Don’t get me wrong, I love Organic Syntheses, but sometimes the lack of a proper rationale is frustrating. In all likelihood, we will never know how this reaction was conceived. No, it is not a misprint, I have triple-checked it.
Org. Synth. 1963, Coll. Vol. 4, 451-453. (Available here.)
How do you think the discussion went? “OK guys, we are going to need about 70 grams of 5-ethyl-2-methylpyridine. What do you say? Heat some acetaldehyde with ammonia, perhaps with a buffer or something? Just a hunch.”
What is the mechanism? 🙂
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They reference the Bayer-Chichibabin reaction. The Chibabin pyridine synthesis https://en.wikipedia.org/wiki/Chichibabin_pyridine_synthesis is a well established synthesis of pyridines from 3 eq of aldehyde in ammonia or some such derivative. Paraldehyde decomposes to 3x acetaldehyde, it is simply used for convenience as acetaldehyde is hard to handle. As it was a known reaction at the time, the OrgSynth simply gives a useful procedure. Mind-bending, I think not.
Thanks for clearing that out. Still, if one had asked you to synthesize the title compound, would you after a thorough retrosynthetic analysis have come to the same conclusion? That is, to mix acetaldehyde with ammonia and heat? If so, then I am fairly impressed… you should work for me 🙂
the Wikipedia article gives mechanism of pyridine synthesis which is related but actually a different mechanism from the above example.
Here we have a different substitution pattern of the product, and 4 molecules of aldehyde (not 3 like in the Wiki mechanism). It is most likely a stepwise cyclodimerisation of crotonaldehyde with 1 molecule of ammonia, and the product aromatizes by C=C migration under the rather harsh conditions