Synlett 2013; 24(6): 752-756
DOI: 10.1055/s-0032-1318302
letter
© Georg Thieme Verlag Stuttgart · New York

Relay Catalysis by a Ruthenium Complex–Chiral Brønsted Acid Binary Sytem for Ternary Reaction Sequence Involving Enantioselective Pictet–Spengler-Type Cyclization as the Key Step

Yasunori Toda
a   Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan   Fax: +81(22)7956602   eMail: mterada@m.tohoku.ac.jp
,
Masahiro Terada*
a   Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan   Fax: +81(22)7956602   eMail: mterada@m.tohoku.ac.jp
b   Research and Analytical Center for Giant Molecules, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
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Publikationsverlauf

Received: 04. Januar 2013

Accepted after revision: 01. Februar 2013

Publikationsdatum:
27. Februar 2013 (online)


Abstract

Relay catalysis for a ternary reaction sequence composed of double-bond isomerization, protonation of the double bond, and enantioselective Pictet–Spengler-type cyclization was accomplished using a binary catalytic system consisting of a ruthenium hydride complex and a chiral phosphoric acid as the chiral Brønsted acid catalyst.

Supporting Information

 
  • References and Notes

  • 12 For details regarding the screening of chiral phosphoric acid catalysts, see the Supporting Information.
  • 13 Representative Procedure for the Relay Catalysis (Table 2, Entry 1) To a dried test tube were added (R)-2 (G = 9-anthryl; 5 mol%, 7.01 mg) and 3b (55.5 mg, 0.20 mmol). The mixture was dissolved in toluene (1.0 mL), and then the atmosphere was replaced with argon. [RuClH(CO)(PPh3)3] (1) (2 mol%, 3.81 mg) was added in portion at r.t., and the tube was flushed again with argon. After stirring at 50 °C for 12 h, the reaction mixture was diluted with sat. aq NaHCO3 and extracted with CH2Cl2 (3×). The combined organic layers were dried over Na2SO4, filtered, and concentrated. After purification by flash column chromatography on silica gel (hexane–EtOAc = 10:1 to 2:1 as eluent), 5b was obtained in 62% yield as a white solid. The ee of 5b was determined by chiral stationary phase HPLC analysis. Compound 5b: white solid; Rf = 0.50 (hexane–EtOAc = 2:1). HPLC analysis Chiralpak IA (hexane–2-PrOH = 90:10, 0.8 mL/min, 254 nm, 30 °C): t R (major) = 10.1 min; t R (minor) = 12.5 min (37% ee); [α]D 26 +24.5 (c 1.1, CHCl3); rotamer (major/minor = 60:40) was observed. 1H NMR (500 MHz, CDCl3): δ = 0.95–0.97 (3 H, m), 1.48 (9 H, s), 1.68–1.80 (2 H, m), 2.64–2.67 (1 H, m), 2.81–2.89 (1 H, m), 3.12–3.14 (0.60 H, m), 3.26–3.30 (0.40 H, m), 3.89–3.91 (0.40 H, m), 4.14–4.16 (0.60 H, m), 4.86–4.99 (2 H, m), 6.59 (1 H, s), 6.64–6.68 (1 H, m), 6.96–6.97 (1 H, m). 13C NMR (125.65 MHz, CDCl3): δ = 10.88, 11.18, 28.44, 28.66, 29.73, 30.09, 37.01, 38.48, 55.24, 56.08, 79.86, 80.19, 113.49, 114.98, 115.10, 128.00, 128.30, 129.29, 129.51, 135.21, 135.46, 154.73, 154.83, 155.38, 155.46. IR (ATR): 3330, 2971, 2932, 2875, 1687, 1656, 1613, 1427, 1232, 1160, 918, 863 cm–1. ESI-HRMS: m/z calcd for C16H23NO3Na [M + Na]+: 300.1570; found: 300.1569.
  • 14 The reaction of the product 5b with ruthenium complex 1 and chiral phosphoric acid 2 under the same reaction conditions (50 °C, 12 h). 5b was recovered quantitatively, and no racemization of 5b was observed.
    • 15a The absolute configuration was determined to be S by optical rotation after derivatization to (S)-1-ethyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline: [α]D 24 –24.2 (c 2.2, CH2Cl2); literature value of S-isomer [α]D 20 –51.9 (c 2.1, CH2Cl2). See: Polniaszek RP, Kaufman CR. J. Am. Chem. Soc. 1989; 111: 4859
    • 15b Compound 5g: white solid; Rf = 0.45 (hexane–EtOAc = 2:1). HPLC analysis Chiralpak IA (hexane–EtOH = 96:4, 1.0 mL/min, 254 nm, 30 °C): t R (minor) = 11.3 min; t R (major) = 21.9 min (53% ee); [α]D 25 +49.7 (c 1.2, CHCl3); rotamer (major/minor = 55:45) was observed. 1H NMR (500 MHz, CDCl3): δ = 0.98–0.99 (3 H, m), 1.48 (9 H, s), 1.71–1.80 (2 H, m), 2.57–2.61 (1 H, m), 2.74–2.86 (1 H, m), 3.09–3.13 (0.55 H, m), 3.23–3.27 (0.45 H, m), 3.85–3.92 (3.45 H, m), 4.16–4.18 (0.55 H, m), 4.84–4.86 (0.55 H, m), 4.96–4.98 (0.45 H, m), 5.53 (1 H, brs), 6.57 (1 H, s), 6.65–6.66 (1 H, m). 13C NMR (125.65 MHz, CDCl3): δ = 10.85, 11.09, 27.64, 27.78, 28.31, 29.57, 29.96, 36.62, 38.33, 55.06, 55.84, 79.22, 79.51, 109.29, 109.63, 114.21, 114.46, 126.52, 126.81, 129.15, 129.51, 144.06, 144.16, 144.95, 145.03, 154.92, 155.04; IR (ATR): 3369, 2969, 2932, 2842, 1683, 1515, 1420, 1364, 1271, 1241, 1111, 932, 863 cm–1. ESI-HRMS: m/z calcd for C17H25NO4Na [M + Na]+: 330.1676; found: 330.1675.
  • 16 Kobayashi and co-workers showed that the Pictet–Spengler reaction of benzaldehyde with m-tyramine using Brønsted acids, such as sulfonic acid and carboxylic acid, gave the corresponding product in low yield, see: Manabe K, Nobutou D, Kobayashi S. Bioorg. Med. Chem. 2005; 13: 5154