But have you ever seen a [3,9]? Can you picture what such a system might look like? Well, Wegner, Kessler, and Neuburger (Basel) would like you to try. As they report in their latest JACS ASAP, they've posited a rather cool [3,9] to explain the products of a tandem IEDDA (inverse-electron-demand Diels-Alder) followed by a cyclopropanation:
|Wegner et. al., JACS 2012|
Even more interesting? The catalyst is a bidentate diboron Lewis acid. That's right: each boron atom grabs hold of a single nitrogen atom in the diazine to promote the D-A (You don't see that every day!) The authors make a tiny squeak at the end of their paper regarding possible future synthesis of 'unsymmetrical' versions of their catalyst. I bet that that'll be tough, given the proposed transition-state binding model - one ring flat, one sticking straight up at the incoming dienophile.
My guess? They'll likely try to hang a helically-chiral group on one side, or else staple a PHOX or Evans auxiliary on.
Time will tell.