Triphenylphosphine → N,N-dimethyl-2-(3-nitro-2-pyridinyl)ethenamine

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Particularly key to an efficient synthesis of 3-AP or 3-AMP is the high-yield introduction of a methyl or vinyl group, preferably vinyl, at the 2 position of the pyridine moiety utilizing a Heck or Stille vinylation reaction or a methylation reaction under Suzuki conditions. These reactions proceed at the 2-position of a pyridine moiety in high yield (generally, greater than 50% and in most instances, greater than 70%). The advantage of the vinylation reaction is that the 2-vinyl group may be readily converted to a 2-carboxaldehyde group for further conversion to the 2-thiosemicarbazone using a high yield ozonolysis reaction conducted in a polar solvent, such as methanol. The preferred vinylation reaction is a Heck or Stille vinylation reaction performed utilizing a reagent selected from among the following: Bu3SnCH=CHR3, (OH)2B--CH=CHR3, ClZnCH=CHR3 and XMgCH=CHR3. The vinylation reaction takes place in the presence of triphenylphosphine (PPh3) and/or tetrakis(triphenylphosphine) palladium [Pd(PPh3)4 ] usually in a solvent such as toluene in the presence of heat. The advantages of the methylation reaction under Suzuki conditions over the two step methylation of the prior art include a greatly improved yield, reduction in the number of steps and therefore handling required and ease in scaling the synthesis up for commercial production. Alternatively, to introduce a vinyl group at the 2-position of 3-nitropyridine, one may advantageously react 2-methyl-3-nitropyridine with dimethylforamamidedimethylacetal (DMFDMA) to produce 2-dimethylamino vinyl-3-nitropyridine compound 3a, followed by oxidation with sodium periodate (NaIO4) to produce carboxaldehyde 4 in an unexpectedly high 84% yield.

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