Many pathways to natural products i

Many pathways to natural products involve steps which remove portions of the carbon skeleton. Although two or more carbon atoms may be cleaved off via the reverse aldol or reverse Claisen reactions mentioned above, by far the most common degradative modification is loss of one carbon atom by a decarboxylation reaction. Decarboxylation is a particular feature of the biosynthetic utilizationof amino acids, and it has already been indicated thatseveral of the basic building blocks (e.g. C6C2N andindole.C2N) are derived from an amino acid via loss of thecarboxyl group. This decarboxylation of α-amino acids is also a PLP-dependent reaction (compare transamination) and is represented as in Figure 2.16(a). This similarly depends on imine formation and shares features of the transamination sequence of Figure 2.15. Decarboxylation of the intermediate aldimine is facilitated in the same way as loss of the α-hydrogen in the transamination sequence.The protonated nitrogen acts as an electron sink and the
conjugated system allows loss of the carboxyl proton with subsequent bond breaking and loss of CO2. After protonation of the original α-carbon, the amine (decarboxylated
amino acid) is released from the coenzyme by hydrolysisof the imine function; PLP is regenerated.

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Many pathways to natural products involve steps which remove portions of the carbon skeleton. Although two or more carbon atoms may be cleaved off via the reverse aldol or reverse Claisen reactions mentioned above, by far the most common degradative modi fi cation is loss of one carbon atom by a decarboxylation reaction. Decarboxylation is a particular feature of the biosynthetic utilizationof amino acids, and it has already been indicated thatseveral of the basic building blocks (eg C6C2N andindole.C2N) are derived from an amino acid via loss of thecarboxyl group. This decarboxylation of α-amino acids is also a PLP-dependent reaction (compare transamination) and is represented as in Figure 2.16 (a). This similarly depends on imine formation and shares features of the transamination sequence of Figure 2.15. Decarboxylation of The Intermediate Aldimine is facilitated in The Same Way As The Loss of alpha-hydrogen in The transamination Sequence.The Nitrogen protonated Acts As an Electron Sink and The
Loss of The carboxyl conjugated System allows Proton with Subsequent Bond breaking and Loss of CO2. After protonation of The Original alpha-carbon, The Amine (decarboxylated
amino acid) is Released from The coenzyme by Hydrolysisof The imine function; PLP is regenerated.