 1 H), 6.42 (ddd, J = 1.4, 7.7, 14.0 Hz, 1 H), 5.10 (t, J = 6.1 Hz, 1 H), 3.69 (s, 3 H), 1.33 (d, J = 6.8 Hz, 3 H). 13C NMR (75 MHz, CDCl3): δ = 150.7 (d, JCP = 11.6 Hz), 146.4, 144.1 (d, JCP = 5.5 Hz), 133.0 (d, JCP = 103.9 Hz), 132.6 (d, JCP = 104.5 Hz), 132.2 (d, JCP = 10.1 Hz) (2 C), 132.0 (d, JCP = 9.9 Hz) (2 C), 131.8 (d, JCP = 2.5 Hz), 131.7 (d, JCP = 2.5 Hz), 128.4 (d, JCP = 12.2 Hz) (4 C), 127.9 (2 C), 126.2 (2 C), 126.0, 125.6 (d, JCP = 11.0 Hz), 117.5 (d, JCP = 15.5), 115.6 (d, JCP = 2.5 Hz), 115.5 (d, JCP = 104.2 Hz), 55.5, 55.4, 24.6. 31P NMR (121 MHz, CDCl3): δ = 37.5. EI-MS: m/z (%): 427 (M+, 30), 412 (100). HRMS (ESI-Orbitrap): m/z [M + H]+ calcd for C27H27NO2P: 428.1774; found: 428.1766. [2-(Diphenylphosphoryl)-6-methoxyphenyl][(1S)-1-phenylethyl][(2E)-3-phenylprop-2-en-1-yl]amine [(S)-9a] To the solution of the phosphine oxide (S)-8a (2.14 g, 5.0 mmol) in MeCN (50 mL) at r.t. were added K2CO3 (3.46 g, 25 mmol) and cinnamyl bromide (1.18 g, 6.0 mmol) in MeCN (20 mL), and the mixture was stirred at 60 °C for 22 h. The mixture was then filtered and concentrated under reduced pressure. The residue was purified by chromatography [silica gel, hexane–EtOAc (5:1)] to give a white solid; yield: 2.29 g (84%, 4.22 mmol); mp 169–171 °C; [α]D 20 +53.1 (c 0.36, CHCl3). 1H NMR (300 MHz, CDCl3): δ = 7.84–7.73 (m, 4 H), 7.47–7.38 (m, 8 H), 7.24–7.01 (m, 10 H), 6.66 (ddd, J = 1.2, 7.7, 13.9 Hz, 1 H), 5.83 (d, J = 15.9 Hz, 1 H), 5.33 (ddd, J = 7.0, 7.0, 15.9 Hz, 1 H), 4.67 (q, J = 6.0 Hz, 1 H), 3.73 (dd, J = 7.5, 7.6 Hz, 1 H), 3.67 (s, 3 H), 3.26 (dd, J = 6.5, 15.1 Hz, 1 H), 1.11 (d, J = 6.7 Hz, 3 H). 13C NMR (75 MHz, CDCl3): δ = 160.1 (d, JCP = 11.5 Hz), 145.3, 143.8 (d, JCP = 4.4 Hz), 137.8, 134.7 (d, JCP = 106.6 Hz), 134.0 (d, JCP = 105.3 Hz), 133.0 (d, JCP = 103.8 Hz), 132.1 (d, JCP = 9.0 Hz) (2 C), 131.1(931) (d, JCP = 2.6 Hz), 131.1(925) (d, JCP = 9.4 Hz) (2 C), 131.0 (d, JCP = 2.8 Hz), 129.5, 128.9, 128.5 (2 C), 128.2 (d, JCP = 12.2 Hz) (2 C), 128.1 (d, JCP = 11.7 Hz) (2 C), 128.0 (2 C), 127.6 (2 C), 126.7 (d, JCP = 12.4 Hz), 126.4, 126.3, 126.2 (d, JCP = 3.3 Hz), 126.0 (2 C), 115.6 (d, JCP = 2.3 Hz), 62.3, 56.9, 55.1, 22.4. 