46 Pyridines
formal aldol condensation, and the 1,6-bond in a formal enamine formation, though the precise order of events is not
known (and may involve the enolised form of the 1,3-dicarbonyl component).
The C 2 N component can also be a primary enamine derived from a 1,3-diketone; in the example, the enamine derived
from pentane-2,4-dione when combined with pentane-2,4-dione gives 3-acetyl-2,4,6-trimethylpyridine.
Similarly, the C 2 N component can be the primary enamine derived from a 1,3-keto-ester; the next example, aimed
at a 4,5,6-unsubstituted pyridine, uses 1,3,3-tri(ethoxy)prop-1-ene (the acetal enol ether of malondialdehyde) as a
synthetic equivalent, because the simplest 1,3-dicarbonyl compound, malondialdehyde, is too unstable to be stored
and used.
Vinamidinium salts, easily prepared from carboxylic acids RCH 2 CO 2 H with POCl 3 and DMF, serve as 2-substituted
malondialdehyde equivalents.
The fi nal variation that we deal with here is the Bohlmann-Rahtz reaction in which the enamino-ketone reacts with
an yne-one as the 1,3-dicarbonyl equivalent. The fi rst step is conjugate addition to the alkyne so this dictates the regi-
ochemistry of combination.