Biomimetic synthesis and HPLC–ECD analysis of the isomers of dracocephins A and B

Viktor Ilkei,
András Spaits,
Anita Prechl,
Áron Szigetvári,
Zoltán Béni,
Miklós Dékány,
Csaba Szántay Jr,
Judit Müller,
Árpád Könczöl,
Ádám Szappanos,
Attila Mándi,
Sándor Antus,
Ana Martins,
Attila Hunyadi,
György Tibor Balogh,
György Kalaus (†),
Hedvig Bölcskei,
László Hazai and
Tibor Kurtán

Full Research Paper
Published 24 Nov 2016

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1. Graphical Abstract
2. Figure 1: Structures of (±)-naringenin, (±)-dracocephins A1–A4 and B1–B4 with the indication of the absolute ...
  
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3. Scheme 1: Proposed biosynthetic route to dracocephins A and B.
  
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4. Scheme 2: Synthesis of (±)-5-ethoxypyrrolidine-2-one ((±)-9).
  
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5. Scheme 3: Synthesis of (±)-dracocephins A and B (±)-2a–d and (±)-3a–d and the elution order of stereoisomers ...
  
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6. Figure 2: a) Chiral HPLC–UV and HPLC–ECD traces of dracocephins A (2a–d) using Chiralpak IC column with the e...
  
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7. Figure 3: Structure and population of the low-energy CAM-B3LYP/TZVP PCM/CHCl₃ conformers (>2%) of (R)-1.
  
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8. Figure 4: Experimental HPLC–ECD spectrum of (R)-naringenin ((R)-1) compared with the Boltzmann-weighted ECD s...
  
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9. Figure 5: Structure and population of the low-energy CAM-B3LYP/TZVP PCM/MeCN conformers (>2%) of (2R,5’’R)-2d....
  
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10. Figure 6: Structure and population of the low-energy CAM-B3LYP/TZVP PCM/MeCN conformers (>2%) of (2R,5’’S)-2a....
  
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11. Figure 7: Experimental HPLC-ECD spectra of (2R,5’’S)-2a (second eluted stereoisomer) and (2R,5’’R)-2d (fourth...
  
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12. Figure 8: Experimental HPLC–ECD spectra of (2R,5’’S)-2a and (2R,5’’R)-2d compared with the Boltzmann-weighted...
  
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13. Figure 9: a) Chiral HPLC–UV and HPLC–ECD traces of dracocephins B1–B4 3a–d using a Chiralpak IC column with t...
  
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14. Figure 10: Structure and population of the low-energy CAM-B3LYP/TZVP PCM/MeCN conformers (>2%) of (2R,5’’R)-3c....
  
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15. Figure 11: Structure and population of the low-energy CAM-B3LYP/TZVP PCM/MeCN conformers (>2%) of (2R,5’’S)-3b....
  
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16. Figure 12: Experimental HPLC-ECD spectra of 3b (first eluted stereoisomer) and 3c (third eluted stereoisomer) ...
  
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17. Figure 13: Experimental HPLC-ECD spectra of 3b and 3c compared with the Boltzmann-weighted ECD spectra compute...
  
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18. Figure 14: Proposed mechanism for the formation of dracocephins A and B (2a–d and 3a–d) starting from (±)-9.
  
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