31P NMR (121 MHz, CDCl3): δ = 27.5. EI-MS: m/z (%) = 543 (M+, 0.15), 438 (100). HRMS (ESI-Orbitrap): m/z [M + H]+ calcd for C36H35NO2P: 544.2400; found: 544.2388. X-ray diffraction analysis: Colorless plate crystals (0.20 × 0.10 × 0.020 mm3) from hexane–CHCl3; monoclinic space group P21, a = 12.4152(3) Å, b = 8.9538(2) Å, c = 13.5274(3) Å, β = 103.4670(10)°, V = 1462.40(6) Å3, Z = 2, ρ = 1.235 g/cm3, μ (Cu Kα) = 1.54178 mm–1. The structure was solved by the direct method of full-matrix least–squares, where the final R and Rw were 0.0327, 0.0875, respectively, for 4982 reflections. [2-(Diphenylphosphino)-6-methoxyphenyl][(1S)-1-phenylethyl][(2E)-3-phenylprop-2-en-1-yl]amine [(S)-6a] To a mixture of phosphine oxide (S)-9a (1.09 g, 2.0 mmol) and Et3N (3.1 mL, 22 mmol) in m-xylene (10 mL) was added HSiCl3 (2.0 mL, 20 mmol) at 0 °C under Ar. The mixture was stirred at 120 °C for 24 h then cooled to r.t. and diluted with Et2O. The reaction was quenched with 2 M aq NaOH, and the organic layer was washed with brine, dried (MgSO4), and concentrated under reduced pressure. The residue was purified by chromatography [silica gel, hexane–EtOAc (50:1)]. to give a white solid; yield 0.530 g (50%. 1.0 mmol); mp 55–56 °C; [α]D 20 +59.6 (c 0.51, CHCl3); rotamer ratio = 20:1. 1H NMR (300 MHz, CDCl3): δ (major rotamer) = 7.66 (d, J = 8.0 Hz, 2 H), 7.37–7.04 (m, 19 H), 6.86 (dd, J = 1.0, 8.1 Hz, 1 H), 6.53 (ddd, J = 1.3, 2.8, 7.6 Hz, 1 H), 6.04–5.94 (m, 1 H), 5.85 (d, J = 15.9 Hz, 1 H), 4.59 (q, J = 6.5 Hz, 1 H), 3.83 (s, 3 H), 3.58–3.42 (m, 2 H), 0.89 (d, J = 6.7 Hz, 3 H); δ (minor rotamer) = 7.66 (d, J = 7.4 Hz, 2 H), 7.37–7.04 (m, 19 H), 6.64 (dd, J = 1.0, 8.1 Hz, 1 H), 6.42–6.39 (m, 1 H), 6.09 (d, J = 16.1 Hz, 1 H), 5.76–5.68 (m, 1 H), 4.81 (q, J = 6.5 Hz, 1 H), 3.75 (s, 3 H), 3.58–3.42 (m, 2 H), 1.50 (d, J = 6.9 Hz, 3 H). 13C NMR (75 MHz, CDCl3): δ = 159.3 (d, JCP = 3.9 Hz), 147.1, 141.9 (d, JCP = 4.2 Hz), 141.1 (d, JCP = 22.0 Hz), 138.7 (d, JCP = 14.4 Hz), 138.5 (d, JCP = 15.1 Hz), 137.6, 134.2 (d, JCP = 20.5 Hz) (2 C), 134.0 (d, JCP = 20.5 Hz) (2 C), 130.1, 129.0, 128.2(8) (d, JCP = 2.0 Hz) (2 C), 128.2(6), 128.2(2) (d, JCP = 1.0 Hz) (2 C), 128.1(4) (2 C), 128.0(9) (d, JCP = 1.0 Hz) (2 C), 128.0(3), 128.0(1) (2 C), 126.9, 126.6, 126.4 (2 C), 126.1 (2 C), 111.9, 61.6, 56.3, 55.1, 23.5. 31P NMR (121 MHz, CDCl3): δ (major rotamer) = −16.0; δ (minor rotamer) = −14.1. EI-MS: m/z (%) = 527 (M+, 8.2), 422 (100). HRMS (ESI-Orbitrap): m/z [M + H]+ calcd for C36H35NOP: 528.2451; found: 528.2441.

