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Synlett 2007(1): 0079-0082  
DOI: 10.1055/s-2006-958434
LETTER
© Georg Thieme Verlag Stuttgart · New York

A Practical and Facile Approach towards Indole Alkaloids: (-)-Mitralactonine

Subhash P. Chavan*, Pallavi Sharma, Rasapalli Sivappa, Uttam R. Kalkote
Division of Organic Chemistry, Technology, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
Fax: +91(20)25902629; e-Mail: sp.chavan@ncl.res.in;
Further Information

Publication History

Received 10 October 2006
Publication Date:
20 December 2006 (online)

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Abstract

An efficient approach to (-)-mitralactonine using Pictet-Spengler cyclisation with a highly functionalised masked aldehyde is described. Sharpless asymmetric dihydroxylation (SAD) is ­utilised to introduce chirality in the key substrate.

Key words

alkaloids - asymmetric - cyclisations - dihydroxylations - thioacetals

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Column: CHIRAL CEL-OD; mobile phase: PE-i-PrOH (90:10); flow rate: 0.5 mL/min; t R (major isomer): 24.8 min, t R (minor isomer): 26.8 min.

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Experimental Details and Spectral Data of Selected Compounds:
Compound 10: The ligand (DHQ)2PHAL (50 mg, 1 mol%), K3Fe(CN)6 (6.4 g, 19.5 mmol), K2CO3 (2.7 g, 19.5 mmol) and OsO4 (0.4 mol%) were dissolved in a 1:1 mixture of H2O (30 mL) and t-BuOH (30 mL) at r.t. MeSO2NH2 (0.62 g, 6.5 mmol) was added at this point. The vigorously stirred mix-ture was then cooled to 0 °C and the olefin 9 (2 g, 6.5 mmol) was added in one portion. After completion of reaction (24 h) it was quenched at 0 °C with Na2SO3 (10 equiv) and then warmed to r.t. and stirred for an additional period of 30-60 min. The organic layer was separated and the aqueous layer was extracted with EtOAc (5 × 50 mL). The combined organic layers were dried over anhyd Na2SO4, filtered and concentrated on a rotary evaporator to give the crude diol which was purified through column chromatography (SiO2) to furnish the diol 10 in 90% yield (2 g, 5.8 mmol); R f 0.4 (PE-EtOAc, 1:1); [α]D -10.38 (c = 0.96, CHCl3). IR (CHCl3): 3431, 1706, 1606 cm-1. 1H NMR (400 MHz, CDCl3): δ = 7.97 (d, J = 8.9 Hz, 2 H), 6.90 (d, J = 8.9 Hz, 2 H), 4.61 (t, J = 5.1 Hz, 1 H), 4.40 (d, J = 11.7 Hz, 1 H), 4.28 (d, J = 11.7 Hz, 1 H), 3.84 (s, 3 H), 3.82 (m, 1 H), 3.37, 3.33 (2 × s, 6 H), 1.82-1.90 (m, 2 H), 1.68-1.77 (m, 1 H), 1.47-1.57 (m, 1 H), 0.98 (t, J = 7.3 Hz, 3 H). 13C NMR (100 MHz, CDCl3): δ = 166.4 (C), 163.5 (C), 131.7 (2 × CH), 122.3 (C), 113.6 (2 × CH), 104.1 (CH), 74.9 (C), 70.1 (CH), 65.8 (CH2), 55.3 (CH3), 53.9 (CH3), 52.9 (CH3), 32.9 (CH2), 25.9 (CH2), 7.4 (CH3). Anal. Calcd for C17H26O7: C, 59.64; H, 7.65. Found: C, 59.50; H, 7.55.
Compound 21: To a well-stirred solution of the diol 20 (0.67 g, 2.5 mmol) in freshly dried CH2Cl2 (10 mL) at 0 °C was added weighed amount of Dess-Martin periodinane (1.5 g, 3.7 mmol) in two portions with an interval of 1 h. The mixture was stirred at 0-5 °C for a total period of 3 h at the end of which the reaction was quenched at 0 °C with the addition of a 1:1 mixture of sat. aq solution of Na2SO3 and NaHCO3. The organic layer was separated and the aqueous layer was extracted with CH2Cl2 (3 × 25 mL). Combined organic layers were washed with brine, dried over anhyd Na2SO4, filtered and concentrated under reduced pressure. Column chromatographic purification (SiO2) of the residue furnished the pure product 21 as a solid in 40% yield (0.26 g, 1 mmol); R f 0.5 (PE-EtOAc, 70:30); [α]D -3.78 (c = 0.58, CHCl3); mp 57-58 °C. IR (CHCl3): 3458, 2923, 1718 cm-1. 