2.2 Enzymatic approach
In 2018, Turner et al. reported a chemo-enzymatic synthesis of
substituted pyrazines using an amino transaminase (S-selective, ATA-113,
Codexis [24]) in the presence of a suitable amine donor, which
mediated the key amination of the 1,2-diketone precursor to
α-aminoketones that underwent oxidative dimerization to the final
product (Figure 2)[25]. In the case of pyrazines, the chirality of
the amine group is irrelevant for the synthesis of the aromatic
heterocycle core. All reactions were carried out at room temperature
with isopropyl amine as the amine donor. Substrates were exhausted after
72 hours and pyrazine was extracted in pure form from the aqueous phase,
but the yield of pyrazine was still moderate at 50-65% for symmetric11, deriving from cyclohexane-1,2-dione and 8 deriving
from diacetyl, and 32% for the non-symmetric 12 from
pentane-2,3-dione (Figure 1). The explanation for by-product formation
requires further studies e.g. identification of possible double aminated
by-products in the aqueous phase or extraction problems. The question
whether dimerization to the pyrazine core occurred in the aqueous buffer
or in organic solvent after extraction remains elusive and needs further
investigations.
On the one hand, diketones can be produced by different chemical steps
from various building blocks (Figure 2). On the other hand, there are
also biological options, since the biological route is known to be via
acetoin 10 (3-hydroxy-butan-2-one, Figure 2). Recent
developments have been made with a ’new’ ThDP-dependent lyase, which is
able to synthesize acetoin building blocks from smaller subunits e.g.
pyruvate and activated acetaldehyde enzymatically. Pohl et al.showed with engineered PDC from Zymomonas mobilis (Zm PDC)
and Acetobacter pasteurianus (Ap PDC) they can obtain
excellent yields of 61-98% from the combinations of arylated aldehydes
with 1) 3-oxobutanoic acid and 2) alkyl aldehydes. At the same time, it
was published how to obtain excellent pyrazine yields from either
acetaldehyde with SucA [ThDP-dependent E1 subunit of the
α-ketoglutarate dehydrogenase complex from Escherichia coli(E.coli )) ], or from pyruvate (3-oxopropionic acid) with
an cellobiose dehydrogenase (CDH )[26].
In industrial processes, cost is one of the determining factors. The
price of ATAs (e.g. ATA-113) is much higher than chemical amination with
simple ammonia. The enantiomeric integrity of the amination is of great
importance e.g. for pharmaceuticals; however, for the production of
planar heterocycles the enantioselectivity of the amination is
redundant, whereas high regioselectivity of the condensation is
essential.
In this context, non-symmetric pyrazines could be synthesized
regioselective with ATA- 113 [25] in an one-pot approach, whereas a
standard chemical synthesis would only allow access to symmetric or a
mixture of non-symmetric pyrazines.