The Org. Lett. paper by Michiko Sasaki and co-workers from Japan contains a reminder of how unusual N-phenyl-1,2,4-triazolinedione (PTAD) is as a dienophile. PTAD is one of the most reactive participants in the Diels-Alder process described to date; attempts to understand its reactivity date back many years. If you are keen to find out more about the strange behavior of PTAD in Diels-Alder reactions, I would recommend the 1998 paper by Houk and co-workers:
http://pubs.acs.org/doi/abs/10.1021/ja982050y
The extreme characteristics of PTAD are seen, for instance, in how it reacts with dienes that cannot easily adopt the requisite s-cis conformation. PTAD engages these molecules, leading to products of 1,4-addition – and this is just one of the pathways that were examined by Houk. Coming back to the Sasaki paper, I liked the comparative study with N-methylmaleimide (NMM): the reaction between one of the allene-containing silyl enol ethers and PTAD goes smoothly at -80oC, whereas NMM clearly does not have “enough” in it even at room temperature. Those adjacent nitrogen lone pairs… Pure magic.