    • For recent examples, see:
    • 10a Li S, Zhang J, Li H, Feng L, Peng Jiao P. J. Org. Chem. 2019; 84: 9460
    • 10b Imrich MR, Maichle-Mössmer C, Ziegler T. Eur. J. Org. Chem. 2019; 3955
    • 10c Hu Y.-L, Wang Z, Yang H, Chen J, Wu Z.-B, Lei Y, Zhou L. Chem. Sci. 2019; 10: 6777
    • 10d Baumann T, Brückner R. Angew. Chem. Int. Ed. 2019; 58: 4714
    • 10e Qiu Z, Sun R, Teng D. Org. Biomol. Chem. 2018; 16: 7717
    • 10f Borràs C, Elias-Rodriguez P, Carmona AT, Robina I, Pàmies O, Diéguez M. Organometallics 2018; 37: 1682
    • 10g Argüelles AJ, Sun S, Budaitis BG, Nagorny P. Angew. Chem. Int. Ed. 2018; 57: 5325
    • 10h Biosca M, Margalef J, Caldentey X, Besora M, Rodriguez-Escrich C, Saltó J, Cambeiro XC, Maseras F, Pàmies O, Diéguez M, Pericàs MA. ACS Catal. 2018; 8: 3587
    • 10i Chang S, Wang L, Lin X. Org. Biomol. Chem. 2018; 16: 2239
    • 10j Naganawa Y, Abe H, Nishiyama H. Chem. Commun. 2018; 54: 2674
    • 10k Szulc I, Kołodziuk R, Zawisza A. Tetrahedron 2018; 74: 1476
    • 10l Yao L, Nie H, Zhang D, Wang L, Zhang Y, Chen W, Li Z, Liu X, Zhang S. ChemCatChem 2018; 10: 804
    • 10m Sartorius F, Trebing M, Brückner C, Brückner R. Chem. Eur. J. 2017; 23: 17463
    • 10n Ogasawara M, Tseng Y.-Y, Uryu M, Ohya N, Chang N, Ishimoto H, Arae S, Takahashi T, Kamikawa K. Organometallics 2017; 36: 4061
  • 11 Palladium-Catalyzed Allylic Alkylation of Malonates; General Procedure BSA (0.15 mL, 0.60 mmol) and the appropriate allylic ester (0.20 mmol) were added to a mixture of [Pd(C3H5)Cl]2 (1.48 mg, 4 μmol), (S)-6d (4.64 mg, 8 μmol), and NaOAc (1.64 mg, 20 μmol) in PhCF3 (0.4 mL) at r.t. under Ar, and the mixture was stirred for 10 min. A dialkyl malonate (0.60 mmol) was added, and stirring was continued for 24 h at r.t. The mixture was then diluted with Et2O and water. The organic layer was washed with brine then dried (MgSO4), filtered, and concentrated in a rotary evaporator. The residue was purified by column chromatography [silica gel, hexane–EtOAc (10:1)]. Dimethyl [(1S,2E)-1,3-Diphenylprop-2-en-1-yl]malonate [(S)-10a]16 Colorless oil; yield: 55.7 mg (86%, 0.172 mmol, 94% ee); [α]D 20 –21.6 (c 0.51, CHCl3). HPLC [Daicel CHIRALPAK AD-H, 0.46 × 25 cm, λ = 254 nm, hexane–i-PrOH (90:10), 0.5 mL/min]: t R = 26.3 min (minor), 33.3 min (major). 1H NMR (400 MHz, CDCl3): δ = 7.34–7.18 (m, 10 H), 6.48 (d, J = 15.8 Hz, 1 H), 6.33 (dd, J = 8.4, 15.7 Hz, 1 H), 4.27 (dd, J = 8.1, 8.6 Hz, 1 H), 3.95 (d, J = 10.8 Hz, 1 H), 3.70 (s, 3 H), 3.52 (s, 3 H). 13C NMR (101 MHz, CDCl3): δ = 168.2, 167.8, 140.2, 136.8, 131.9, 129.1, 128.7, 128.5, 127.9, 127.6, 127.2, 126.4, 57.7, 52.7, 52.5, 49.2. EI-MS: m/z (%) = 324 (M+, 13).

