Decarboxylation Reactions
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 utilization
of amino acids, and it has already been indicated that
several of the basic building blocks (e.g. C6C2N and
indole.C2N) are derived from an amino acid via loss of the
carboxyl 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 hydrolysis
of the imine function; PLP is regenerated.
β-Keto acids are thermally labile and rapidly
decarboxylated in vitro via a cyclic mechanism which
proceeds through the enol form of the final ketone
[Figure 2.16(b)]. Similar reactions are found in nature,
though whether cyclic processes are necessary is not
clear. ortho-Phenolic acids also decarboxylate readily
in vitro and in vivo, and it is again possible to invoke