1H NMR (200 MHz, CDCl3): δ = 4.58 (t, J = 6.8 Hz, 1 H), 3.80 (d, J = 11.5 Hz, 1 H), 3.62 (d, J = 11.5 Hz, 1 H), 3.02 (d, J = 6.8 Hz, 1 H), 2.78-2.96 (m, 4 H), 2.06-2.18 (m, 1 H), 1.67-1.97 (m, 4 H), 0.91 (t, J = 7.3 Hz, 3 H). 13C NMR (100 MHz, CDCl3): δ = 206.8 (C), 81.5 (C), 49.3 (CH2), 43.2 (CH2), 40.4 (CH), 30.2 (CH2), 30.1 (CH2), 29.6 (CH2), 25.2 (CH2), 7.4 (CH3). Anal. Calcd for C10H17ClO2S2: C, 44.68; H, 6.37; Cl, 13.19; S, 23.86. Found: C, 44.45; H, 6.29; Cl, 13.09; S, 23.72. MS (ESI): m/z = 291.09 [M+ + Na].
Compound 24a: A mixture of acetate 22 (0.2 g, 0.65 mmol), MeI (0.9 g, 0.4 mL, 6.4 mmol) H2O (0.2 mL) in MeCN (10 mL) was refluxed for 24 h under a nitrogen atmosphere. After completion of reaction most of the MeI and volatile solvents were removed in vacuo and the residue was extracted with CH2Cl2 (3 × 10 mL). The organic layer was washed with NaHCO3, brine, dried over anhyd Na2SO4, filtered and concentrated under reduced pressure. The crude product was used in the next reaction without further purification because of the lability of the compound. The mixture of the crude aldehyde (assuming 100% conversion) and tryptamine (0.10 g, 0.6 mmol) was stirred in absolute EtOH under nitrogen atmosphere for 0.5 h. The resulting solution was acidified by the addition of 2 M ethanolic HCl at 0 °C and the mixture was further stirred for 12 h at r.t. After this time, the mixture was diluted with EtOAc and H2O. The aqueous solution was made alkaline by the addition of excess of NaOH. The organic layer was separated and the aqueous layer extracted with EtOAc (3 × 10 mL). The combined organic layers were washed with brine, dried over anhyd Na2SO4, filtered and concentrated under reduced pressure. Column chromatographic purification (SiO2) of the residue rendered the tetracyclic compounds 24a and 24b (3:1) in a combined yield of 40% over two steps (0.073 g, 0.25 mmol); R f 0.5 (24a), 0.4 (24b) (PE-EtOAc, 60:40). 24a: [α]D -108 (c = 0.13, CHCl3) {Lit.2 [α]D -156 (c = 0.64, CHCl3)}. IR (CHCl3): 3409, 3018, 1716, 1440 cm-1. 1H NMR (500 MHz, CDCl3): δ = 7.83 (br s, 1 H, NH), 7.52 (d, J = 7.8 Hz, 1 H), 7.35 (d, J = 8.0 Hz, 1 H), 7.19 (m, 1 H), 7.14 (m, 1 H), 3.82 (br s, 1 H), 3.60 (br d, 1 H), 3.26 (d, J = 11.2 Hz, 1 H), 3.18 (dd, J = 4.8, 11.4 Hz, 1 H), 2.99-3.06 (m, 1 H), 2.86 (dd, J = 3.2, 13.2 Hz, 1 H), 2.79 (br d, J = 12.2 Hz, 1 H), 2.70 (dd, J = 3.8, 11.4 Hz, 1 H), 2.57 (d, J = 11.2 Hz, 1 H), 2.23-2.31 (m, 1 H), 1.84-1.91 (m, 1 H), 0.84 (t, J = 7.6 Hz, 3 H). 13C NMR (100 MHz, CDCl3): δ = 211.5 (C), 136.3 (C), 132.6 (C), 126.9 (C), 121.9 (CH), 119.6 (CH), 118.3 (CH), 111.1 (CH), 108.7 (C), 79.4 (C), 65.9 (CH2), 59.7 (CH3), 51.8 (CH2), 42.8 (CH2), 30.7 (CH2), 21.9 (CH2), 6.9 (CH3). Anal. Calcd for C17H20N2O2: C, 71.81; H, 7.09; N, 9.85. Found: C, 71.62; H, 6.95; N, 9.66. MS (ESI): m/z = 285.33 [M+ + 1].

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Column: CHIRAL CEL-OJ-H; mobile phase: PE-i-PrOH (90:10); flow rate: 0.5 mL/min; t R (minor isomer): 48.6 min, t R (major isomer): 79.0 min.