    • For examples, see:
    • 12a Qiu Z, Sun R, Yang K, Teng D. Molecules 2019; 24: 1575
    • 12b Zhang L, Xiang S.-H, Wang J, Xiao J, Wang J.-Q, Tan B. Nat. Commun. 2019; 10: 566
    • 12c Yamamoto K, Shimizu T, Igawa K, Tomooka K, Hirai G, Suemune H, Usui K. Sci. Rep. 2016; 6: 36211
    • 12d Feng B, Pu X.-Y, Liu Z.-C, Xiao W.-J, Chen J.-R. Org. Chem. Front. 2016; 3: 1246
    • 12e Xu J.-X, Ye F, Bai X.-F, Zhang J, Xu Z, Zheng Z.-J, Xu L.-W. RSC Adv. 2016; 6: 45495
    • 12f Mino T, Ishikawa M, Nishikawa K, Wakui K, Sakamoto M. Tetrahedron: Asymmetry 2013; 24: 499
    • 12g Hoshi T, Sasaki K, Sato S, Ishii Y, Suzuki T, Hagiwara H. Org. Lett. 2011; 13: 932
    • 12h Cheung HY, Yu W.-Y, Lam FL, Au-Yeung TT.-L, Zhou Z, Chan TH, Chan AS. C. Org. Lett. 2007; 9: 4295
  • 13 Palladium-Catalyzed Allylic Alkylation of Indoles; General Procedure PhMe (0.2 mL) was added to a mixture of indole or a substituted indole (0.2 mmol), the appropriate 1,3-diarylprop-2-enyl acetate (60.6 mg, 0.24 mmol), (S)-6a (6.3 mg, 12 μmol), [Pd(C3H5)Cl]2 (2.2 mg, 6 μmol), and KOAc (39.3 mg, 0.4 mol) at r.t. under Ar, and the mixture was stirred for 18 h at 40 °C. The reaction was quenched with H2O, and the mixture was diluted with Et2O. The organic layer was washed with H2O and brine then dried (MgSO4), filtered, and concentrated in a rotary evaporator. The residue was purified by column chromatography [silica gel, hexane–EtOAc–Et3N (20:2:1)]. 3-[(1R,2E)-1,3-Diphenylprop-2-en-1-yl]-1H-indole [(R)-11a]4 Yellow solid; yield: 48.0 mg (74%, 0.146 mmol, 97% ee); mp 118–120 °C; [α]D 20 –35.3 (c 0.19, CHCl3). HPLC [Daicel CHIRALPAKIB, 0.46 × 25 cm, λ = 254 nm, hexane–EtOH (99:1), 0.7 mL/min]: t R = 56.3 min (major), 63.9 min (minor). 1H NMR (400 MHz, CDCl3): δ = 7.98 (br s, 1 H), 7.43–7.15 (m, 13 H), 7.04–7.00 (m, 1 H), 6.90 (d, J = 2.4 Hz, 1 H), 6.73 (dd, J = 7.4, 15.8 Hz, 1 H), 6.43 (d, J = 15.9 Hz, 1 H), 5.12 (d, J = 7.3 Hz, 1 H). 13C NMR (101 MHz, CDCl3): δ = 143.3, 137.5, 136.6, 132.5, 130.5, 128.5 (2 C), 128.4, 127.1, 126.8, 126.4, 126.3, 122.6, 122.1, 119.9, 119.4, 118.7, 111.1, 46.2. EI-MS: m/z (%) 309 (M+, 100).
  • 14 CCDC 1938333 contains the supplementary crystallographic data for compound (S)-9a. The data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

    • For examples, see:
    • 15a Brown JM, Hulmes DI, Guiry PJ. Tetrahedron 1994; 50: 4493
    • 15b Sprinz J, Kiefer M, Helmchen G, Reggelin M, Huttner G, Walter O, Zsolnai L. Tetrahedron Lett. 1994; 35: 1523
    • 16a Mino T, Asakawa M, Shima Y, Yamada H, Yagishita F, Sakamoto M. Tetrahedron 2015; 71: 5985
    • 16b Tanaka Y, Mino T, Akita K, Sakamoto M, Fujita T. J. Org. Chem. 2004; 69: 6679