Plant Alkaloids Atallah F. Ahmed

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2. Water-insoluble polypeptide alkaloids (~ 80% of total alkaloids). They contain a cyclized tripeptide fragment bonded to lysergic acid via an amide linkage. Based on the nature of the three amino acids, these structures can be subdivided into three groups: Ergotamine gota e ggroup oup -Ergoxine go e group g oup -Ergotoxine goto e ggroup. oup Hydrolysis Atallah F. Ahmed, products PhD 2010 of Ergotamine 23 The amino acids involved in the tripeptide part are alanine, valine, leucine, isoleucine, phenylalanine, proline, and β-aminobutyric acid, in various combinations. All contain proline and an αhydroxy-α-amino acid. Ergotamine group by hydrolysis (-)-lysergic acid + proline + phenylalanine + alanine pyruvic acid + ammonia. Ergotoxine group by hydrolysis ( (-)-lysergic ) lysergic acid + proline + valine dimethylpyruvic acid + ammonia. Atallah F. Ahmed, PhD 2010 24 5/30/2010 12
The isomeric series derived from (+)-isolysergic acid (C-8 epimer of lysergic acid) as ergometrinine, ergotaminine, ergoxinine, and ergotoxinine groups g p are less medicinally y active. Ergot alkaloids related to (-)-Iysergic acid have the suffix –ine levorotatory more medicinally active. Ergot alkaloids related to (+)-isolysergic acid have the suffix –inine dextrotatory less medicinally active. Conversion of -ine ine -inine inine isomer is done by refluxing its methanolic solution or treatment with alc. alkali. The conversion of -inine -ine isomer is done by refluxing its AcOH solution or treatment with alc. H 3PO 4. Atallah F. Ahmed, PhD 2010 Since (+)-inine isomers are less active, a decrease in potency may occur during long storage of the active (-)-ine alkaloids. The fresh active alkaloid show blue fluorescence under UV light UV maxima at 318 nm, while alkaloids suffered from excessive light or UV exposure (in dil acidic soln.) change into lumi-alkaloids (addition of H 2O into 9,10-double bond) UV maxima at 285 nm (no fluorescence) inactive alkaloid. H C 8 O R 25 N CH CH3 Ergot-ine Alkaloids Ergot-inine Alkaloids (e.g. ergometrine) (e.g. ergometrinine) Hydrolysis y y lysergic y g acid isolysergic y g acid Optical activity Levorotatory (−) Dextrotatory (+) Pharmaco. activity active inactive + hot alc. alkali Isomerized to ergot-inine − + hot H3PO4 − Isomerized to ergot-ine Stability Unstable in light and Atallah radiation F. Ahmed, PhD 2010 stable 26 HO 10 5/30/2010 13
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Plant Alkaloids Atallah F. Ahmed

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