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Synlett 2008(12): 1894-1896  
DOI: 10.1055/s-2008-1078502
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

First Enantioselective Synthesis of a Hydroxyindolizidine Alkaloid from the Ant Myrmicaria melanogaster

Naoki Toyooka*a, Dejun Zhoua, Hideo Nemotoa, Yasuhiro Tezukab, Shigetoshi Kadotab, Tappey H. Jonesc, H. Martin Garraffod, Thomas F. Spanded, John W. Dalyd
a Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, Sugitani 2630, Toyama 930-0194, Japan
Fax: +81(76)4344656; e-Mail: toyooka@pha.u-toyama.ac.jp;
b Institute of Natural Medicine, University of Toyama, Sugitani 2630, Toyama 930-0194, Japan
c Department of Chemistry, Virginia Military Institute, Lexington, Virginia 24450, USA
d Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, DHHS, Bethesda, MD 20892, USA
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Publikationsverlauf

Received 4 April 2008
Publikationsdatum:
19. Juni 2008 (online)

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Abstract

The first enantioselective synthesis of the recently reported ant alkaloid 1 has been achieved starting from commercially available lactam 3 in seven steps and 25% overall yield. The proposed structure of the natural product was confirmed by comparison with synthetic 1 and its absolute configuration established as 3S,5R,8S,9S.

Key words

hydroxyindolizidine - venoms of ant - Myrmicaria melanogaster - chelation control - new ant alkaloid

9

Spectral Data of 1
IR (neat): 3482, 2956, 2872, 1513, 1457, 1378, 1234, 1130, 1054, 970, 826 cm-¹. ¹H NMR (500 MHz, CDCl3): δ = 0.90 (3 H, t, J = 7.2 Hz), 0.91 (3 H, t, J = 7.2 Hz), 1.17-1.49 (11 H, br m), 1.53-1.62 (4 H, m), 1.68-1.86 (3 H, m), 2.25 (1 H, t-like, J = 9.8 Hz), 2.40 (1 H, m), 2.75 (1 H, t-like, J = 8.5 Hz), 3.03 (1 H, d, J = 10.3 Hz), 3.74 (1 H, d, J = 9.8 Hz). ¹³C NMR (75 MHz, CDCl3): δ = 14.27 (q), 14.50 (q), 19.16 (t), 22.98 (t), 25.92 (t), 26.67 (t), 28.75 (t), 28.98 (t), 32.18 (t), 37.83 (t), 39.43 (t), 60.43 (d), 64.07 (d), 65.45 (d), 70.06 (d). MS: m/z (%) = 239 [M+], 196 (100). HRMS: m/z calcd for C15H29ON: 239.2103; found: 239.2121. [α]D ²6 -48.92 (c 0.62, CHCl3).

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Spectral Data of 9
Mp 39-40 ˚C. IR (KBr): 3343, 2955, 2933, 2859, 2784, 1466, 1378, 1259, 1207, 1194, 1158, 1123, 1062, 1019 cm-¹. ¹H NMR (500 MHz, CDCl3): δ = 0.89 (3 H, t, J = 7.3 Hz), 0.91 (3 H, t, J = 7.3 Hz), 1.18-1.36 (8 H, m), 1.42 (1 H, m), 1.45-1.64 (4 H, m), 1.80-1.84 (2 H, m), 1.92-2.08 (3 H, m), 2.15 (1 H, br), 2.66 (1 H, t-like, J = 8.5 Hz), 3.43 (1 H, br). ¹³C NMR (75 MHz, CDCl3): δ = 14.24 (q), 14.53 (q), 19.55 (t), 22.88 (t), 27.91 (t), 29.22 (t), 29.68 (t), 30.87 (t), 34.08 (t), 37.51 (t), 39.37 (t), 62.10 (d), 63.75 (d), 72.87 (d), 73.05 (d). MS: m/z (%) = 239 [M+], 182 (100). HRMS: m/z calcd for C15H29ON: 239.2103; found: 239.2127; [α]D ²6 -53.55 (c 1.05, CHCl3).

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For proof of identity of (-)-1 with the natural ant alkaloid (10a, see ref. 5), a Shimadzu QP-2010 GC/MS equipped with an RTX-5 column (30 m × 0.25 mm i.d.) was used employing a program of 60 ˚C to 250 ˚C at 10 ˚C/min. Here, both synthetic (-)-1 and natural product 10a coeluted and had identical mass spectra. Synthetic (-)-1 also had a retention time and mass spectrum identical to the first eluting isomer, (±)-1, of the mixture of diastereomers synthesized in a nonstereoselective manner by Jones et al.5 For the determination of the absolute configuration of 10a, an HP 5890 GC with flame-ionization detection was used with He carrier gas and a head pressure of 20 psi. This was fitted with a chiral permethylated β-cyclodextrin column (SGE, 30 m × 0.22 mm i.d., 0.25 µm film thickness) operated with a program of 100 ˚C at a rate of 1 ˚C/min. Using these conditions, the Myrmicaria melanogaster ant hydroxyindolizidine 10a and (-)-1 each coeluted with the slightly more slowly eluting enantiomer (156.5 ˚C) of the (±)-1 racemate present in Jones’ synthetic mixture, whereas the (+)-1 enantiomer, from Jones’ synthetic mixture eluted at 156.0 ˚C.