J Am Chem Soc. 2014 Feb 19;136(7):2735-8. doi: 10.1021/ja412444d. Epub 2014 Feb 10.
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Why nature eschews the concerted [2 + 2 + 2] cycloaddition of a nonconjugated cyanodiyne. Computational study of a pyridine synthesis involving an ene-Diels-Alder-bimolecular hydrogen-transfer mechanism.