A couple of weeks ago, I finished teaching the synthesis of pyridine and its derivatives in my 4th year synthesis class. Whenever I present this material, I can’t help but appreciate the value of N-oxidation. While there are N-oxides of other heterocycles (thiazole N-oxide stands out for its interesting properties), nowhere else do I feel the same level of enthusiasm about N-oxidation than in the case of pyridine. And new applications attributed to the role of N-oxides keep coming out! The one that attracted my attention comes from the lab of Nuno Maulide. This chemistry, described in a recent Angewandte paper, documents thermal modification at the 3-position of pyridine. According to the authors, the reaction likely involves a series of [2,3]-sigmatropic rearrangements. I think this “merry-go-round” sort of activation is amusing. Given the value of 3-substituted pyridines in drug discovery, this simple reaction should find many applications.
Intermediate D is reminiscent of intermediates seen in the Buchner ring expansion. Given the highly reversible nature of the Buchner reaction, it would be interesting to see the computed energies of azepine intermediates.
I think you are making a great point here!
DMF gradually decomposes at reflux (154C) so heating a react mix in wet DMF to 200C, especially in microwave (known for local overheating effects) is a rather messy way of generating dimethylamine plus carbon monoxide and dimethylamine formate. It would be better if they had used propiolic ester with a thermolyzable ester group, i.e. tBu or PMB = in that way maybe they could use a less problematic solvent like chorobenzene, and decrease the ratio of propiolic ester from four equivs down to two equivs or less
This is true, DMF is a messy solvent for thermal activation, especially under such harsh conditions.
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Actually Dimethylacetamide is much, much better behaved at high temperature and its boiling point is not greatly different than DMF. I always use DMA instead of DMF at a reaction temperature above 120-130 C.
Thanks, I think this is a good point.