I don’t want you to think that I have been reading only Diels-Alder literature of late, but I found the recent JACS paper from Michael Doyle’s lab to be quite remarkable. The authors report a mild conversion of diene-tethered diazo compounds to the corresponding [4+2] cycloaddition products. Prior to the Doyle work, diazo compounds were not known to partake in cycloadditions of this kind. When I read stuff like this, I inevitably ask myself: how many times such a reaction happened (unbeknownst to the experimentalist) in the past? You have to agree that people must have studied metal-catalyzed intramolecular cyclopropanation processes in which a diene was evaluated as the carbenoid acceptor. But then maybe no one bothered to evaluate dienes in that capacity. Otherwise, how can one possibly imagine that a room temperature pathway to the Diels-Alder adducts shown below has remained veiled for so long? There is some gold-based catalysis described in this paper as well, but it is the room temperature transformation in chloroform that is surprising.

http://pubs.acs.org/doi/abs/10.1021/jacs.5b12877